PRV drain on a hot water boiler

I always thought they must terminate within 6” of the ground but I see code check references it at 18”.
Can someone clear this up for me please.
Is it that a TPRV for the WH is 6” and a boiler PRV is 18”, from the ground ?

Or only if it’s a Boiler and indirect WH?
Thank you

A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. … As the fluid is diverted, the pressure inside the vessel will stop rising.

The T & P valve (temperature and pressure) valve opens at a pre-determined pressure(say 150 lbs) OR a pre-determined temperature (say 210 degrees) to protect a system from burst and to protect people from scalding. It doesn’t have to be both conditions at the same time, either one alone will open it.

I believe the difference is in relation to the 18" height requirement of the WH in garages. The 6" would still apply, which is why in some references you will see the generic 6" to 24" statement (18" + 6" = 24"). Sometimes confusion sets in when a drain pan is used beneath the WH.


Thank you all for responding. But sorry I’m still confused. In my area we have a lot of boilers. So a boiler with a PRV valve, does that have to discharge within 6 inches from the ground or is within 18 inches of the ground still ok? I tried researching it and it always keeps referring back to the TPR valve for a water heater. Thanks

Note: Marcels post was spot on for the statement that he made…

With that being said… many inspectors confuse the terms PRV and TPRV. They ARE NOT the same, and in discussions or reports, the terms should never interchanged. I suspect that is happening here.

Now with that being said… A Boiler and a Water Heater both have a TPRV with a drain tube attached to them. They both have tha same requirements for discharging to within 6" of the floor or the drain pan (if equipped). If the unit (either) is elevated to 18" above the floor, the TPRV drain pipe must still extend to EITHER within 6" of the floor or 6" of the drain pan (if equipped). The preference is within the drain pan, but many drain pans do not allow extra access for the drain pipe to discharge to it (too tight fitting to the unit), thus the extending to the floor level.

IMO, I very rarely see a PRV with a discharge pipe at all, and they are typically installed above the WH or Boiler.

Note… a standard PRV should not be confused with a SE Pressure Reducing Valve… (a whole nuther beastie)!

Hope that helps.

Yes it does.

Thank you


…And yes, there are many, many variations of the above!

I know this is a really old thread, but I thought I’d mention that in the class I’m taking now, they say water heater TPR valves should terminate no more than 6" from the floor (IPC 504.6) and boiler PR valves should terminate no more than 18" from the floor (IRC M2002.4). I’ll be asking the instructor about this later, but I wonder if it has to do with the water temps. The water heater could be releasing 210 degree water whereas I believe most boilers have limiters that shut them down around 190, so if the pressure relief valve discharged, the discharge could not be as hot as water heater discharge. Also, the discharge from the boiler will be at a MUCH lower pressure!
(I realize my speculation may be wrong.)

IMO, it has more to do with the possible location/placement of the appliance in regards to potentially causing harm to a person in close proximity when it discharges. WH’s are often in areas that humans frequently occupy, ie. Laundry Rooms, whereas a Boiler is typically located in a Utilities Room/Basement/etc. and is hardly ever even seen by the occupant of the home, and rarely so close as to be harmed if exposed to the super-heated steam when discharging.

The codes for water heaters and boiler TPRV has nothing to do with equipment location LOL :joy: They are often located in the same room. Think about it, a boiler is a closed system operating at 12 psi (for a modern boiler).

Please post the instructors comments.

Boilers use PRVs, water heaters with a tank use TPRVs – they’re not the same thing. Per code, the discharge pipe for a boiler can be up to 18" above the floor if not piped to a floor drain. From there, follow the installation instructions that came with the boiler, the requirements vary. Some want you to put it at least 6 inches above the floor so you could put a bucket or some such under it :slight_smile:

Great, Peter, look forward to what he says… :thinking:

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I asked my course instructor why it’s no more than 6" in one instance, but no more than 18" in the other instance. And he said three things:

  1. The TPR valve is more likely to discharge than the boiler PR valve
  2. These codes are referring to different pieces of equipment (TPR vs PR)
  3. IRC & IPC don’t address every issue the same way

Peter… the code is NOT perfect! This is one of those cases where you need to apply your own logic… in both cases you can have scalding water release from the pipe… in both cases you can get scalded. There is no reason to pipe a boiler’s discharge pipe high. In fact, all manuals of a modern boiler will instruct the installer to pipe it in a way where it will not cause injury should there be a release. Hydronic boilers can easily be 200F when PRV is triggered.


I agree with your instructor, he/she made some excellent points I didn’t think of.