Manifold vacuum not actuating pressure switch

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Old 12-04-16, 04:10 PM
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Manifold vacuum not actuating pressure switch

Our downstairs furnace, a 1988 Carrier 395BAW060120, has stopped lighting its 6 burners, though the draft inducer is running and the pilot comes on.

The vacuum in the manifold is somewhere between 0.13" and 0.15" wc, but the FS6001-248 air pressure switch seems to want 1/4" before it will ignite the burners. I couldn't find any pressure specification online for this switch. The tube from the manifold to the switch is neither leaking nor blocked. I took photos of the manifold interior by poking my cell phone inside, which showed no soot or condensation anywhere.

When I hold a piece of cardboard against the draft induction motor's 5-blade fan it plays middle D very slightly flat, which is 293 Hz, corresponding to (293/5)*60 = 3516 rpm. This is slightly over the rated 3300 rpm, so it seems to be running at full speed.

To see what draw the draft inducer was capable of, I removed it from the furnace and ran it on the bench. I improvised a manifold as a 6"x6" sheet of styrofoam with a hole in the middle for the manometer and placed it 1/4" from the intake side of the blower, where it drew 1/2" wc. I didn't see any deformities in the blower.

To see if the flue to the roof was blocked somewhere I removed it and let the induced draft blow straight out the top of the furnace. This didn't increase the manifold vacuum at all.

With the draft inducer running I reversed the NO and NC (brown and yellow) wires to the air pressure switch, as one way to make it appear to have detected vacuum when it hasn't. The burners started wthin a minute of the pilot coming on, a couple of minutes later the main blower came on with its usual flow, and downstairs warmed quickly.

For now I'm leaving the furnace unplugged until I sort this out.

QUESTIONS

1. Which one is at fault, the 0.14" wc manifold vacuum or the 0.25" wc switch? For now I've ordered a used FS6001-248 switch to see if it opens and closes at the same pressures as mine. But I'd really like to diagnose the problem properly before I buy too many parts or we might have been better off buying a new furnace, though I hate throwing out otherwise serviceable things if they can be fixed.

2. Where does the air for the draft inducer come from in this unit? When it was running (with the gas off) I couldn't feel any air flow anywhere at the front of the unit or near the burners, not even with a feather. Is it drawn from the back, or where? If there's any obstruction before the air reaches the draft inducer wouldn't that increase the vacuum? There's definitely no obstruction between the draft inducer and the tube where the pressure switch taps into the manifold. Could there be a leak somewhere in the heat exchanger? Wouldn't that admit combustion products into the house? I put a CO2 meter and a CO detector beside a heating vent in the living room and ran the furnace for 30 minutes. The CO2 rose from 580 to 770 ppm (for comparison the naked gas burner in our fireplace quickly pushes it to 2000 ppm) while the CO detector didn't go off, so a leak if any would surely be a small one.
 
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Old 12-04-16, 04:40 PM
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It sounds like you have holes in the heat exchanger.

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Old 12-04-16, 06:14 PM
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Thanks, Houston204. After convincing myself that everything except the heat exchanger is ok, holes in the heat exchanger sounds like an excellent diagnosis. I'll order a new furnace.

Incidentally it occurs to me that the draft inducer will suck room air into the combustion chamber rather than vice versa, preventing combustion products from entering the room air. I'd been thinking that the point of the draft inducer was to induce a draft, but didn't understand why that was necessary. If the real point is to prevent combustion products contaminating the room air even when there are holes in the heat exchanger, that seems an excellent reason to have a draft inducer!

Again many thanks for your diagnosis.
 
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Old 12-05-16, 12:04 AM
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The high CO is one problem but the pressure switch is not your burner problem.

You mentioned the pilot was lighting. The pilot won't light unless the pressure switch has proven. Your problem is a dirty pilot light.... possibly the orifice. The pilot light needs to be big and bushy in order to generate enough heat for the pilot sensor to close and the burner to light.
 
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Old 12-05-16, 11:51 PM
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Houston204: Thank you, I've ordered a used pressure switch. If it doesn't work any better than my present one I'll go with your (very plausible) diagnosis of holes in the heat exchanger even though I can't see them, and order a new furnace. If however it fixes the problem then I'll keep the furnace until 2038 on the ground that 50-year-old furnaces should be replaced.

Pjmax: Sorry, can't agree. The pilot orifice was the very first thing I checked and it was clean as a whistle and produced a good big flame even after I disconnected the vacuum tube between the manifold and the air pressure switch. When I "lie" to the furnace that the air pressure switch is working correctly the furnace then works like new.

Maintainers of this blog: after I log in and type my reply, your system tells me I'm not logged in and deletes my reply. Even your Autosave feature can't bring my reply back. This has happened several times and has been wasting a lot of my time. If I'm the only one with this complaint then I'll concede that I'm an idiot.
 
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Old 12-06-16, 12:05 AM
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To all repliers: why do you think I have any CO problem when I said that my CO detector gave no alarm?
 

Last edited by Vaughan Pratt; 12-06-16 at 12:07 AM. Reason: sp
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Old 12-06-16, 03:15 AM
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Most big box store CO detectors don't go off until levels have reached a point where your health is already affected. See the chart below. This is to prevent "nuisance" alarms.
I don't recommend these types of detectors. Search for "low level CO detector". I use these in my house.
 
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Old 12-19-16, 08:31 AM
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Thanks, all. The new furnace is a 5-burner 80% AFUE 2-stage ICP. It went in easily, only took a couple of hours to swap with the old one. Took longer to route the thermostat!

Once the old furnace was outside I could see the six heat exchangers. It was too bright to look for holes using a flashlight and they certainly weren't visible with the naked eye. However the combination of a draft inducer running at full speed and inducing a strong draft on the bench, along with a low vacuum and no evident blockage in the flue, does seem to leave holes as the only remaining possibility, and a likely one given its 28 years of otherwise reliable service.

Thanks for the tip about low level CO detectors, roughneck77. I'll certainly look into that. But how many cases have there been in recent years of CO leaking through heat exchangers with holes? I'm having difficulty imagining how that could happen as long as the draft inducer is producing enough of a vacuum to allow the burners to start.
 
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