wiring in programmable thermostat


  #1  
Old 10-18-01, 09:22 AM
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I have electric heat ( formerly baseboard, but in-wall fan units replaced them last year. ) The circuit box has two 240 volt circuits ( 20 amps ea )that supply the heaters with power. This means that there are two romex(?- 1 black, one white, one ground, each wrapped in white plastic coating) coming into the thermostat box in the wall. There are three heating units ( two @ 1000 watts, one at 750 watts) so three romex wires go out of the thermostat box. Until recently, there was a relay connecting the incoming power to an old low-voltage (broken) thermostat and the heaters. I recently replaced the relay and the low voltage thermostat with a directly wired in 2 pole non-programmable Cadet thermostat. I couldn't figure out how to do it with both incoming romex's ( kept blowing the main circuit breaker in the box), so an neighbor who does electrical for a utility company capped off one romex in the thermostat box in the wall, said it was not needed, and hooked up the Cadet with one incoming romex wire ( 1 black, one white, one ground ). When the heaters stay on for more than 5 minutes at a time, the circuit breaker turns itself off, but the thermostat is not hot when this happens. Why does this happen? What is the correct way to wire in this type of thermostat? This is important to know because if I replace it with a programmable one ( see below ), this thermostat is going to go into my SO's condo, which is the same heating set-up as mine. We need to install it correctly.

Since I am gone for most of the day would like to put in a programmable unit that economizes yet turns up the heat automatically in anticipation of my return home. From what I have learned in the installation process so far, I think I need a 2-pole -- clue me in otherwise if I am wrong....Can I install the programmable thermostat using both sets of romex so I do not have a drain on one circuit, and the other one not used at all? Is this an issue --do I even need the second one? If I install a 2-pole programmable, do you have a recommendation as to which one, AND can you instruct me as to how to install it so no circuits are adversely affected?


 
  #2  
Old 10-18-01, 04:34 PM
Wgoodrich
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You need both circuits to carry the heating elements you spoke of. Your elements pulls more than 20 amps.

You need to buy a relay 20 amp rated 4 pole. The control coil should be 24 volt rated. Your progamable thermostat will need 120 volts fed to a transformer stepping the voltage down to 24 volts. Then run power to the programable thermostat that should also be rated 24 volts. run the white wire through to one side of the control coil on the relay that should be installed inside the furnace. Connect the other wire coming from the 24 volt transformer to one contactor in the thermostat. Run the wire left going to the furnace to the other side of the thermostat. Then the end of that last wire 24 volts still should be connected to the empty screw of the control coil of the relay. The two 240 volt 20 amp circuits should be directly connected to the big screws of the 20 amp rated relay. then connet a set of wires on the oppisite side of the relay to one set of elements and then jumped to the second set of elements. Then the second circuit or the two screws left connect wires to the third heating element.

You are dropping the voltage going to the progammable thermostat to the voltage the thermostat is rated. This thermostat is not designed to carry the load of the three heating elements but only carry the load of the magnetic coil operating the relay that has the 4 contact points for the higher voltage.

Hope this helps

Wg

 
  #3  
Old 10-20-01, 09:37 PM
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How do I figure out the amps the 3 heaters draw? "You need both circuits to carry the heating elements you spoke of. Your elements pulls more than 20 amps".

Is it possible to use a double pole thermostat in this system if the thermostat is rated for 4000 watts?

Thanks again for your help.
 
  #4  
Old 10-21-01, 03:41 AM
Wgoodrich
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Programable thermostats usually are designed to carry the load of the coils of a relay NOT THE LOAD OF THE HEATING ELEMENTS. This would make it logical in my mind that if you are going to end up with a programmable thermostat you should set up for that regardless whether the thermostat that you are using now could carry the load or not.

YOu said you are carrying 2 - 1000 watt elements and 1- 750 watt element and want to control it by one 4000 watt thermostat. Then you speak of controlling the same electric heating system in a condo.

Before we proceed I want to explain a couple of things. All the elements that you are talking about is no more heating capability than a portable heater 240 volt rated. The come as high as 3000 watt portable heating size.

A condo normally is about 1000 square feet. if you calculated with 1.5 as an adjustment of heating degree days for the bread basket area of the US you would multiply the square feet of the condo times the hieght of that condo most commonly being an average of 9' due to cathedral cielings in a couple of room. Even if you calculated with 8' you would have 8000 feet of area to heat. Multiply that with the 1.5 heating degree day adjustment you would have 12 kw heating required. That is 12,000 watt heating elements required to heat that condo in most likly guess.

Unless you are heating a bath room with the 2 - 1000 watt and 1- 750 watt heater you don't have the right sizing of these elements. Are you trying to heat a room with these heating elements or a house? This is what is concerning me.

The answer to your question is that you are carrying 2,750 watt worth of load with the elements your are naming on your 4000 watt rated thermostat if you really have 2750 watts total. Question is why are you kicking a 20 amp 240 volt rated breaker with a 11.45 amp load?

Then you are saying that you are kicking a 20 amp 240 breaker in 5 minutes of running these heating elements that pull only 11.45 amps. Now if these elements are 120 volt rated instead of 240 volt rated then I could see that 20 amp breaker kicking. That breaker should not be kicking of in 5 minutes if you truly have 2750 watts worth of heat. I am suspecting your are reading the size of the heaters wrong.

Are these small wall mounted heaters like what you would find in a bathroom or an electric furnace heating an apartment?

What size room are these heaters serving, a single room or whole house?

Curious

Wg
 
 

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