# boiler keeps overpressurizing



## 907plumber (Mar 13, 2010)

Okay, I went out and looked at an old boiler that had several leaks and a bad pump. This is an old weil Mclain, blue cover, natural gas.

I ended up replacing the expansion tank, It had a 15, I put on a 30. Water was present in the original one. added a taco airscoop. I replaced the pump with a taco 007 IFC. Replaced the fast fill and replaced the leaky unions. I also replaced the t&p Guage and the pressure relief valve.

The pump is always on, even when there is no call for heat. Im not real up to snuff on Taco zone valves and these older relay-transformer combinations. I would really like to rewire this thing but so far I have been unsuccessful in my attempts.

But now there is a problem. The boiler is over pressurizing. This happens when the zone valves are shut off. The boiler shuts down at 180 degrees but the temp continues to climb to 220 degrees until circulation is restored. It only takes about 2 seconds for the boiler pressure to go from 20 to 40 when the zone valves shut off.

I have verified the fast fill is set correctly. I put it down to 10psi. I verified the expansion tank is at the proper pressure. 

There is really no where to purge on this system. It has Hy-vents out the wazzoo, and I added the airscoop. Hindsight I should have at least cut in a t and a shut off on the return to purge from.

The boiler sounds clean and normal.

Boiler is probably 20 years old or more I would guess. I was pushing for a new install but didnt get that far. Course now that i was the last one to touch this boiler its my problem now... 

Thanks again guys for the help. An awesome resource this forum is.


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## OldSchool (Jan 30, 2010)

The boiler TT contact have to be wired in with or interlocked with the zone valves.

this way the boiler can only fire once the zone valve is in the open postion

So.... the thermostats in the home control zone valve ... and zone valve controls boiler on/off

Look at zone valve instructions and it will show how to wire


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## Eric (Jan 10, 2011)

I would pull the check out of the 007 since you have zone valves on the system. Is the expansion tank on the baseboard side of the zone valve or boiler side? I had a call for a similar issue couple of years ago and it had to do with the double checks ( pump and zone valve) isolating the xpansion tank from the boiler. 

Another guy did it, and we removed the check which fixed it. 

If you can't find directions with the zone valves you should be able to on the manufactures site.


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## Plumbworker (Oct 23, 2008)

is this piped direct supply and return? if soo sounds like you needs a pressure by pass piped in


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## RealLivePlumber (Jun 22, 2008)

I would put the connection to the expansion tank back where it was, ahead of the backflow preventer. 

I'm just sayin...


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## 907plumber (Mar 13, 2010)

I did take the check out of the pump last night. Also I got the wiring figured out. I wired it to be a cold boiler and also got it to where the pump only comes on for when there is a call for heat. 

I sat there for a few cycles and everything seems fine.

*If I had to decide what fixed it I would say it was taking the check out of the pump. 

* Ihave the expansion tank as well as the pump on the return side*. *


The only significant change I did to the original installation was install that IFC pump and add an airscoop. 


Thanks guys. Hopefully we can call this a success story!


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