Line Voltage Thermostat Calibration

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aline

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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.
 
Line voltage thermostats are notorious for being inaccurate. One manufacturer gave up putting numbers on the thermostats completely a few years ago, they just labled a section of the dial "comfort zone". Trying to calibrate one of these things is an invitation to drive yourself crazy! Aso the position of the thermostat in the room will have a great effect on the accuracy of the reading.
 
aline 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.
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If this is a typical rotary-knob baseboard-heat type of wall thermostat, there is only a knob. Where is he getting that 70 deg. reading from, with the knob at 60?

Remember, a thermostat can maintain a given temperature, +/- its tolerance, where the thermostat is located. As Haskin said, ignore the numbers and set the control wherever comfort is reached.
 
Even the differential on a line voltage thermostat is great. I think it's around 5 degrees.

If this is the typical Marley Electric (Fahrenheit, Mears, etc...), the left hand Allen screw is only an early make/break for positive disconnect in the 'off' position, and does nothing for the calibration.

The right hand contact is the operating contact, for purposes of controlling the load. A good digital thermometer and very slight tweaking of the right hand Allen-screw (fraction of a turn) either way will calibrate the stat much better than the factory calibration. I've pretty much had to do this to nearly every line voltage stat for the last few years.

For the fussy customer, or the higher-end install, it's still advisable to use a relay with a T87 or similar stat.
 
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.
 
What I meant by turning on at 75 deg. and shutting off at 65 deg. is in reference to rotating the dial not the automatic on & off cycle of the thermostat.

With the room temperature at 75 deg. rotating the dial past the 75 deg. mark turns the heater on. Rotating the dial back to the 65 deg. mark turns it off.

I'm not sure how the thermostat will work as far as turning the heat on and off. If you have it set at 75 deg. I wouldn't think you would want it to get down to 65 deg. before turning the heat on. That seems like a large temperature swing.

Having the dial set at 70 deg. and having it turn on at 65 deg. and off at 75 deg. would make more sense.
We don't have a lot of electric heat around here and I don't have that much experience with it.
 
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