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allenzachary 12-26-2006 08:38 PM

Furnace will not stay on
Just replaced inducer blower. Prior to replacement of inducer, unit did not react to thermostat at all.

Now Inducer blower comes on, about thirty seconds later a relay clicks, then the furnace flame begins burning. Another 30 seconds or so, the blower comes on that delivers the forced air to the house.

Heat stays on for 1 to 5 minutes, then shuts off well before thermostat temperature is attained.


Columbia, South Carolina, Average Winter high 56, Average Winter low 37.

Ranch home on slab, HVAC in attic. Natural gas supplied A/C and furnace in the same unit, by Inter City products Corp Model DGHH075A012IN, at least 12 years old. Honeywell CT3400A electronic programmable thermostat, same age.

Acefurnacefixer will say to call a pro, any other suggestions?

#CARRIERMAN 12-26-2006 09:19 PM

Hi allenzachary

I will go over a few possibilitys, First and foremost. Is the filter clean?. If this is a yes. Reading your earlier post I will bet you have a voltmeter. In the middle of the furnace will be a black limit about 2"x 3.5" with two leads coming off of it. Put a meter lead on either side and see if this limit shows voltage when the furnace shuts off. If it does you could have a partially plugged evap coil, blower not running up to speed, etc. With your back ground you should have no trouble determining this. If you find you have a limit opening during your heat cycle. Then I would reccomend having the pros come in and fix your problem. If this is not the problem if you will give me a little more detail we will nail it down.

Good luck

allenzachary 12-26-2006 10:08 PM

I have filters in the two returns, one at either end of the house. I'm embarassed to say that the filters are not terribly clean. Shame on me. I'm heading to the 24 hour store to get replacements.

I've attached a picture of what I believe is the limiter you mentioned. I couldn't get a reading across it as my multimeter's battery died, (when it rains, it pours). When I disconnected one of the leads to get to it, though, it engaged the main blower fan (terminology? the fan that forces the air through the vents) and nothing else. I also noticed in the photo that there is soot surrounding the "limiter" as if it either burnt up or smoke was forced out through the tiny gap between it and the furnace. Is this normal, as an accumulation over time, or symptomatic of a sudden failure?

The (new) inducer blower is blowing constantly whether the whole system is runing or not.

I'm off to Wal-Mart to get filters and a battery for my meter. (I believe that's the single advantage of having Wal-Mart infest my town... hardware late at night).

I'll update you with a voltage measurement when I get back.

Thanks for your help.


allenzachary 12-26-2006 10:16 PM

1 Attachment(s)
oops...the picture.

allenzachary 12-26-2006 11:21 PM


Filters replaced with sparkly clean new ones...

Tested limiter with furnace off, (circulation fan off, no flame), and it shows 0 volts.

Could this have something to do with "cleaning" the unit. I'm embarassed to say that I've never cleaned the furnace or even thought about it.

Meanwhile, the inducer blower continues to run, as it has since I replaced it around 6PM today. Perhaps that is why the old one was running constantly.

I await your response with baited breath, (and chattering teeth)


Aceinstaller 12-27-2006 12:25 AM

try turning your thermostat down or off, then check to see if the inducer motor is still running.

this motor will run when the t-stat is calling for heat, but if it's running when there is no call for heat then you either have some miswired components or a bad circuit board.

if it shuts off when you turn your t-stat down, then turn it back up and post exactly what the furnace is doing as such:

1. inducer motor starts yes/no

2. ignitor glows, or spark ignition is working yes/no

3. main gas valve opens and burners light yes/no

4. burners stay lit for longer than 30 seconds yes/no

5. main blower motor starts and supplies home with air yes/no

knowing which spot in the furnace cycle that you circle "no" will allow me to help you efficiently solve your problem.


allenzachary 12-27-2006 07:36 AM

Inducer motor shuts down about 30 seconds after setting t-stat to "off."

The answers to your troubleshooting questions have changed since last night. Last night the system cycled on and off in short bursts and got the house marginally warm. The burners shut off in less than 30 seconds each time, thus:

Last night:

1) yes 2) yes 3) yes 4) no 5) yes

I tried the inducer motor experiment this morning around 5AM. Since then the answers have changed:

1) yes 2) no 3) no) 4) no 5) no

Weird, huh?

Thanks again to all for assistance.


#CARRIERMAN 12-27-2006 08:14 AM

Hi allenzachary

On the board you connected the inducer too should be a ATO fuse. Check the fuse and make sure it is ok. Use your voltmeter on the Ohm scale. Unplug your furnace, disconnect one side of each of the limits. Test across the limits with your meter. If any of the limits show infinite, that limit is open. There is now doubt that you have a 24 volt problem, we just need to locate the source. It could be a resetable limit that is open. Let us know and we will help you figure it out. You could have a pressure switch not holding. The problems listed on the previous response would point more to that being your source. To determine this, get a jumper wire. Locate the pressure switch. Start the unit up, place the jumper wire across the pressure switch terminals. If the unit fires up and runs. Replace the pressure switch.

Good luck

allenzachary 12-27-2006 09:47 PM


Looked carefully at the PC board...there is no fuse of any type as far as I can see. No blades, no glass fuses.

The only limit I see is the one in the photo in the previous post. It reads 0.5 ohms. When I disconnect one of the leads on the limit, the circulation fan comes on, so there is clearly some kind of effect the limit is having. My guess is that it's working as it is designed.

I also tried the jumper on the pressure's where the fun starts. When I jumped between the leads with the unit powered up, it began the normal heat cycle; the flame fired up and about 20 seconds later, the circulation fan came on. When I removed the jumper, the system shut down. It seems that this is the source of the problem.

Now the questions come:

1) Do you agree that the pressure sensor seems to be the culprit?

2)There are two pressure sensors mounted on the same bracket. I only bypassed one of them. The one that I bypassed has two vacuum tubes connected to it, the other has only one. Do I need to replace both or just the one that worked when it was bypassed? Would a photo help?

2) Are pressure switches as particular as inducer blowers? Are there many types/models/brands of pressure sensors? Would I get tthem from the same HVAC suppliers as the inducer blower?

Thanks again....


caseywa 12-28-2006 12:55 AM

Just my two-cents, if I may...allenzachary, you said on your previous post:

"2) On the old unit, there is a condensate line connected in the top left quadrant along with the drain line connected to the bottom. The new unit has a nipple in each the top left and top right quadrants. Am I to connect the line to the top left? Would I have to punch through the nipple?"

On my inducer like yours the bottom clear plastic tube is the condensation drain and the top left hose is to the pressure switch.

Is it possible that either the top left nipple is not going to the pressure switch or that this nipple is not "drilled" out into the inducer housing?

Will the pressure switch not being able to sense air pressure from the inducer cause these symptoms?

allenzachary 12-28-2006 05:09 AM


That was my though as well, so I took the inducer off and disassembled it. Alas, it was "drilled through."

All the research I'm doing is saying the problem is not the pressure switch. They are nearly impossible to cross reference and even harder to find available to buy. If they often went bad, they would be much more prevalent (like our old friends, the inducer blowers-everyone stocks and sells them).

Although I find sleeping temps in the upper 50's quite to my liking, (along with the lack of a heat bill), my wife and daughters are growing tired of being "cold" while Daddy struggles with the monster in the attic. I may give in and call a pro. I'm at my wit's end.

I know to my bones the answer is something incredibly simple and it will cause me great pain to hand over $100+ to a guy who says, "Oh, you didn't do this," and watch him flick a switch or connect a wire. The only problem is I don't know what incredibly simple solution may be.

Today is my last chance. My wife said this morning "So, today you'll call the heating guy?"

Frustrated in SC.

PS: Did you get that blower wheel to BDNPA? I also have a spare for him.

#CARRIERMAN 12-28-2006 08:24 AM

Hi allenzachary

We just had this happen on a brand new unit that someone installed. We were able to fix the problem with replacing just the one. But before you do you need to make sure you are not ponding water in the exhaust first. If there is no problem there, you should be good with just replacing the pressure switch. And yes they are mostly unit specific.

Good luck

askme233 12-28-2006 08:48 AM

I'm a newbie here, but Allen Zachary is my son's name so I figured I would post.

I also had similar problems that frustratingly would change somewhat over time. Same difficulty starting (staying started) that if I played with it I could get it to work.

We thought it was various sensors (bad flame sensor?), checked them all. In the end, and after much frustration, it turned out to be the main control unit. A simple LED light on the unit indicated a fault. replacing the unit ($100) cured all of the problems.


zloy 12-28-2006 09:22 AM


Originally Posted by allenzachary (Post 27879)
Heat stays on for 1 to 5 minutes, then shuts off well before thermostat temperature is attained.

I have a similar problem. The difference is I have not changed anything recently (I just bought a house a couple of months ago), I have Concord. In my case, it shut off when I try to heat above 65-67F. It held last night 63F, but shut off in the morning insted of going to 72F.

If I switch off - switch on the heater then it works for 5-15 min and shuts off again. A simple LED light on the heater indicates the "system lockout" fault. It started this Tuesday.

Any suggestion?

Thanks for your help.

caseywa 12-28-2006 10:31 AM blower wheel is damaged and was about ready to tilt. Get ahold of Brad about your wheel. I have his direct email and will get a message to him now.


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