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

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?

Posted Jul 22, 2014 10:35 PM ET
Edited Jul 23, 2014 4:38 AM ET


1 Answer

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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.

Answered by Martin Holladay, GBA Advisor
Posted Jul 23, 2014 5:26 AM ET

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