Hand-considered hypotheses, 4701–4800
Building services, written one at a time. Air in ducts and the fans that move it; water in closed loops and in domestic pipes; the drain inside the building; the chiller, the tower and the refrigerant; ventilation and what it removes; the body’s account of a room; light and the eye; and the controls that decide all of it. Each entry carries the logical form chosen for it, printed above the claim, so that the variety of argument can be checked against the text rather than taken on trust. None has been tested; no citation is given because none is being invoked, and every quantity offered as an estimate is marked as one.
Air, ducts and fans
Test-defined quantity
H4701. What is called a duct’s leakage class is a property of the test as much as of the duct: the flow through a crack of fixed geometry passes from viscous to inertial within the range of pressures used, so the exponent is not constant and the inferred leakage area moves with the test point. A three-point test separates the two, because better sealing moves the whole curve while a change of regime bends it, and only the fitted exponent tells them apart.
Misidentified index
H4702. Sizing a run by equal friction equalises loss per metre, not total pressure at the terminals, and the index run is then often the branch with the most fittings rather than the longest one. The symptom is a starved terminal fed by generous ductwork, and the test is a total-pressure traverse at the terminals rather than a static reading at the fan.
Inference bias
H4703. Install a terminal box within a few diameters of an elbow and its airflow reading is biased, because the coefficient it uses was established on a straight inlet and the velocity profile it now sits in is not the one that produced it. The bias does not announce itself as a fault: it shifts the minimum flow the box holds, so the ventilation intended for the zone arrives only if the box was calibrated where it was installed.
Masking by control
H4704. Under static-pressure reset the fan raises speed to hold setpoint, so a fouled filter or a slipping belt is invisible on the pressure gauge and appears only as a slow rise in fan power. Trending power against damper position separates the two causes, and pressure alone cannot, which is why a regime that watches only the setpoint misses the failure until the fan is at full speed.
Non-scaling offset
H4705. Affinity laws assume that everything measured at the fan scales with speed squared, but the transmitter’s zero offset and the duct’s own stack pressure are additive terms that do not scale at all. Below about a third of design speed (an estimate) those terms become a large fraction of the reading, so the inferred pressure rise is too high and the inferred efficiency too low, and a trend fitted with a cube through the whole range will report a false decline at the bottom end.
Sublinear response
H4706. Cold jets do not drop in proportion to their supply velocity, because spreading dilutes momentum and buoyancy per unit volume together, so more air is a weaker remedy for a dumping diffuser than the throw curves suggest. The test is a two-flow series at fixed supply-to-room temperature difference: the drop point should move less than the flow ratio, and if it moves more, the diffuser’s own geometry is setting the separation.
Selection effect
H4707. Reported savings from duct sealing are drawn from buildings selected because a screening test found them leaky, so the published mean describes the screened population rather than the building stock. A randomised trial that seals half of an unscreened sample would give the expected saving. The gap between the two numbers is the cost of the screening.
Boundary ownership
H4708. Leakage into a return plenum is not a loss at all, because the escaped air is drawn back through the coil, while the same leak in a duct outside the envelope is a total loss of cooled air. Two ducts with identical leakage class therefore carry different penalties, and a leakage number quoted without the path it leaks into has no energy meaning.
Non-area scaling
H4709. Breakout noise is quoted per unit length or per unit area, but the radiated power is set by the panel’s coincidence frequency and radiation efficiency, which depend on the smaller cross-sectional dimension rather than on the surface. A flat rectangular duct and a round one of equal area therefore do not radiate equally, and a silencer chosen on insertion loss per metre can underperform in the flat duct.
Angle-dependent ceiling
H4710. Lining a duct more thickly stops helping at a point set by the grazing angle at which sound travels along it, because the wave penetrates only a fraction of a wavelength into the lining. The ceiling is set by the lowest frequency that must be attenuated, so a thicker lining chosen without regard to the angle buys weight and not insertion loss.
Cycle integral
H4711. Cycling changes what a coil dehumidifies, because the coil is left wet at every shutdown and returns that moisture when the fan restarts. The net removal depends on the fraction of the cycle the coil is wet and the fan is running. A unit with an excellent steady apparatus dew point can therefore be the poorer dehumidifier.
Optimisation threshold
H4712. Filter change points set by pressure drop are not cost-minimising points, because loading at constant speed reduces airflow and can reduce fan power while loading at constant pressure raises it. The correct change point is therefore earlier than the pressure limit under constant-pressure control and later under constant-speed control, and a single maintenance rule cannot be right for both.
Hydronic circuits and pumps
Authority traded for pump energy
H4713. Differential-pressure reset is credited with pump savings, but the pressure it removes is the margin that gave the critical valve its authority: at low load that valve sits wide open and its characteristic no longer controls, so the zone’s temperature is set by the reset schedule rather than by the valve. The other valves gain authority as the critical one loses it, and the loop that was stable at design is a different loop at part load.
Reversal at a boundary
H4714. In a primary-secondary plant the decoupler reverses at part load, and from that moment the secondary supply is fed by primary return rather than by the chiller’s leaving water. The supply sensor sits on the wrong side of the bridge during those hours, so the recorded temperature is the one the plant is trying to correct and not the one it is delivering.
Self-cancelling margin
H4715. Head margin chosen at the pump does not survive installation: the extra pressure moves the operating point out along the system curve, raising flow and lowering the coil’s temperature difference. The margin is consumed by the system. The lower difference then brings the chiller to its minimum flow at a lower load, so oversizing costs part-load stability as well as pump energy.
Externally imposed bound
H4716. What limits a chilled-water pump’s turndown is not the motor or the drive but the evaporator’s minimum flow, below which the refrigerant control becomes unstable and the machine trips on low temperature. The pump may be perfectly capable of going slower, and the plant’s real minimum load is a property of the chiller’s bundle rather than of the pump curve.
Stratification threshold
H4717. Thermoclines are usually credited to insulation, but the thickness of the transition layer is set by the inlet’s Richardson number, so a tall tank with a strong inlet jet mixes as thoroughly as a short one. Insulating the shell cannot restore a thermocline the inlet destroyed, and the test is to vary inlet velocity at fixed geometry and watch the outlet temperature’s decay.
Reference-point error
H4718. Discharge pressure is often held at a gauge value, but the quantity that drives flow through a coil is the difference between supply and return, and the gauge value includes whatever fill pressure the expansion tank imposes. Two buildings with identical coils but different fill pressures need different discharge setpoints, and a schedule copied between them over-pressurises one and starves the other. The control variable should be the differential at the index coil, and the gauge reading is a proxy that works only when the fill pressure is the design one.
Degeneracy
H4719. Roughness and diameter are degenerate when only the total drop at one flow is known, because many pairs produce the same drop. A second flow separates them, since roughness enters the friction factor at high Reynolds number while diameter enters both the velocity and the factor, and a design that infers internal condition from a single test is fitting one equation to two unknowns.
History dependence
H4720. Cavitation is treated as a steady margin, but the nuclei that start it come from air dissolved at filling, so a system that cavitates in its first week may run quietly later without any change to the pump. The relevant variable is the water’s air content at the suction, and a test that fills, runs and re-tests separates a genuine suction deficit from a commissioning artefact.
Conditional independence
H4721. Pressure independence has a floor: below a minimum differential a pressure-independent balancing valve behaves as a fixed orifice, and when the pump is reset downward the branches nearest it keep their flow while the far ones lose theirs. The valves that promise independence therefore fail at exactly the moment the reset is meant to save energy.
Release point missed
H4722. Vents are placed at the highest point, where air collects, but air is released where the pressure is lowest, which is the pump’s suction, and a vent at the top cannot remove it before the impeller. A separator at the suction sits downstream of the release point and upstream of the pump, and a system with only a high-level vent passes bubbles through the pump and removes what survives.
Nonlinear indicator
H4723. Strainer pressure drop rises with the inverse square of open area, so a basket half blocked shows about four times the clean drop. The indicator is therefore least sensitive exactly where the remaining margin is smallest. A schedule keyed to a pressure threshold cleans late.
Domestic water and drainage inside the building
Storage-delivery distinction
H4724. Legionella control is judged at the calorifier because the temperature there is easy to measure, but the risk lives in the branch, whose surface temperature is set by insulation, ambient and draw pattern. A cylinder held well above the disinfection threshold can coexist with a colonised dead leg, and only a sample taken at the branch after a stagnation period tests the thing that matters.
Steady-test blind spot
H4725. Certification tests a thermostatic mixing valve on steady flows, but its sensor has thermal mass, so a step in draw passes a temperature excursion before the valve responds. The fail-safe claim is a claim about the transient, and a test that steps the flow from full to a trickle, rather than holding each condition, is the only one that exercises it.
Capacity for a rate
H4726. Trap seals are quoted as a depth, but the depth is a capacity and the event that empties it is a rate: the stack’s transient reaches the water through the outlet leg, whose length and friction delay and attenuate it, so two traps of equal depth can differ in survival. The catalogue gives the capacity and not the rate, and the failure is decided by the rate.
Set-point mismatch
H4727. Arrestor precharge is set at installation against fill pressure, but the pressure it sees varies with the expansion tank’s state and the relief valve’s setting. An arrestor correct at fill is undersized at the top of the range, and the peak it was meant to absorb reappears precisely when the system is hottest.
Wrong objective
H4728. Equal return temperatures are not equal waiting times: the riser loses far more heat per unit length than a short branch, so a loop trimmed to a uniform return leaves the long branch cold and the short one hot. The correct objective is the time to temperature at each outlet. Balancing to that will not produce uniform return temperatures, and balancing to uniform return will not produce equal times.
Safeguard as pressure source
H4729. Backflow protection can become the path for backflow: a heater downstream of the check valve turns the closed system into a pressure source, and the expansion tank’s location decides which side of the valve sees the higher pressure. Placed on the heater side the tank gives the expanding water somewhere to go; placed on the supply side it leaves the valve to be forced open by the expansion it is meant to contain.
Assumption-dependent rating
H4730. Low-flow fixtures are rated at a supply pressure of a few tenths of a megapascal (an estimate) that many gravity-fed installations never reach, so the flow in service is lower than the label and the spray or flush is worse than the rating implies. The saving attributed to the fixture is then partly a saving of performance, and a building with a low residual pressure can use less water than specified while failing the test the specification assumed.
Residence-time constraint
H4731. Sizing greywater storage against the driest spell makes the tank large and its turnover slow, and slow turnover is what lets the residual disinfectant decay and the biofilm establish. A smaller tank that overflows in wet weeks stores less and delivers better quality, so sizing for yield alone trades a water saving for a biological risk.
Symptom misreading
H4732. An undersized vent stack does not announce itself with a gurgle; it throttles the air behind the slug and the fixture drains slowly, which reads as a blockage. The tell is that the slow fixture is the one furthest from the stack and that rodding changes nothing. The test is a pressure measurement in the vent, not another rodding eye.
Off-design shear
H4733. Minimum-slope rules are written to give a self-cleansing velocity at the design flow, but that velocity is computed for the pipe running near full, and a main sized for a future peak runs at a shallow depth for most of its life, where the hydraulic radius is small and the shear on the invert falls below the transport threshold. The grade that cleans a branch at its design flow therefore silts a main at its actual flow, and such rules are not conservative in the direction their users assume.
Range-end selection
H4734. Meters are selected by maximum flow, but accuracy at the low flows that dominate consumption is set by starting flow. A meter sized for the peak therefore under-reads the trickle that makes up most of the volume. The error is systematic and one-signed, so reported consumption is low by an amount that grows with the ratio of peak to average demand.
Chillers, towers and refrigeration
State-dependent limit
H4735. Minimum load is quoted as a fraction of nominal capacity, but the limit is a minimum refrigerant flow and a minimum lift, so it falls as the condenser water gets colder. A machine that cannot go below a stated fraction on a warm day may run stably at a smaller one on a cool day, and a staging sequence that assumes a fixed minimum starts a second machine it does not need.
Wrong decision variable
H4736. Deciding the sequence on load alone is right only while the machines are equal and the condenser water is fixed; with unequal machines and a floating condenser the cheaper sequence is set by lift, because the larger machine’s minimum flow forces it to cycle where the smaller one is efficient. A load-only sequence is therefore right for part of the year and wrong for the rest.
Limiting component
H4737. Tower approach is quoted at design range and flow, but at low load the range shrinks, the fan reaches its minimum speed, and the water is driven colder than the chiller wants, so the machine either cycles the fan or loses its expansion-valve differential. The limit on condenser-water reset is the tower’s turndown and the chiller’s minimum lift rather than the approach the tower can achieve, and a schedule written from chiller efficiency alone asks for water the plant cannot use.
Mean hides a local extreme
H4738. With a zeotropic blend the evaporating temperature glides from inlet to outlet, and the controller displays a mean or saturated value that the coil never sees at either end. A coil selected on the mean can reach its surface-temperature limit at the inlet first. The limit then appears at the distributor rather than at the coil’s average.
Interior optimum
H4739. Lower condenser water saves compressor work and costs tower fan work, and the two curves cross at a temperature set by the tower’s size rather than the chiller’s. A schedule that drives the water to the coldest the tower can make is past the optimum, and the plant’s total power has a minimum that no single machine’s efficiency curve reveals.
Metric blind to a constraint
H4740. Combined efficiency, which adds cooling and heating output, hides the constraint that decides a heat-recovery machine’s usefulness: the recovered heat must be above the distribution temperature, and heat below it is unusable however much of it there is. A machine that looks excellent on the combined metric can be worthless in a building whose heating loop needs a higher temperature.
Ambient-dependent limit
H4741. Variable-refrigerant part-load advantage is quoted from the inverter’s range, but at low ambient the condensing pressure falls until the expansion valve lacks the differential it needs, and capacity is lost exactly where the advantage was claimed. The limit is a pressure difference rather than a speed, and it moves with the outdoor temperature rather than with the indoor load.
Lagging signal
H4742. Leaving-water temperature is a late signal by construction, because the plant’s thermal mass and the sensor’s location delay it until the lead machine is already at its limit. The valve position or the bypass flow moves first. A sequence that waits for the temperature starts the second machine only after the supply has sagged by a few tenths of a kelvin (an estimate).
Threshold
H4743. Air-cooled capacity is rated at a condenser airflow the installation may not deliver, because the unit sits in a recess or against a wall and its own discharge recirculates. The result is not a gradual derating but a sharp loss above a recirculation fraction, and the test is a temperature traverse across the coil face rather than a single ambient reading at the unit’s side.
Placement-limited detection
H4744. Threshold is only half of a refrigerant detector’s performance; the other half is placement. A sensor where the gas pools detects a high-level leak only after the room has filled. A sensor at the breathing zone detects the same leak at a concentration the occupants have already met.
Boundary omission
H4745. Counting the evaporator alone cannot close a chiller plant’s energy balance: the condenser’s heat rejection, the tower’s evaporation and the pump work must be counted together, and a test that omits them reports a coefficient the balance cannot reproduce. The missing terms are the compressor’s shell losses and the pump heat, and a plant that looks compliant on the chiller’s own numbers can fail the building’s meter.
Ventilation and indoor air
Proxy failure
H4746. Demand-controlled ventilation uses carbon dioxide as a proxy for occupancy, but generation per person depends on metabolic rate, so a gym and an office at the same concentration need different outdoor-air rates. The controller cannot tell them apart without a second signal, and a system tuned in an office under-ventilates the gym at the same reading.
Interface misplacement
H4747. Displacement ventilation assumes an interface above the occupied zone, but the interface’s height is set by the plumes’ buoyancy as well as by the supply flow, so a room with strong sources can have its interface below the occupants’ heads. The supply flow that satisfies the heat balance is then not the flow that keeps the interface up, and a design checked only on temperature has checked the wrong variable.
Metric's missing variable
H4748. Effectiveness measured by dosing the supply and reading the extract describes the room’s mixing, not the occupant’s exposure, because a contaminant released at the floor and one released at the ceiling have different histories in the same airflow. The same system can score well and still deliver a poor dose to a seated person. The source’s location is the variable the metric leaves out.
Definition artefact
H4749. Temperature efficiency for a heat-recovery unit is computed from supply and exhaust temperatures, but the ratio is defined for balanced flow, and with unequal airflows the number depends on which stream is the capacity-rate minimum. The same core therefore reports different numbers at different flow ratios, and a commissioning sheet that compares them is comparing test conditions rather than cores.
Sign reversal across modes
H4750. Filter loading is assumed to cost energy, but at constant speed a loaded filter reduces airflow and can reduce fan power, while at constant pressure it raises power, so the sign of the effect flips with the control mode. A maintenance policy imported from one mode to the other will therefore change filters too often or too late.
Transport-limited capacity
H4751. Steam must be absorbed before the duct’s first obstruction or it condenses on the wall, and the absorption distance grows as airflow falls, so a humidifier at rated output can wet a duct at part load. The usable capacity is set by the duct’s geometry rather than by the boiler, and a unit selected on its rated mass flow will be selected on the wrong number.
Impossibility of balance
H4752. One stack cannot balance a multi-storey building: the neutral plane fixes which floors draw air in and which exhaust it, and the floors nearest the plane receive the least. A stack sized to give the top floor its design flow therefore starves the middle. The driving pressure is a function of height, so the imbalance is geometric rather than a matter of tuning.
Context transfer
H4753. Wind pressure coefficients measured on isolated models in a smooth boundary layer do not transfer to a city canyon, where the flow is channelled, the leeward pressures differ, and the canyon can even reverse the sign of the difference on some faces. A natural-ventilation design validated on an open site is not validated for its own site, and the error is not a constant factor.
Extrapolation failure
H4754. Clean-air delivery rate is a chamber quantity, but in a room the position relative to the occupant decides the exposure, so a unit behind a sofa delivers its rated rate to the wrong air. The equivalent air change it is credited with is a property of the installation rather than of the appliance, and the test that matters is a concentration measurement at the breathing zone.
Averaging error
H4755. In the return duct a carbon-dioxide sensor reads the mixture of every zone, so it responds to average occupancy rather than to the crowded room. The zone that needs air most gets the same share as the empty one. The error grows with the diversity of occupancy across the zones.
Path over rate
H4756. Hood capture is tested with make-up air supplied so as not to disturb the plume, but in a real kitchen the make-up enters at the nearest opening and can cross the hood’s face. The hood that passes can fail in service, and the make-up’s path, not the hood’s exhaust rate, is the variable that decides capture.
Protection-induced hazard
H4757. Frost protection that cuts the supply flow leaves the exhaust larger, which depressurises the dwelling, and the protection that saves the core creates the pressure difference a radon membrane was meant to avoid. The two requirements conflict for the hours the protection operates, and a design that specifies both without ordering them has specified neither.
Thermal comfort
Misattribution
H4758. Cold complaints beside a window are assigned to the glass, but the sensation comes from the downdraught the glass generates. Blind position separates them: if the complaint follows the blind and not the glass, the remedy is the air path. If it follows the glass, no amount of warm air will fix it.
Index insufficiency
H4759. Predicted mean vote is a whole-body index, and it says nothing about the vertical gradient or the radiant asymmetry a seated person feels, so a room can be neutral on the index and still be rejected. The index is a necessary condition for comfort and not a sufficient one, and using it as a compliance test requires the local terms to be checked separately.
Survivor selection
H4760. Field studies find that people tolerate wider ranges than the index predicts, and the gap is credited to adaptation, but the surveyed population is a survivor set, because those who found the building intolerable have moved or left. Part of the adaptation the statistics reveal is therefore selection, and a longitudinal study of the same people before and after a move would separate the two.
Asymmetric lag
H4761. Thermal sensation lags operative temperature, and the lag is not symmetric, because warming is sensed by cold receptors falling silent while cooling is sensed by them becoming active. A room cycling about a mean is therefore judged differently from a steady room at the same mean, and the difference is a function of the cycle’s period rather than of its amplitude alone.
Lost direction
H4762. Ceiling warmth and floor coolness at the same asymmetry are not equally uncomfortable, because the ceiling affects the head and the floor the feet and the body’s sensitivity is not uniform. A single limit is therefore conservative in one direction and permissive in the other. The direction of the asymmetry is lost in the metric that governs it.
Fluctuation-weighted limit
H4763. Draught is assessed from mean speed, but the sensation comes from the fluctuation, so a low mean with high turbulence is felt as a higher steady speed. Two rooms at the same mean can differ in complaint rate by a factor set by the turbulence, and a limit written on the mean cannot distinguish them.
Endogenous input
H4764. Clothing insulation is an input to the index and an output of the building: people wear more when the weather is cold and the building is underheated, so a fixed value makes the predicted neutral temperature a function of the assumption. The seasonal movement of clothing is one reason field neutral temperatures track the outdoor running mean, and an index that holds clothing fixed cannot reproduce it.
State history
H4765. Metabolic rate is treated as a step that follows activity, but heat storage makes skin temperature and sensation lag by minutes, so a person entering a cool room after exercise feels cold at a core temperature the index calls neutral. The transient is not a correction to the steady index but a different problem, and the variable that matters is the skin’s history rather than the instantaneous metabolic rate.
Geometry mismatch
H4766. Globe temperature at one height is an integral over the instrument’s own geometry, and a seated body integrates the radiant field differently in a room with a hot window at eye level and a cold floor. The single value is an average no part of the occupant experiences. The local asymmetry is the term the globe cannot supply.
Saturation of a benefit
H4767. Moving air extends the comfort range through convection and evaporation, and in humid air the evaporative part saturates while the convective part continues, so the speed at which a fan stops helping is lower in a humid room. A design that specifies fan speed from a dry-climate test over-promises in the tropics.
Treatment confound
H4768. Operable windows are reported to make buildings more comfortable, and the difference is credited to air movement or adaptation, but a window also gives the occupant a means of acting. A trial with a dummy control connected to nothing would separate the comfort of the air from the comfort of the agency, and it is rarely run.
Wrong surface
H4769. Floor heating is limited by surface temperature, but the temperature the occupant feels is at the sole of the shoe, and shoe insulation and contact resistance set the difference. A barefoot test therefore sets a limit for a condition nobody occupies, and a floor at the limit can feel cold in a thin sole and warm in a thick one.
Lighting
Instrument-defined property
H4770. Flicker is quantified at the lamp, but the meter’s time constant and the amplifier’s limited bandwidth attenuate high frequencies, so a lamp with content above the meter’s bandwidth reads as flicker-free. The claim then becomes a claim about the instrument. Two laboratories with different meters can disagree about the same lamp without either being wrong.
Non-monotonic efficiency
H4771. Dimming by current shifts the spectrum and lowers the junction temperature at once, so efficacy does not fall monotonically with output. There is an output at which it is highest and it is not full power, which means a dimmed installation can be more efficient per lumen than the same installation at full output.
Average versus extreme
H4772. Glare is computed from average luminance over the emitting area, but a small source’s peak can be several times its average and the sensation responds to the peak. A bright chip behind a diffuser can therefore produce more glare than the rating of the fitting that contains it predicts. The rating is blind to the source’s internal structure.
Metric's chosen points
H4773. Daylight autonomy is computed on a grid the designer chooses, so the number describes the chosen points as much as the room. A room can look compliant because the grid avoided the corner the occupants use, and the grid’s resolution is the design decision the number conceals.
Factor drift
H4774. Maintenance factors date from when lamp lumen depreciation dominated, and for LEDs that term is now small while dirt remains, so a factor inherited from fluorescent practice is dominated by the wrong term and misstates the maintained level in both clean and dirty rooms. A design that applies the old composition overstates illuminance where the air is dirty and understates it where the air is clean.
Wrong plane
H4775. Circadian effectiveness depends on light at the eye, but designs are checked at the workplane, and a downlight that meets the desk level can deliver very little vertical illuminance at a seated occupant’s eye. A room can be bright on the drawing and dim to the circadian system, and the two quantities are related only through the luminaire’s distribution.
Condition-dependent specification
H4776. Colour temperature is quoted at the rated condition, but the phosphor shifts as the junction warms, and the shift is larger for warm-white than for cool. A fixture measured cold is warmer in a plenum, so the colour consistency of an installation is a function of its thermal design rather than of binning alone.
Missing reference term
H4777. Glare depends on the source’s luminance relative to the eye’s adaptation, but the unified glare rating has no term for the window or the bright wall that sets the adaptation. The same luminaire is comfortable in a dim room and glaring in a daylit one. The metric cannot express that because the missing term is the room.
Specification versus perception
H4778. Dimming resolution is quoted as a percentage of full output, but the eye’s contrast sensitivity sets a threshold and the driver’s minimum current sets the floor, so a driver fine at the top can show a visible step at the bottom. The resolution a specification rewards is therefore not the resolution a room reveals.
Placement selection
H4779. Occupancy-sensor savings come from buildings where the sensors could see the occupants, and coverage depends on mounting height, sensitivity and partitions, so the published savings describe the best placements. A retrofit that follows the same specification in a partitioned office saves less without any component failing.
Silent recalibration
H4780. Daylight sensors are calibrated against the view they have at commissioning, and a later partition or a stack of boxes changes the view without changing the sensor’s output at a given daylight level. The control then dims when the room is bright and leaves the lights on when it is dark. Nothing reports a fault, because the sensor is working as calibrated.
Rating condition
H4781. Emergency duration is certified with the battery at a comfortable ambient, but the battery sits in a warmer void where its capacity fades faster, so the rated duration describes a new battery in a cool room rather than the installation at the end of its maintenance interval. A capacity check at the installed temperature after the interval is the test that matters, and a commissioning test cannot substitute for it.
Wrong controlling variable
H4782. Light trespass is regulated through cut-off and aiming, but sky glow is scattered by the atmosphere and depends on spectrum, so a blue-rich fully cut-off fixture can contribute more to the sky than a warmer one that is not. The two effects are governed by different variables, and a specification written only on cut-off addresses the near field while leaving the far field unaddressed.
Proxy versus outcome
H4783. Lighting power density is computed from connected load at full output, so a low-density scheme without controls can use more energy than a higher-density scheme with good dimming. The code’s metric is a proxy for the peak and not for the energy. A design that optimises the proxy can miss the outcome the code was written to protect.
Datasheet condition
H4784. Efficacy is quoted at a junction temperature a real installation does not hold, and the derating is larger for warm-white and high-colour-rendering products, so a datasheet comparison can reverse at the temperature both will run at. The ordering of two fixtures is therefore a property of the ceiling rather than of the catalogue.
Controls and sequences
Tuning mismatch
H4785. Integral time is tuned to reject the disturbance commissioning can create, usually a step in setpoint, but the building’s real disturbance is a slow load change, so the loop that looks well tuned on a step can hunt at part load. The oscillation is the loop passing a disturbance whose period it was never tuned for.
Sensor proxy
H4786. Mounted near a return grille, a thermostat reads return air, which is a mixture that has already passed the occupants, so the setpoint controls a supply-air proxy and the room’s temperature is set by the duct layout and the doors. The sensor’s position is part of the control law, whether or not anyone wrote it down.
Deferred cost
H4787. Widening a deadband is treated as free energy, but the space must be recovered at the start of occupancy, when the plant is least efficient and energy is most expensive. The saving during the float and the cost during the recovery are both real. Whether the deadband saves depends on the part-load curve and the demand charge, not on the deadband alone.
Averaged parameter
H4788. Optimum start learns a warm-up rate from previous mornings, but the rate depends on outdoor temperature and stored heat, so one learned value is an average that mispredicts at both ends of the season. The coldest morning is where the prediction matters and the one the learning has seen least often, since such mornings occur a handful of times a year (an estimate).
Mutual assumption
H4789. Supply-air reset and static-pressure reset each assume the other is fixed, so running both makes them interact: warmer air needs more flow, more flow needs more pressure, and the pressure reset then removes it. The pair converges to a point neither reset would choose alone. The interaction is invisible in either loop’s own trend.
Aliasing
H4790. The historian’s interval is often longer than a terminal unit’s cycling period, so the recorded oscillation is an alias and its apparent period belongs to the interval rather than to the loop. A diagnosis of hunting made from such a record can describe the historian, and a one-minute record would show a different and possibly steady signal.
Feedback through occupants
H4791. Raising outdoor air in response to carbon dioxide can create the draught that makes occupants open a window, which lowers the concentration and lets the controller cut back, so the loop oscillates with the occupants as its actuator. It is stable in simulation because the simulation has no occupants in it.
Identified parameter
H4792. Fitted from a step response, a time constant absorbs the furniture, the carpet and the internal mass, so the constant belongs to the experiment as much as to the structure. The same building reoccupied with different furniture shows a different constant. A model calibrated empty mispredicts the warm-up of the full building.
Null claim
H4793. In one zone with a linear plant and an exact model, a well-tuned proportional-integral loop and a model-predictive controller have the same optimal input. A comparison that reports a difference is therefore reporting a difference in the model, the horizon or the tuning. Give both the same model and the same horizon, and the gap should vanish.
Impossibility
H4794. One temperature sensor cannot distinguish a zone warmed by the sun from one warmed by people, because both raise the same variable while the correct responses differ: the first calls for cooling, the second for ventilation. The controller’s choice is a guess about an unmeasured cause, and no tuning removes the ambiguity, only biases it toward the more common cause.
Time-scale separation
H4795. Staging on the instantaneous load cycles, because the load’s noise has a shorter period than the plant’s response and the controller acts on the noise before the plant answers the previous command. The signal that should drive staging is the load averaged over the building’s time constant, which is a different number from the one the sensors report at any instant.
Boundary solution
H4796. Choosing the supply-air temperature that minimises a variable-air-volume system’s energy is usually described as an interior optimum, but the coil turns sharply near the dew point, so the optimum is usually at a constraint: either the leaving air is saturated or the boxes are at their minimum. A model that assumes a smooth trade-off reports an interior point the plant cannot hold.
Fault masking
H4797. Damper stuck open, sensor biased: the residual at the zone is the same and integral action removes it either way, so temperature alone cannot separate them. A step in setpoint can. The stuck zone does not move at all, while the biased sensor moves with the step and keeps its offset.
Conservation violation
H4798. Command more zone airflow than the fan can deliver and nothing fails visibly: static pressure falls, every zone receives less than commanded, and the temperatures may still be met if the loads are low. The building is then under-ventilated while appearing to be controlled. The violation is a mass balance no single zone’s sensor can detect.
Interval-actuator mismatch
H4799. Tuning on historian data assumes the actuator moves within the historian interval, but a valve’s stroke can take longer, which lengthens the loop’s effective dead time. The loop then looks well tuned in the trend and oscillates in reality, and the discrepancy is a property of the actuator rather than of the tuning.
Closure argument
H4800. Sized for one day, a plant is controlled for all the others, so the sequence is the part of the design exercised for almost all of the building’s life, and a plant optimal at the peak can be unstable at part load. The sizing and the sequence have to be argued together, and a design that reports only design-day performance has reported the one condition the controls will rarely see.
Status and plan
One hundred entries, numbered in sequence. Each was written on its own, with the argument shape chosen before the subject, so that no two share a form; the label above each claim is there to be checked against the sentence beneath it rather than accepted as a taxonomy.
The physics stays inside the building: the fan curve and the duct’s pressure budget, the valve that loses authority when the reset removes its margin, the trap and the stack, the chiller’s lift and the tower’s approach, the interface of a displacement system, the eye at the workplane and the eye at the window, and the loop that decides all of it on a schedule written before the building was occupied. No claim here concerns a brand, a regulation, or a person.
If one of these is worth an afternoon, it is H4712 — that a loaded filter can reduce fan power at constant speed and raise it at constant pressure, so the same pressure drop carries opposite energy signs. It needs one filter, one fan, two control modes and a power meter.
Incoprea Research · hand-considered hypotheses 4701–4800 · 3 November 2026 · ~/Desktop/Nov3/