Water Pressure Mystery - please help me diagnose

[profhoff]

I have a closed system with a MoenFlo and a LifeSource filter and two tankless water heaters with no expansion tanks. Home was built new 10 years ago, high end custom, and used as a second home. No one living in home.
We have observed 3 problems via MoenFlo data that have stumped our plumber. Can anyone help me diagnose problem?
#1) no one living in house right now. Last week or so, avg PSI is 80 and we notice it's ~85psi at 8am and ~75psi by 6pm. nobody is running any water. called plumber.
#2) plumber comes. he replaced PRV. then a new problem emerged. He ran hot and cold water for 10 minutes at 4-5gpm. After shutting off faucets, within 5 minutes went from low 60s PSI to 95 PSI. Plumber stumped.
#3) because of issues #1 and #2, we shut off main valve. PSI fluctuating and goes up or down anywhere from 5-15 PSI with no water running and main valve OFF.
New plumber coming Monday. How can I help him efficiently problem solve and diagnose? Theories?

 

[profhoff]

Husband says to add this:
Municipal water > Water Shutoff > Pressure Reduction Valve > Life Source Water Filter > Moen Flo > house plumbing

House has two tankless hot water heater that only heat when water is running.

 

[Twowaxhack]

I wouldn’t assume the Moen Flo is accurate. I’d check with a known working gauge.

 

[Twowaxhack]

While I watched the pressure rise I would open a faucet enough to let it drip at a fast rate and see if the pressure stopped rising. If it didn’t stop rising with the faucet dripping then it’s not any type of thermal expansion.

I’ve had homeowners forget or didn’t know they had a small tank type heater in series with a tankless.

 

[GReynolds929]

With two tankless water heaters, I'm gonna guess it's a large house with multiple baths. Is there a recirculation loop installed? If so the heaters will circulate and heat the water in the loop which will create thermal expansion. Either the existing expansion tank or thermal expansion control has failed or there is not one installed. This is the first thing I would check as it sounds like thermal expansion to me.

@Twowaxhack beat me to it. Lol

 

[Twowaxhack]

With two tankless water heaters, I'm gonna guess it's a large house with multiple baths. Is there a recirculation loop installed? If so the heaters will circulate and heat the water in the loop which will create thermal expansion. Either the existing expansion tank or thermal expansion control has failed or there is not one installed. This is the first thing I would check as it sounds like thermal expansion to me.

They say the water heaters only heat while using water. So that would exclude thermal expansion.

But maybe the piping is being heated some other weird way……like running parallel to a hydronic line, etc.

 

[profhoff]

MoenFlo is accurate as it matches plumber readings on his gauge. We don't have expansion tanks or any other tanks other than 2 tankless water heaters. One tankless is in the garage and the other is in the attic above the primary bath. Yes, 5 baths. big house.

Also, no recirculation loop installed.

I can't see how piping is being heated in some other weird way. No hydronic line.

 

[GReynolds929]

Some tankless water heaters have an internal buffer tank that is kept hot and recirculate internally. Not saying that's what's happening here but just a thought.

 

[Twowaxhack]

Install an expansion tank. They’re cheaper quicker than chasing your tail even if it doesn’t work.

Check that PRV again, just because it’s new doesn’t mean it’s working properly.

Verify the gauge being used is accurate.

 

[profhoff]

The plumber actually installed a SECOND new PRV using just that logic.

 

[RS]

You need an expansion tank, as temperature changes, volume changes.