I recently installed three double pole line voltage thermostats in a customer’s home. The line voltage is 240volts.

The customer says that one of the thermostats reads a temperature of 70 deg. F even though he has it set at 60 deg. F. This thermostat is in a single gang box with two heating elements run to it. The heating elements are imbedded in the ceiling plaster. This is the only thermostat that has two elements run to it. The others have only one element.

I don’t see how controlling the two elements with this thermostat would make any difference on the calibration of the thermostat. From what I can see the thermostat should just monitor the room temperature and open and close the line voltage contacts for the heating elements. The heating elements are wired in parallel with each other. I wouldn’t think you would want them wired in series. Either way I don’t see how this would have any effect on the calibration of the thermostat.

The thermostats are supposed to be factory calibrated with a + or – 2 deg. F accuracy. There are two calibration pots on the thermostat that are sealed with a potting compound.

Does anyone have any experience with the calibration of line voltage thermostats?

Have you ever had to calibrate these in the field?

These are simple manual, non-programmable rotary dial thermostats made by King model HET-2R.

I wouldn’t think calibrating these in the field would be necessary.

I could just try replacing it with another one but was wondering if anyone else has had this problem with line voltage thermostats and what they did to correct it .

I have one sitting here at my desk right now and it’s reading a temperature of 68 deg. F. When I rotate the dial clockwise I can here it click on at about the 68 deg mark and when I rotate it counter clockwise it clicks off at about 62 deg. F. mark.

Maybe the one I installed is just not calibrated properly.

I though I’d see if I could get some input from someone before I go out to look at it.

His daughter told me she thinks he just likes to tinker with things too much and keeps turning the dial up and down and won’t leave it alone.

Any help is much appreciated.

I would replace it, it could have been knocked around during shipping.

Maybe the ceiling panels are heating the thermostat hotter/quicker than the air since they are radiant (radiant needs a mass to heat, won't heat the air directly). Also, you need a known good thermometer or temp probe for your meter to see what the room temp really is, then you can go off that. I don't think the cheap little thermometer built into the front of the t-stat cover is a highly accurate instrument. Start with your accurate thermometer, and then see how far off you are. Leave the t-stat alone for at least 6 hours to let the room temp stabilize.

You said "The customer says that one of the thermostats reads a temperature of 70 deg. F even though he has it set at 60 deg. F. This thermostat is in a single gang box with two heating elements run to it. The heating elements are imbedded in the ceiling plaster. This is the only thermostat that has two elements run to it. The others have only one element."
Just for clarity the stat is set at 60 and the temp reading is 70? Is the heat on or off when it is showing 70? If the heat is off then there is no problem unless the stat also controls cooling. If the heat is on and it is 70 degrees in the room there is either a calibration problem or the stat is broke. From your customers statement it appears he thinks the room temperature should be 60 even if it is warmer outside. There can be lots of reasons the room is warmer than the stat setting without there being a problem with the heating. Now if the stat is set at 70 and it is 60 in the room that is a heating problem otherwise he has an AC problem.

In which case a dollar to a pinch of snuff he's probably had the thing to bits already to see how it works! I have seen these things 'recalibrated' by mispositioning of the dial gear-ring relative to a central pinion.

I've found thermostats to be notoriously inaccurate in controlling actual room temperatures - the vital thing is to have them positioned in the right place. Put the stat in a 'cold spot' in the room and it will always lag the actual temperature.
No need to wait hours for a test, just use a standard thermometer reading, after a few minutes to stabilise, then test the stat by your 'click' test- quite accurate enough to see if it really is 10 deg out.
Since time is money, I'd change it anyway for the one on your desk, he sounds like a fusspot.

Alan

I was kind of thinking that somehow the dial may have been "repositioned" in such a way, where the "60°" mark would actually be equal to "70°" ambient temperature, but after seeing the quoted text below, I would say 99.5% chance this is the "Problem"!



Line Stats aren't the most accurate things, but they do have at least +/- 5% tolerances with something like a 3° Differential.

As was mentioned before, radiant heat will heat up objects directly, not the enviromental air directly.

This seems to confuse some people with radiant heat strips (ceiling mounted, encased in the Plaster of the "Lid"), as they do not feel warm air immediately.
As a result, they start spinning the Line Stats' dial up and down - eventually calling for a service tech to check things out.

One of the disadvantages to recessed radiant heat strips is when one has to be replaced. The ceiling (Lid) will require patching and texture.
Typical failures come from Toggle Bolts and Lag Screws going through the cable - commonly occurs when someone wants to hang something from the ceiling, encounter "a tough spot" in the plaster, then find out later there is no heat in that room!

Scott

Thanks for the replies.

I ended up calibrating the thermostat.

When I got there it was turning on when I rotated the dial to the 65 deg mark and shutting off when the dial was rotated t the 60 deg mark. The actual temperature was reading 75 deg. I adjusted the calibration using the allen head adjustment pot. It now kicks on at 75 deg. and shuts off at 65 deg.

As I increased the upper set point the span increased as well. There didn’t seem to be any way to decrease the span separately but I think it’s going to work fine. The real test will be next winter when it gets cold again.

I’ll just have to wait and see if he’s happy with it.