Thermostat With Variable Differential Set-Point

[JohnZ]

I had posted many months ago about trying to find a thermostat which would allow me to change the temperature set-point differential between "on" and "off". My Reznor forced-air gas power-vented ceiling-hung unit heater has been controlled by a conventional home furnace thermostat, and they typically are pre-set for a 2* differential, to minimize temperature fluctuations in a home furnace application. This results in frequent cycling in my garage application, and I have been manually turning the thermostat "on", set for 75*, and turning it "off" when it shuts down, then turning it "on" again when it gets down to around 65*-66* several hours later. None of the thermostat manufacturers (Honeywell, White-Rodgers, etc.) were able to help me with a thermostat with anything other than a standard 2* set-point differential, and suggested expen$ive programmable industrial controllers ($300-$400 and up).

I found the solution the other day, and thought I'd share it. Ranco makes a whole line of single-stage electronic temperature controls, that run on either 120V or 24V, with or without an output for remote temperature indication:

http://www.rancoetc.com/ranco-etc112000000-p-102.html

I bought the 24V model without the remote output feature ($61.99) to match the existing standard thermostat control wiring, and it's easily programmed for any temperature drop set-point you like up to 30* differential. Now I can set it for 75* with a drop to 65* before it turns on again and not have to keep fiddling with the thermostat. It will also control an external load, but I don't need that feature.

 

[Major Ramifications]

If you hold down the alt key and type 248, you will get a degree symbol like this: °

My programmable White-Rodgers that I bought about 6 years ago has a jumper on the back that you can cut to increase the differential, but I still don't think it will go over 2°. Your solution was simple and reasonably economical, congrats.

 

[MrCrewcab]

Johnson controls make one as well and i think it might be a little bit cheaper A419

 

[njsteve]

I been having the same issues with my gas heater. Saw this old post and order the part last week. Should be here this week. I'll post after I get it install.

 

[njsteve]

I been using the Ranco for the last two weeks. It works just as JohnZ said it would. I used to set my old thermostat at 70 and would cycle on and off. With the Ranco I set it at 68 with a 10 degree differential. The way the weather has been the Ranco has already paid for itself.

 

[fuji0030]

My programmable White-Rodgers that I bought about 6 years ago has a jumper on the aback that you can cut to access the differential, but I still don't anticipate it will go over 2°. Your band-aid was simple and analytic economical, congrats.

_________________
Thermostat

 

[Keith_MN]

Sorry for digging up an old post but I wanted to thank JohnZ for sharing this solution.

I have a ridiculously over sized unit heater in my garage (48 kBTU output for a one car garage) and have been looking for a solution for a while to keep it from short cycling so bad.

I was about to implement my home brew solution of two standard thermostats in series with a latching relay that would give me the differential I need by the difference in set points between the two stats when I came across this thread and saw this much more elegant solution.

Doing a little bit of homework I found that Honeywell, White Rodgers and Johnson controls all make something very similar to the Ranco that JohnZ recommended.

I selected the Ranco based on the fact that the user interface seemed to be the simplest to use. Most of these controls were designed to be set once and left alone to run. However, in the home garage environment, I knew that I would always be dinkering with the set point and differential. The Ranco menu for adjusting these parameters was the simplest.

I installed the control last week and have been running the heat all weekend for painting. It is working perfectly. I set it for a 10 degree differential when I was working and pushed it up to 15 degrees while the paint was drying.

So, thank you JohnZ for sharing, I appreciate it.

I do have one question though. I am a bit concerned about the possibility of someone accidentally switching the control from heating to cooling mode. If this were to happen, and the ambient temperature was higher than the set point, the garage would quickly turn into a sauna. The only thing that would limit the heat is any over temperature protection built into the unit heater. The reason this could happen is that there is only one output relay on this control. The same contacts are used for both heat or cool, depending on the application, and the mode is selectable via the user interface. It is not like a standard thermostat that has separate connections for heat and cool.

Did you do something to limit this risk? I am thinking about installing a manually resettable thermal switch in series with the thermostat that would open in the 90-100 degree range. However, I have not found a switch like that with such a low set point.