I have a Comfortmaker RPJ II (Enviro plus 90) gas furnace connected to a programmable thermostat that works and cycles on and off fine when inside temp is lower than set target temp - exhaust blower cuts on, pressure switch engages, igniter glows, valve opens, pilot lights, valve opens, jets ignite, and blower happens somewhere in there.

The trouble comes in when inside temp is higher than the set target temp. The furnace starts to cycle on and off continually. The exhaust blower comes on and off cycling. I believe that it will continue until the temperature drops below target heat setting, cycle through one time properly, and start the troubles again when the inside temp is higher than the set target temp.

When inside temp is higher than the set target temp, the furnace shoud be idle, but the exhaust blower cycling on and off. Please let me know what is the problem? Somebody told me the manual thermostat will fix the problem.

It could be in the thermostat or more than likely in the thermostat wiring that might be shorted. It could also be a short circuit in the furnace wiring or a defective circuit board. With so many different areas that could cause this type of problem and if you do not have the proper tools and meters to check for this problem you may be better off calling in a professional to have a look.

I got the following suggestion from another reply. What is your opinion?



Replace your "power robbing" thermostat withn a better quality unit. What is happening is during the "off" cycle the thermostat is robbing power from the furnace and storing it. After awhile the triac in the stat discharges the power to the furnace and there is enough power to energize the relay to bring on the venter. Once the relay is energized there is not enough power left to hold it closed so it shuts down and starts "robbing power" again.

1) Replace stat
2) Place a 22k 2 watt resistor across W and C
3) Put in an isolating relay between stat and furnace.

In your original post there was no mention of a brand or exact model of thermostat so the first thing I mentioned was to check out the thermostat or the wiring.

It is possible that if the thermostat is power robbing it could cause a problem on some equipment so the suggestions should possibly work.

The cheapest would be the resistor but I always used a 250 ohm 10 watt but see below for example. Try this first then if it works OK if not go to next solution.

REMEDY:

Note: 2 resistors may be used at the same time.

Not applicable with Heat Pump systems.

Thanks a lot for your reply. I forgot to specify the thermostat. The thermostat is made by Honeywell (32207626-002 REV B). I opened the cover and this thermostat has terminals R, RC O, W, Y, and G. Do I have to connect Y and G with a resister for the heating mode? I really appreciate your pdf file. Your pdf file shows to connect Y and C. Also you suggested to use 250 ohm resister than 22k ohm. Which resister will be better for the Honeywell thermostat? Again I appreciate your help.

Actually both resisters should work. Put resistor in the furnace on on the thermostat itsef. The post shows Y and C try it and see if it helps post back with results. You can always try W and C.

I have a problem that may be similar to yours. I too have the RPJ II furnace, but mine is working OK. The problem I'm having is with my new Thermostat.

My old thermostat worked great, it was an old Janitrol. It was a "power robbing" type, but my DMM says it only used 10mA max. Again, no problems.

I switched to a new Hi-Tech stat from HAI called the Omnistat2. It connects to my PC and has a color changing background display. Very nice unit. However it can draw up to 60mA at 24VAC.

Now here is the strange part. As soon as I disconnect the fan wire and gnd from the old stat (with everything else connected to the new stat) the 24VAC power kind of shuts down; the new stat powers up for about a second then powers down then back up and repeats. The 24VAC supply becomes "weak" and can't power the stat. For some reason it will only work with the old stat connected to the fan wire. If I disconnect the fan wire completely or connect it to the new stat the 24VAC is not strong enough to power the new stat.

Is this something that commonly happens with these furnaces and power robbing stats? In all the reading I've done here I've not seen anyone mention the fan wire connection as having any relation to their problem.

Thanks.

Quote:

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Originally Posted by hvac1000

Actually both resisters should work. Put resistor in the furnace on on the thermostat itsef. The post shows Y and C try it and see if it helps post back with results. You can always try W and C.

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Looks like, it works with a resitor in the furnace. I put a 22K 2W resistor between W and C in the furnace. So far, I have ot experienced the short cycling of the exhaust blower when the room temperature is higher than the set temperature of my thermostat. I guess this proved that the power robbing thermostat was the cause of the short cycling for my furnace "Comfortmaker RPJ II". Thanks for your help.

Works every time.

What I ended up doing was running the ground wire. Now it sources as much current as I need without any problems.

One thing about the resistor is it wastes power, something I'm trying to avoid.

The infinitesimal amount of power that resistor controls or consumes and dissipated by heat is not a concern of a professional HVAC tech. Our job is to get the system to work. On the energy savings side of things most equipment is improperly installed and the new 13 SEER A/C units are running at 11 SEER and the 95% furnaces are running at 90% or less because of improper instillation and failure to perform a professional check out with the proper tools and test equipment. Most service techs do not have all the test equipment necessary and many that do have no idea how to use the equipment properly to start with.

While it is nice to play games with thermostats and controls to try and save .10 cents a year most US residential and commercial structures are burning up the utility bills with improperly installed equipment.

These are just the facts. All the new system controllers we are currently testing work great and the computerized remote control products are proving dependable in our testing once the bugs were worked out but none of the new controls are proving any great energy savings as compared to the proper adjustments of the new high efficiency products on the market. We consider all the new control systems including the Surveyor main frame internet remote control network products to be just icing on the cake as compared to properly operating equipment.

Hey no doubt on the energy savings. The resistor power dissipation is insignificant compared to a new high efficiency unit with good installation. Did not mean to imply otherwise.

My desire to have a hi-tech thermostat is for the geek factor. I'm connecting my entire home to my PC. Controlling the lighting, irrigation, thermostat, security, home theater, etc.

Thanks again for the wiring color info...

I understand. You are playing with your toys and that can be fun. I come from the serious side of the tracks so there is the difference. Enjoy the toy.

Quote:

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Originally Posted by than2402

...The thermostat is made by Honeywell (32207626-002 REV B).

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I have this same thermostat and found this post based on the model #. However it is connected to a heatpump and the problem with the cycle is slightly different. We have two heatpumps and identical problems with both.

The problem is at some point the heatpump will be running, but the air handler will not be--I have the fans set to auto.

It seems to happen when cooling while hot outside AND heating while cold outside. When experiencing this in the winter (as now is happening, heating with cold outside), I have gone outside to look at the units and a couple of times found the coolant tube covered in ice.

Switching the fan setting to 'on' does not resolve the issue--it seems to need to go through a cycle of some type before coming on. I've even disconnected housing and primary electronics, removed the batteries and replaced them.

The setting in the thermostat's NVRAM for 'heatpump' (#16? I'm just recalling from fuzzy memory, but I know I've changed the setting) does not make a difference.

And of course the Honeywell website's 'support' is non-existent for this model. Really? No knowledgebase or FAQs?