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How Can Replacing A Furnace Make Your Customer Less Comfortable?

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Let’s say you have to replace a customer’s trusty old furnace because it’s finally at the end of its life (the furnace, not the customer!). You go and take a look at the furnace and find its capacity. Then you go back and tell the customer that they have a furnace rated for 60,000 BTU per hour and talk to them about some of the options.

You could replace the old one with one that has the same efficiency, say 80 AFUE, although the natural draft furnace in the photo here would be lower. Then you show them the price of a high efficiency furnace, which is 95 AFUE. It’s only $400 more and you convince them that since they’re going to have the furnace for another 15 or 20 years, they’ll quickly get that money back in savings on their gas bill (a harder sell these days with such low gas prices).

So you get the job to install the 95% furnace. The old one, remember, was rated at 60, 000 BTU/hour of input capacity. You look in your product catalog and find that the high efficiency model comes in a 46,000 BTU/hr model or a 69,000 BTU/hr model. There’s not a 60,000 BTU/hr model, and you don’t want your customer to freeze, so you install the 69,000 BTU/hr model.

Hmmm. See a problem here? Most of us in who work with furnaces think in terms of input capacity. The 69k model is a little bigger than the old 60k model you’re replacing, but not too much bigger. That can’t be so bad, right? Actually the oversizing you end up with is worse than the 9k difference in input capacities, and if you’re reading this, you’re probably sharp enough to know where this is heading…and sharp enough to avoid this problem in the field. But are your sales people and installers?

The relevant capacity of the furnace isn’t the rate of BTU input. It’s how quickly those BTUs go into the home, the output capacity. Let’s look at what happened to those numbers when you replaced the old furnace. You started with 60,000 BTU/hr at 80% efficiency, so the old furnace supplied 60,000 x 0.8 = 48,000 BTU/hr. The new furnace has an output capacity of 69,000 x 0.95 = 65,550 BTU/hr. To summarize the numbers:


Input: 60,000 BTU/hr

Output: 48,000 BTU/hr


Input: 69,000 BTU/hr

Output: 65,550 BTU/hr

So really, your new furnace is about 37% larger in terms of its ability to provide heat to your home. If you had started with an older furnace, perhaps only 60% or 68% efficient, the amount of oversizing would be even worse.

How does that affect your customer? The furnace will be on for shorter periods of time. If they live in a high performance home with a well-insulated and air-sealed building enclosure, they may not notice much of a difference. If they live in an older home with air leakage and insulation problems, they may notice that they’re less comfortable. The furnace comes on and blasts them for a few minutes, possibly making them uncomfortably warm in some parts of the home. Then it goes off, and they feel the cold walls and the drafts.

If you’re replacing a furnace, size the new system properly. An accurate Manual J heating and cooling load calculation is one of the best ways to do it. Even if you just base the new system on what was there before, though, don’t forget to account for the effect of increased efficiency on the output capacity. Bigger isn’t always better. When it comes to heating and cooling a home, it’s often worse.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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