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residential service
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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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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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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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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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residential service
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Discussion Starter #6
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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residential service
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Discussion Starter #7
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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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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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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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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It don't matter smells, you will figure it out. Just let us know the outcome.
 

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

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J. McCabe Plumbing Inc.
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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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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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J. McCabe Plumbing Inc.
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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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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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