Thermostat for Steam Heat

sh500

I currently have a Honeywell RTH2520 thermostat and sometimes I notice that the boiler turns on again to continue providing heat and turns off as soon as the thermostat reaches the set point. I was looking at possibly changing it to a T8775A1009 in case it better handled steam heat and also because it seems that it would work with just the R,W wires without a battery. This one also had a CPH setting of 1 whereas the RTH2520 minimum Cycles per hour is 2. But it has since been discontinued.

Is the digital T87 round digital worth getting from somewhere or is there a better non-mercury thermostat that is best for steam heat that does not require batteries?

Thanks.

ethicalpaul

> I notice that the boiler turns on again to continue providing heat and turns off as soon as the thermostat reaches the set point.

Isn't that what thermostats do?

KC_Jones

Are you sure it's the thermostat shutting it down and not one of the safeties, like the pressuretrol?

sh500

ethicalpaul said:

> I notice that the boiler turns on again to continue providing heat and turns off as soon as the thermostat reaches the set point. Isn't that what thermostats do?

Right, but I thought some of the non programmable ones handled steam heat better to force a longer cycle. 

sh500

KC_Jones said:

Are you sure it's the thermostat shutting it down and not one of the safeties, like the pressuretrol?

I haven't checked the pressuretrol directly. But I know when the boiler first turns on cold,  it runs about 20 min before it turns off and then turns on again after a few minutes. So it doesn't seem that the pressure limits are set improperly. 

Jamie Hall

It's not that the pressure limits are set improperly -- although they may be. It's that the boiler is reaching those pressures and the pressure switch is doing what it's supposed to do. A thermostat won't behave that way.

sh500

I guess what I was hoping to achieve was a way to take advantage of the fact that the boiler turned on for a call for heat. Turning on for 1 degree seems like it is inefficient. A little more run time in that cycle could probably make more efficient use of the already heated water/steam in the radiators. Even if it results in a temperature overshoot.

I'm not sure how the cycles per hours come into play. But currently at 2, which is the lowest my thermostat allows, would it be better with a thermostat that can be set to 1 perhaps?

ethicalpaul

They don't really force a long cycle. What they seem to do (there may be exceptions) is to increase the "swing" of the temperature from its low point (where the call for heat starts) to its high point (where the call for heat would end).

Settings like "steam" or "hot water" have an increased "swing" range and settings like "forced air" have a smaller "swing" range, and the result is usually a longer call for heat for "steam" which is generally a good idea (within reason).

I believe that Cycles Per Hour (CPH) is just a kind of lie they told to make it easier to understand. But again, maybe there are some thermostats that do indeed force a certain number of cycles per hour. I don't know everything.

As far as your thermostat goes, yes I think if you believe your calls for heat are too short then a setting of "1 CPH" would be better. Some thermostats even let you program the actual "swing" amount.

Another thing for you to check is: where is the thermostat located? If it's too close to a radiator, it is going to shut off prematurely. Make sure it is away from radiators and not influenced by other sources of heat or cold.

No thermostat that I can imagine is going to keep asking for heat if the temperature it senses is much above the setpoint.

SteamingatMohawk

Does your system have a device like a Cycle Guard with an Intermittent Level Test function? If so, it is designed to turn off the flame every so many minutes and check the water level in the boiler, then turn the flame back on. The CG400-2090 series works on 24 volts and has a 20 minute test cycle time with a 90 second level check time.

https://www.supplyhouse.com/Hydrolevel-45-410-2090-CycleGard-CG400-2090-Low-Water-Cutoff-24V-Steam

The unit with a 10 minute and 90 second cycle apparently has been discontinued.

sh500

Yes there is a cycle guard among all the safety devices. 

Thanks for the info. It has been helpful. 

I'm not sure if there will be much benefit going to the older discontinued T8775A1009 , other than not having to replace batteries. 

JaymeHart

On my Ecobee, I can set a minimum time for the boiler to run.  Default is 5 min.  I moved it to 12 min for my setup.

My previous Honeywell thermo shortcycled so much.

After the ecobee install, my heating costs dropped 10-15%.  Much more comfortable too.  I am quite happy with the change.

I also swapped the Honeywell switching relay for a better Taco control board, which supports a C wire.