Burnham v8 - Fluctuating pressure

[MikeDSVT]

Hi Guys - I am new here and looking for advice.

The PRV was blowing off every know and again the past week for the first time. After reading up I determined the expansion tank was bad. There was water out the bottom when I checked pressure. I thought the pressure was to high so I lowered to 12PSI and it became waterlogged. Lowering of the tank seemed to have brought the boiler PSI back to 18-20psi max. The pressure would drop and then rise when the boiler fired.

I changed the expansion tank and purged my system and zones. The pressure still fluctuates between 12-20 when the heating system is running and calling for heat to the zones. I verified the auto fill was not the issues by shutting it off and verifying no pressure change while idle, in addition auto fill off the pressure still fluctuates when running but no more than psi.

At this point I thought I was fine. However when domestic water is called for the boiler fires when water temp is 160ish and shuts down around 180-185ish. When running the domestic the boiler seems to struggle to get the water back up to temp and the pressure builds to 38psi and water runs by the PRV due to it being 30psi, but doesn't blow off and dump much water.

My thoughts at this point is the tankless coil for the domestic water has a pinhole leak.

Thoughts?

 

[MikeDSVT]

I tested the line water pressure before at 38-39 psi using a gauge and set the new expansion tank pre charge psi to 39psi to match.

 

[Sponsor]

 

[MikeDSVT]

So i think i screwed up. I should have set the new expansion tank to 12-15 psi.

It should have been 39 if I was installing on a hot water heater. Now the thermal expansion cant go into the tank until after 39psi, so it causes the Pressure Release Valve to leak.

 

[Jadnashua]

Yep, the expansion tank needs to be set to the expected pressure of the nominal system it is installed in. And, realize that you cannot reliably adjust that pressure while there is pressure in the heating circuits of the boiler and you need a valve open when pumping it up for the displaced water to go.

If the tank is good AND it is sized properly for your system, the pressure should not vary more than a few pounds between running and not running the boiler as things heat up and cool down. If it rises, either the autofill valve or shutoff to it is defective, or you do have a leak in your heat exchanger. 40psi for your potable water is fairly low, but any differential can drive that pressure up through a leak.