There's no downside to installing a PRV other than an extra piece of hardware and the cost. If you do, install it so it can easily be replaced.

If you do, you'll create a closed plumbing system and a thermal expansion device is required... you mentioned an expansion tank.

I like to keep a pressure gauge with a tattletale needle installed to monitor for pressure variances cause you'll know if the PRV needs to be adjusted or has failed.

could there already be one out at the meter?

There could be but if there is it is usually the property of the water authority and they frown on homeowners touching them. Call them and ask.

If there is there is no reason you can't install your own at the water service inside entrance to your home.

Cleverchimp,

You seem to know what your water pressure is already - so I'm not sure you need a gauge.

The maximum working pressure of any residential system is 100 psi.
You are coming close to that.

Yes, I would install a PRV. Running that close to the max + water hammer is hard on your plumbing.

I'm not sure that an expansion tank is required for a tankless heater, since the heater isn't on unless the water is running. - anyone know a definitive answer on that?

You mentioned you have a circ pump. Be sure you insulate all of the hot water lines and at least use a timer. On demand is even better. The cost of running a circulating pump in wasted heat if you don't insulate the pipes and use a timer is just crazy.

This post was edited by jakethewonderdog on Mon, Sep 29, 14 at 10:31

A standard pressure gauge won't register transient pressure spikes where the gauge with a tattletale needle does. That offers a check of proper PRV operation and thermal expansion problems at a glance.

Dramatic pressure fluctuation could indicate a PRV problem or the need for a thermal expansion device.

Lurker,

Do you know if code requires an expansion tank with a tankless heater when it's a closed system?

A phone call to your local plumbing authority will get you the answer.

For water supplied by a pipeline, I strongly suggest a PRV. Water main pressures vary more than one might realize. New line planned for part of my rural area, the first ever for this area. Will not pass my house but serves connecting road.
I looked over the preliminary engineering data and found that the line will have places with pressure as high as 105 PSI and as low as 20 PSI, depending on elevation and usage patterns at a given moment. The 20 would be higher elevations with heavy hydrant use, such as fire. Ideally, hydrants will have 25 or more.
The PRV does not boost pressure, but does limit the maximum pressure within your house.

Jake, a tankless does not require an expansion tank. There is just not enough vol in the system to warrant it. That said this always comes down to the Authority Having Jursidiction (AHJ). They may require it.

The basics on expansion tanks is that when you heat water from 32-212* it will expand in vol approx 4%. With the vol in a 40, 50 or 80 gal tank that expansion can trigger the relief valve.