# Hydronic water heaters blowing relief valves-



## Tim Whistler (11 mo ago)

Okay, bear with me on this one. We have installed probably 25-30 A.O. Smith direct vent hydronic water heaters with side taps in condo buildings here in the Maryland area. Three or four of them have had a serious issue with the relief valve blowing out very large amounts of water, so they are blowing due to overheating, not pressure which would only give a short burst of water. We have checked the gas valves and even replaced one and those do not seem to be the problem. Water pressure at these buildings is 65 PSI to 75 PSI and we are using the larger st-12 potable water expansion tanks pumped up to match the building pressure. I used to set the tank thermostat about 80% of the way up so the heating system would be nice and hot but we have been setting them cooler, around 140 degrees to 150° max for the heating system and it has helped a little bit but a couple of them have still blown. The mixing valves for potable water of course are set at 120-125F.
A.O.Smith thought it was heat stacking due to covid and people washing their hands much more often - but why then JUST the hydronic water heaters? 
One of my guys thinks it may have to do with the heat loop returning cooler water down near the thermostat, next time we'll tie the return to the drain valve instead I guess, and see if it makes any difference. 
Has anyone else had this problem!? A couple of these customers are super upset, and we're very lucky none of the drain pans have overflowed or caused any damage yet. 
Thanks, 
Tim Whistler 
Tim Whistler Plumbing LLC 
timwhistlerplumbing.com


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## Tim Whistler (11 mo ago)

Sorry, forgot the pics. Couple of recent ones....


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## skoronesa (Oct 27, 2015)

You're using these as space heaters? Check the manual to make sure you have them piped properly. I would also let a couple psi out of those expansion tanks, don't quite match your building pressure.

Are the pans piped to drains?


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## OpenSights (Mar 23, 2015)

I work on that type of system, but the complex provided their own heaters, State. Never had that problem, and the maintenance guys would crank them all the way up in the winter.

Here’s a thought, semi unrelated, but possible. Because of Covid, Rheem couldn’t get their normal gas control valves so in order to provide heaters they bought garbage valves from France. I have another control valve to replace this week. It could just be bad parts.


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## Sstratton6175 (Jan 10, 2021)

What’s the rating on the T&P? Sometimes when those heaters are specifically used for hydronic heating they have a T&P with a higher rating. Some dumb kid at the factory could have installed the wrong ones in the unit all day long I’d also double check the expansion tank size with AO Smith even an st-12 may be too small.


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## Papaw U (11 mo ago)

Hello to all,
I have an on going issue that i could really use help with. My old water heater that had been in for approx. 15years starting leaking through the pressure relief valve(pop off valve). I replaced it, and it still leaked. I was advised to replace the water heater. I did replace it with a performance model by Rheem (50 gallon electric). The new water heater also "leaked" / dripped through the pressure relief valve. I was then advised to check my incoming or line pressure. I did that and realized my pressure regulator valve was bad. I replaced it and got a regular incoming line pressure of approx. 40PSI. The intermittent dripping from the water heater continued. I called the manufacturer, and they sent me a new pressure relief valve. I changed it out and had the same result. I called the manufacture again, and they replaced the entire water heater. This was installed and it still intermittently dripped. I called the manufacture again, and they have now sent another pressure relief valve. I replaced it and the dripping continues. It is intermittent and not constant. I have also checked the water temperature coming out of a faucet, and the thermostat settings on the water heater. The thermostats are set to approx 120degrees. The water temperature coming out of the faucet is between 115 and 120 degrees. I placed a pressure gauge on the drain valve to see what it shows. I left it on over night. While the dishwasher was running. The pressure went from about 40psi up to 80psi, and then returned back to 40psi (no dripping). After two of us took showers this morning (with the gauge being on there over night) the pressure climbed up to between 140-145, and started dripping. Had to leave for work. Anyone have any ideas or recommendations?


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## Blue2 (12 mo ago)

Papaw U said:


> Hello to all,
> I have an on going issue that i could really use help with. My old water heater that had been in for approx. 15years starting leaking through the pressure relief valve(pop off valve). I replaced it, and it still leaked. I was advised to replace the water heater. I did replace it with a performance model by Rheem (50 gallon electric). The new water heater also "leaked" / dripped through the pressure relief valve. I was then advised to check my incoming or line pressure. I did that and realized my pressure regulator valve was bad. I replaced it and got a regular incoming line pressure of approx. 40PSI. The intermittent dripping from the water heater continued. I called the manufacturer, and they sent me a new pressure relief valve. I changed it out and had the same result. I called the manufacture again, and they replaced the entire water heater. This was installed and it still intermittently dripped. I called the manufacture again, and they have now sent another pressure relief valve. I replaced it and the dripping continues. It is intermittent and not constant. I have also checked the water temperature coming out of a faucet, and the thermostat settings on the water heater. The thermostats are set to approx 120degrees. The water temperature coming out of the faucet is between 115 and 120 degrees. I placed a pressure gauge on the drain valve to see what it shows. I left it on over night. While the dishwasher was running. The pressure went from about 40psi up to 80psi, and then returned back to 40psi (no dripping). After two of us took showers this morning (with the gauge being on there over night) the pressure climbed up to between 140-145, and started dripping. Had to leave for work. Anyone have any ideas or recommendations?


hit it with your purse


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## Papaw U (11 mo ago)

Blue2 said:


> hit it with your purse


I did and it has already cost me too much.


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## Blue2 (12 mo ago)

Papaw U said:


> I did and it has already cost me too much.


Maybe call a plumber. It’s a really simple job that you are over complicating.


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## OpenSights (Mar 23, 2015)

Papaw U said:


> Hello to all,
> I have an on going issue that i could really use help with. My old water heater that had been in for approx. 15years starting leaking through the pressure relief valve(pop off valve). I replaced it, and it still leaked. I was advised to replace the water heater. I did replace it with a performance model by Rheem (50 gallon electric). The new water heater also "leaked" / dripped through the pressure relief valve. I was then advised to check my incoming or line pressure. I did that and realized my pressure regulator valve was bad. I replaced it and got a regular incoming line pressure of approx. 40PSI. The intermittent dripping from the water heater continued. I called the manufacturer, and they sent me a new pressure relief valve. I changed it out and had the same result. I called the manufacture again, and they replaced the entire water heater. This was installed and it still intermittently dripped. I called the manufacture again, and they have now sent another pressure relief valve. I replaced it and the dripping continues. It is intermittent and not constant. I have also checked the water temperature coming out of a faucet, and the thermostat settings on the water heater. The thermostats are set to approx 120degrees. The water temperature coming out of the faucet is between 115 and 120 degrees. I placed a pressure gauge on the drain valve to see what it shows. I left it on over night. While the dishwasher was running. The pressure went from about 40psi up to 80psi, and then returned back to 40psi (no dripping). After two of us took showers this morning (with the gauge being on there over night) the pressure climbed up to between 140-145, and started dripping. Had to leave for work. Anyone have any ideas or recommendations?











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## asapmarty (Nov 19, 2012)

Tim, I found this in A O Smiths installation manual for a combi installation. I am providing this just in case. I do not know how you are controlling flow to the hydronic heating circuit or if the controls-valves are in proper working order. A properly installed flow control valve is required to prevent thermosiphoning. Thermosiphoning is the result of a continuous flow of water through the air handler circuit during the off cycle. Weeping (blow off) of the temperature-pressure relief valve (T&P) or higher than normal water temperatures are the first signs of thermosiphoning. I am not saying this is the problem, just a possability.


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## MACPLUMB777 (Jun 18, 2008)

asapmarty said:


> Tim, I found this in A O Smiths installation manual for a combo installation. I am providing this just in case. I do not know how you are controlling flow to the hydronic heating circuit or if the controls-valves are in proper working order. A properly installed flow control valve is required to prevent thermosiphoning. Thermosiphoning is the result of a continuous flow of water through the air handler circuit during the off cycle. Weeping (blow off) of the temperature-pressure relief valve (T&P) or higher than normal water temperatures are the first signs of thermosiphoning. I am not saying this is the problem, just a possibility.


 WOW 20 POSTS IN 10 YEARS ! ! !


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## Debo22 (Feb 15, 2015)

When all else fails, throw some more parts at it


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## Sipp (Jul 14, 2016)

Tim,
Are these connected to air handlers? If so, I have seen the check valve in the air handler system failing and cause the water to gravity circulate. This can cause the system to build up temperature and cause the relief valve to release.


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