# T&P sizing for Tankless Heaters



## SewerRatz (Apr 25, 2009)

Quick question about sizing T&P or even just pressure relief valves. I know you need to size it to the BTU rating of the heater. Recently I came across a multiple unit tankless install and the relief valves where rated at the total combined BTU's of all of the heaters. In this case there where 5 heaters rated at a max of 250K BTUs Normally I would see a relief valve rated just above the units BTU's but in this case each unit had a 2 Mill BTU relief valve installed.

So the question is do you size it to the unit it is serving or the total combined BTU's of all the units?


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## 1703 (Jul 21, 2009)

How would that be different than say 4 100 gal/199K btu water heaters piped together?

Those reliefs don't get changed.

Or atleast I didn't think they needed to be changed.

I dunno. I didn't answer your question.


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## ILPlumber (Jun 17, 2008)

How can that single relief be in the hottest water on the outlet of each unit?

Are there valves between it and the units?

Each heating device gets a t&p valve.


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## 1703 (Jul 21, 2009)

I thought he meant each unit had a 2 mil. Rlf vlv.


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## SewerRatz (Apr 25, 2009)

Colgar hit the nail on the head. Each unit has a 2 mil relief valve. Here is a couple of pics for the reason I am asking. I been asked to put new ones in but I want to know if the 2 Mill BTU relief valves are needed or should a valve rated for the 250K BTU heater all it needs. There is a tag from the local fire department stating the system passed a boiler inspection. I wish I took a picture of that tag.


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## ILPlumber (Jun 17, 2008)

Oh OK. Valve should be sized for the invidual appliance it protects.


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## SewerRatz (Apr 25, 2009)

ILPlumber said:


> Oh OK. Valve should be sized for the invidual appliance it protects.


So these valves are an over kill. Thank you guys. I was getting worried that I installed mutple units wrong in the past.


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## Tankless (Jun 12, 2008)

ILP is correct. The original installer / designer goofed. 250K per unit.


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## rickmccarthy (Jul 20, 2009)

I am not entirely sure about this for some of the other brands but I do know that you do not use a T&P on a Rinnai Tankless it is simply a Pressure Relief Valve has nothing to do with Temp. Their Requirement is as follows:

ANSI Code requires the addition of an approved pressure relief valve

Valve must be rated not more then 150psi and no less then the water heaters maximum Btu input


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## SewerRatz (Apr 25, 2009)

rickmccarthy said:


> I am not entirely sure about this for some of the other brands but I do know that you do not use a T&P on a Rinnai Tankless it is simply a Pressure Relief Valve has nothing to do with Temp. Their Requirement is as follows:
> 
> ANSI Code requires the addition of an approved pressure relief valve
> 
> Valve must be rated not more then 150psi and no less then the water heaters maximum Btu input


 In this case each heaters maximum btu input is 250K , but for some reason the installer/designer felt the need to total the BTUs for all 5 units and size each units relief valve for the total BTU rating which is 1.25 Mil Btus so they installed 2 Mill Btu relief valves.


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## rickmccarthy (Jul 20, 2009)

Wow I would be surprised if they worked the way they wanted them to. Sounds way oversized to the point of impractical and dysfunctional


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## smellslike$tome (Jun 16, 2008)

What is the pressure rating for each valve? If it is an aggregate pressure as well then that is an enormously dangerous situation. Is there not a particular correlation between pressure ratings, temperature ratings (in general, I know there is not temp component here) and btu ratings? Are you saying that each valve had a pressure capacity of 750 psi? Surely not? Do they even make such a thing?


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## Redwood (Sep 8, 2008)

rickmccarthy said:


> Wow I would be surprised if they worked the way they wanted them to. Sounds way oversized to the point of impractical and dysfunctional


No they would work just fine as long as the temperature and pressure ratings were correct....

Just at a slightly higher cost than a properly sized T&P...


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## Protech (Sep 22, 2008)

Actually, I think you have that backwards. Since there is not heating of water in a closed static system the possibility of over pressure is pretty slim. I could however see a thermister malfunction and allow the burner to run full bore even though there is only .5gpm running. This would boil the water and the t/p would open and blow the pressure of as well as boost the flow thru the heater to cool the water back down to acceptable temps.

I challenge anyone on this forum to come up with a scenario where the heater could cause a static pressure buildup above the t/p blow off point Keep in mind those heat exchangers contain less than a gallon of water.



rickmccarthy said:


> I am not entirely sure about this for some of the other brands but I do know that you do not use a T&P on a Rinnai Tankless it is simply a Pressure Relief Valve has nothing to do with Temp. Their Requirement is as follows:
> 
> ANSI Code requires the addition of an approved pressure relief valve
> 
> Valve must be rated not more then 150psi and no less then the water heaters maximum Btu input


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## smellslike$tome (Jun 16, 2008)

Ok. 

I see static pressures all the time that exceed 150 psi on tanks due to failed prv. The t&p is either open with continuous flow or if it's only marginally higher than 150 psi or right at it it will cycle on and off. No thermal expansion, just whatever the main pressure is at that location.


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## Protech (Sep 22, 2008)

huh? could you run that by me again? :001_huh:



smellslike$tome said:


> Ok.
> 
> I see static pressures all the time that exceed 150 psi on tanks due to failed prv. The t&p is either open with continuous flow or if it's only marginally higher than 150 psi or right at it it will cycle on and off. No thermal expansion, just whatever the main pressure is at that location.


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## smellslike$tome (Jun 16, 2008)

Granted this is not the sort of hazard occasioned by pressure due to heating water and I've never heard of anything much beyond 200 psi which is not enough to blow anything up but it is certainly enough to start causing damage I would think, or at least I would never want to find out.


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## smellslike$tome (Jun 16, 2008)

Ok I didn't read carefully enough. No nothing comes to mind in the way of static pressure increase _caused by the heater._


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## Protech (Sep 22, 2008)

:001_huh:..................


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## Tankless (Jun 12, 2008)

Actually Protech brings up one of the first things I learned about tankless systems. Standing water on constant flame....a.k.a. A tanked heater. 

One of the other things I first learned is that there is no T in the TnP valve, so it's just a pressure relief valve....Ok, fine. But why is there no Temp stick required for tankless (i know some locals still require it, but the manufacturers do not)? 

The answer is there is more than enough redundent systems built into tankless units that should a malfunction occure the unit will shut down on its own. This was apparently proven to the powers that be and allowed to be installed as such. Now, I have personally seen a few old tankless units spike up to 180ish which of course were installed completely wrong...none the less it got really hot.

As for pressure......I HAVE to ASSume that a thermal situation could occure in a perfect storm. This would be something like a massive failure in system components on a closed system where flame was applied to the STANDING water which is now under constant flame. That, now over heated water would never make it to a TnP, just within the heat exchanger and about a foot or so in either direction of it. Closed system, boiling water (about 4/10ths of a gallon) you will have thermal expansion. Is it enough to cause damage? Call Myth busters or come out here and I'll supply the heater and we can make a day out of it. This is my personal reasoning for the required Pressure relief valve...as well as protecting the DWS from a pump / city surge. $.02


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## rickmccarthy (Jul 20, 2009)

I actually wrote it verbatum out of the Rinnai service manual I will try and scan and post for proof


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## TheMaster (Jun 12, 2009)

smellslike$tome said:


> Granted this is not the sort of hazard occasioned by pressure due to heating water and I've never heard of anything much beyond 200 psi which is not enough to blow anything up but it is certainly enough to start causing damage I would think, or at least I would never want to find out.


Water boils at 212 degrees at atmospheric pressure. The higher the pressure the higher the boiling point. So with the water heater under pressure the temp can go higher without the conversion to steam. In a closed system as the temp rises so will the pressure until the tank fails and ruptures. When the tank ruptures the rapid pressure drop allows the water to be converted to steam. This rapid exspansion of water to steam is what causes the explosion. Also if the prv failed and the system stayed closed now you have a thermal exspansion problem for sure because your exspansion tanks not workign anymore with that kinda pressure off the main.


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## Protech (Sep 22, 2008)

Solar water heaters frequently stagnate in the low 300s and do not produce steam because of the higher pressure.

As far as T/Ps on tankless heaters go, I'd be more worried about high temps that high pressures from thermal expansion. That small volume of water is going to produce so little pressure rise from thermal expansion that it's more or less negligible.

However, I could see disastrous results from the lack of a high temperature dump. Suppose that the burner malfuctions and is allowed to run full bore for some reason. Some one opens a faucet at very low flow. Let's say 1 gpm. The flow sensor kicks the burner on which instantly starts heating the water to 250F. When the 250F water reaches the pressure dropped zone (the outlet side of the faucet cartridge) it will flash boil (explode) possibly injuring anyone near by.

So yeah, temperature relief is a good idea. Pressure is kinda irrelevant in a tankless situation.



TheMaster said:


> Water boils at 212 degrees at atmospheric pressure. The higher the pressure the higher the boiling point. So with the water heater under pressure the temp can go higher without the conversion to steam. In a closed system as the temp rises so will the pressure until the tank fails and ruptures. When the tank ruptures the rapid pressure drop allows the water to be converted to steam. This rapid exspansion of water to steam is what causes the explosion. Also if the prv failed and the system stayed closed now you have a thermal exspansion problem for sure because your exspansion tanks not workign anymore with that kinda pressure off the main.


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## Tankless (Jun 12, 2008)

That's a bit of Disneyland right there. I won't say it's impossible, but....it's impossible for that to happen.



> The flow sensor kicks the burner on which instantly starts heating the water to 250F. When the 250F water reaches the pressure dropped zone (the outlet side of the faucet cartridge) it will flash boil (explode) possibly injuring anyone near by.


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## TheMaster (Jun 12, 2009)

Protech said:


> Solar water heaters frequently stagnate in the low 300s and do not produce steam because of the higher pressure.
> 
> As far as T/Ps on tankless heaters go, I'd be more worried about high temps that high pressures from thermal expansion. That small volume of water is going to produce so little pressure rise from thermal expansion that it's more or less negligible.
> 
> ...


 I would think with a faucet running 1 gpm the 250 degree water would automatically convert some/most to steam in the pipe well before the outlet at the faucet depending on the pressure drop in the system from the faucet opening. It would act like a steam generator IMO. With the faucet running the system is OPEN and the pressure will drop and with this pressure drop you will get steam at a lower temp. I'm not stating this as fact,just a theory. I'm not a scientist. In a closed system(the most dangerous) you would get a high spike in pressure before you get the temp relief to pop off. In an open system you would get the high ass temps without so much pressure and it would convert to steam. If i had to protect a system like we are talking about with either a temp relief or a pressure relief i would choose the pressure relief valve. Without pressure nothing blows up.


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## Protech (Sep 22, 2008)

I think you mean highly improbable, not impossible.



Tankless said:


> That's a bit of Disneyland right there. I won't say it's impossible, but....it's impossible for that to happen.


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## Protech (Sep 22, 2008)

If the distribution system is sized correctly, you will only get a few pound drop from pipe friction losses at 1gpm. Since that drop is unlikely to drop the boiling point below 250 (unless the pressure was already within a few pounds of the boiling point) the water will not boil and will only boil at the point where the system pressure drops dramaticlly (similer to how refridgerants act when they cross the throttling device in refrigeration, air conditioners and heat pumps)



TheMaster said:


> I would think with a faucet running 1 gpm the 250 degree water would automatically convert some/most to steam in the pipe well before the outlet at the faucet depending on the pressure drop in the system from the faucet opening. It would act like a steam generator IMO. With the faucet running the system is OPEN and the pressure will drop and with this pressure drop you will get steam at a lower temp. I'm not stating this as fact,just a theory. I'm not a scientist. In a closed system(the most dangerous) you would get a high spike in pressure before you get the temp relief to pop off. In an open system you would get the high ass temps without so much pressure and it would convert to steam. If i had to protect a system like we are talking about with either a temp relief or a pressure relief i would choose the pressure relief valve. Without pressure nothing blows up.


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