Indirect water heater coil bleeding pressure into boiler??

Found a huge puddle of water on the basement floor yesterday, saw that the relief valve on the Burnham boiler was trickling water. Like an idiot, I just assumed the valve was bad, went out and bought another one and replaced it. Brought everything back up and called it done.

2 hours later, same thing, tickle, huge puddle.

Of course then I did the right thing and looked at the boiler pressure, it was about 33PSI, relief valve is a 30 PSI unit.

First checked my expansion tank, it has the proper 12 psi on the bladder (w/zero pressure on the water side). Boiler pressure does not spike up when the burner fires. If I relieve the pressure in the boiler, it would very slowly creep back up, even with the burner shut down and not firing. It will go from 20psi to 30 in about an hour.

So next I assumed the back feed water supply regulator was bad and valved off that line. Still the same thing. I scratched my head for a while, then thought that the coil in the indirect water heater might be leaking and supply pressure in the tank is forcing water back into the boiler. So last night I shut down the supply valve to the water heater and this morning the boiler pressure hadn't budged.

SO...

Before I go dropping $1000+ for a new indirect water heater, I wanted to run it by the forum here and see if my diagnosis makes sense? Is this common? I can't see what else it could be.

Is the coil in the heater replaceable? (SuperStore SS-40). Doesn't look like it. I'm looking at the HTP SSU45 as a replacement ($950) and would install it myself.

Thanks for any tips, advice, help before I go lighting up the torch....

JH

 

You are correct since the water pressure did not go up when you isolated the tank. This assumes the only thing the valve isolated was the tank. High pressure goes to low pressure. High pressure is the domestic side of the tank low pressure is the boiler water coil in the tank.
The only water can get into the boiler without a DHW coil in the boiler is the Pressure reducing valve (water feeder), water feed bypass valve or the IWH coil.

 

Look into the tank warranty.
Many IDHW tanks have decent length warranties.
You may be covered for a new one, or a prorated portion of a new one.