Thermostat Response Speed: Fast, Normal, Slow?

I picked up some new Robertshaw RS3110 T-Stats that have the option of fast (1 degree), normal (2 degree), or slow (3 degree) differential. What is the preferred differential for use with coal boilers?

The boiler won't care, use whatever makes you the most comfortable (likely the 1 degree setting).

The lower the differential, the more likely you won't notice the house cooling between calls for heat. 1 degree is the typical setting. 2 degrees is where you might start to feel the house cool a bit before a call for heat. 3 degrees might be comfortable and leads to the lowest cycling times for the heating equipment. Runs longer but not as often for calls for heat. Somewhere in there comfort and efficiency coexist.

I have a stoker stove so it's a little (maybe very) different, but when I had mine on three the house would freeze then cook. Massive overshoot followed by small idle fire, then overshoot, then small idle, etc. I keep mine on one. My thinking is that since coal is slower to respond than oil or gas that one would work better. Since a stove or boiler is always running the heat can be kept much more consistent. Three degrees seems more conducive to an oil/gas furnace - or maybe a shoulder month setting for us.

My propain furnace used to do that too.. It would get cold in the house, then the thermostat would call it to come on and it wouldn't shut down until it was too hot. Constant Yo Yoing of temp in the house. I don't use a thermostat at all now, I like the steady heat output of a manual controlled appliance much better. But I realize that probably isn't an option for a stoker, or could it?

Lightning wrote:I like the steady heat output of a manual controlled appliance much better. But I realize that probably isn't an option for a stoker, or could it?

I'm not sure exactly how Larry's stoker works, but I change my timers depending on the weather to keep a bigger (much bigger, lately) idle fire. In the shoulder months I decrease the size of the idle fire. Sort of a manually run Coal-Trol. I'm finding that I am burning much less coal this way.

Horace wrote:I change my timers depending on the weather to keep a bigger (much bigger, lately) idle fire. In the shoulder months I decrease the size of the idle fire. Sort of a manually run Coal-Trol. I'm finding that I am burning much less coal this way.

Wow, that's fascinating! What a great idea

I did an experiment last night. I turned my aqua stat setting down to 150 low, 170 high. As the outside temperature dropped through the 20's into the teens the boiler had to run almost continuously. At one point the house temp dropped 3 degrees. I did not adjust the feed rate. The house eventually came back up to temperature after a couple hours. It "appeared" I was burning a lot more coal at those settings. Heat transfer in hot water to hot air coil is much more efficient at higher temperatures. I suspended the test and turned the temps back to where I had them. 170 low, 190 high. Those settings worked in sub zero temps. I can always go higher if I ever need to. Just an interesting aside.

Horace wrote:

Lightning wrote:I like the steady heat output of a manual controlled appliance much better. But I realize that probably isn't an option for a stoker, or could it?

I'm not sure exactly how Larry's stoker works, but I change my timers depending on the weather to keep a bigger (much bigger, lately) idle fire. In the shoulder months I decrease the size of the idle fire. Sort of a manually run Coal-Trol. I'm finding that I am burning much less coal this way.

EXACTLY how we need to run the Kaa-2 boiler. I now see it works for other stokers as well. Oliver

Horace wrote:

Lightning wrote:I like the steady heat output of a manual controlled appliance much better. But I realize that probably isn't an option for a stoker, or could it?

I'm not sure exactly how Larry's stoker works, but I change my timers depending on the weather to keep a bigger (much bigger, lately) idle fire. In the shoulder months I decrease the size of the idle fire. Sort of a manually run Coal-Trol. I'm finding that I am burning much less coal this way.

No need for that sort of thing with a Coal Gun. The output is fixed, and there is no timer. The boiler simply maintains a set temperature and runs the fan as needed to maintain that temperature. It is impressive how fast those units go from sleeping to a roaring fire.

Rob R. wrote:No need for that sort of thing with a Coal Gun. The output is fixed, and there is no timer. The boiler simply maintains a set temperature and runs the fan as needed to maintain that temperature. It is impressive how fast those units go from sleeping to a roaring fire.

Aside from a few ashing parameters, the only thing you can change on a Coal Gun is the boilers internal water temperature maintenance setting. Mine is currently set to turn the fan on at 170 degrees, and shut the fan off at 180 degrees. The 10 degree differential is fixed. If the L4006 aquastat that regulates this was to be replaced with an L6006 that had a variable differential, the fixed 10 degree dif could be modulated (expanded or contracted).

The Coal Gun gravity stokes via ashing. It is a lot like a fan assisted hand fed hopper equipped boiler that shakes itself down occasionally and automatically instead of having to be manually shaken. I have 250 lbs. of coal sitting in the hopper above my fire. As ashes fall out the bottom of the fire tube (the ash grate mechanism simply shaves them off the bottom of the fire tube via a knife edge that moves back and forth), fresh coal falls by gravity alone from the hopper into the fire tube. Nothing is hindering this fall. No flaps or cut-offs separate or isolate the coal in the hopper from the fire tube.

The old Honeywell mercury mounted on the wall here has a anticipater setting. If I remember correctly, the AA130 tells you what setting in the owners manual. That is where it is set. Or it was the Honeywell owners manual setting for coal or for steam? Can't remember. It's set at 70. I would think it slows down the reaction time on the downfall and speeds it up on the upfall?