Honeywell CT87N Tstat Issues?

Rizz861

I'm having some issues with a carrier furnace model 5ASTA045-08 using a Honeywell CT87N tstat. I was called for a no heat call a month ago. I was told the tstat was replaced so I didn't bother to check it. There was no error codes, and furnace was operating as it should when I was there. I did the usual checks: gas pressure in was good, outlet was 3.5IWC as required, and flame signal was strong after cleaning flame rod. Pressure drop on gas with manometer was less than 1IWC. Temperature rise was right in the middle of listed temperature range. I also checked for loose wires and /or bad grounds. I cycled it with the tstat a few times and everything checked out. I get a call back the other day. This time I saw the furnace kick out. I noticed it appeared like an end of a heat call, so burners kicked off, blower ran for the allotted time, and then turned off. I decided to make sure all safeties would give an error code, so one by one with the furnace running, I unplugged them to see that I'd get the blinking led codes. This worked for every safety, pressure switch, rollouts, high limit, etc. I now decided to take the tstat out of the equation at the furnace and jumped it. I'd seen the furnace kick off 3 times prior, and now that I jumped it, the furnace didn't kick out. Where I'm stumped is this tstat was brand new, and an exact replacement of the old one. The old tstat had the same issue, but apparently it was in use for awhile before this started to happen. I'm thinking it's the tstat or maybe the control board because I'm getting no codes, gas valve is good, flame rectification is good, and temp rise is good. Also, filter is brand new and going the right direction. I didn't ohm out tstat wires bc r and w obviously had continuity because the tstat can control furnace. I'm wondering if I should check resistance of wires to ensure nothing was knicked too? Do any of you know if there are any known issues with this particular tstat?

Jamie Hall

Not particularly — although anything is possible. However, the R and W wires from a thermostat to the furnace are simple: if R and W is a closed circuit, the furnace should turn on. If it's open, the furnace should turn off. And inside the thermostat it is a simple switch, either on or off.

So… check that. Turn the thermostat up — furnace on. Back down, funace off. No joy? Take the thermostat off the wall and connect R and W together. Furnace on. Disconect, furnace off. No joy? Go down to the furnace and jumper R and W at the control. Furnace on? Take jumper off, furnace off. No joy?

Somewhere in that sequence the furnace should behave. If it failes the first test, suspect the thermostat. Second test, suspect the wires. Third test — control board.

HomerJSmith

Sometimes things that are married together are just incompatible, kinda like my friend and his wife. Did you check the dip switch settings on the thermostat for the heat source you are using?

dko

Is a resistor installed between W and C?

Using a battery operated stat or use of common wire prevents/solves so many issues.

It is possible for power stealing stats to damage the control board with all that cycling. Which could explain why it worked before and not now.

109A_5

Hello @Rizz861,

In situations like that I'd be very curious as to what the voltage is across the R and W wire conections at the control board or very close to the control board, especially when it is acting up. Before touching much of anything. Touching stuff temporarily heals poor conections and makes for callbacks. Good quality clip leads so you don't have to hold the meter probes and put pressure on the control board. Think of it as a divide and conquer point to give you troubleshooting direction.

When the unit acts up, with the thermostat calling;

If the Voltage is at 24 volts or too high or too erratic to keep the control board interested in the call, thermostat wiring or the thermostat.

Voltage remains low (maybe less than 2 volts), safety limits, loose wiring inside the unit, poor connections of the wires conecting to the control board, control board power, control board.