DuCane not heating the house to t-stat set temperature.


Old 12-11-12, 03:59 PM
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DuCane not heating the house to t-stat set temperature.

Hello all.
The wife informed me the other night while I was at work that the furnace would fire up and run for a few minutes and then shut off. And it was only about 62 in the house when the thermostat was set to 68.
I told her to turn on her electric blanket and Id look at when I could.
Well, after poking around online, and running some of the tests Ive read about, Im still stumped.
Its almost like its short cycling, like my well pump was a few months ago before I replaced the pressure switch. Itll run for 3-4 minutes, then shut off for awhile, and then run for 3-4 minutes again.

The furnace in question is a DuCane, Model CMPE075U3.
Not sure of the age, but we have been in the house for 7 years.

Here is everything I have looked at/tested:

Checked all the vents and returns, a little dirty, but none are blocked.

The wife recently replaced the filters, Ive checked those, they are the pleated kind, but new and clean. I even pulled them, and it still shuts down after a few minutes of run time.

The tstat itself is only about 3-4 years old. Ive replaced the batteries, and turned it up to 80, and no change.

I jumped the R and the W wires at the furnace itself (taking the tstat out of the equation), and it still shut down.

Removed the flame sense rod, cleaned it with emery cloth, still shuts off.

Removed the HSI, it looks good. Checked it with a volt meter, and its reading around 82 ohms.

Also, I forgot to add, that when it shuts down, I am getting 3 flashes from Im guessing the control module?? At times its flashing the normal "heartbeat", and after it shuts off, Im getting 3 flashes, pause, 3 flashes, pause, etc....

Im running out of ideas, the wife is cold and not too happy.
Anyone have any ideas??
Short of calling an HVAC to look at it, since I havent got the money. And that will be my last resort.

This site was alot of help when I replaced the pressure switch on my well, and now Im hoping for the same results to get my furnace fixed.

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Old 12-11-12, 04:15 PM
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Usually an index of diagnostic codes can be found on the insider of the blower compartment door. Check that and post the cause of the codes you describe.

Your description of what is happening is vague.

Take the cover off thge burner compartment. Describe in sequence and in detail what happens when you turn up the thermostat.
Old 12-11-12, 05:08 PM
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OK, Ill try and describe it as best as I can....
The furnace "clicks", the fan kicks on, and runs for awhile, the HSI glows hot, the burners kick in, and runs for 2-3 minutes, then it shuts off.
It doesnt shut off when it reaches the temp set at the tstat, it just shuts off after running for 2-3 minutes. And the fan keeps running for about 2 more minutes.
It takes a break of 5-10 minutes, and it starts all over again.

The diagnostic codes youre talking about, Im guessing are the flashes, coming from the control module?
There are 3 flashes after it shuts down.
Other wise its the normal "heartbeat" series of flashes.

I found a list of codes online for DuCane furnaces, and it said:

3 Flashes:
Pressure switch or aux. limit was still open 30 seconds after the inducer was energized. System is in 5-minute delay mode, with inducer off. After 5-minute delay, new ignition sequence is initiated. Note: SmartValve On/Off switch in OFF position during a call for heat generates this flash code.

1. Move SmartValve control switch to ON.
2. Inducer and inducer wiring.
3. Inducer [IBS] switch open.
4. Low line voltage power supply.
5. Airflow proving switch operation, tubing, wiring.
6. Obstructions or restrictions in appliance air intake or exhaust flue system that prevent proper combustion air flow.
7. Other devices (such as auxiliary limits) in the airflow proving switch circuit.

Is that the info youre looking for?
Old 12-11-12, 08:55 PM
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I did a quick Google search and found:

Hello, check the small clear tubing that goes from the pressure switch to the inducer motor, sometimes water can drain back into it preventing the pressure switch from closing also make sure the tubing is clear of any restrictions.

Last edited by hvactechfw; 12-12-12 at 03:21 PM. Reason: removed link
Old 12-12-12, 09:55 AM
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EnclosedVisions, thanks for the idea.
I just tried that. Disconnected all the tubing coming out of the pressure switch, blew thru them all and they were all clear.
You did give me an idea. I am also going to check the exhaust vents too, and make sure they are clear.

Will keep everyone informed.
Old 12-12-12, 12:52 PM
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These guys go through manually bypassing the pressure switch to determine if it is working or not. The guy with the furnace got it to work by: disconnecting the two wires going to the pressure switch and then switching on the furnace - he waited for the burners to light then he shorted the two wires together - and Al-la-peanut-butter-sandwiches - his DuCane worked without the three flashing error. The above link goes step-by-step.

Hope it helps.

Last edited by hvactechfw; 12-12-12 at 03:20 PM. Reason: removed link
Old 12-12-12, 04:24 PM
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Bypassing a pressure switch can be very dangerous. A mod removed the link, but really he should have deleted your post.

Usually when a safety (pressure switch, limit, etc.) prevents a furnace from firing, there's a reason why.
Old 12-12-12, 06:56 PM
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This was not intended as a fix, but instead as a diagnostics procedure. The three blink directly point to the pressure switch. This Pressure switch is simply an pressure indicator. The inducer still receives its normal start and stop signals. Bottom line this procedure will tell you TRUE or FALSE if the pressure switch is operating as intended.

Since I am in a dangerous area. How about you tell us how you would go about fixing this problem? I am always up for learning.
Old 12-12-12, 07:13 PM
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This is a smart valve. There are many documented issues with them..

IMO call in a pro.

If there are any other posts that may affect health and safety I would consider closing this thread.

From that point on please seek a professional.
Old 12-18-12, 11:02 AM
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Thumbs up

Problem solved...
I changed the pressure switch.

I used the diagnostic tip from the post that was deleted (I wont mention what the diagnostic tip was). And my furnace ran flawlessly for a test run. I didnt perform the diagnostic tip test run for an extended time, so there was never any danger.

Ordered and replaced the pressure switch, and my furnace now works correctly.
Thanks to EnclosedVisions for the tip that got me headed in the right direction.
A $45.00 pressure switch that I can replace myself was cheaper than a smart valve and calling in a pro IMO.

And thanks to all who replied.

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