I recently purchased a new house with a natural gas Ducane CMPA075ua3 furnace, and I am not sure if it is working correctly, please help!
I turn the heat down to 60 when I go to work, when I come home 8 hours later, I turn the heat up to 70.
Here is the problem.
The furnace turns on, and heats the house to 66, then the flame and blower motor shut off for about a half hour, the exhaust inducer fan stays on.
The furnace will then re-ignite and heat the house to 67, the flame and blower motor shut off for about a half hour , exhaust inducer fan stays on.
And it will consistently follow this pattern till the house reaches 70.
-I checked the voltage at the high limit switch, and the switch remains closed (I dont think the furnace is overheating)
-I have never had to reset the flame rollout switch either.
- I tried it with and without a filter, and all registers open, made no difference.
**** of note, when I moved in, the makeshift filter, was completley clogged.

If I turn the thermostat from 60 to 65, it will heat directly from 60 to 65, without no interuptions. I dont think there is an anticipator circuit in the cheap honeywell thermostat.

All other timed start up, and cool down cycles work correctly.

It is like there is some type of timer circuit????

Thanks for any help, Gene

 

Is it getting enough air? Are there any filters that need to be changed? Is there an outside vent?

 

When it shuts down, is the board or Smartvalve giving you any kind of blink code? I'm suspecting you could be getting a vent code.

 

I think it is getting enough air because it will heat steadily from 60 to 66, I tried with and without filter, made no difference. There are no "diagnostic led" on the board.
I dissambled the motor housing, and the back auxillary limit switch is totally covered in dust.
How do I check the auxillary limit switch without removing the motor?

 

If you can find the source of power to the aux limit & where the power goes, you can check it there.
If the aux limit was covered with dust & if you have A/C there's a good chance the A/C coil is also full of dust & blocking air flow. Are you sure of that model number? I could find nothing on the web for that model number.

 

No ac. The auxillarly and limit switch are no opening when it shuts off. That is the model number inside the case on the factory sticker. There is a 72 degree temp difference between the inlet and outlet air.
I am curious if the blower motor is overheating???
Thanks for the help, Gene

 

A temperature rise of 72* sounds high but without being able to find any specs I can't say for sure.
If the motor is hot to the touch when the burners shut down, there's a good chance the motor is overheating.

Is this furnace vented with metal or plastic pipe?

 

The furnace is vented with plastic pvc. According to the spec sheet inside the furnace "air temp rise 40-70 degrees f.
I am in the process of checking the temp of the motor when it shuts off.
Will post later.

 

The motor is not overheating, stays at 76 f, when running.
Back to the drawing board.
Gene

 

With a spec'd rise of 40-70*, the 72 isn't far out of line. If you want to get it within spec, probably increasing the fan speed one step would do it.

 

I have noticed a pattern of operation. When heating from 65 to 68 degrees.
1st cycle 18 minutes run time
2nd cycle 14 minutes run time
3rd cycle 10 minutes run time
4th cycle 5-7 minutes run time

It takes about 5 heat cycles to heat the house from 65 to 68.
Its like there is a timer on the furnace??

 

https://www.youtube.com/watch?v=tYwR9qL4lnk

According to the guy who made that video, cycling can be due to a dirty flame sensor. He cleaned it with a new $5 bill. I think that you should use a $100. You might get better results.

 

If the problem were a flame sensor, it wouldn't run for more than a few seconds. If you want to clean the sensor, just to make you feel better, use either fine steel wool or a Scotch Brite pad. DO NOT use sand paper or emory cloth.

With each run cycle getting shorter, it indicates either a condensate drainage problem or the furnane getting too hot. My money would be on the condensate.

Can you post some pictures of the furnace with the front door off & a few of the vent system?

 

This model does not have a flame sensing rod.

 

I'm still leaning toward a condensate problem.
Any luck with those pictures?

 

Ok, I will get some pics tonight.
Thanks, Gene

 

 

Here are some pics of my furnace and condensate trap. I found different directions to remove the condensate drain hose from the inducer fan, and fill it with 12oz of water, this seemed to make the furnace run much better, but didnt completely fix the problem.
Thanks, Gene

 

Something which I think will help is to get about a cup of bleach into that trap. Before adding the bleach, pour hot water through which ever hose goes into the trap. I believe you have a bunch of muck in the trap which isn't allowing it to drain properly. The hot water will help to cut the muck & the bleach will kill of the growing bacteria which causes the muck.

 

Good point, I will try that and get back to you this weekend.
Thanks, Gene

 

Man, you dont need bleach or any of that... a 90% heater has a full drain system an trap. Remove all hoses an blow an clear them out. Reconnect them to a port an blow that out. Flush your pump with bleach an hot water, you will see the small 3/8 an pump will look brand new. An a heads up, your inducer looks like its leaking with all that rust. Why not get a professional, an take care of the entire heater from the drains, sensor, gas pressure, an combustion test. Its called a tune up on your heat. A heater should be serviced every year, an this wont happen.

 

I just moved into this house 3 months ago, and there are no service records for the furnace. I called all the local HVAC guys, and none were familiar with Ducane furnaces. I noticed the leak under the inducer, and will get it fixed.
Thanks, Gene

 

Just FYI, inducers of that design are notorious for leaking condensate.
I don't know if yours can be taken apart or not but I suspect it can. In the vast majority of cases, the inducer housing can be taken apart, cleaned, dried, the seam sealed with red silicone, put back together, & reinstalled. Many "service" companies will tell you you need a new inducer at several hundred dollars when for the cost of a tube of silicone & less than an hour's labor it can be repaired.