Brand: Carrier

 brand option: The brand is not listed it is:

 type: hot air furnace

 fuel: natural gas

 location: basement

 problem: controls

 email address: panther@mts.net

 S1: Hello,
 Thank You for taking the time to answer my question.

 I have a milli-volt system with a central air unit that has been added at a
 later date. I have 5 wires running out of the furnace to the stat. 2 from the
 gas valve, 1 from the fan (green), and 2 from the air conditoner/coil. there is
 one transformer that is supplying power for the air conditioner. On the
 transformer are 5 terminals labeled R G C Y W. This system uses an old mechanical
 stat (white rodgers) with the mercury in a glass bubble, and the heat
 anticipator on the top side of the temp coil.

 My question is:
 I have changed the stat to a electronic programmable stat. Once installed I
 could no longer call for heat. I have found that I will have to install a 24vac
 relay to activate the milli-volt gas valve. (from what I can see the stat does
 not act like a dry contact, it needs voltage). Can you show me what terminals I
 can connect the 24vac to so the stat will trip the relay calling for heat? (on
 the stat it is Rh & W).
 I need the connections at the furnace.
 My second problem is, again once all wires are connected at the stat (with the
 stat "off"), I am showing 24vac across the line going to the air
 conditioner. This should be "no volts" until I call for
 "cool". The stat instruction manual mentions a possible resistor
 having to be added but only states "refer to a local contractor".

 If you would happen to have a wiring schematic showing how to change this system
 to an energy efficient electronic stat I would love to hear from you. You can
 email me at panther@mts.net or call  XXX-XXX-XXXX evenings (please reverse the
 charges)
 or XXX-XXX-XXXXdays.

 PS the furnace is a Crane Sunnyland model CT-112

 Thank You, have a good day.
 Kevin Ewasiuk

 I know exactly what you are getting at.  It is almost easier to do than it is to explain it. Use the transformer in the air conditioning unit (if that is where it is) (connect RH and RC together), use W and C to go to the furnace to operate the new relay (for heat) that you added to the furnace that switches the millivolt circuit on the gas valve instead of the thermostat doing it. Or W and the common side that activates the fan relay if you have a forced air furnace with air conditioning added. Then the dry contacts of the new relay operate the millivolt system on the gas valve. Do not use crimp connections for solid wire. Solder or use stranded pigtails with wire nuts. If there is no transformer on this system (no air conditioning) then one must be added. Make sure the limit controls work when you are done!!!

 I can go into the details of explaining why your new thermostat won't work on millivolt but this fix is needed when you have problems with the millivolt system not working (on a mechanical thermostat too). Then if you replace the gas valve with a 24 volt one you are ready to go.

 The air conditioning should have Y as one side of the contactor and C as the other R is the hot side of the transformer that goes through the thermostat to Y to activate the contactor and at the same time G is activated to run the fan cooling not heating). http://toad.net/~jsmeenen/wiring.html

    The reason that your electronic thermostat may not work with your millivolt system is that many electronic thermostats (programmable or not) use triacs which are electronic switches to control the signal back to the equipment. While they will work with 24 volts AC they will not work with the 3/4 of a volt that the thermopile puts out and if they did since it is DC. If they did work then you would have another problem that triacs will not shut off with DC running through them.
 
    Another reason that you can't use your electronic thermostat with a millivolt system is that some use what is called a "power stealing" technique. The have a small transformer that takes a sample of the 24 volt system current to operate the relays on the stat so it will not run down the batteries. If they didn't do this the batteries would only last a month or so at best.

    I do believe that White Rodgers makes a programmable thermostat that is designed to operate a millivolt system and has a "6" terminal instead of a "W" terminal. It also uses 3 "AA" batteries to run the relays. Which will last longer when used on a 24 volt AC system

 Read more that I have written on this subject at the freegas page
 and the question page .
To find out more about fan and blower motors click here
To find out what controls the fan in the heat mode or proper settings
To get parts look to Grainger or Johnstone.
To solve a problem use the link to the form below.
 Scott Meenen