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How do I figure the size of an air conditioner and a gas furnace for a mobile home?

zeb49Gir | Posted in General Questions on

I am having a new outside air conditioner and a new 2 stage gas furnace put in. My mobile home is from about 1977. It has 1440 sq. ft., floor (out flow vents) & ceiling in flow vents. The present duct work is inaccessible. There is an old 4 ton elec. A/C unit and an old – maybe 100,00 BTU gas furnace. Neither one is efficient. My place is probably not very air tight Don’t know about the insulation. I think I live in zone 3C (about 2+ hours N. of San Francisco on Hwy 101; About 60 miles inland from the coast-Ukiah.) Should I replace the A/C with a 13 SEER 4 ton model and a 2 stage, 95%, variable speed 88,000 BTU gas furnace? Do I need a 4 Ton A/C unit? What about the furnace?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joann,
    To size a furnace and an air conditioner for a mobile home, an HVAC contractor should follow the same steps necessary to size equipment for a regular home. The contractor (or an energy consultant) needs to perform a heating load calculation and a cooling load calculation; the method usually used to do this is called the Manual J method.

    This steps should be a routine part of every HVAC contractor's job. Unfortunately, many HVAC contractors don't know how to perform Manual J calculations.

    For more information on these issues, see:

    Saving Energy With Manual J and Manual D

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    Calculating Cooling Loads

    Two other points worth mentioning:

    1. Your existing equipment is almost undoubtedly oversized.

    2. It may make more sense to install 2 new ductless minisplit units in your mobile home rather than a new furnace and a new central air conditioner.

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