I have a Slant Fin Monitron EH electric boiler set up with 3 zones as my heating system. A couple of weeks ago the expansion tank exploded on me. It literally blew in half at the seam which scared me half to death in the process. I shut the power off to it and called a plumber who came out and replaced the expansion tank, he filled the system and gave me a crash course on filling it and bleeding the air out of it. He also adjusted the aquastat and the high point is set now at 180 before he left.

Everything seemed fine for a little while of course but I had to wonder why the pressure valve did not open if the pressure got high enough to blow the expansion tank apart at the seam so I tested it, it opened and then would not close. I shut it down and drained the system a bit and it would leak constantly when I tried to fill it again so it obviously needed to be replaced, I also had to replace the 60A breaker on it because that had pretty much fallen apart. Obviously the pressure gauge is not accurate as it was reading 10 PSI when the system was pretty much empty.

Replacement parts installed, system filled, air bled out, pressure valve not leaking, so I start it up only running 1 zone and everything seems ok for a while. I find that the zone 2 valve wasn’t completely closed even though the thermostat was off so I turn that thermostat up enough to get it a call for heat and then shut it off and the valve closed. I check the boiler a short while later and the temperature gauge is up over 220 F which seems insanely high to me when the high point is set way below that so I shut it off again, I turned on the thermostat for zone 3 and that ran some of the cooler water from that zone through it that brought the temperature down. Would that zone 2 valve being open when it wasn’t supposed to cause the water to get hotter than it should or could the temperature gauge be inaccurate as well OR is the aquastat also faulty?

If the temperature gauge is accurate I just don’t understand how the temperature would go that high unless the aquastat also needs to be replaced. This system has been a nightmare for the last few weeks, I feel like I have the boiler from The Shining in my house, any advice is greatly appreciated.

 

You have a defective aquastat and when you have it replaced you want to have an additional high limit aquastat installed. It can be just strapped on the hot water out line on the boiler. It gets set for typically +/-200° as a standalone high limit safety shutoff.

The relief valve should be replaced too.

 

Thanks, I did replace the relief valve, no issues there anymore. I don't know much about these systems, what should I have for a temperature window? I just lowered the high limit again down to 160, it was around 180 and sometimes it's shutting off, sometimes it isn't. I'd assume it's not safe to run if it's around or above boiling temperatures?

 

It doesn't appear to matter where it's set. The temperature is going well over the set point.

 

I too find it hard to believe that the system relief valve did not trip first saving the expansion tank from bursting, unless the expansion tank had taken on water, lost its air cushion permitting system pressure to rise rapidly even with modest temperature risess, and also rusting out and becoming the weak spot for bursting.

 

On every hot water boiler I serviced there was always at least 2 temperature controls controlling the boiler. Both devices were installed into or as close to the boiler as possible so they would always sense the hottest water. One device was the normal operating control and the other device was a manual reset high limit. Both were tested during a service call or any time I was there for service. Not to alarm anyone but I did see a case when all operating and safety devices failed by improper service or neglect and the house with the exception of the basement disappeared . There were no survivors.