3 short questions about gas furnace (forced air)


Old 10-16-10, 08:04 AM
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3 short questions about gas furnace (forced air)

First question is about the fan motor oil. It states to use 7 drops of a certain oil once a year. Would it be O.K. to use 3 in 1 oil instead.

Second question. I like to use my insert at times but the heat does not get to the back bedrooms. Any problems running the motor for extended periods of time? say for about 6 hours?

Finally, a service man last year told me not to use the expensive pleated filters, since they make the furnace work harder. Is that true? I really like the way they collect the junk in the air, compared to the cheaper $1.00 filters.

Many thanks in advance.......
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Old 10-16-10, 08:22 AM
Join Date: Sep 2009
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Best to use electric motor oil available at hardware stores. 3 in 1 is a brand name. Seven drops is a lot of oil for a fractional horsepower motor.

Furnace fan motors are rated for continuous duty.

Pleated filters have higher pressure drop, and may reduce the furnace performance due to reduced air flow. They do not require the furnace to "work harder."
Old 10-16-10, 09:50 AM
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Pleated filters have an extended filtering area so the pressure drop across them would normally be less when compared to a non-pleated filter with the same micron rating. That stated, most non-pleated filters are not much more than screens to catch the the really big particle (like tree leaves) rather than actually "filter" the air. The problem would arise if the pleated (or high micron rating) filter was so restrictive that it lowered the airflow through the furnace and caused the blower motor to exceed its rated temperature (the blower motor is cooled by the air passing over it to the blower inlet) or the temperature rise across the furnace heat exchanger was too great causing the heat exchanger to overheat.
Old 10-16-10, 02:40 PM
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No need to use the high end air filters like 3M. They load up fast, and cuts down on your air flow. Can over heat the motor and/or heat exchanger.

The filters I suggest is the 3-pack NaturalAire that is sold at Home Depot. They are a step up above from the cheap fiberglass filters.
Old 10-16-10, 04:02 PM
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For reasons that I won't bore you with, my last forced-air furnace had room to install extra-thick or mulitple filters. I put a cheapo "chunk" fiberglass filter first, then followed by a pleated filter downsteam.

If that wasn't enough, I pre-coated the "chunk" filter with a spray, possibly 3M, that was billed as an electrostatic attraction - probably just some kind of sticky stuff?

Anyway, I swear that combination was the cat's meow. I would change the cheapo filter twice (per year, or so) before changing the pleated filter. And the furnace (and air conditioner) worked fine.
Old 10-16-10, 04:11 PM
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That's a good way to do it, Mike. I've been running four-inch thick "box filters" for about two years now with absolutely no problem and my return air duct is a typical minimally sized one. My feeling is that if a person is going to just use an el cheapo filter they may as well just get a piece of 1/4 inch hardware cloth and never bother with it again.
Old 10-17-10, 09:07 AM
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Sorry furd, I can't agree with your criticism of inexpensive filters.

Your comment that they are primarily good for catching things like leaves is a wild exaggeration.

The primary purpose of furnace filters has been to protect the fan, motor, heat exchanger and ductwork from getting loaded up with dust and dirt. Those who have seen furnaces operated for lengthy periods of time without a filter know the inexpensive filters are highly effective at doing that job.

Inexpensive filters aren't designed to filter out pollen, smoke allergens or other things that might affect people adversely. They aren't designed to do that and they don't do it.

For most people, cheap filters are fine. If your family has allergy or other lung ailment issues, more elaborate filtering may make sense.

Personally I use cheap filters and they serve me well. I have no puminary issues and the cheap filters keep my equipment clean in an entirely satisfactory way.
Old 10-17-10, 05:39 PM
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I believe the biggest, if not the only reason behind those big thick box filters is that they use the same paper media, but the box holds a larger area of pleating. That if you unfolded it, it be like 25 square feet or whatever (compared to a standard say almost 3 square feet one), making it so you really do not have to change the filter as often. That is the idea of pleats in a filter (besides giving it ruffled potato chip-like strength). Even the 1 inch ones - if there were no pleats in that paper media, you'd be changing the filters 4 times (or whatever) more often than you already have to. The 4 inch thick one really makes them last a long time. I think in one rental, we did not have to(even though we maybe could have) change that thick box filter for a couple-few years.

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