Primary / Secondary relief on water heaters

[Mulligan Stew]

Somewhere long ago I learned to install a Primary/Secondary T/P valve system when it wasn't possible to direct the T/P vent pipe down from the valve provided on the water heater.
The existing conditions: The relief valve on the HWH was below the ground level that existed outside the building. There is no floor drain in the room. Since you cannot vent up from the valve to get above ground outside (because it creates a condensate trap) you simply drop the vent down to terminate just above the floor. (This is the Secondary 150 PSI 210 F) Then, you install a 125PSI T/P in the hot water piping above the water heater at a level that allows you to terminate it's vent above the ground level outside. (This becomes the Primary T/P). If there is an overtemp or pressure situation the Primary blows off outside first. Maybe leaving a drip inside or none at all.
Was this wrong?

 

[GReynolds929]

Yes that is the proper method.

 

[Sponsor]

 

[Mulligan Stew]

Yes that is the proper method.

Thanks for having a look. Much appreciated. But for the life of me I can't find it written anywhere in the UPC code book.

 

[John Gayewski]

I don't think your scenario is correct. Why would a water heater that is overheating get hot out in the piping network? It would only be hot at the heater. Pressure would travel and you would might get a drip farther away but no hot water would go out into the system.

The answer is you need a drain. There are other systems for cutting the gas off in an overheat /over pressure situation where the gas is hooked to the water line but I've never used this.

 

[Mulligan Stew]

I don't think your scenario is correct. Why would a water heater that is overheating get hot out in the piping network? It would only be hot at the heater. Pressure would travel and you would might get a drip farther away but no hot water would go out into the system.

The answer is you need a drain. There are other systems for cutting the gas off in an overheat /over pressure situation where the gas is hooked to the water line but I've never used this.

The Primary valve is located within 24" of the water heater. I would think the pressure at least would transfer. But I do see your point and it's a good one.

 

[John Gayewski]

The Primary valve is located within 24" of the water heater. I would think the pressure at least would transfer. But I do see your point and it's a good one.

If it's that close then yeah. The water heater manufacturer should say how close it needs to be.

 

[Fitter30]

Really don't understand the secondary relief 125 lb. Even if you had 10' of height that's only 4.3 lbs. The pressure drop across the secondary valve at full flow would be at least that much or more. The other possible problem is bug like wasps building a nest in the end of the pipe.

 

[Reach4]

Thanks for having a look. Much appreciated. But for the life of me I can't find it written anywhere in the UPC code book.

I see this as two things. UPC 608.5 Discharge Piping. covers the normal T&P output

https://sbcc.wa.gov/sites/default/files/2020-03/2018 UPC Insert Pages_final.pdf ( the WA amendments) says

Exception: Where no drainage was provided, replacement water heating equipment shall only be required to
provide a drain pointing downward from the relief valve to extend between two (2) feet (610 mm) and six (6) inches
(152 mm) from the floor. No additional floor drain need be provided.

In cold places it is the norm to just let the T&P drain to the floor with no drain, under the belief that high rate discharge is rare and much better than an explosion.

The 125 psi pressure relief I see an additional bonus thing-- the discharge piping requirement having already been met. So if it is not done in a way prohibited elsewhere in the code, I see it as a bonus that makes it less likely that the floor gets flooded with hot water. I am not a pro. My thoughts on this may be wrong.

 

[John Gayewski]

An educated guess. The t&p would need to be max18"from the heater s that's the minium length of pipe where metal piping is required at the heater. To me it doesn't make much sense that the code would allow a t&p on the portion that is allowed to be plastic pipe.

 

[Reach4]

An educated guess. The t&p would need to be max18"from the heater s that's the minimum length of pipe where metal piping is required at the heater. To me it doesn't make much sense that the code would allow a t&p on the portion that is allowed to be plastic pipe.

I think the T&P valve has a probe that extends into the WH to sense temperature.

 

[wwhitney]

Really don't understand the secondary relief 125 lb.

My take: UPC 608.4 requires a pressure relief valve to be set at at most 150 psi. So the 150 psi T&P valve at the water heater satisfies the UPC requirements, and given the Washington state exception to UPC 608.5, the discharge piping from it can terminate at the below grade floor level for existing installations, so that takes care of the code requirements.

But still, you don't really want water discharged there if possible. A T&P valve at a higher elevation is not going to be too useful for temperature control, as the temperature in the tank may differ significantly from the temperature at the higher elevation. But the pressure at the higher elevation will be the same as the tank, adjusted for elevation. So really the higher relief valve is just being used as a bonus pressure relief valve, like Reach4 said.

As long as the elevation difference is not too great, the higher 125 psi valve should provide pressure relief before the lower 150 psi valve activates, and the higher valve output can be piped to exterior grade by gravity. So the lower valve is just working as a temperature relief valve and backup pressure relief valve.

A related question is then in various failure modes, what usually triggers a T&P valve, temperature or pressure? For the inadequate expansion compensation in the system, it's pressure. For the case of a runaway heating source, not sure; I guess it also depends on how much expansion compensation the system actually has.

Cheers, Wayne

 

[johnbarstow]

You’re spot on about the UPC requirements — the 150 psi T&P valve meets code, and the Washington state exception allows below-grade termination for existing setups. The higher 125 psi valve works as extra pressure relief, with the lower valve handling temperature and backup pressure relief.

For Utilitech water heaters, their T&P valves are designed to respond accurately to both pressure and temperature changes, ensuring safe operation. Proper installation — whether below grade or to exterior grade — keeps the system running smoothly. Let me know if you need anything else!

 

[Mulligan Stew]

My take: UPC 608.4 requires a pressure relief valve to be set at at most 150 psi. So the 150 psi T&P valve at the water heater satisfies the UPC requirements, and given the Washington state exception to UPC 608.5, the discharge piping from it can terminate at the below grade floor level for existing installations, so that takes care of the code requirements.

But still, you don't really want water discharged there if possible. A T&P valve at a higher elevation is not going to be too useful for temperature control, as the temperature in the tank may differ significantly from the temperature at the higher elevation. But the pressure at the higher elevation will be the same as the tank, adjusted for elevation. So really the higher relief valve is just being used as a bonus pressure relief valve, like Reach4 said.

As long as the elevation difference is not too great, the higher 125 psi valve should provide pressure relief before the lower 150 psi valve activates, and the higher valve output can be piped to exterior grade by gravity. So the lower valve is just working as a temperature relief valve and backup pressure relief valve.

A related question is then in various failure modes, what usually triggers a T&P valve, temperature or pressure? For the inadequate expansion compensation in the system, it's pressure. For the case of a runaway heating source, not sure; I guess it also depends on how much expansion compensation the system actually has.

Cheers, Wayne

Thanks Wayne and yes it makes sense that the 125 T/P relief valve really only handles an emergency pressure situation. Due to distance from the tank. Thanks for the detailed breakdown.

But since over pressure is the most common reason in my area (we often get fluctuations from our utilities) I'll keep installing this way when necessary. I've come in after customer's tanks blow off and it ain't pretty. The only reason I even questioned it is that a home inspector flagged it for a customer who's going to sell their house. You know...Home Inspectors.

 

[Fitter30]

I think the T&P valve has a probe that extends into the WH to sense temperature.

That probe haven't seen one even 8" long. Still i can't see a relief valve is set by spring pressure. How can that change by elevation? What's in going on in the vessel can see the boiling point change by internal pressure and not by elevation. On top of a cook stove elevation matters for boiling point but below temp is temp 120° is still 120°.

 

[Reach4]

Thanks Wayne and yes it makes sense that the 125 T/P relief valve really only handles an emergency pressure situation. Due to distance from the tank.

I think "emergency" overstates it. In the past that was sometimes used as the primary thermal expansion pressure control. But the downside is that it consumes water. If have city water, you should have a thermal expansion tank. If that tank is under-sized, expect some water release.