PRV expansion tank

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Plumber69

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Very few houses here have pressure reducing valves. If you have a incoming pressure of 90 PSI reduced to 60 and no expansion tank what are the consequences? What would it peak at PSI?
 

Reach4

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If there is a check valve in the path, you need a thermal expansion tank, regardless of whether there is a PRV.

If you have a PRV without the bypass, you need an expansion tank, because that acts like a check valve in addition to its pressure reducing.

If you know there will be no check valve, and you used a PRV with the "bypass" feature with your 90 psi coming in, you would not need a thermal expansion tank.
 

Bannerman

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Most PRVs are not equipped with a high pressure bypass unless specifically noted. Even when equipped with a high pressure bypass, the pressure downstream will be only lowered to the upstream pressure which at 90 psi, continues to be greater than recommended for a residential plumbing system.

Water cannot be compressed. With no provision to absorb water expansion caused by cold water being heated within the WH, the system pressure will rapidly rise usually until the WH Thermal Pressure Relief Valve begins to weep to provide some relief. The amount of expansion will be conditional on the temperature of the incoming water, the amount of temperature rise, and the quantity of water being heated.

Various tank manufacturers provide a thermal expansion tank sizing calculator such as this:. https://www.watts.com/resources/planning/etp
 

Sylvan

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Only one home did I have to install a PRV as the street pressure was over 100 PSI which I reduced to 50

The T&P did not seep or discharge as the only place I do install a check valves is on the sewer ejectors and sump pumps and return hot water lines


Regarding "What would it peak at psi?

That would depend on temperature pressure relationship

The hotter the water the higher the pressure in most cases

A 40 gallon water heater with 80 PSI line pressure will build to a pressure of 145 PSI with a temperature increase of 16°F. (Brandford White 2019)

That's a 65 psi increase occurring from a 16°F temperature increase, or roughly 4 psi increase for every 1°F of temperature increase.
 

LLigetfa

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What would it peak at psi?
That is a "how long is a piece of string" question for which the answer is YMMV. Buy a pressure gauge that has a tattler hand on it and monitor it for the high peak over time.

Often, things such as ice makers, humidifiers, RO filters, etc. will consume small amounts of water, bleeding off some pressure. Also, pressure spikes may overwhelm some toilet fill valves which may relieve excess pressure.
 

Sylvan

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General Minimum and Maximum Pressure Recommendations

Faucets: ASME codes require supply fittings to function at a minimum of 20 psi to 125 psi maximum (this includes metering and self-closing faucets).

Gravity Toilet Tanks: ASME/CSA codes require a minimum of 20 psi to a maximum of 80 psi static.
 

jadnashua

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Unless something is leaking, the pressure due to expansion will exceed the incoming 90-psi, or try to, so your pressure during/after heating water will get to 90-psi, and stay there until you use some water. If your PRV has a bypass, it should stabilize around that 90-psi, but that still exceeds the residential maximum recommended, i.e. the reason you put it in in the first place! Where I live, after a cold spell in the winter, my incoming water can be literally just above freezing. That means, it's got to rise nearly 100-degrees, and I've got a fairly big tank, so without an ET...there would be leaks every time the WH ran.

Do yourself a favor and put in an expansion tank...in combination with the PRV, it will keep the pressure nearly constant over time...yes, it will still go up some, but it won't spike like it would without one...and, if you go with one a little larger than the minimum, and the pressure rise will also be smaller.

In a really tight plumbing system, after a PRV has been installed, your T&P valve should open. Often, something leaks and a common one is a toilet fill valve...you might hear it, but since any excess water will just go down the drain as the bowl continues to fill, you may not notice. A dripping faucet may be big enough to prevent the pressure from spiking.
 
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