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If installing a pressure reducing valve, is there any sense in installing a one-way check valve in series, or would it be redundant? (i.e. does the PRV also act as a one-way valve?)

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City supply, or well/intake? Where are you considering installing the check valve, and the pressure reducing valve? You mention in a comment you have an expansion tank -- if you're on city supply, why? – gregmac Oct 9 '11 at 17:23
City. Close to the stoptap. Expansion tank because hot water expands (and it's not supposed to be pushing back out to the mains supply). – UpTheCreek Oct 9 '11 at 17:47

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First, no, they don't function as a check valve. They limit inflow by dynamically changing the valve opening so that when less pressure is on the out side, the valve opens up fully, and too much pressure on the out side causes the valve to shut. In a no pressure situation (where water could go backwards), it won't be impeded at all.

Assuming that this is for your residential plumbing, you generally wouldn't install a check valve, because you want to be able to drain the system at the shut-off if you ever need to do any modifications.

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Thanks - good to know. Problem is I have a backflow issue (which seems to be caused by a combination of fluctuating supply pressure and an expansion tank). This is causing incorrectly high meter readings. I don't think draining the system would not be a problem, as I have a tap in the basement lower than the shut of point. – UpTheCreek Oct 9 '11 at 11:07
A PRV won't help you if the city supply is irregular - PRV's help to keep the pressure from exceeding a certain point in your system, but won't prevent it from going too low, so in your case a check valve, after the shutoff and before your low point tap, is your better option. – The Evil Greebo Oct 9 '11 at 11:12
The city supply is fluctuating between about 4Bar-4.2Bar . Since it never goes below 4, I think setting the PRV to e.g. 3.8 should solve the problem. I want to put a check valve in too - but I don't want to soley rely on this... there is already a check valve before the expansion tank - but whether this is not able to react to small backflows quickly enough, or whether it's just plain broken I don't know. – UpTheCreek Oct 9 '11 at 11:20
+1 Greebo. I think your answer is right, just wanted to provide a link to the PRV with backflow bypass and my own experience with these. – BMitch♦ Oct 9 '11 at 12:29
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Many PRVs also include a backflow bypass, but since this is explicitly described in the product specs, it implies that not all valves include this functionality. And you may be able to find a combination device out there with both capabilities.

Here's an example with a backflow bypass which you can see more details about in the manual.

Without evidence that your device has both capabilities, then all you can assume is that it will prevent a backflow when your inside pressure is below that of the municipality.

I will note that in my experience, I've had the pressure inside exceeded my hot water tank's TPR valve until I installed an expansion tank with only a PVR installed, but it's possible that the municipality has a backflow preventer on their meter. I'm fairly certain my supply pressure doesn't exceed the TPR on it's own (my old valve failed, and the TPR never tripped, but after I fixed my last leaking toilet and faucet, the TPR started to go since there was nowhere left to expand).

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