Short-cycling - Old Chronotherm T8085A1004

PEvans

It has been a while but I'm back.

This is a two-pipe steam system with two zones controlled by thermostat-operated steam valves.

One zone is controlled by a Chronotherm T8085A1004 thermostat. The thermostat calls for heat and all is good but now after about 4 minutes the valve closes, indicating that the thermostat is not calling anymore. This is a pure waste of fuel, no radiators get heated in that time. (I know for the system is not shutting off on pressure. I have stood there and watched the valve close with pressure just off the zero mark on the low-P gauge.)

I know this thermostat has a "heat anticipator" feature. It is set at the highest amperage setting if I'm reading it right.

This thermostat might be missing some of the dogs it is supposed to have. The clock is also non-operational.

The other zone has a regular round Honeywell thermostat that seems to work fine. Last winter I ran for a while with the Chronotherm set to a very high temperature to keep the valve open and disconnected the valve position switch wires to keep it from starting the burner, so basically running the whole system from the thermostat on the other zone.

If I should just replace the Chronotherm what you recommend?

Jamie Hall

It is possible that the anticipator on the Chronotherm has failed -- it happens. I'd just get a nice battery powered, perhaps programmable, digital thermostat from the Big Box and replace it.

JUGHNE

There was that exact tstat in a church that the anticipator had failed.
The boiler just sat there and would cycle on and off every 10 minutes or so.

It must have done this for years because a storeroom next to the boiler room was overheating.
They installed an exhaust fan.

Since changing to a new tstat and other improvements, the fan has not run.

Never mind, I digress, I would recommend a Honeywell 5000 or 6000 Pro series if available.

Your old tstat may have had 4 wires run to it, you could use one for a "common" .

EdTheHeaterMan

Old Chronotherm thermostats with clock motors have to take into account the heat that is generated by the clock motor. When that clock motor failed, the thermostat is no longer accurate. The heat anticipator may also be problematic on the old thermostat. I would just install a new digital thermostat and set the "cycles per hour" at 1 (or 2 at the highest) in the thermostat set up programing mode

PEvans

Thanks, everyone.

Is there a reason why not Nest other than the price?

This spot has power for the clock. For cycles, I know from running the Jamie Hall test that my system takes about 40 minutes for the pressure to begin to rise, so if I understand Ed's guidance my system should cycle no more than once in an hour.

PEvans

Update: FWIW I did find the "Smart Thermostat with CPH Options" thread which was pretty helpful. I am not a Nest hater, in fact I was an early adopter and have done work with the the company. However, it is certainly the case that the Nest does not have much user configurability.

That thread is almost 3 yrs old and seemed pretty inconclusive in terms of thermostat recommendations to fit the OP's requirements.

mattmia2

It kind of comes down to the mass of a steam system is such that it really like setbacks, it is sort of like trying to run setback on a radiant system. The system stays at around the right temp to replace the heat loss all the time, in the case of steam instead of it being a outdoor reset of supply water temp the thermostat and the anticipator cycle the system to keep it averaging at that temp. When you recover from a setback everything gets hotter than it normally would and it overshoots for a bit.

PEvans

I ended up installing a new Honeywell RTH9598 thermostat, and the system isn't short cycling anymore, so again, thanks, all.

I decided I did not need fancy occupancy or phone proximity sensing since I am not going to set back the system much. This thermostat's settings include one specific for steam heat and you can edit the time-based schedules via the internet or a smart phone app. No support for external temperature sensors. So we'll see how well it does.

I moved my substantive notes to the "what thermostat will deal best with boiler recovery times and outdoor reset" thread in the thermostat forum.