Intermittant suction from inducer to pressure switch ???

[bigmatt]

So today I've learned quite a bit about gas furnaces and also found/joined this forum (which seems great) - quite productive! Now if I can just get my heat going...

I've got a Tempstar 5000 gas furnace which is firing up the burners for around 15 seconds, then they go off, after a short bit they come on again and burn for maybe 5 seconds. Over and over.

I've narrowed down the problem - the suction on the draft inducer motor tube port starts out good, but after a few seconds of the gas burning it seems to peter out. It seems to be heat related somehow, I think, perhaps. If I suck on the tube it runs fine. If I short the two wires coming off the pressure sensor it runs fine. The airflow on the exterior vent seems appropriate, though I don't have a baseline to judge it. It feels steady but not overpowering. Certainly not as much volume of air as from a dryer vent.

The other, probably related, issue I had last winter and this one is that the draft inducer motor vibrates quite loudly. If I tug on the motor and torque it either down, right, or up it is silent as can be. It seems the blades inside the assembly are slightly touching the housing and causing the loud noise/vibration and torquing it a bit frees them up.

So... any ideas how to resolve it? I'm guessing replace the DIM assembly is the best approach unless there's something else I should try first.

Any idea what would cause the suction to be intermittant??

thanks!

edit: whoa! just noticing prices for inducers online, quite hefty. Let's try to fix this thing! If not, where's a good source for this thing?

[Grady]

Without being able to install a manometer in the sensor line, we are guessing if the problem is the inducer or the switch gettting stiff or something else. A faily common problem is some sort of restriction (often water) in the exhaust. One can't tell the difference just by feel or listening. The trapping of water is quite common where there is not much slope to the exhaust pipe or if there are any 'bellies' in it. If there's any chance this might be the case (or any other type of restriction) I suggest taking the pipe down & cleaning/inspecting it.

[goldstar]

The short gas on cycle usually indicates a flame sensor which needs to be cleaned.

[bigmatt]

Thanks, Grady. I took the exhaust pipe off the assembly and there was a bunch of rust and metal debris - the little blades on the fan were completely rusted and approx. 50-60% completely gone. I can't believe it was still sucking enough air. Got a replacement assembly locally for $80 and am back in business. Everything else appears in good shape.

When I moved into this house 6 years ago the dryer was venting into the crawlspace - I don't think the fan liked the humidity.

Thanks a bunch.

[xpogi]

Originally Posted by bigmatt

...
When I moved into this house 6 years ago the dryer was venting into the crawlspace - I don't think the fan liked the humidity.
....

Don't mean to alarm you but you aught to have the furnace's heat exchanger checked. It probably wasn't "humidity" that ate up your inducer. It probably was the residual bleach and fabric softener chemicals and they may have done a number on your heat exchanger too.

When those chemicals are burned they create nasty corrosive agents not to mention phosgene gas. All installation and owner's manuals for furnaces warn about installing a furnace near a laundry room not to mention virtually venting the dryer into your combustion air inlet. That's scary!

[Grady]

Xpogi is absolutely right about the laundry products being hard on equipment, not to mention the humidity.

[bigmatt]

Wow, thanks for the advice. When I had the DIM assembly off what I could see through the opening was a little bit of rust but it didn't seem like enough to worry about and didn't go deep, i.e. it was worst right by the opening, but not really that bad, superficial. Do you think I should inspect deeper somehow? Is the heat exchanger typically detachable/inspectable/replaceable?

Would a carbon monoxide detector by a vent tell me if there was a compromise?

thanks.

[xpogi]

Originally Posted by bigmatt

...
Would a carbon monoxide detector by a vent tell me if there was a compromise?

Couldn't hurt. I have one by mine!
The worse thing that could happen would be hole in your heat exchanger or it could become completely blocked and if either of those things should happen hopefully your roll-out switch would act to shut off the furnace.
My expertise with furnaces is limited to design, I don't do service work. I am not sure how to go about inspecting a heat exchanger but I am sure someone else here can help.

[Grady]

Inspection of the heat exchanger is something best left to the professional. One almost give away of a bad heat exchanger is if the flames blow or otherwise change when the blower comes on. Usually by the time that happens, the heat exchanger is in really bad shape. Electronic combustion analysis can often detect a defect in the heat exchanger as well. If there is an increase in the oxygen or decrease in the CO of the exhaust gasses when the blower comes on, this is a very strong indication of leakage.

[ecman51`]

Grady,

Can anyone buy a meter to detect this?

[SeattlePioneer]

It takes experience and good judgement, and honesty, to do a good job of inspecting a heat exchanger. Unfortunately, a lot of heating contractors have repair services in a large part to scare people into buying new furnaces.

On the other hand trying to do your own inspection when you don't know how to do it isn't any better an option.

Finding an honest and competent repairman is one of the tasks homeowners need to be able to do. Recommendations from firends, neighbors and relatives to outfits that have given them honest and fair service is a good way to start.

[ecman51`]

Originally Posted by SeattlePioneer

It takes experience and good judgement, and honesty, to do a good job of inspecting a heat exchanger.

If a homeowner has the opportunity to watch such a repairman work - what should the homeowner look for to see that the repairman is inspecting the heat exchanger correctly, and simply not taking some (possibly unclear)shortcut?

[Grady]

In my opinion a proper inspection is multi faceted. The inspection is usually preceeded by a complete cleaning of the heat exchanger & exhaust system. After the cleaning is done, combustion tests are taken & a visual inspection of the easy to see parts done. Upon completion of this if there is any doubt as to the integrity of the heat exchanger, any disassembly needed to gain complete access to the heat exchanger is done.

SeattlePioneer: Any disagreement or additions?

[SeattlePioneer]

Originally Posted by ecman51`

If a homeowner has the opportunity to watch such a repairman work - what should the homeowner look for to see that the repairman is inspecting the heat exchanger correctly, and simply not taking some (possibly unclear)shortcut?

Interesting question, Ecman.

I used to work at a natural gas utility as a repairman. If we turned the gas on for people and left a hazardous condition in a piece of equipment that injured or killed someone, the company could be expected to be sued for damages.

On the other hand, you didn't want to spent all day tuning up every piece of equipment in someone's house.

So the company's standard was that when lighting up any piece of equipment the burners would be lit and inspected for proper operation. Furnaces and such would be checked for fan interference when the blower fan turned on ---which would be a possible indication of a seriously cracked heat exchanger.

Any old oil furnace converted to natural gas would be checked for carbon monoxide in the combustion gasses. Any equipment where there was doubts about it operating properly was inspected for making CO.

Any equipment found to be making 100 PPM CO or more in the undiluted combustion gasses would either be repaired or adjusted to produce less than that, or shut off and red tagged.

The basic idea was to inspect the equipment for safe operation, and if the equipment was making less that 100 PPM CO in the combustion gasses, it wasn't going to be a hazard even if there was a heat exchanger crack or whatever.

Mostly I relied on that kind of test and standard together with a visual inspection of the heat exchanger. If I could find cracks I was always glad to write it up and red tag the furnace.

However, the lengths to which a lot of heating contractors go to hunt for cracks seemed like a greater interest in selling people new equipment rather than protecting people's safety and health.

[ecman51`]

Recently, I had the rare opportunity to closely look at both the primary and secondary heat exchangers all the way around, in an older high efficiency updraft Coleman in a 4-plex property.

It was remobved and replaced because the fire dept. was out twice in one year due to tenants CO detector went off and fire dept. got higher readings throughout building AND natural gas odors as well.

The heating contractor caled in said that particular furnace was faulty and also commented that the condensate water smelled suspect.

I did not buy it. I believe incompetant HVAC company made error, and I told the landlord. That is why I was so interested in carefully evaluating the furnace myself, when it was out and outside where I coud really see in it good, from every angle. I could see all the way around, over and under each exchanger, and every weld seam. They were stainless steel upgrades and all looked brand new. !

I found no footprints out in the snow the next day, to hint that the HVAC firm took a look at the buildings's fresh air intake and venting system!

The fresh air intake to the 4- furnace's room is right next to the exhaust gas pipes!, and I have witnessed steamy exhaust vapors on real cold evenings wafting into the fresh air intake when several furnaces were going at once. !

I told landlord my theory is that all that CO is going back into furnace room and is being sucked into heat duct through gap where you slip the furnace filter in and also under the furnace where they set the furnaces up 2 inches off the ground and cut out the bottom of each furnace and put that 1/2 inch thick fiber filter material over the cutout and laid sheetmetal ontop. I believe the original installer got clever and did this to let in fresh air mix everytime the blower came on.

Each apt. is positively air charged, severely (door to back hall in each apt wil fly open requiring effort to close when furnace is running!, and I think that is how they are being overcharged, since more air can get into the blower than what is being returned.

I talked to HVAC pro at supply house about this and person said I can do a smoke test. But that it would have been better if that had been done when furnace was hooked up and hot, in case secondary heat exchanger seams open up when hot. But I still plan on smoke testing. I also am going to call Coleman and ask if they have had any noted failure of their secondary heat exchanger seams. The primary heat exchanger cannot leak as it is a barrel like a water heater and no flaws are found at any weld. But the secondaries are sandwiched together somehow, and that seam could be suspect. But I doubt anything is wrong, and am looking to confirm my theory about the exhaust gases getting back into the furnace room via fresh air intake port.

Last year I made plywood cover that makes it harder for exhaust air to find the intake port. This had not been put on this year. HVAC person told me there is code requirement for distance between the exhaust pipe(s) and fresh air intake. Obviously that makes sense. I forgot to ask person what that distance is, and will recontact this person. But I KNOW they are too close. And like I said, I can see the exhaust gases going into the fresh air intake port.

............................

BTW, I just got back a bit ago ----from working on a furnace. One-time limit switch went out.

[SeattlePioneer]

The installation manual of direct vented equipment or that designed to be vented out near the furnace air intake usually specifies the design of sunch pairs and clearances to windows and other vents. These clearances are usually satisfactory, in my experience.

In my years as a gas company repairman, I was occasionally called out to inspect heat exchangers condemmed by heating contractors. Usually I found no evidence of cracks when I inspected them, and rather commonly when the heating contractor was there they's say "Oh, that piece of rust looked like a hole/crack" or whatever.

I tended to become somewhat cynical about such things. I used to suggest that repairman tended to look for cracks in heat exchangers as if they were the hundred dollar bill they'd receive as a kickback if they found one.


Yes, such things can be hazards, but I found simple neglect of furnace maintenance and plugged chimneys and such to be far more common hazards than cracked heat exchangers.