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FunWithTools 11-02-2012 11:52 AM

Insufficient flow from exhaust inducer - won't actuate pressure switch
I have a Heil Quaker high-efficiency furnace which is believed to be about 20 years old. Two years ago I had the exhaust inducer unit replaced by a professional. Last winter the furnace failed to light so I began investigating. On inspection I could hear what sounded like sand sized particles flying around in the inducer housing. At that time I discovered that the port feeding the vacuum switch was blocked and cleared it with a straightened paper clip. The furnace began working again for the remainder of the winter, despite the continued noise inside the inducer.

This winter, the inducer again fails to actuate the vacuum switch though I no longer hear the particles inside the inducer housing. I disassembled the inducer housing to clean it anyway, but found no debris. I assume they eventually worked there way out. The motor is operating when reassembled and air can be heard at the port but there is not enough vacuum to trip the pressure switch. Sucking on the switch hose does actuate it, so presumably the switch is fine.

While feeling for leaks at the hose connections, I discovered that a slight pinching or the hose between the furnace and the inducer motor seems to increase the air speed inside the inducer housing allowing the vacuum pressure switch to actuate. I'm guessing this is the result of the venturi effect when the hose is restricted. Can anyone identify what the actual problem may be here, and what the solution would be?

Doc Holliday 11-02-2012 12:07 PM

May be that the flu pipe (exhaust pipe) is blocked. That's where the debri enters the inducer. Remove the pipe from the furnace and open hand smack it a few times, see what falls out.

If you can get on the roof SAFELY than I'd remove the cap from the pipe and check in there. That pipes needs to be clear for the inducer to have enough suction to close the pressure switch.

phuz 11-02-2012 01:27 PM

I had bees build a hive on the inlet to mine, and the pressure switch wouldn't make, so when I disconnected the PVC inside by the furnace, it lit right up. I went outside and smacked the pipe and broke the hive apart and vacuumed everything up and it was good to go.

FunWithTools 11-02-2012 02:03 PM

Thanks Doc and Phuz! The more I read, the more I begin to understand how this system works. It seems like the flow of air through the inducer housing plays a large roll in how much vacuum is generated, not just how much air is generated by the fan blades.

I think it's going to be a pain in the arse to detach the flu pipe for cleaning, since my furnace is inside a closet and the PVC pipes are all glued together, but I think you're right Doc Holliday, the debris probably entered from the roof. Every time I've opened up the ceiling to do something, I get black, sand-like particles from the asphalt roofing dropping down. I never connected that to the sound in the inducer until I read your comment.

HVACTECH96 11-02-2012 05:14 PM

check condensate trap.

FunWithTools 11-02-2012 06:23 PM


Originally Posted by HVACTECH96 (Post 1043428)
check condensate trap.

While investigating the exhaust flu on my furnace I discovered that there is a condensation/water trap on the bottom end of the flu. There is water in the bottom of the trap and an over-flow tube that rises up out of the water and drains out the side of the trap into an open drain pipe. There is also a weighted Styrofoam float in the trap.

What conditions should I be checking for in the trap? Should water always be in the trap to act as a barrier again exhaust fumes venting through the overflow tube into the house? What conditions are incorrect?

how 11-02-2012 09:08 PM

Are those hoses just going between the ID assy and the PS or are they also T'ed elsewhere?

HVACTECH96 11-02-2012 09:13 PM

plugged trap that doesn't alow condensate to drain properly, causes water to backup inside secondary HE.

FunWithTools 11-02-2012 11:31 PM


Originally Posted by how (Post 1043573)
Are those hoses just going between the ID assy and the PS or are they also T'ed elsewhere?

There is a single vacuum hose running from the nipple on the inducer to the pressure switch. See photo:

FunWithTools 11-02-2012 11:35 PM


Originally Posted by HVACTECH96 (Post 1043576)
plugged trap that doesn't alow condensate to drain properly, causes water to backup inside secondary HE.

I rinsed algae out of the trap. The trap and overflow tube now have no restrictions, however the pressure switch still does not get actuated. See photo:

how 11-03-2012 12:18 AM

If that picture is of your furnace..A mid efficency with a single hose from ID to the PS, and the furnace will work if you pinch that hose, then this story doesn't make sense.
The only thing that pinching the hose should do, is decrease the air flow vaccum to the PS.

OK in re reading your first post I see that you are probably talking about a flexible hose that carries the exhaust gases from the exchanger to the inducer. (I Have not seen that myself) This would explain how you were increasing the vaccum on the line to the pressure switch.
If so, that may still mean that your PS is failing. (A small hole in the PS diaphram that the increased vaccum is overcoming)
We normally check this with a manometer to see if the vaccum on hose to the PS is enough to trigger the threashold of the PS.
If the vaccum is over the PS threashold and it's not switching then the problem is the PS. If the vaccum is under the PS threashold then we have 20 other possibilities to explore.

yuri 11-03-2012 02:07 PM

That is a high efficiency unit and the particle noise you are hearing is usually/probably water drops being carried over in the exhaust and flung thru the exhaust fan. that is not unusual and won't harm it as it has a plastic impeller and is designed to sling some water. you may have a faulty pressure switch or cracked heat exchanger or other draft problem. has no intake pipe as it has open burners/takes air from the house. may need a pro with a manometer to check the draft on the switch and find out its rating in inches WC if it is not stamped on the switch sticker.

FunWithTools 11-03-2012 03:11 PM

My current pressure switch is a HQ1005254TR.
Here is a photo:
How can I determine how many inches of wc this switch should require?

yuri 11-03-2012 03:50 PM

Bummer, you can't on those. does not have the "WC on the label and tri delta went out of business or stopped producing them. replaced by honeywell unless someone has old stock. a Heil dealer with old manuals may be able to find out but not easy to dig up that info. sorry.
A heil supplier should be able to cross reference it and hopefully the new switch has the rating on it.

how 11-03-2012 06:25 PM

I'm not getting this...
The op photo on post 9 is not my description of a high efficiency furnace!

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