thermostat problems?

trivetman

My steam boiler started cycling on and off quickly...cycling every few seconds. I noticed it for the first time yesterday. Prior to yesterday, a cycle would last minimally 10-15 minutes.

I noticed the green light indicating heat (see pic) on the thermostat module in the basement going on and off in the same pattern. I assume this means the boiler is just doing what the thermostat is telling it to do. Is there a way to adjust/fix the thermostat or is this a sign it's busted?

mattmia2

Sounds like what happens when switching power supplies fail.

ratio

The white wire from the thermostat is the heating call, check it with a multimeter. ≈24VAC (generally, to ground) is a heating call, ≈0 isn't. That'll tell you if it's originating in the module or thermostat.

My guess is the thermostat, FWIW.

trivetman

Assume for the moment its due to the heating call fluctuating. Is there a way to fix the thermostat or just replace it?

pecmsg

Take the Wi-Fi module out of the loop and see if the boiler stays on

ratio

What kind of thermostat is it? If it's a normal, nothing-special thermostat, sounds like it's got issues of some kind. If it's a fancy programmable (Like IFTTT, not just scheduling), more likely a software/programming issue.

What changed when this first started?

Hap_Hazzard

Replace it with one of these:

trivetman

It is a fancy wifi/programmable model installed by the prior owner. I’ve never done anything with it but turn the heat on and off. I have an email into the thermostat manufacturer.

Nothing has changed recently, not even the weather.

What i think is happening is the house temp was right on the line between calling for/not calling for heat. Ive always assumed a thermostat has some sort of minimum cycle so that once the heat is called for, that call lasts at least a few minutes to avoid a short cycle caused by small fluctuations. Is that not the case?

Hap_Hazzard

trivetman said:

Ive always assumed a thermostat has some sort of minimum cycle so that once the heat is called for, that call lasts at least a few minutes to avoid a short cycle caused by small fluctuations. Is that not the case?

No, it isn't not the case.

In other words, your assumption is correct. The difference between the on and off temperatures is called the "swing." Some thermostats allow you to adjust it directly, while others use a confusing concept known as CPH, or cycles per hour.

In your case, however, I don't think these settings are the issue. No possible user setting whould cause the thermostat to cycle as rapidly as you are describing. Something is broken.

Since this appears to be an IOT device, I wouldn't rule out the possibility that it's been hacked. Check to see if there's a way to do a hardware reset that resets the device to its original factory settings, and disconnect it from the network, because a hard reset will set the access credentials to their default values, then go through the setup procedure as you would for a new unit. If that doesn't work there's probably a hardware problem, and you should consider replacing it with something that only does what you need it to do. (See my previous comment.)

trivetman

Per the thermostats tech rep: their thermostat doesn’t control for cycles/swing. It calls for heat strictly based on the set point and air temp without consideration for cycle. It makes sense to him that the call for heat could fluctuate like that, particularly when the outside temp is mild so the heat loss from the house is slow.

Seems a little bizarre - right??

My AC unit is using a separate Honeywell thermostat which I can wire the boiler into. Guess thats the next step.

ethicalpaul

Their tech rep is insane. There is no properly working thermostat that should be dithering on calling for heat every few seconds, no matter what the outside temperature is. Just replace it with a simple thermostat since you don't use the programming anyway.

mattmia2

It still has to have some sort of hysteresis. The on temp and the off temp can't be exactly the same, even if it is 1/4 or 1/2 of a Fahrenheit degree differential, it has to turn on and off at different temps, otherwise it would be both turning on and off at the same time.

You could do something like turn the setpoint up a few degrees and see if it stays on solidly. If it does the exact same thing you know it is something else other than differential (probably a power supply with a cap that has developed a high ESR, maybe one that converts between 2 low voltages, probably the supply that runs the coil for the relay for the heat call, when the relay closes the failing supply can't supply that much current so the voltage drops and the relay opens, the supply voltage goes up without the load of the relay coil, the current in the coil drops the voltage again and the cycle repeats)

trivetman

It does stay on without issue when the setpoint is a couple degrees above the house temp.

Regardless of whether the thermostat is malfunctioning or just bad design, I’ll be using a different thermostat shortly.

mattmia2

Despite what their tech support told you, sounds like it lost the setting for hysteresis.

trivetman

For When I rewire to the new thermostat...my vaporstat is currently wired on the hot side of the 24V. Does it matter if its on the hot side or the return white wire?

I was thinking about the power supply to the thermostat being interrupted and if its using that power internally

ratio

Say, it isn't a Filtrete 3M-30 programmable touchscreen, is it? I just condemned one last week for that very thing, waffling around the setpoint calling for heat 5-90 seconds(!!!) on, 1-3 minutes off. You could hear the relays clicking in it!

trivetman

Nope. Its a bayweb

exqheat

Go on line for manufacturers instructions. If you cannot fix it and the rep told you it is not designed properly, get rid of it.