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  1. #1

    Incorrect installation of air return side effects.

    We have had many problems with our Lennox heat pump over the last 7 years (coolant leaks, capacitors, fan motor, compressor, etc...). We had a sales person out to give us a bid to replace it. He noticed that the return air on the furnace didn't match the specifications in the installation manual. We have a 5 ton unit and he said that there should have been two return air options (one from two sides OR one side and one from the base). He also said the the the return air from the side should have been large enough for 20 x 25 x 1 filter (It's smaller with two angled 16 x 25 x 1 filters). The furnace is a Lennox G60UHV(X) Series. Can these discrepancies cause problems with the heat pump?

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,035
    Quote Originally Posted by DWAJDA View Post
    We have had many problems with our Lennox heat pump over the last 7 years (coolant leaks, capacitors, fan motor, compressor, etc...). We had a sales person out to give us a bid to replace it. He noticed that the return air on the furnace didn't match the specifications in the installation manual. We have a 5 ton unit and he said that there should have been two return air options (one from two sides OR one side and one from the base). He also said the the the return air from the side should have been large enough for 20 x 25 x 1 filter (It's smaller with two angled 16 x 25 x 1 filters). The furnace is a Lennox G60UHV(X) Series. Can these discrepancies cause problems with the heat pump?
    Oh yes, not having enough free air filter area can cause extreme problems in the system.
    For example, these ultrahigh efficiency coils on a 5 ton system at 1700 CFM a wet coil can be as high as .49 inches of water column pressure drop.

    That is very close to the .5 inches that many air handlers are rated at, add to that a large return air filter drop plus all of the other pressure drops in the system and you're going to end up way over 1 inch of static pressure, with a 5 ton system, that will cause extreme airflow shortage problems and be extremely hard on many of the components in the system.

    Manual D says, size for 300-fpm when using a cheap media throwaway filter; using a 5-Ton low 1700-CFM divided by 300-fpm is 5.66 sf of free-air-area.

    Two 16X25 is 800-sq,ins., 800 / 144 is 5.55 sf *.65% free-air-area is only 3.6 sf of free-air (Ak) it needs 5.66 sf of free Ak to achieve 300-fpm through the filters; it needs +1200-sq.ins., of physical filter area to get to around 300-fpm; that is more than (2) 24X24 which is a physical 1152-sq.in sized RA filters.

    If you're staying with 1" deep filters you'll need the 300-fpm factor to keep pressure drops reasonable with 1" thick pleated filters.

    Whenever possible, a Manual D should be run on all duct systems...

  3. #3
    Thank you so much for the explanation. We will get it corrected! Thanks again!

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