Problem with air intake on 90+ Furnace
I am looking for some suggestions on a very intermitent problem I having with my own 7 year old furnace. Starting this fall then the furnace would not kick on due to the pressure switch being on. Each time I would remove the rubber lines going to the pressure switch and it would start up. I suspected a bad pressure switch, so I replaced it.
The problems began again with the new pressure switch. So I disconnected the 3″ PVC line feeding fresh air to the combustion chamber. As soon as I cracked the supply line the pressure switch would go off and the furnace started up. So I focused on the 3″ air supply line. I hooked up the shop vac and sucked air through (to clear the pipe) , and then I hooked up my shop dust collector and really moved some air though the pipe 1200 CFM.
The exhaust and supply lines use a concentric kit on the outside of the house. Both are free from obstruction (birds, bugs, nests…) The line has about a 20′ run with two 90s (within mfg specifications).
I could pull the existing supply line out, but it is above a finished basement with a really nice drywalled ceiling (it is the only line under this part of the basement).
The only option I think I have to to run a new supply line and “Y” it in to the existing line. Does anyone have any other options?
Thanks much,
Mike
Replies
Man, I had much the same problem for 15 years - you got a Carrier system? Our problems may be different, but here was my issue a month or so ago - http://forums.taunton.com/tp-breaktime/messages?msg=96776.1
Good luck!
Forrest
Did you check the rubbler hoses? 1/4" or so that that they were supported and not kinking?
If it was working I can't see why you would have to replace the supply.
Yeah, my guess would be that the supply line is kinking closed. Keep in mind that it's not just the vacuum of the induction fan but added to that any pressurization of the house. And the classical fresh air admittance setup will produce that pressurization.
(Try leaving a window open a crack.)
You have a blockage in your vent pipe. When your furnace first kicks on a exhaust fan
goes on first to vent all gases. If there's anything obstructing that flow your pressure switch wont trip
It does'n have to be much, a lot of time it 's about a cup worth of water, this happens when
the pipe isn't pitched correctly. When the warm air hits the cold air from outside it condensate and getting trapped in the pipe. Next time it happens try removing the vent at the furnace then see if the furnace will start up. DON'T LET THE FURNACE RUN UNVENTED
zzzzz raises a good point. It could be water in either pipe. On the intake side your vacuum is strong enough to overcome it, but the induction fan isn't. On the output side, once you remove the intake pipe you add the force of the circulating fan to the induction fan, possibly creating enough pressure to overcome a water trap.
I can guarantee you -- unless the rubber pipe is fully supported for its entire length there will be low spots that collect water.
OK, maybe I misinterpreted the rubber hose reference. Is the line rigid PVC its entire distance on both sides? If so, check for low spots in both lines, places that would collect condensation. The lines should be peaked in the middle so that they drain dry to one end or the other, with no unsupported sections longer than about 3 feet.
If your view never changes you're following the wrong leader
Thanks for all the comments. Here is my game plan:
I’ll wait for some cold temps. It has been way too warm here for mid January. (yes I ski…)
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Once it fails to start up, I’ll try opening a window to see what affect that might have.
I’ll check the 2” exhaust line and induction blower.
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Then I’ll take the exhaust line off and use the shop vac to pull any water through. If there is excess water in the pipe I should be able to pull some of it out.
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If all this does not work then I’ll run another 3” PVC supply line to the furnace and ‘Y’ it into the existing pipe.
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If that does not work I break down and call someone else…
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Forest,
The furnace is not a Carrier it is Nordyne. I have installed a bunch of these over the past few years. It has been working great for the first 6 years so I suspect something environmental changed. I don’t believe it was a defect with the manufacturing since it work fine for years. Your story was quite an adventure… Just as an FYI. I rigged up a line (rope) that connects to the supply line so my wife can pull the rope to crack the supply line and start the furnace. Not as involved as sucking on a vacuum tube from the kitchen , but still not right…
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ClaysWorld,
I checked the rubber hoses for the pressure switch. They were not kinked, cracked and did not contain any debris.
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Dan,
I will check the supply line for debris and moisture. I also like the idea of trying to leave a basement window open a crack. I build this house and it is pretty tight. The hot water tank is direct vent, so there is a power vent unit pushing air out of the house when it is on. The hotwater tank uses ‘inside’ air for combustion. PS – Both the supply and vent pipes are Schedule 40 PVC so there should not be any sags in the line.
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Zzzz,
Similar to Dan’s comments, I will take the exhaust line off and use a clean shop vac to pull air through to see if there is excess water in the pipe.
If a line goes unsupported for 10-15 feet there can be a sag, even with sch 40. (I suspect the temperature cycling accelerates the tendency to sag over time.) I've fixed a couple of furnaces by just propping up the exhaust line in the middle. In one case the furnace guys had been out a dozen times, and I fixed it with a piece of coathanger wire.
If your view never changes you're following the wrong leader
I'm not sure if this would adversly affect the pressure switch but I had a problem with my concentric vent fitting coming loose. I think it was from expansion and contraction of the vent itself. Worked fine looked fine but was leaking. In my case it was drawing exaust gas back for combustion. Garett
pull the air intake blower and make sure that's clear...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
Even though the exhaust line is within specs, I'm thinking it may be close enough to the limit to be a problem. Does your conc vent go through the roof? Are the two 90's long sweeps or short? What portion of the line is in an unconditioned space, are we talking too much force required just to expel air along the vent, or when water lays in a bad slope, or ice/frost build up in cold weather? Can you cut the concentric vent off in the attic and run separate exhaust/vent through the roof? The installer did put the exhaust through the middle of the concentric vent, and intake down the side??
I had problems with a unit under construction, only the basement was heated, 2" abs through bungalow's roof kept freezing up, hooked it into plumbing DRY vent stack for a while, worked fine, left T on stack just in case ..
Does the internal air pressure of the house affect these systems, I thought the pressure sensors only monitor the pressure/blockages inside the exhaust and intake supplies?
Edited 1/11/2008 8:47 am ET by wane
I think you are real close but doesn't the + draft = the function of the pressure switch?
and if so then maybe depresurizing the cabinet relases the switch. Then if so maybe #7
Pressure Switch And Auxiliary Pressure Switch If Required:
A normally open switch used to prove draft on an induced draft furnace. Two-stage furnaces have dual pressure switches, one for high fire and one for low fire. If the furnace were to cycle on the pressure switch, the burners would light, go out, and then the furnace would start the ignition sequence again. This cycle would repeat indefinitely. If the furnace would not prove draft at all, the induced draft motor would continue to run and the ignition sequence would never start.
The auxiliary pressure switch is used to sense draft on the secondary heat exchanger. Most of the time, if the auxiliary pressure switch is tripping, the secondary heat exchanger is trapping water.
If the unit is cycling on the pressure switch, check for:
1. Vent pipe blockage.
2. Water in the pressure switch or tube.
3. Leaks in hoses.
4. Blockage in chimney.
5. If 90+ furnace, blockage in intake or exhaust pipe.
6. Induced draft motor amps (as the bearings go bad, the motor slows down and draws excessive current).
7. Secondary heat exchanger drain blockage.
8. Pinched hose or drain line.
And I forgot to add the above attribute to http://www.achrnews.com/CDA/Articles/Service_and_Maintenance/89e03e048b16a010VgnVCM100000f932a8c0____
Edited 1/11/2008 1:12 pm by ClaysWorld
Clay,
I think you might be right on the drain blockage. I did double check all the supply and vent PVC lines over the weekend and they are not sagging or holding excess water.
The lower cabinet of the furnace has a loop of clear tubing which acts as a trap for the water leaving the furnace. The hose is not kinked, but it could be desinged better. I wonder if this hose intermitently restricts the water from leaving causing the the backup. I'll probably run a longer hose and smooth out any turns.
It is going to get nice and cold this coming weekend so the furnace will be running a bit more. I suspect I'll be able to run down this problem and investigate the drainage problem.
Thanks for your suggestion.
Michael
The contractor who does my HVAC installations provides a tee in the air supply which has a cap that can be removed to allow air from in the house to supply the furnace. He did this because in his experience some locations such as near water allow the intake to frost up and restrict the air flow to the point that the furnace will not operate. IIRC I have only had to remove a cap a couple of times in the past 14 years.
I'm in South Dakota near a couple rivers and it does get cold and frosty here.
Depending on the ? humidity around the furnace and such here's what to check.
I've had this problem on 2 different furnaces over the last 2 years and it makes it real hard to diagnose.
The symptom is getting close to identifying it with a couple of paths it can take, but the easy one if your lucky. Take the spade terminals on the sensors to pressure valve and as many as you want to check.Also the other end to the control module.
Turn off electric first please. Slip the terminal off and back on.If any corrosion could be adding to failure to contact. Take note as to the feel of the seat of the contact, you may or not be able to feel a bad connection.
I had one that had a leaking relief valve on the HWH next to the furnace causing the moisture problem and the other one had a drip down the side of the furnace from a humidifier. And I got no help from the flash code.
I check the drainage in lower blower unit. I installed a 90' elbow and eliminated any potential restriction in the line. The furnace worked great for two days then I got the Pressure Switch failure again.
The furnace and basement is really dry so I don't think it is humidity.
Each time the furnce would not fire, I could crack the supply line and get it to work. I will probably work this weekend on adding an existing 3" supply line to the furnace.
Thanks for all your suggestions...