Best T-Stat Differential, Is It 1Deg,, 2 Deg., or 3 Deg.?

I have replaced 3 of my homes 4 thermostats with Robertshaw RS3110 thermostats. I'm not sure what the differentials were for all of my old T-Stats, but the new ones allow settings for differentials of 1 degree (fast), 2 degrees (normal), and 3 degrees (slow). One of my old T-Stats was an old Honeywell mercury switch type, which I'm almost certain had a fixed 1/2 degree of differential. I don't have a lot of history to go on yet, but even with my new T-Stats set at their fastest 1 degree of differential setting it seems as if they are triggering the boiler far less often that I recalled from the old T-Stats (which leads me to imply that all of them were probably fixed at only 1/2 degree of differential). To make a long story short, should I leave them at 1 degree, or mess with the 2 and 3 degree differential setting options?

PS: The factory default for the new T-Stats was 2 degrees. I changed them to 1 degree while installing them.

Something is wrong. Why would anyone set a thermostat to 3 degrees differential? The purpose of a thermostat after all is to hold a set temperature as closely as possible.

To hold a temperature as close as possible the thermostat has to anticipate the lag in time before the heating unit actually delivers heat to the room, and then to compensate for any overshoot as radiators for example cool.

To do this a small resistance that is adjustable is built into the thermostat. When the thermostat calls for heat this resistance is energized and depending on how it is set supplies a small amount of heat to the thermostat sensing element to anticipate the heat actually being delivered to the room. If that resistance puts too much heat in, then short cycling will occur; not enough and too wide a temperature swing. I suspect this is what is happening when you adjust that differential is that you are really adjusting the heat anticipation which will differ with each installation. The right setting is the one that holds the set temperature the closest.

The new thermostats may operate on different principles that I am not familiar with but that is how your old round Honeywell worked which was a pretty damn good thermostat.

franco b wrote:Something is wrong. Why would anyone set a thermostat to 3 degrees differential? The purpose of a thermostat after all is to hold a set temperature as closely as possible.

To hold a temperature as close as possible the thermostat has to anticipate the lag in time before the heating unit actually delivers heat to the room, and then to compensate for any overshoot as radiators for example cool.

To do this a small resistance that is adjustable is built into the thermostat. When the thermostat calls for heat this resistance is energized and depending on how it is set supplies a small amount of heat to the thermostat sensing element to anticipate the heat actually being delivered to the room. If that resistance puts too much heat in, then short cycling will occur; not enough and too wide a temperature swing. I suspect this is what is happening when you adjust that differential is that you are really adjusting the heat anticipation which will differ with each installation. The right setting is the one that holds the set temperature the closest.

The new thermostats may operate on different principles that I am not familiar with but that is how your old round Honeywell worked which was a pretty damn good thermostat.

Franco, from what I have read, what you have described is how the old bi-metallic T-Stats operated. The mercury ones had 1/2 degree of differential. The digital ones (or at lest the ones I installed) work only on temperature differential.

The longer temperature differentials are supposed to be of benefit if you are experiencing "short cycling". But at only one degree of differential, I'm already experiencing longer times between cycling than I was previously used to.

lsayre wrote:The longer temperature differentials are supposed to be of benefit if you are experiencing "short cycling". But at only one degree of differential, I'm already experiencing longer times between cycling than I was previously used to.

That may change as the weather cools. In this mild weather the room cools much slower and longer cycling times would be expected. That adjustable differential seems to equate to heat anticipation.

Every space has it's own unique heating load. Some are naturally drafty some tight, some lots of glass some not so much...then the heating unit have their unique response patterns to all variables in their design and installation details.
But...

For coal boiler such as a AHS & AA a higher differential setting maybe better if the heat calls are giving shorter run times.
These boiler run best if heat calls are both fewer and heavier, as compaired to frequent and shorter. Less unburnt in the bucket. Just need to be mindfull that if you get carried away there can come other problems...boiler heat over-temps and needing to activate the heat dump zones.

McGiever wrote:Every space has it's own unique heating load. Some are naturally drafty some tight, some lots of glass some not so much...then the heating unit have their unique response patterns to all variables in their design and installation details.
But...

For coal boiler such as a AHS & AA a higher differential setting maybe better if the heat calls are giving shorter run times.
These boiler run best if heat calls are both fewer and heavier, as compaired to frequent and shorter. Less unburnt in the bucket. Just need to be mindfull that if you get carried away there can come other problems...boiler heat over-temps and needing to activate the heat dump zones.

Should I take it that you feel I should put them back to their factory default setting of 2 degrees differential?

T87 with the anticipator setting. Just ordered a NOS one with the mercury switch. My old one has a lot of dust in it and just ain't what it used to be, 40 years old, can't complain.

mozz wrote:T87 with the anticipator setting. Just ordered a NOS one with the mercury switch. My old one has a lot of dust in it and just ain't what it used to be, 40 years old, can't complain.

NOS = new old stock?

Yup, you know, box is shelf worn, part inside is new. Guy on Ebay has 5 of them. I used to set mine at 70 and not touch it all winter. Then in the summer, sometimes the heat from the basement would make the first floor rather hot. so I would turn it all the way down and open some windows. It's got lots of dust in there on those coils and on the mercury switch. Now when I turn it to 70, it gets 75 in here, try to turn it back down and it seems to stick, then very low the house is too cold. With coal, I found out to set it and forget it. The amount of coal you would save with a nighttime setback or otherwise, would hardly be able to be measured,with a auger at least. Your anticipator setting, if you have one, should be set to the model of aquastat you use. Thermometer on my desk never varied more than a degree or 2 in the winter and I run steam too.

Also, watch those new fancy digital types. There was one with batteries, if they went dead, the thermo went max, people came home to their wallpaper peeling off the walls. Do a google search, the stories were enough for me to say never.

lsayre wrote:

McGiever wrote:Every space has it's own unique heating load. Some are naturally drafty some tight, some lots of glass some not so much...then the heating unit have their unique response patterns to all variables in their design and installation details.
But...

For coal boiler such as a AHS & AA a higher differential setting maybe better if the heat calls are giving shorter run times.
These boiler run best if heat calls are both fewer and heavier, as compaired to frequent and shorter. Less unburnt in the bucket. Just need to be mindfull that if you get carried away there can come other problems...boiler heat over-temps and needing to activate the heat dump zones.

Should I take it that you feel I should put them back to their factory default setting of 2 degrees differential?

Yes, try 2

I doubt any old t'stats had 1/2 sec. The 1/2 sec. came about for use w/ air to air heat pumps, and were supplied w/ the heat pump package. Heat pumps need short frequent cycling.
Like *Mozz* said, with age t'stats get less accurate.