This is an image of the main water shut off valve. I am a new inspector in training. Want to know if it is necessary to have pressure relief valve to maintain positive water pressure flowing in from public water to the main holmes water 
supply?
It’s not necessary.
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Thank you for the feedback, always appreciated.
It is only needed if the water pressure is over 80 psi. And if there is a prv installed there needs to be an expansion tank as well. As the prv makes it a closed system and requires the expansion tank. I see prv’s installed on brand new houses without the expansion tank, this can really cause lots of issues.
Thank you. This platform is amazing.
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How would a pressure relief valve maintain posative water pressure?
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Why would a water supply system need an expansion tank if a PRV is installed? Most homes have a PRV at the meter with no expansion tank. You know why? There is no expansion of the water.
Now install a tank water heater and you have created expansion. An expansion tank is required. 90% of my inspections have tankless water heaters. I rarely see an expansion device on the system and there are zero issues.
So what are you seeing that’s so problematic?
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Water always expands when heat is applied to it.
Much less expansion than with a tank, sure, but it does. And the manufacturer recommends the expansion tank, too.
(erroneous citation removed)
It also happens to be part of my states SOP to report a missing expansion tank when a PRV is installed.
You are misquoting. You are quoting from the potable water system from one sentence not the sentence I referenced with a Rinnai tankless water heater. So where is the expansion tank on the manufacturers installation manual for Rinnai? You would think the leader in the industry would know a thing or two.
An expansion tank is not required in the current IRC for tankless water heaters unless it is used for space heating. The installation instructions from the manufacturer also state the same information.
You gotta love the fact that Rinnai illustrates components in their water heater drawing that are not referenced in the key 
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thank you, that was hasty. I see the recirculating lines. I was once having a conversation with a local inspector about this very thing and they mentioned manufacturers require it, besides our state’s regulations…
I agree that real world implications are certainly much less than with a tank water heater. But I also commonly see those baseball sized expansion tanks on tankless’ that are not being used for space heating. Because, of course, there is some expansion. The higher the GPH, I assume the higher the expansion. The higher the starting PSI is, the more implication that expansion has.
And of course I’m not going to try and precisely assess those variable factors when I’m inspecting, so I just say “closed water system = required expansion tank”
In your opinion, a tankless absolutely never needs an expansion tank unless used for space heating?
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If you see recirculating lines with a tankless water heater a thermal expansion device is recommended.
Can you explain how there is some expansion in the potable water system on a tankless water heater. Keep in mind the water heater only operates when a valve is open. The valve I am referring to could be a lavatory valve or a kitchen sink faucet. When a valve is open the pressure in the pipe is reduced greatly. When a valve is open you have now turned a static closed system into an open system. Of course I’m talking about a standard tankless water heater used to heat water in a potable water distribution system.
Here is an experiment you can perform. The next time you inspect a tankless water heater put a pressure gauge on the cold water flush port. Note what the pressure is. Now open a faucet in the house and tell me if that pressure went up or down.
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I meant control pressure. How would you know how much pressure is coming in through your public water to the house? I don’t see anything that tells me the water pressure is over or under 80psi. The home has an electric water heater. It is funny, I’ve been in the floorcovering industry for over 25 years. Becoming a home inspector is difficult. Well, the lingo is difficult. Hope I make sense. Remember Im a newbie 
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You’re asking good and valid questions so that’s the important thing. Best of luck to you.
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Very valid point. I knew I could rely on getting more information than I had, from you, Martin.
Isn’t there a general concern to be had regarding water heaters that heat water more than they should be? TPR valves aren’t needed if the device is operating as intended, but…?
Edit: nah, just reaching for possible explanations. That has nothing to do with expansion tanks
Very good point. Thank you for the added understanding… another there is still expansion, you’ve convinced me that it doesn’t affect plumbing joints. But I’ll report it for now because of TREC.
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I like the way you think things through Michael
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Now I get it! I got it yay thanks! So simple but complex.
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And I recommend paying ~$15 for a decent one filled with liquid. The air filled ones get blown out easily imo. And get at least two. Home Depot trips during the work day aren’t as fun.
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I have never once seem a prv at the meter in my 30 years. Maybe your area is different. There are lots of houses I inspect where the water pressure is over 80psi. I recommend a prv and expansion tank be installed, as they should be installed together per local code. Even if there is a tankless water heater.