honeywell RTH221B1021 wiring


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Old 10-21-14, 07:24 AM
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honeywell RTH221B1021 wiring

I have 3 wires coming in. red,white and blue. I have tried different combinations but cannot get it to work. The blue seems to be common. When i use a volt meter and touch red to blue, i get 24 volts. when i touch white to blue, i get 11 volts. Any ideas? Here is a pic
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Old 10-21-14, 07:49 AM
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When i use a volt meter and touch red to blue, i get 24 volts. when i touch white to blue, i get 11 volts. Any ideas?
How many volts between R and W? The R terminals receives 24V from the transformer "hot" side. The common from the transformer does not have any voltage on it. The W terminal receives 24V from the R terminal. When there is no call for heat you should see 24V between R and W terminals. You do not have a C terminal on your thermostat to connect the common wire from the transformer.
 
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Old 10-21-14, 07:57 AM
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there is 11.8 volts between the red and white wire
 
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Old 10-21-14, 08:02 AM
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Are those the only two wires connected to the thermostat? What type of heat system do you have?
 
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Old 10-21-14, 08:07 AM
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the old one has all 3. it was an old mercury one. I believe the blue was connected to C but this new one does not have a C terminal. This is a gas furnace with baseboard water heat.
 
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Old 10-21-14, 09:04 AM
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You should definitely be get 24V across R and W terminals. The thermostat is probably defective. Did you get 24V across R and W terminals on the old mercury thermostat?
 
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Old 10-21-14, 09:12 AM
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It's brand new. it's 11.9 volts even when not hooked up to the thermostat. (red & white) As far as the old one, i'm not sure. I disposed of it.
 
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Old 10-21-14, 09:30 AM
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I have seen brand new thermostats bad right out of the box. When testing, are you placing the voltmeter leads on the R and W terminals?
 
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Old 10-21-14, 09:37 AM
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yes and unhooking the thermostast and placing right on the wires
 
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Old 10-21-14, 10:40 AM
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Placing the voltmeter between the wires doesn't tell you anything. The white wire is the return back to provide 24V to energize the heat relay and has no voltage until the R terminal on the thermostat provides it when a call for heat is made. If you remove the red and white wires from the thermostat and twist them together does the heat come on?
 
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Old 10-21-14, 04:56 PM
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i tried shorting the red and white wires together and no heat.
 
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Old 10-21-14, 05:17 PM
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If the heat doesn't come on when shorting the red and white wires together there are issues other than the thermostat. Have you tested for 24V coming from the low voltage side of the transformer?
 
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Old 10-21-14, 08:20 PM
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I believe the original thermostat you want to replace is probably what is known as a series 20 three-wire thermostat (attached image). To upgrade to a more modern thermostat you will need to purchase a thermostat that supports 3-wire zone control. These thermostats are typically a lot more expensive than a standard thermostat. Two examples of thermostats capable of series 20 wiring are:

Non-programmable - TH5110
Programmable - TH6110
 
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Last edited by firedawgsatx; 10-21-14 at 09:01 PM.
 

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