Thinking of adding a pressure reducing valve (PRV) to reduce my home's water pressure. I understand that I'd need to add an expansion tank to the incoming cold water line of the domestic hot water heater (DHW).

My question is should I size the expansion tank for the system pressure before installing the PRV or after installing the PRV? By sizing for original pressure I have the capacity to handle expansion in the event the PRV fails.

Also related question... Do I pressurize the tank to the system pressure before or after the PRV for the same reason of possible PRV failure?

I'm probably just going to go with a larger expansion tank that will handle the current incoming pressure since it's not much more expensive and having a larger tank won't hurt anything but I'm curious to know the answers and reasoning.

Here are the stats:

Current pressure around 92 PSI Desired pressure 65 PSI (maybe a little more like 68 PSI) HW capacity 40 gallons HW temperature 125F

  • What is the incoming pressure? What will you reduce that to?
    – Ecnerwal
    Jun 6, 2015 at 17:16
  • @Ecnerwal just over 90psi current, guess reduce it to 65 psi? Jun 6, 2015 at 17:36

1 Answer 1


PRV (or pressure regulator) failure should be a rare event.

One problem with pre-pressurizing the tank based on the incoming pressure is that it will accept essentially no water until you reach that pressure, so that seems like a poor idea. Whether the incoming pressure will be so far above the functional pressure that it will pop the expansion tank bladder will depend on the pressures in question and the tank. I suppose the approach to take would be to figure out how far to pre-pressuirize it to prevent popping in the event of regulator failure, but that is not actually covered in the usual installation instructions.

If there is a very large difference in pressures, you might want a relief valve on the low-pressure side. This will of course depend on your local system pressure and how far down you are reducing it.

Responding to 90/65 psi comment - this is probably well within the capacity of normal expansion tanks. A typical well tank is expected to cycle over a 20 psi range, but the actual volume in the bladder changes (20/40 PSI holds more water than 40-60 or 50-70 with correct pre-charge pressure.) Given typical upper working pressure above 100PSI, you should be fine pre-pressurizing to 63 PSI (or 2 PSI below whatever you actually set to) and not pop the bladder (it can work at 20/40, so at 63 it can hold enough water to go to 126, even though that's an atypical use-case; so 95 should be just fine.)

A 100PSI relief valve would still be a smart move (i.e. don't make the T&P on the hot water tank the only relief in the system) to protect against a higher than expected input pressure and regulator failure, but might be moderately paranoid (I get that way in systems design, at least where it does not cost too much.)

  • Thanks. I added more variables to the question but this answer was very helpful. Even at 130F temperature the smallest tank will still have enough capacity for about 95PSI incoming pressure if precharged to 65. When you say add a pressure relief valve on the low side, do you mean in addition to the T&P valve on the WH? Jun 6, 2015 at 18:02
  • 1
    I do. This is standard fitment on well systems; probably paranoia for municipal, but being municipal, they can change things up without you knowing about it. On the plus side, without the temperature element or the hot water exposure, these seem to be much less failure prone than T&P valves.
    – Ecnerwal
    Jun 6, 2015 at 18:07
  • It can be anywhere on the system right? Maybe 10-15' of piping away is a laundry sink and if I install it there can have it drain into the sink. Would that work? Jun 6, 2015 at 18:15
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    Yes, it will work anywhere on the system so long as it's not valved off from the system - laundry sink should be fine. Odds are it will sit there and do nothing, but it's good to have, IMHO. About $11.
    – Ecnerwal
    Jun 6, 2015 at 18:19
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    I prefer fixed pressure for this application, less to go wrong. aquascience.net/fittings/index.cfm?id=1160 Not a good idea to reduce the output size, but fine to use el-cheapo PVC for it, or PEX if you like. Rarely used, never drunk from, any water that touches it is headed for the drain.
    – Ecnerwal
    Jun 6, 2015 at 21:46

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