A pressure relief valve opens at a set pressure to protect a boiler, tank or water line, and the right one depends on the system: a 30 psi ASME valve like the Watts 174A or Cash Acme F-30 for a hot water heating boiler, a CSA-listed thermal expansion valve like the Cash Acme FWL-2F for a water heater line, and an ASME pressure-only valve like the Cash Acme F-82 for storage and well tanks. A water heater still needs its own temperature and pressure (T&P) valve. Match the setting to the label on the equipment and the BTU rating to its output.
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This page covers pressure-only relief valves. For the T&P valve on top of a water heater, use our T&P valve replacement guide by size.
Relief valve types
Three valves get called “PRV” on a job. Know which one you need before you shop.
| Valve | Opens or works on | Where it goes | Example |
|---|---|---|---|
| Pressure-only relief valve | Pressure at its set point | Hot water boilers, storage tanks, well systems, thermal expansion on supply lines | Watts 174A, Cash Acme F-30, F-82, FWL-2F |
| Temperature and pressure (T&P) relief valve | Pressure or temperature, whichever comes first | Water heaters and hot water storage tanks | See our T&P replacement guide |
| Pressure reducing valve (regulator) | Lowers street pressure to a set outlet pressure | Main water line after the meter | See our water pressure regulator guide |
A pressure reducing valve is not a safety device. If you need one, start with our water pressure regulator guide.
Boiler relief valves (30 psi)
A hot water heating boiler needs an ASME-rated pressure relief valve. Watts’ spec sheet ES-174A-740 (afsupply.com) describes the 174A as a bronze-body ASME relief valve for pressure protection only on hot water heating boilers, in 3/4 in to 2 in sizes with female inlet and outlet. Settings run from 30 to 150 psi, and the maximum temperature is 250°F. The 3/4 in valve at 30 psi is rated 650,000 BTU/hr, tested and rated by the National Board of Boiler and Pressure Vessel Inspectors to ASME requirements.
Watts gives two sizing rules: the valve’s BTU rating must exceed the boiler’s heating output, and the valve must be set to relieve below the boiler’s maximum working pressure. Both numbers are on the boiler’s rating plate. If the output is bigger, Watts’ 740 iron-body valve has an expanded outlet and starts at 925,000 BTU/hr in the 3/4 x 1 in size.
Buy the Watts 174A 3/4 in 30 psi from a Watts dealer or heating supply house; we found no in-stock new Amazon listing for that setting on October 14, 2026.
Cash Acme’s F-30 (cashacme.com) does the same job: a 3/4 in ASME safety relief valve set at 30 psi for hot water space heating boilers, water supply heaters and storage tanks. It is for hot water service only, not steam. The 22887-0030 is FIP x FIP. The F-30F (24797-0030) is the lead-free version.
Check the Cash Acme F-30 22887-0030 on Amazon →
Skip these on a steam boiler. Steam needs a steam safety valve, and that is a job for a licensed heating contractor.
Thermal expansion relief on water heater lines
When a check valve, backflow preventer or pressure reducing valve closes the water system, heated water has nowhere to expand. Pressure climbs every time the heater runs. Cash Acme lists the FWL-2 (cashacme.com) for exactly this: pressure-only relief for hot water supply systems where thermal expansion is likely. It mounts on the tank or in a tee, on the hot or cold side of the heater, and reseats on its own.
The FWL-2 spec sheet (cashacme.com) lists ANSI Z21.22/CSA 4.4 certification, a 150 psi standard setting (75 and 125 psi available), a 250°F maximum and a CSA rating of 200,000 BTU/hr for the 3/4 in valve. The 1/2 in FWL-2 is CSA-rated at only 15,000 BTU/hr. The forged, lead-free FWL-2F in 3/4 in at 150 psi is SKU 24799-0150.
Check the Cash Acme FWL-2F 24799-0150 on Amazon →
This valve does not replace the heater’s T&P valve. Most plumbers fix thermal expansion with an expansion tank first, and use a relief valve as the backup or where a tank will not fit. See our water heater expansion tank guide.
Storage tank and well tank relief
Cash Acme’s F-82 (cashacme.com) is a pressure-only ASME safety relief valve for hot water space heating boilers, water supply heaters and storage tanks. It comes in 3/4 in to 2 in with female inlet and outlet, and the 3/4 in valve is sold at 30, 50, 75, 100, 125 and 150 psi (SKUs 23341-0030 to 23341-0150). The F-82 spec sheet (cashacme.com) rates the 3/4 in valve at 717,000 BTU/hr at 30 psi and 1,462,000 BTU/hr at 75 psi, and lists certification to ASME Section XIII for Section IV (HV) use and lead-free certification to NSF/ANSI/CAN 372.
On a private well, plumbers often add a relief valve at the tank tee when the pump can build more pressure than the tank is rated for. Cash Acme does not list well systems by name, so confirm with the tank and pump makers. Pick a setting above the pressure switch cut-out and below the tank’s maximum working pressure. With a 40/60 switch, a 75 psi valve clears the 60 psi cut-out, as long as the tank label lists a maximum working pressure above 75 psi. Our guide to 30/50 vs 40/60 pressure switches explains the settings.
Buy the Cash Acme F-82 23341-0075 from a Cash Acme dealer or plumbing supply house; we found no in-stock new Amazon listing for the 75 psi setting on October 14, 2026. Amazon does carry other F-82 settings, so check the psi printed on any listing against your tank before you buy.
Cash Acme also sells the FW and FWC (cashacme.com) adjustable valves (15 to 200 psi, 125 psi from the factory). They are non-code valves for things like pump relief. Do not use one where code calls for a listed safety valve.
How to choose one
- Name the system. Boiler, water heater line, storage tank or well. That decides between ASME, CSA (ANSI Z21.22) or both.
- Read the label. Find the maximum working pressure and the heat output (BTU/hr) on the equipment’s rating plate.
- Pick the setting. It must be below the equipment’s maximum working pressure and above normal operating pressure.
- Check capacity. On a boiler or heater, the valve’s BTU rating at that setting must exceed the equipment’s output. On a well tank, the risk is pump pressure, not heat, so ask the pump or tank maker what relief capacity the valve needs.
- Match the threads. Measure the tapping or fitting size. Do not reduce the inlet.
- Buy lead-free for any valve on drinking water. Look for NSF/ANSI 372 on the spec sheet.
Discharge piping
The Watts spec sheet sets the basics: the discharge line must be the same size as the valve outlet and must pitch downward from the valve to a safe place for disposal. The valve is for emergency relief only, not an operating control.
Local code adds more rules, such as approved pipe materials, where the line may end and how close to the floor. Use the same rules as a water heater T&P line, which our T&P guide covers, and ask your inspector. Never cap, plug or thread a cap on the end of a relief line.
When a relief valve drips
A relief valve that drips is telling you something. Find out what before you replace it.
- Check pressure. Put a gauge on a hose bib or boiler drain. Our water pressure gauge guide lists gauges that hold the peak reading.
- Boiler: if the gauge reaches the setting when hot, look at the expansion tank and the fill valve. A waterlogged expansion tank is a common cause.
- Water heater line: pressure that spikes after the heater runs points to thermal expansion. Add or replace an expansion tank.
- Normal pressure but still dripping: the seat may be dirty or worn. Watts says to trip the lever at least once a year to keep the waterways clear. If a valve will not reseat, replace it.
Call a pro if a boiler relief valve opens, if pressure keeps climbing or if you smell gas near the equipment. A licensed plumber or heating contractor should find the cause.
Pressure relief valve FAQ
What is the difference between a pressure relief valve and a T&P valve?
A pressure relief valve opens only on pressure. A temperature and pressure (T&P) valve opens on either high pressure or high temperature. A water heater needs a T&P valve. A hot water heating boiler usually gets a pressure-only relief valve, often set at 30 psi.
What pressure should a relief valve be set at?
Below the maximum working pressure of the tank, boiler or heater it protects, which is printed on its label. Residential hot water boilers commonly use 30 psi valves. On a well system, the setting also has to sit above the pressure switch cut-out so the valve does not open every pump cycle.
Why is my pressure relief valve dripping?
Either the system pressure is reaching the setting or the seat is dirty or worn. Check the pressure with a gauge. Thermal expansion on a closed water system, a failed expansion tank or a boiler fill valve set too high are common causes. Fix the cause before you replace the valve.
Can I put a shutoff valve between the relief valve and the tank?
No. Nothing should be able to isolate a relief valve from the equipment it protects. Watts also says a relief valve is for emergency relief only and must not be used as an operating control.
How often should a relief valve be tested?
Watts says to trip the lever at least once a year to make sure the waterways are clear. Put a bucket under the discharge, lift the lever briefly and let it snap shut. If it will not reseat or keeps dripping, replace it.
Can I replace a boiler relief valve myself?
The swap is simple threaded work, but you must drain pressure from the system and match size, setting and BTU rating. Gas boilers and any system you are unsure about are a job for a licensed plumber or heating contractor, and some areas need a permit.
How we chose
This is a document-based guide from a licensed plumber’s point of view. We picked one valve for each common home system and checked the maker’s spec sheets and product pages for sizes, settings, BTU ratings and listings. The Watts sheet was read from distributor-hosted copies of the Watts document because the Watts product pages did not load. Sources were read October 14, 2026; sellers and availability change. We do not publish prices or claim hands-on testing. More about our selection method and Plumbing Picks.
Primary sources
- Watts ES-174A-740 spec sheet (PDF, distributor copy) (afsupply.com): 174A and 740 application, sizes, settings, 250°F max, BTU ratings, ASME/National Board, discharge and sizing rules.
- Watts ES-174A-740 spec sheet (PDF, second distributor copy) (literature.neuco.com): 174A 3/4 in at 30 psi 650,000 BTU/hr; 740 3/4 x 1 in 925,000 BTU/hr; annual lever test.
- Cash Acme F-30 product page (cashacme.com): 3/4 in, 30 psi, ASME, hot water only, SKUs 22887-0030 and 24797-0030.
- Cash Acme F-82 product page (cashacme.com): sizes, 30 to 150 psi settings, SKUs, application.
- Cash Acme F-82 spec sheet (PDF) (cashacme.com): BTU/hr ratings, ASME Section XIII for Section IV (HV), NSF/ANSI/CAN 372.
- Cash Acme FWL-2 product page (cashacme.com): thermal expansion use, tank or tee mounting, SKUs and settings.
- Cash Acme FWL-2 spec sheet (PDF) (cashacme.com): ANSI Z21.22/CSA 4.4, 150 psi standard, 250°F, CSA BTU ratings.
- Cash Acme F, FW and FWC page (cashacme.com): non-code adjustable valves, 15 to 200 psi.
- Cash Acme relief valve listing (cashacme.com): relief valve families.