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Thread: Thermostat transformer

  1. #1
    Retired tool & Die and Mechanic Giles's Avatar
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    Default Thermostat transformer

    I have an old Heil natural gas central unit. It is about twenty years old and is only used as a backup heat source.
    The pervious owner did not use it in the twelve years that he owned the home and could not tell me anything about it.
    The thermostat was defective so I installed a good T87F thermostat. The T87F has a heat anticipator adjustment that I can't seem to get right. The 24v transformer is actually producing 29.68 volts. This is app. 1.40 amps and the honeywell setting only goes to 1.2 amps.
    Even with the anticipator set to 1.2 the heat cycles are to frequent and too short. In fact, adjusting the anticipator has little effect on the cycling period.
    Should I replace the transformer with a new 24v or purchase a new thermostat or both?

  2. #2
    DIY Member cattledog's Avatar
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    Giles--

    You will have significantly more control over the system if you use a digital thermostat with adjustable cycles per hour (cph) and internal control logic for pulse width modulation of the on/off times within the cycle.

    I think if you want to see if you can get the currrent thermostat with heat anticipator working you can use an external power resistor to drop the current to the level required by the anticipator. Be sure to size the resistor correctly for the heat dissipation. You can always change the 24v transformer later if you want to stick with the Honeywell.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Is that open circuit voltage or closed (i.e., when the thermostat is calling for heat)? Having the voltage a little high isn't too uncommon when it is sitting there open, under no load. WHen the thermostat is closed, calling for heat, the current through it is what pulls in relay(s) and potentially other things in the heating system. This added load usually drops the voltage level some.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Retired tool & Die and Mechanic Giles's Avatar
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    Quote Originally Posted by cattledog View Post
    Giles--

    You will have significantly more control over the system if you use a digital thermostat with adjustable cycles per hour (cph) and internal control logic for pulse width modulation of the on/off times within the cycle.

    I think if you want to see if you can get the currrent thermostat with heat anticipator working you can use an external power resistor to drop the current to the level required by the anticipator. Be sure to size the resistor correctly for the heat dissipation. You can always change the 24v transformer later if you want to stick with the Honeywell.
    Would a digital be more tolerant of this higher voltage?
    Somthing that is strange and not necessarly related to my problem is that I have a new White Rodgers digital thermostat for my main heat pump and the two thermostats sometimes have a 2-3 degree difference in their readings. other times , they are close. They are mounted a few inches apart.
    For example--The temperature in my home was 68 degrees by both thermostats. I turned the heat up on the gas and when it cut off it registered 72 degrees and the W/R digital registered 69 degrees.
    House was definately warmer then 69 degrees.
    I think I will go with digital as you suggested.--THANKS---

  5. #5
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by Giles View Post
    The 24v transformer is actually producing 29.68 volts.
    30v[open circuit voltage?]/24v [full load voltage] = 1.25 = pretty poor voltage regulation, but maybe this is normal for an HVAC 'frmr.

    You can check it with a 14 ohm, 80w or 60w resistor or four paralleled 56 ohm, 20w resistors. With a 40VA rating and this load you should read 24.0vac with input voltage at the rated value. Using the open circuit voltage with some calculations you can check other 'frmrs with the same resistor.

    Depending on the cost of the 'frmr you may want to put an inline fuse in the secondary circuit.
    Last edited by Thatguy; 11-27-2009 at 02:11 PM.

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