Steam System Not Shutting Down Properly

Perplexed

Hi, I have a gas-fired steam heat system, without a tankless coil. When I turn on the system heat, all seems to work properly;. The rub is on shutdown.

 Once the thermostat is satisfied, the system shuts down, but within 5 minutes, the system is again turning on. I have replaced the 24v thermostat, but the symptoms are the same. In order to get the system to shut down, I cut the power to the thermostat. Any ideas?

Hopefully you can help...

Thanks.

jpf321

um...t-stat settings?

please tell us your exact brand and model of t-stat. if possible link to the online manual for it. it's very possible that you have an incorrect t-stat setting somewhere .. like CPH or Gas/Elec or something. when it goes back on after 5mins .. does it ever shut off after that?

Jamie Hall

Your thermostat

seems bonkers.  As jpf says, maybe a wrong setting somewhere.  Or it may be flat out defective.  Tell us more about it...  I assume you have checked, and the thermostat is really turning off?  Since it is supposed to enable the boiler, and nothing else is, if it's really turning off, there are only two possibilities: either it's turning back on again, which it shouldn't do that quickly, or somewhere there's another circuit in parallel with it which is turning on -- and there shouldn't be, since you don't have a tankless coil nor, presumably a hot water loop with an aquastat or anythihng like that.

Perplexed

Perplexed

For many years, I have used the old Honeywell T852a thermostat. That thermostat was in place, working fine for over twenty years.

This problem has begun to happen and, two days ago, I replaced the T852A with a Honeywell T8775A, 24v digital round thermostat.

The operating symptoms are the same as with the older stat. I can turn the system on from the thermostat, but once it has satisfied the thermostat and shut down, it will cycle back on within five minutes and continue that until I flip the switch to cut off power to the thermostat.

BTW, there is an aquastat within the system.

Thanks for any ideas...

DavidK_2

Maybe you have a wiring problem.

I had this problem with my doorbell. Seems the cloth insulation had failed, and sometimes the bell would go off by itself.



Maybe your thermostat wiring has an intermittent short in it someplace.

Jamie Hall

It does sound

like a wiring problem of some kind, as DavidK suggests.  Quite possibly the insulation on the thermostat wires is worn -- or equally possibly, some wretched little rodent has decided it tasted good.  The way to check that is at the boiler: when the boiler comes back on, unwanted, disconnect the thermostat wires from the control (careful here; your dealing with live wires -- granted, only 24 volts, but "only" 24 volts can kill) and see if it turns off.  If it does, it's the wires or the thermostat.  A better way, if you don't like dealing with live wires (can't blame you) is to turn off the power, put an ordinary switch into one of the thermostat wires (on) and turn the power back on.  When the thing misbehaves, turn the switch off and see what happens.  Either way, if the thermostat circuit is broken, the boiler shouldn't run.  If it does, it's in the boiler controller and out of my field!  If it doesn't, replace the wiring to the thermostat and see if that helps.

Tinbrook

Perplexed

Please check the heat anticipator setting in the thermostat. It sounds like it's set too low which would cause it to shut the system off prematurely resulting in short cycling. To set it properly, find out what the current draw is of the connected control: zone valve, circulator, etc. and set the heat anticipator to the same value. If it's set too high, the room will get too warm followed by too cold and long run cycles.