New Honeywell Thermostat- short cycling or normal?
#1
09-22-23, 05:12 PM
New Honeywell Thermostat- short cycling or normal?
I just installed a new Honeywell round thermostat (CT87N) to replace a 20+ year old Honeywell round thermostat that was failing due to the furnace running beyond it's anticipated shut off time (ex: setpoint at 70, furnace still running at 75 when it would nomally shut off at 72). So I installed the new one, all settings for my oil fired warm air furnace verify with the instructions, wires are tight and screwed in inside the thermostat and not loose when I pull on them.
The issue is when I run it, the furnace seems to shut off after about 5 minutes (heat+fan) if I set the setpoint just above the room temperature and then it barely gets to the setpoint or a little below, where with the old one, I'd expect it to run about 15 minutes and go a degree or two past the setpoint. So I tried to increase the setpoint temperature a few degrees beyond the room temperature. The furnace ran longer but still stopped just below the setpoint (set at 72, stopped at 71.5). Thermostat seems level so I'm wondering if there's an issue with this new thermostat or something I've done wrong? By the way, new filter was installed too.
Thanks in advance for any help. Everything else with the furnace has not changed between the old thermostat and the new one.
The issue is when I run it, the furnace seems to shut off after about 5 minutes (heat+fan) if I set the setpoint just above the room temperature and then it barely gets to the setpoint or a little below, where with the old one, I'd expect it to run about 15 minutes and go a degree or two past the setpoint. So I tried to increase the setpoint temperature a few degrees beyond the room temperature. The furnace ran longer but still stopped just below the setpoint (set at 72, stopped at 71.5). Thermostat seems level so I'm wondering if there's an issue with this new thermostat or something I've done wrong? By the way, new filter was installed too.
Thanks in advance for any help. Everything else with the furnace has not changed between the old thermostat and the new one.
#2
09-22-23, 08:25 PM
Group Moderator
This is now an electronic thermostat.
Since it's not mechanical it no longer needs to be 100% level.
That stat should be compatible with your system.
How many wires in your setup ?
Heating and cooling ?
What were they attached to on the old stat ?
Like red on R - white on W - etc.
Since it's not mechanical it no longer needs to be 100% level.
That stat should be compatible with your system.
How many wires in your setup ?
Heating and cooling ?
What were they attached to on the old stat ?
Like red on R - white on W - etc.
af66
voted this post useful.
#3
09-23-23, 05:26 AM
Thanks for responding. It's a 4 wire system. Red, Blue, Green and Yellow. So, I've jumpered the red wire with the R and Rc connections on the thermostat and attached the other wires where they're supposed to go. Last night I checked them (specifically the red wire since I had to squeeze it in with the metal jumper) to make sure nothing was loose and everything was secure.
On the old one, the old Honeywell round with the mercury switch and heat anticipator, the wires were attached the same way (the old stat had a wire jumper from the R to Rc instead of the current metal one but that's the only change I can see with the wiring), and I may be getting thrown off because this one has no heat anticipator and I'm wondering if the setting on that one may have been set incorrectly. It would overshoot the set by 2-3 degrees.
On the old one, the old Honeywell round with the mercury switch and heat anticipator, the wires were attached the same way (the old stat had a wire jumper from the R to Rc instead of the current metal one but that's the only change I can see with the wiring), and I may be getting thrown off because this one has no heat anticipator and I'm wondering if the setting on that one may have been set incorrectly. It would overshoot the set by 2-3 degrees.
#9
09-23-23, 03:05 PM
Group Moderator
When re-reading your problem.....
it sounds like the old stat is shot which is likely with those old warhorses.
If changing the cycle time doesn't help.... make sure the thermostat is not in the direct air flow path.
Make sure the hole behind the stat is fairly sealed so hot air can't come up the wall from the basement. You can seal it with a little piece of insulation.
it sounds like the old stat is shot which is likely with those old warhorses.
If changing the cycle time doesn't help.... make sure the thermostat is not in the direct air flow path.
Make sure the hole behind the stat is fairly sealed so hot air can't come up the wall from the basement. You can seal it with a little piece of insulation.
af66
voted this post useful.
#10
09-24-23, 08:06 AM
So in doing a little more research both on a HVAC site and on the Honeywell site, it sounds like this “short cycling” is now the new normal for their thermostats. Honeywell claims their thermostats have a -1/+1 degree swing and the HVAC site I was on said they are designed to cycle 6x per hour (5 minutes on/ 5 minutes off) to maintain a comfortable temperature. My question is: all those excessive cycles must be bad for the internal components of the furnace? Rather than running continuously, you’re now igniting the furnace a few extra times an hour..
Houston204’s advice is what I’m going to try. Apparently setting that 2nd switch to “on” will increase the cycle time to 9 minutes from what I’m reading. Not really thinking this is a great mod by Honeywell since the old mercury ran for 15 minutes and cycled 3x/hour on really cold days. In any event, thanks to all for your help!
Houston204’s advice is what I’m going to try. Apparently setting that 2nd switch to “on” will increase the cycle time to 9 minutes from what I’m reading. Not really thinking this is a great mod by Honeywell since the old mercury ran for 15 minutes and cycled 3x/hour on really cold days. In any event, thanks to all for your help!
Last edited by af66; 09-24-23 at 08:26 AM.
