3 install questions Honeywell ct87k thermostat

Old 12-29-12, 11:23 AM
Thread Starter
Join Date: Dec 2012
Posts: 1
Upvotes: 0
Received 0 Upvotes on 0 Posts
3 install questions Honeywell ct87k thermostat

I spent my lifetime DIY'ing but I am a new homeowner and I'm finding that there is a whole new level of DIY that I didn't even know existed. Quite a humbling experience.

I replaced an old dial mercury thermostat with a Honeywell CT87k thermostat today. After guessing on which 3 wires from the wall go into which of the 3 holes in the thermostat I think I have the combination that is correct (I did not know that I was supposed to record the wiring positions before I removed and discarded the old thermostat). My first attempt was connecting the Red to the R, the Yellow to the Y, and the green to the W. This didn't seem to do anything. Next I tried a combo that seemed to make thing work backwards but I'm not sure. My final combo seemed to work.

3 questions:

1) The system seems to be running fine but am I taking some sort of risk by doing my trial and error method of guessing which wires go where?

2) What should I set the 2 cycle switches at? I have a Burnham oil burning furnace that heats my hot water as well (side note: this is costing me a fortune w/a cold Maine winter and a drafty 1850's house). I have baseboard heat. I'm thinking that it should be either OFF-OFF (#3 below) because I burn oil but then again maybe it's OFF-ON (#2 below) because I'm heating hot water with it. Both seem logical The manual gives these options:

1. Steam or gravity ON-ON
2. Hot water, 90% eff warm air, heat pump OFF-ON
3. Gas/oil warm air OFF-OFF
4. Electric warm air. ON-OFF

3) Why is it that I have to set the heat on significantly higher than the current room temperature to get it to turn on? e.g. if it's 60 degrees in the house I will have to turn the dial up to 66 to get it to turn on.

Thanks in advance for any help that you can provide!
Old 12-29-12, 01:59 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Received 38 Upvotes on 30 Posts
With your heating system I suspect that you only need two of the three wires. First step would be to check how the wires are connected at the boiler's aquastat. If standard wiring protocol was used in the initial installation only the red and the white wires would be used. You MAY find that two of the wires at the aquastat are connected to one terminal and if so it was for an ancient thermostat that required a third wire for "heat anticipation" to prevent room temperature overshoot. If this is what you find then disconnect the wires and use ONLY the red and white.

Your system configuration (at the thermostat) would be option 2.

Your third problem may not even be a problem. Does this happen immediately after you have re-installed the thermostat to the wall? If so it is because the thermostat is reacting to the heat it gained from being in close proximity to your body. If it happens all the time then it signifies a thermostat way out of calibration.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: