comfortmaker RPJII furnace short cycling - http://www.hvacmechanic.com/ Forums

Original message

popandcranky

65.184.170.106

"comfortmaker RPJII furnace short cycling" , posted Fri 22 Feb 19:14

Wrote in earlier about a suspected thermostat problem. I replaced a Hunter digital programmable thermostat with a mechanical mercury switch type and my furnace quit its continuous short cycling and works correctly. With your advice I have recently tried a Honeywell CT8775C thermostat and the furnace starts cycling on and then off again before the igniter starts to glow in exactly the same way as before.
Some one mentioned the therm was robbing power from the furnace - just enough to cut on and then it would shut down and they offered some wiring solutions. One was a 22K 2 watt resistor across W and C - but I don't have a C and the other was an isolating relay between the stat and the furnace, but this stat gets its power from the furnace. Can someone shed some more details on these add on options and any other possible info for solving this problem? for more info follow this thread:
http://www.hvacmechanic.com/forums/resservice/messages/27574.htm

Replies:

frost

76.196.58.143

"Re(1):comfortmaker RPJII furnace short cyclin" , posted Fri 22 Feb 20:48

When you say it short cycles; do you mean: The fernace runs for a few mins and then shuts off, or it tries to start and then shuts down before it has time to ignite. How old is the unit and you know if it has an induce draft motor (exhust fan motor), this info will help me, to help you

Don't try to fix whats not broken!

popandcranky

65.184.170.106

"Re(2):comfortmaker RPJII furnace short cyclin" , posted Fri 22 Feb 22:29

The furnace is from 1998. By short cycling I mean: the exhaust blower cuts on, the air pressure switch engages, and before the igniter is triggered the units shuts down. there is a short pause of 30 seconds, and then this happens again and again until you turn the stat to off. All these symptoms go away completely when the mechanical thermostat is installed. Here is the original post if that might help:
http://www.hvacmechanic.com/forums/resservice/messages/27574.htm

thank you for your time.

mgussler614

75.180.43.90

"Re(3):comfortmaker RPJII furnace short cyclin" , posted Sun 24 Feb 16:31

sounds like the pressure swich is opening before ignitor is coming on either the inducer motor orfice is pluged up or a clogged exhaust pipe or the board is going bad better call a pro

guz 1

popandcranky

65.184.170.106

"Re(4):comfortmaker RPJII furnace short cyclin" , posted Sun 24 Feb 20:45

Why, then, would the symptoms go away when a mechanical thermostat is installed?

mgussler614

75.180.43.90

"Re(5):comfortmaker RPJII furnace short cyclin" , posted Tue 26 Feb 22:16

you reset the power to the board when you turned of the furnace to put on the new tsat

guz 1

popandcranky

65.184.170.106

"Re(6):comfortmaker RPJII furnace short cyclin" , posted Wed 27 Feb 07:58

Each time the stat was swapped out the AC switch disconnect supplying power to the furnace was set to "off" first, then to "on" once stat was installed, then the thermostat was set from "off" to "heat" then a temperature was selected higher than room temp - is that the same as a board reset?
the funny thing is that the furnace will try to cycle on and off even when the digital stat is set HIGHER than the room temp, but this does NOT happen at all when the mechanical one is in place (three different digitals tried - one mechanical). I was just trying to get a little more control over the temperature with a digital stat and this is what I get.