High pressure between Navien and Tank

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djdavenport

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I'm a little baffled. I have a Navien NCB 240 that feeds a 10 gal electric buffer tank. I noticed that the T&P valve on the tank was dripping a little, so I checked the pressure, and it was spiking at about 140 psi. It goes from normal to the high pressure whenever the element in the electric HW kicks in. The contractor had not put in an expansion tank, so I figured that would solve it.

But, today, in checking, the high pressure only occurs BETWEEN the Navien HW outlet and the buffer tank (and probably the whole DHW system, although I've only checked the pressure at the tank.) Not on the cold water inlet side, (so my plan of putting in the expansion tank on the main right after the PRV would have done nothing. That pressure is normal.) I am guessing that there is some kind of check valve in the Navien that is not allowing the high pressure that builds up in the buffering tank back through. Is that a logical conclusion?

As far as a remedy, should I put the expansion tank on the hot (and high pressure) side?

Thanks for your advice.
 

Reach4

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As far as a remedy, should I put the expansion tank on the hot (and high pressure) side?
That would then be your only choice. Your conclusion is quite logical. Although I am not sure what you mean that you only checked the pressure at the tank. If you dribble water from a hot faucet that would keep expansion pressure down on the hot. By your observations, dribbling water from a cold socket will not keep the hot pressure down.

Therm-X-Trol and Watts thermal expansion tanks are speced to 200F maximum operating temperature. http://www.amtrol.com/media/documents/thermxtrol/MC8520_08_14_TXT_Sizing_Chart.pdf http://media.wattswater.com/2915054.pdf

You can connect to a line farther from the WH, as long as there is not a valve that would be closed while the WH is operating. The flow is very slow, even though the tanks come with 3/4 NPT threads. The big pipe size is just for mechanical reasons and to make connections to 3/4 inch fittings easier.
 
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djdavenport

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Did a little more sleuthing. Turns out, they had installed a check valve on the line from the Navien to the buffering tank--I imagine to isolate the recirc pump that feeds in and out of the tank. But, it is most definitely its own closed system. Or subsystem. So, I think you're right. I better tee it into the hot side. The ET will easily to be two or three feet away, so hopefully the heat will not be too much of an issue.
 
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