Gas boiler won't shut off - Slantfin S-90 DP

Multiman

Gas fired for baseboard hot water Slantfin S-90 DP. No heat call. Replaced thermostat, worked fine, but then tenant said ran constantly, would not turn off. Replaced thermostat again, ran fine, turned off appropriately, and within hours, same problem, runs constantly, won't turn off. Any ideas?

109A_5

With the system actively not shutting off I would carefully disconnect one of the thermostats wires to see if it shuts off, prove it is or is not the thermostat. There could be a staple shorting the wires out and moving the thermostat to change it changes the symptoms briefly.

Basically some electrical troubleshooting is needed. Do you have a multimeter ? Sometimes visual and mechanical inspection is all that is needed.

https://slantfin.com/wp-content/uploads/2015/11/Sentry-Installation-and-Operating-Instructions-119.pdf

mattmia2

does it have zone valves?

Multiman

I don't think so, because it doesn't do domestic hot water, and it's just a one loop baseboard hot water system for one floor.

Multiman

It runs if we short connect the thermostat wires in the basement, to cut out the thermostat. Tech replaced the brain on it, saying that he thought the aquastat was shorting out the thermostats. But when he replaced the brain, and we tried a new thermostat, still not working. Grrrr! I am wondering if we should put in a wireless thermostat.

Multiman

Oh, and it had gone completely cold after last attempt with another thermostat, ran a bit, then would no longer turn on from thermostat and no heat.

bburd

That sounds very much like a problem with the wire between the boiler and the thermostat.

Multiman

And the final verdict is… 3 crappy new honeywell dial thermostats (most likely).

The boiler ran appropriately when the wire to the thermostat was jumped by shortcircuiting after it left the boiler and before the thermostat.

Young tech figured (I think wrongly) that it was the aquastat brain, repeatedly short-circuiting the replacement thermostats. He replaced the aquastat unit, and we then put in yet another new honeywell round thermostat… and it didn't work. Still worked from the short circuit jump near the thermostat.

So he went out to the truck and got a simple digital honeywell thermostat… and it seems to be working fine now.

So my kid handyman, who originally thought it was just that we needed a new thermostat appears to have been right. And because of 3 in a row POS honeywell round simple dial thermostats, it cost me what I'm sure is going to be an insanely expensive service call with installation of a new aquastat brain for the furnace.

coby

https://forum.heatinghelp.com/discussion/comment/1872776#Comment_1872776

did he have the dipswitches set right

EBEBRATT-Ed

@Multiman that's why a real tech would use a meter and test things. Shooting the parts cannon seldom works and gets expensive.

It would be unlikely to get multiple bad thermostats.

But, hopefully the problem is fixed

mattmia2

although i have occasionally run in to a store having a bad batch of something on the shelf so they are all bad but this seems more like a configuration or wiring or not enough power issue

Multiman

Replaced the thermostat, all seemed okay, and then failed again - no heat again, even with new thermostat.

109A_5

@Multiman the other thread you added your issue to stated "Controller Always On", the issue you added is "I am having the exact same problem on the same WM boiler with integrated hot water. Stopped responding to thermostat, constantly on.".

Now "But it has very quickly gone right back to no heat, still supplying domestic hot water."

Here you state "no heat again, even with new thermostat."

So the symptoms have changed from your original post(s) ?

So now you are trying to solve a different issue ?

I'm getting a bit lost with what is going on.