How to Size a Mobile Home Furnace

Sizing up a furnace for your mobile home is important to your comfort and pocketbook. A too-small furnace will struggle to heat your space effectively, and paying for too much capacity isn't ideal either (manufacturer Trane says that a capacity of up to 40 percent over your nominal requirement is fine). You ​could​ simply check the capacity of your old furnace, contact the manufacturer for a recommendation, or call in a professional to do an assessment. If you'd rather work it out for yourself, there are several ways to size a mobile home furnace.

The quickest way to estimate the correct size for your mobile home furnace is by working from its square footage. If you don't have that figure from when you purchased your mobile home, you can just multiply its length and width to get the number. Typical single wides can range from 500 to 1,200 square feet, for example, and double wides average 1,000 to 2,200 square feet.

Next, you'll need to multiply that by the recommended heating capacity — usually measured in BTUs — for your climate. According to propane installers Kauffman Gas, that can range from 25 to 30 BTUs per square foot in mild climates to 45 BTUs per square foot in areas with sterner winters.

Multiplying the square footage by the recommended number of BTUs will give you the total heating capacity needed. A 1,200 square-foot mobile home at 45 BTUs per square foot works out to 54,000 BTUs, for example. That's still not your final number because furnaces aren't 100 percent efficient. Most are about 80 percent efficient, so you'd actually need a furnace of 67,500 BTUs' capacity or more in this example (unless you've got the budget for a higher-efficiency model).

That method gives you — at best — a ballpark estimate. That's because your mobile home isn't an empty box but a home with walls, ceilings and fixtures that take up much of the space. To get a better read on the actual space you'll be heating, go through your mobile home and measure the spaces on a room-by-room basis (it's faster with an inexpensive laser measure than with a conventional tape measure).

Excluding the space taken up by interior walls, ceilings, floors and appliances will give you a smaller square footage number and reduce the likelihood that you'll buy more furnace than you need. That being said, it's still a rough measure because it leaves out several variables that can affect your heating needs.

Professional installers use a much more detailed set of criteria, codified by the Air Conditioning Contractors of America in a technical document called ​Manual J Residential Load Calculation​. You can buy your own copy if that's the kind of thing you geek out on, but at over 600 pages, it's a dense read.

You probably don't need to know all of those details if you're not in the business. A professional Manual J calculation might ultimately involve hundreds of data points, but a relatively small handful account for most of the impact on your heating and cooling needs. You can get a pretty good estimate of those by choosing an online Manual J calculator and entering the measurements it asks for. It's a stripped-down version of the more detailed apps used by the pros and takes into account crucial factors like the type of insulation and windows you've got (spoiler: they're usually better in newer homes), your location and the number of people living in your home.

Heating capacity is the most important part of sizing a furnace for your mobile home, but physical size is important and sometimes overlooked. Most mobile homes have a relatively small furnace enclosure, so before you go furnace shopping you should carefully measure both your existing furnace and the space it occupies. If your chosen furnace won't fit the available space, and you haven't budgeted for a major renovation, the time to find this out is ​before​ you make your purchase.