The Value of A High Efficiency Home Furnace

"Is a high efficiency furnace worth it"? That's the question posed by Kim, one of our readers. From a technology standpoint, we scanned this overview by "" and decided there are too many options to offer a yes or no answer. A few efficiency points added to millions of home furnaces would result in a large cumulative cutback in greenhouse gas emissions; and, hopefully, much money saved. But, is it worth that outcome to discard millions of perfectly good furnaces, taking a chance on newer technology, of unknown reliability? Could there be unintended consequences? Lets start with an assumption that your existing furnace burns natural gas or propane, is quite old, and operates near or below the low end of the "name plate" efficiency range of modern furnaces. Unfortunately, if you burn oil, and have no access to natural gas, there is not much can be done in the way of high efficiency furnace technology. If thats' the case, skip to the next post!If you have access to gas, numbers first. Get three or more directly competitive bids. Ask each bidder to give you one estimate for a "budget" system, of high reliability, but low capital cost; and another bid for a high efficiency furnace. Ask for detailed line items in the bids, and be certain to ask that the exact Annual Fuel Utilization Efficiency (AFUE) rating be included on each furnace listed.

If a bidder gives you line item details on one system and not the other, tell him to try again. Rule of thumb: unresponsive bidders are the unlikely to provide reliable cost estimates.

Choose bid(s) you like: one high and one low. What is the installed cost difference between them? Is it hundreds; or is it thousands? If it's a few hundred, the decision can rest on reliability.

If the installed cost difference is thousands, you'll want to know that the payback period is relatively short for the high performing furnace. OEM furnace makers usually supply guidance on expected cost savings. Check their web sites.

For a 'back of envelope' projection of annual fuel savings of the high-end model versus the low-end model, figure out what your fuel bill was last year. Although even the "budget" system likely could do substantially better than your existing furnace, use your actual fuel consumption for a "conservative" baseline. Considering that the weather and fuel prices are both fairly unpredictable, and likely to be less predictable in the future, this is about as good as you'll be able to do with fuel cost basis for your comparison.

Calculate the incremental annual savings of the high-end system by direct proportion. Here's a hypothetical example:

79% AFUE budget system
-------------------------- * Last year's bill = lowered fuel bill.
97.% AFUE high end system

0.814 * $2,400/yr = $1,954 (sample calcultion of fuel expense last year with new high efficiency v.s. the new budget system for the duration)

Incremental savings with the high end system would have been just over $400/last year.

If you can find a furnace model comparable to your existing one and it has a modern AFUE rating on the sticker (likely to be under AFUE 60), run the calculation again to see what the improvment would be from your existing, to a new high end model. (In the hypothetical example calculated savings would have been close to $900/year assuming a 60 AFUE number for the existing furnace.)

Looking over the process diagram for the high efficiency condensing gas furnace (see illustration), one of the odd things, both environmentally and aesthetically, is that the existing chimney would be abandoned, as only a small plastic pipe is needed to discharge the cool, dry exhaust. Tearing down the chimney would be an added cost that would certainly change the look of a building -- some would say detrimentally. You'd at least want to see that any exterior chimney bricks be re-used. Or just have it plugged and left in place.

The plastic pipe used for exhaust has has to be corrosion resistant and very well could be vinyl: not an issue for this writer but some may find it 'compromising.' Also needed is a condensate discharge pump and line. Apparently the high efficiency gas furnaces discharge several gallons of combustion produced water per day. Because natural gas should be relatively free of metals, it may be possible to at least consider using the condensate discharge as a feed for things like toilet flushing, depending on corrosivity.

Bottom line: if the investment pays for itself in 5 to 10 years and you can afford the cost, its all good for the worlds climate from the first time you use it until end of its operating life.