# tankless relief valve



## smellslike$tome (Jun 16, 2008)

I just completed my third tankless install in the last 2 years. Obviously I don't have a wealth of experience with these although I look forward to selling the snot out of them in the future.

I finished last night around 9:00 p.m. (didn't start until 12:30 p.m.). I get a call this morning from the ho stating that water is pouring from the relief valve. Pressure was fine pre install but I had to relocate the existing prv in the crawl space and now I'm wondering if it should have just been replaced it whether it was bad or not? I moved it back upstream about 6' on a galvanized system in order to access 3/4" piping. Yes I'm nervous about this install on galvanized but was able to inspect the interior surfaces of the galvanized and it was remarkably clean. Of course there was evidence of light surface rust but no full blown decay of the pipe. There remained about 10' - 15' of 3/4" galvanized piping upstream of my supply connection to the wh which was run in pex and of course the service from the meter to the house is most likely galvanized. I'm thinking that the relocation of the old exisiting prv may have shook loose some scale or something and has rendered the prv ineffective. 

Could another possibility could be thermal expansion? I have not experienced this with either of the other two units I have installed (Rinnai, Takagi, and this was my first Noritz). I read something on Noritz's website about thermal expansion the other day before I installed it but dismissed it not understanding how you could have thermal expansion with a tankless. Today as I think about it more I think I can conceive of a possible scenario and want to see what anybody thinks.

Ok, here it is. Hot water is being used in the house and as soon as demand ceases, the burner shuts off but their is residual heat built up in the heat exchanger which continues to heat the water for a short time, which creates thermal expansion past 150 psi, which opens the relief valve, which could (if flow is large enough) constitute a hot water demand since it is downstream of the heater, which will cause the burner to turn on, which creates an endless cycle.

Am I wrong? Has anybody witnessed this? It seems silly to me to install an expansion tank for a tankless water heater, but now suddenly maybe not quite so silly.

I will be returning in the morning to check everything out but am hoping to get some input from anybody that has a lot more experience with these than I do.


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## user4 (Jun 12, 2008)

Anytime you have a closed system you need an expansion tank on the heater.


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## Ron (Jun 12, 2008)

When you get back there throw on a pressure gauge at of near the tankless unit, test the pressure prior to the test, if you see a spike in the pressure at start, and it reads 100psi or better, replace the PRV valve cause it has more then likely failed, if that is fine then start running the test, get it to fire up and then shut if down, watch the gauge, you will know real soon if the tankless is indeed causing expansion to take place.


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## user4 (Jun 12, 2008)

Ron The Plumber said:


> When you get back there throw on a pressure gauge at of near the tankless unit, test the pressure prior to the test, if you see a spike in the pressure at start, and it reads 100psi or better, replace the PRV valve cause it has more then likely failed, if that is fine then start running the test, get it to fire up and then shut if down, watch the gauge, you will know real soon if the tankless is indeed causing expansion to take place.


Whenever you heat water it expands, it doesn't matter that you are not storing it, it still has to expand somewhere, which is impossible in a closed system.


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## Ron (Jun 12, 2008)

We have thousands of homes here that have PRV's valve on there service line, and I can contest that there is very few and that's not many, that have an expansion tank attached to there system, expansion tanks are needed only under extreme pressure problems.


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## smellslike$tome (Jun 16, 2008)

This is an issue that I feel like I am pretty in tune with. Of all plumbing tasks under the sun, pressure problems are the ones that I will correct more than any other plumbing task. The only problem is that I don't have nearly as much experience with pressure problems with respect to tankless installs since I have only done a few of them to this point.

The problem is so pervasive here that I automatically check house pressure on every call whether asked to or not (I'm never asked to). I explain to the ho before I check it that there are two things that control pressure in their home, prv and thermal expansion. I explain what the allowable parameters are and what it means if they are found to be above maximum. I explain how they can be at proper house pressure with respect to their prv and still be at risk from high pressures (thermal expansion).

The problem in this instance is that the pressure was fine pre-install but I failed to re-check it post install (it was late, dark, and I was very tired, and I didn't think to re-check it). The second problem is that I would have thought that there should be no need for an expansion tank in a system that shuts off the burner the moment demand stops, ... except for the possibility of residual heat being transferred from the heat exchanger to the water after demand has ceased as I described in my earlier post. This is a possibility I had not accounted for. What I would like to know is is this a reasonable conclusion and has anybody experienced this specifiacally or should I look somewhere else?


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## smellslike$tome (Jun 16, 2008)

Ron The Plumber said:


> We have thousands of homes here that have PRV's valve on there service line, and I can contest that there is very few and that's not many, that have an expansion tank attached to there system, expansion tanks are needed only under extreme pressure problems.


Run this test. Next time you go into a home with a prv, ngwh and no expansion tank, connect your gauge to the wh drain, run a tub or shower w/normal bathing temps, after 5 - 10 minutes turn the water off and go watch the gauge. It is astonishing how quickly it will climb. For a tanked water heater, especially gas, in a closed system, there should always be a properly charged exp. tank present.


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## Ron (Jun 12, 2008)

I have installed a few tankless in my time, and have never had the need for an expansion tank, you talked about you moved the prv to the crawl space, disruption in the old system could have, (not saying it did, I'm not there to test things out), caused the prv to clog causing it to fail, if it was me, I'd be thinking more towards the prv valve, before I would think of an expansion tank, as you stated, there was no prior problem, so why would there be one now, a normal tank heats water in excess of what that tankless will ever do.


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## user4 (Jun 12, 2008)

I can't answer that specifically, as it has been code here for years that you have to install an expansion tank on the water heater in a closed system, just like a boiler always gets an expansion tank because they always have a PRV on them and boilers have no real storage to speak of. I have seen water heaters with the flue baffles in them crushed from a combination of a closed system and bad TPRV. Water expands when heated, that expansion has to go somewhere, sometimes there is a fixture close enough that the pressure forces itself through the fixture, usually a ballcock, but in the end physics don't lie and you can't compress water.


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## Ron (Jun 12, 2008)

smellslike$tome said:


> Run this test. Next time you go into a home with a prv, ngwh and no expansion tank, connect your gauge to the wh drain, run a tub or shower w/normal bathing temps, after 5 - 10 minutes turn the water off and go watch the gauge. It is astonishing how quickly it will climb. For a tanked water heater, especially gas, in a closed system, there should always be a properly charged exp. tank present.



Well I can can say is 25 years at this and I my have installed one or two expansion tanks in the time frame and they were under special conditions.

There not needed in each and every home.


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## Ron (Jun 12, 2008)

It don't matter smells, you will figure it out. Just let us know the outcome.


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## WestCoastPlumber (Jun 17, 2008)

Expansion tank may take care of the situation. Before I install the expansion tank, I would replace the valve to make sure it is not faulty, maybe clean the PRV, galv is nasty stuff to take apart, no matter how careful you are, things break loose all the time.

I personally would never install a tankless on a galv system. NEVER.


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## 22rifle (Jun 14, 2008)

Ron The Plumber said:


> Well I can can say is 25 years at this and I my have installed one or two expansion tanks in the time frame and they were under special conditions.
> 
> There not needed in each and every home.


Most meters in the areas I have worked are now installed with a check valve. 

Many codes require an expansion tank if you install a PRV.

So where is the water going in your systems when there is thermal expansion?


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## Plumberman (Jul 7, 2008)

Busting the tank if it is a tank heater....


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## Ron (Jun 12, 2008)

Expansion takes place that's a fact but it's not hitting or exceeding the 150psi that's for sure.


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## A Good Plumber (Jun 18, 2008)

Smells, I've installed about 40 tankless water heaters over the past 2 years. Never have I had to install an expansion tank. The code requires an expansion tank on any closed loop hot water system, but a tankless water heater does not heat the water unless there is a flow of water, at which point it is no longer a closed system. A boiler or tank type water heater heats the water even though there isn't any flow.

I am a member of the local chapter of IAPMO and this very question was asked of all the inspectors present. They informed us that because of the reason I described above, an expansion tank is not required by UPC code.

Ron is right, it's a failed PRV.


BTW, I've had no problems with any one of the installations.


Change out the PRV


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## 22rifle (Jun 14, 2008)

Ron The Plumber said:


> Expansion takes place that's a fact but it's not hitting or exceeding the 150psi that's for sure.


OK. The UPC says anything over 80 is not legal. So I use that as my guide. If it goes over 80 we got a problem and it needs correction.


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## Ron (Jun 12, 2008)

22rifle said:


> OK. The UPC says anything over 80 is not legal. So I use that as my guide. If it goes over 80 we got a problem and it needs correction.


Yes by installing a prv valve, problem solved, factory sets them at 50 PSI


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## A Good Plumber (Jun 18, 2008)

22Rifle...Keep in mind, it's not the tankless that's causing the system to build pressure, cause the tankless is not heating water, thus no thermal expansion, it can only be a failing PRV.


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## 22rifle (Jun 14, 2008)

Ron The Plumber said:


> Yes by installing a prv valve, problem solved, factory sets them at 50 PSI


I am sorry Ron. I am not making myself clear and I apologize. I don't mean to be busting your chops. I am just trying to figure out where you are coming from in case I can learn something new.

So you install a PRV set at 50 psi. Now the water heater kicks on. The pressure is going way over 50 psi. and most likely over 80 psi. In your scenario, where is that pressure going? Do you guys installs 75 psi relief valves in your water lines?


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## 22rifle (Jun 14, 2008)

A Good Plumber said:


> 22Rifle...Keep in mind, it's not the tankless that's causing the system to build pressure, cause the tankless is not heating water, thus no thermal expansion, it can only be a failing PRV.


What? The tankless does not heat water? Gimme my money back! 

Kidding man! I know exactly what you meant. Just had to razz you a bit.

Seriously, I am not talking about the OP's case. I am talking in general here. Trying to understand where Ron is coming from.


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## smellslike$tome (Jun 16, 2008)

22rifle said:


> Most meters in the areas I have worked are now installed with a check valve.
> 
> Many codes require an expansion tank if you install a PRV.
> 
> So where is the water going in your systems when there is thermal expansion?




??? Out the t&p valve. Does no one but me experience this with a regular tanked water heater?

If I were unscrupulous I could sell probably 2 water heater installs each week because people call in a panic because water is leaking out onto their floor from the water heater. The leak point is nearly always the t&p and is the direct result of either high pressure (failed prv) and/or thermal expansion (failed or non-existant exp. tank). Now the ho doesn't know the difference and it would be easy enough to sell them a new wh, exp. tank, and prv except for the fact that it would almost always be unnecessary to replace the wh under these circumstances. It is however necessary to replace the prv if it is bad and to install an expansion tank if there is none or it has failed.

Think about how many houses you go to in which the relief line goes through the floor or out a wall. These people have no idea they are even losing any water at all. They are paying to heat water only to have it discharged through the relief line because of thermal expansion. Oh yeah and t&p's don't always open when they are supposed too either. I have seen them go as high as 165 psi before opening. The t&p almost always is just doing it's job though and doing it correctly. Very rarely will I find a t&p that is bad and when I do it is nearly always some physical damage.

It has just become automatic with me to perform this test and have this discussion with the ho in every house I go into. Once I have explained everything and shown them that they have high pressure and if they then don't want to do anything about it, fine, that's up to them but I am going to make certain that they understand that they are throwing water and energy away and wearing out their fixtures prematurely.

Now, back to the tankless. Looking back on it, and being of the opinion that a working prv should never be touched after about a year or two, not even for adjustment, I'm betting that it has failed and when I test it in the morning I will find just that. If thermal expansion was a problem with a tankless system then I would expect that they would have very clear references and warnings contained in the install instructions. I have not found any mention of it for Noritz and don't remember any reference to it for Rinnai or Takagi. Believe me I would not miss an opportunity for add on sale of an expansion tank if I had any idea that it would be necessary. 

I will find out in a few hours.


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## Ron (Jun 12, 2008)

22rifle said:


> I am sorry Ron. I am not making myself clear and I apologize. I don't mean to be busting your chops. I am just trying to figure out where you are coming from in case I can learn something new.
> 
> So you install a PRV set at 50 psi. Now the water heater kicks on. The pressure is going way over 50 psi. and most likely over 80 psi. In your scenario, where is that pressure going? Do you guys installs 75 psi relief valves in your water lines?


The relief valves on the heater are the required 150psi, the pressure created during the heating phases on a tanked heater is not great enough to exceed the 150, if it don't exceed, the system will handle the expansion, I don't know the science behind how much pressure will build under heated water in a closed system, but it might have some thing to do with developed length within the system, the shorter the developed length the less capacity for water to pressurize in it, if the DL is short then an expansion tank could be needed. I'm just talking off the top of my head on what I think is taking place inside the closed system. It may all seem crazy to you that here we hardly install expansion tanks, but believe me, it's not being done here. 

It's just not a big issue in residental homes here.


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## smellslike$tome (Jun 16, 2008)

A Good Plumber said:


> Smells, I've installed about 40 tankless water heaters over the past 2 years. Never have I had to install an expansion tank. The code requires an expansion tank on any closed loop hot water system, but a tankless water heater does not heat the water unless there is a flow of water, at which point it is no longer a closed system. A boiler or tank type water heater heats the water even though there isn't any flow.
> 
> I am a member of the local chapter of IAPMO and this very question was asked of all the inspectors present. They informed us that because of the reason I described above, an expansion tank is not required by UPC code.
> 
> ...




This is the response I was looking for! Thank you. 40 installs with no expansion tanks is enough to satisfy me.


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## 22rifle (Jun 14, 2008)

Thanks Ron. I respectfully disagree with you when you seem to say it's OK as long as it doesn't go over 150 psi. I don't want to see it going over 80 psi. like the UPC requires. 

Do this sometime. On a system you know has no leaks, put a pressure gauge on a gas water heater and turn the gas up enough to kick it on. I think you will be amazed.

I forget, do you use copper or PEX?


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## 22rifle (Jun 14, 2008)

smellslike$tome said:


> This is the response I was looking for! Thank you. 40 installs with no expansion tanks is enough to satisfy me.


Yeah, we all got off onto tank systems. Sorry about that man.

A tankless never heats any significant amount of water unless the water is flowing which automatically relieves the pressure.


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## Ron (Jun 12, 2008)

Pex always.

That's fine if you disagree with me, was not trying to make you agree with me, I'm just letting you know what it is we do here.


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## 22rifle (Jun 14, 2008)

Ron The Plumber said:


> Pex always.
> 
> That's fine if you disagree with me, was not trying to make you agree with me, I'm just letting you know what it is we do here.


Sure. It's cool. I wasn't trying to do anything but understand you.

I wonder how much PEX gives as opposed to copper.


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## Alan (Jun 18, 2008)

We always do because code requires it, and actually have solved a myriad of leaky t&p valves with expansion tanks.

:whistling2:


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## smellslike$tome (Jun 16, 2008)

PRV is the winner but still not without one more thumb in my eye. Ran the test and it's definately the prv so I crawl under the house and change it out. Crawl back out and check the pressure, ... 100 psi! Oh man! What have I missed? So I'm standing there scratching my head and finally decide well it must be a bad prv right out of the box. So I grab another one off the truck and back under I go. Changed it out again (love doing stuff twice especially when there is a cs involved), crawl back out, recheck pressure, 55 psi . I could probably sink a destroyer with all the prvs I've changed out (well maybe a canoe) but this is the first time I've ever had one bad right out of the box.

Anyway, mystery solved and I have concluded, until shown otherwise, that an expansion tank is not necessary for a tankless water heater.


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## Ron (Jun 12, 2008)

I never had a bad prv out the box, good catch, glad the problem got solved. :thumbsup:


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## A Good Plumber (Jun 18, 2008)

_"Anyway, mystery solved and I have concluded, until shown otherwise, that an expansion tank is not necessary for a tankless water heater."_

Excellent!....Good Job!....Another mystery solved.


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## GrumpyPlumber (Jun 12, 2008)

*I have one conflicting point from a job about a year ago.*

*Ron makes a valid point regarding expansion vs size of piping.*
*If you look at the pressure ratings of copper pipe, you'll see that the smaller the pipe size, the greater the pressure it can handle.*

*A tank type water heater will expand at the sides and absorb much more thermal increase in density than 3/4" or 1/2" pipe.*

*Last year I had a customer that complained of vibrating pipes when certain fixtures where opened, but it wasn't all the time...he couldn't understand why it was sporatic, also the relief on his tankless would spill a few drops when this happened.*

*He had a PRV, it was working, pressure was 60.*

*He tried to show me, but when he opened the laundry, shower valve or flushed the toilet, nothing happened.*

*His wife used the KS upstairs as we spoke, then it occured to me to check again...voila, the relief dripped!*

*An expansion tank solved the problem.*

*I didn't get into deeper investigation, it still seems odd that the opened hot water fixture didn't relieve the pressure but the tank worked, customer happy.*


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## pipepimp (Jun 19, 2008)

some things to think of ,is this a tankless in a boiler? does the the water service have a check valve? if so put a expantion tank on the hot water. ask your self why was the prv going off in the first place ..things to look for bad heating expantion tank ,feeder valve not working right , and my favorite a leak in the tankless coil will cause the prv to blow. to test shut off cold h2o going into coil wait 24 hrs to see if the prv blowes.


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## Plumber Jim (Jun 19, 2008)

The reason you may see alot of houses in your area with a PRV and no need for an expansion tank is because they may have a thermal bypass. which only works if the city pressure is less than the pressure on the house side of the PRV.


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## HonestPlumb (Jan 25, 2015)

I am in NJ and in my area street pressure varies from 75#'s to 175#'s. We have to install a Thermal Expansion tank with every tank water heater, as most inspectors require it. I have talked to one, who interprets the code as if the reducing valve has a by pass, and the street pressure is lower than 150#'s a tank is not required. Due to the excess pressure will go out to the street. The majority of inspectors do not go with that though, so we install them on every tank heater.


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