The Pressure and Temperature Numbers on Every Residential Call
Why this matters
Most residential callbacks that are not leaks are a number nobody wrote down: a relief valve that drips because the house sits at 95 psi, an expansion tank that does nothing because it shipped at 40 psi into a 70 psi house, a shower that scalds because the heater went in at the factory setting and the valve's limit stop was never touched. This card is the shelf's single source for those numbers. The other cards on this shelf use these values and point here instead of restating them, so if a figure below disagrees with another card, this one is the owner.
The pocket card
RESIDENTIAL NUMBERS - read the code as adopted
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STATIC PRESSURE AT FIXTURES
max 80 psi static IPC 604.8 / UPC 608.2
over 80 -> PRV, set and re-read static
IRC floor: 40 psi static at the entrance
EXPANSION TANK PRECHARGE
= measured static house pressure
set with tank empty, before it is piped live
WATER HEATER SETPOINT
commonly 120 F at the tank
hotter storage only with a mixing valve
SHOWER AND TUB OUTLETS
120 F max, ASSE 1016 or 1070 device
IPC 424.3 + 424.5 / UPC 408.3 + 409.4
RELIEF VALVE (T and P)
relieves at 150 psi or 210 F
psi setting not above tank working rating
PUBLIC HAND SINKS (light commercial)
tempered, 110 F max, ASSE 1070, IPC 416.5
Every code section on the card is cited as adopted by your jurisdiction, from the 2021 editions of the IPC and UPC, and a local amendment beats both. The IRC equivalents for one- and two-family dwellings sit in Chapters 25 to 33 of the IRC 2021, as adopted, and say the same thing in substance on the residential lines above; the public hand sink line is commercial and has no IRC counterpart.
Static pressure: what the 80 is measured against
The rule. IPC 604.8 and UPC 608.2 both require an approved pressure reducing valve where static pressure exceeds 80 psi. The IRC sets the same ceiling and adds a floor of 40 psi static at the building entrance. Static means no water moving anywhere in the house: every fixture closed, no irrigation zone running, no ice maker filling, no toilet refilling.
Where to read it. Screw a liquid-filled gauge onto a hose bib or the washer box hot or cold outlet, both downstream of any PRV. Reading at the hose bib upstream of the PRV tells you street pressure, which is useful for the PRV decision but is not the number the code limits. On a house with no PRV the two are the same reading.
Why 80 and not 100. Fixtures, supply stops, braided connectors and appliance solenoid valves are made to live below it, and the relief valve on the heater sits at 150. A house at 95 psi static spends its life closer to the relief setting than to the code ceiling, and the first thermal expansion event of the night takes it the rest of the way.
Dynamic is a different number. With fixtures flowing, pressure drops through the meter, the PRV and the piping. A house can read 65 static and 40 at the far shower with the tub filling. Low dynamic pressure is a sizing or restriction problem (see 'Pipe Sizing for Water Supply Reference'); it is never fixed by turning the PRV up past 80.
Thermal expansion: why the static reading lies overnight
A gauge read once at 10 a.m. misses the most important pressure in the house. Water expands when heated: a 50 gallon tank brought from 50 F to 120 F grows by a little over half a gallon (water density falls from about 62.41 to about 61.71 lb per cubic foot across that range, roughly 1.1 percent). In a closed system, meaning a PRV without a thermal bypass, a check valve or a backflow preventer stops that water pushing back into the main, half a gallon has nowhere to go and pressure climbs until something gives. Usually the relief valve gives, a few ounces at a time.
How to see it. Use a gauge with a drag needle (a second pointer that stays at the highest reading). Install it, note static, let the heater recover from a draw, and read the drag needle the next morning. A house that reads 65 static and drags to 140 has a closed system with no working expansion control.
Precharge equals static. The expansion tank's air side must start at the measured static house pressure, set with the tank empty of water. Ship-from-factory precharge is commonly 40 psi; into a 65 psi house that tank is a third full of water before the heater ever fires and has little room left to take expansion. Sizing the tank is its own card: see 'Expansion Tank Sizing for Closed-Loop Domestic Water Heaters'. Setting it is a HowTo on this shelf: see 'How to Install a Thermal Expansion Tank and Set Its Precharge'.
Order matters. Set the PRV first, re-read static, then set the precharge to the new static. A tank precharged to the old street pressure before the PRV was adjusted down is wrong the moment the PRV is.
Temperature: three numbers that get confused
The tank setpoint. Commonly 120 F, and most new heaters ship near there. The dial on a gas control or the setting of an electric thermostat is not a calibrated reading; measure it at the nearest tap after a full recovery, with a probe thermometer in a stream run until the reading stops climbing.
The delivered temperature. Expect it to read a few degrees off the setpoint and to drift between draws. Short repeated draws on a gas tank can stack hotter water at the top than the setpoint suggests. That is why the code does not trust the tank to protect the bather.
The outlet limit. Shower and tub-shower outlets are limited to 120 F maximum by a device at the point of use: an individual shower valve listed to ASSE 1016 (pressure balancing, thermostatic or combination) under IPC 424.3 and UPC 408.3, and a bathtub or whirlpool filler limited by an ASSE 1070 device or the valve's own listed limit under IPC 424.5 and UPC 409.4. The limit stop inside a pressure balancing valve ships set for a typical inlet temperature and has to be adjusted on site against the real hot supply. An ASSE 1017 master mixing valve at the heater tempers the whole house; under the editions most jurisdictions run it does not by itself satisfy the point-of-use requirement for a shower, so check the adopted text before relying on it.
Why 120. Commonly cited burn-time data (originally from Moritz and Henriques) puts a full-thickness burn on adult skin at roughly 5 minutes at 120 F, about 30 seconds at 130 F and about 5 seconds at 140 F. Children and older adults burn faster at every one of those temperatures. Ten degrees on the dial is the difference between a shower that is uncomfortable and one that sends a toddler to a burn unit.
The case for hotter storage. Some owners and some health guidance prefer storing at 140 F to suppress Legionella growth in the tank. That is a legitimate choice only with a mixing valve on the outlet and the point-of-use devices above in place, so the stored temperature never reaches a fixture.
The relief valve's two ratings
A residential temperature and pressure relief valve, listed to ANSI Z21.22/CSA 4.4, is commonly rated to open at 150 psi or 210 F, whichever comes first. The pressure setting may not exceed the tank's rated working pressure on its label, and its BTU or kW relieving capacity has to equal or exceed the heater's input. Its temperature element has to sit in the hottest water at the top of the tank (the codes and the listing commonly say within the top 6 in), which is why the valve stays in the factory opening.
A relief valve that drips is most often doing its job against a closed system or high static, not failing. Replacing it without measuring both numbers above is the standard callback; see 'The Relief Valve That Started Dripping After the Meter Change' for a full case. A relief valve spraying hot water means the heat source goes off first (gas control to off, or the heater's breaker off), then the cold supply is closed, and both stay off until static pressure and the overnight peak have been measured and the cause is found.
Gas numbers, where your license covers gas work
Gas appliance inlet and manifold pressures come from the appliance rating plate and the maker's instructions, not from a habit. As typical values only: natural gas appliances are commonly rated for about 7 in w.c. at the inlet with the manifold near 3.5 in w.c., and propane appliances commonly about 11 in w.c. at the inlet with a manifold near 10 in w.c. Read the plate before the manometer, and if the plate says otherwise the plate wins. Piping and venting rules come from the IFGC or NFPA 54, as adopted. Gas work is only within your scope where your license covers it, and every leak check is done with a leak detection solution or a combustible gas detector, never a flame.
Worked example: one house, every number
A 1990s two-story, city water, a new 50 gallon atmospheric gas heater going in. The customer's complaint is a drip from the relief valve discharge "since the meter was changed."
- Street pressure. Gauge on the front hose bib, upstream of the PRV: 112 psi.
- House static. Gauge on the washer box cold, downstream of the PRV, everything off: 94 psi. The PRV is either set high or failing. Over 80, so it is adjusted.
- Closed system confirmed. The new meter came with a dual check, which closed a system that used to push expansion back to the street. Drag needle from the previous night (gauge left by the tech who quoted the job): 150 psi, which is the relief setting. The valve was opening every night.
- PRV adjusted. Turned down and re-read with everything closed: 64 psi static, held steady for 5 minutes. Within 40 to 80.
- Expansion tank precharge. The tank shipped at 40 psi. With the tank empty and off the system, air added to 64 psi on a tire gauge, matching the new static from step 4, not the 94 from step 2.
- Heater fill and fire. Tank filled and purged at a hot tap before lighting, per the shelf rule that a heater is filled before it is fired. Setpoint left near the factory mark.
- Delivered temperature. After full recovery, kitchen hot tap: 126 F. Turned down one mark, re-read next recovery: 121 F.
- Outlet limit. Master shower, full hot on the pressure balancing valve: 117 F after the limit stop was rotated one notch. Under 120, passes. The tub filler on the hall bath read 119 F full hot, passes.
- Overnight check. Drag needle left installed, reset to 64. Next morning: 76 psi. Under 80, and well clear of 150, so the tank is sized and charged correctly.
What the numbers proved. The drip was not a bad relief valve: 94 psi static plus a newly closed system put the overnight peak at the valve's 150 psi setting. After the PRV went to 64 and the tank was charged to 64, the overnight peak sat at 76, a 12 psi rise.
What would have changed the answer. If static at step 4 could not be brought below 80 because the PRV would not hold a setting, the PRV is replaced before the tank is charged, since the precharge depends on its output. If the drag needle had still climbed past 80 with a correctly charged tank, the tank is undersized for the heater and the static, which is the sizing card's job, not a reason to raise the precharge.
How to verify you got this right
- Static was read downstream of the PRV with every fixture and appliance closed, and the figure is on the ticket.
- The expansion tank precharge on the ticket equals the final static reading, not the first one.
- A delivered temperature at a tap and a full-hot reading at every shower or tub outlet you touched are both written down, each at or under its limit.
- A drag-needle overnight reading exists for any call that started as a relief valve drip.
References
- IPC 2021 Sections 604.8, 416.5, 424.3 and 424.5, as adopted by your jurisdiction; UPC 2021 Sections 608.2, 408.3 and 409.4, as adopted.
- IRC 2021 Chapters 25 to 33, as adopted, for one- and two-family dwellings; IFGC or NFPA 54, as adopted, for gas.
- ASSE 1016, ASSE 1070 and ASSE 1017 product standards, binding through the code sections that reference them; ANSI Z21.22/CSA 4.4 for relief valves.
- See related: Expansion Tank Sizing for Closed-Loop Domestic Water Heaters; How to Install a Thermal Expansion Tank and Set Its Precharge.
- See related: The Relief Valve That Started Dripping After the Meter Change; Pipe Sizing for Water Supply Reference.