The Value of A High Efficiency Home Furnace
by John Laumer, Philadelphia on 06.22.06
“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 “HomeTips.com" 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.
High Efficiency oil burners. Efficiency is a relative term. There are newer burners whith much higher efficiency ratings. I myself have an old burner that is at the bottom of its efficiency plate, and that is when it was new. I recently investigated replacing my boiler with a Buderus boiler. http://www.buderus.net/ It improves efficiency in a number of ways. One of the primary ways is not maintaining the water temprature when not in heating mode. It is a completely computer controlled system that takes into account the outdoor temprature and the indoor temprature when deciding how hot to make the water. The point is there is something you can do to increase your efficiency burning oil. In the end we have not replaced our boiler and are instead saving for a geothermal system.
In a conventional furnace, combustion air is drawn from the house, creating a negative pressure. Most older houses leak and fresh air infiltrates around doors and windows. When you get a sealed combustion high efficiency furnace, combustion air is drawn from outside and your house does not suck in all of that fresh air, so you should install a heat recovery ventilator when you install the furnace to give you fresh air.
Also, many furnaces do not have a condensate line, but exhaust the moisture through the flue. I once installed one with the exhaust under a deck (not legal now but then it was new) and the deck became a skating rink every winter. With my own high efficiency furnace clouds of steam are seen all winter so don't stick it on the front of your house.
I am thinking about replacing the furnace in the attic with a high efficiency condensing model. Two out of the three contractors said that was not recommended, while one said it was doable, but the cool exhaust pipe exiting the attic would likely have icicles. Could someone tell me whether it's true or not? should do get the condensing models, or just higher efficiency (like 87%)ones?
thank you.
Here here to David's saving for a geothermal system. I'd suggest he go one step further and put in an underground thermal energy storage (UTES) geothermal system. Instead of just drawing out the earth's constant temp of 10 deg cel in a UTES system you actually put thermal into the ground, raising its temp up to 20-22 deg cel or more. When you have lots of thermal energy (say the summer time when the living is easy) you "bank" that thermal energy for the colder winters. Before anyone thinks this is sci fi, I'm actually building 4 townhouses with this system right now in Halifax, Nova Scotia, Canada. We expect completion to be about Sept of this year (2006). If all goes well, our system will achieve efficiency over gas burner heat somewhere in the range of 70-80%.
An old no-longer-used chimney is a great place to run pipes to a solar collector on the roof.
Is $2,400/yr for heating your house with natural gas normal? Over the four months we had the heat on in our house (December to March), we spent about $130/month, or about $500 total. This year it might be closer to $700, since we'll also be living there for October and November.
I must be doing something really right, because my house is also about 100 years old, leaks air like a sieve, and only has single-pane windows.
==== author's response follows ====
There really is no "normal" as home size/type, location, weather, and fuel pricing are so variable. Some homes use the furnace boiler as a heat exchanger to heat a hot water tank for domestic consumption for example.
The trouble with high efficiency systems is that consumers assume that if they buy a high efficiency system they will save energy.
The inefficiencies lie when a high efficiency system is connected to a duct distribution system that is old or inadequate. In many cases our company has discovered that as much as 50% of the heated or cooled air is not reaching the rooms that are conditioned.
A new breed of HVAC contractor is now providing actual air diagnostics on homes to determine the actual efficiency of the total heating or cooling system in a home. This diagnostic work includes measuring air flow, temperature and humidity variations from furnace to room, a system's static pressure and more.
Only a system that is completely in balance will achieve 90% or more efficiency. Most consumers will continue to throw energy dollars out the window.
We bought a house last summer (1991 build). There is our first winter here.
Is it Ok to have Dec-Feb Gas bill $450-500/mon?
2 floor house + f. basement
Outside perimeter: 33.9 x 58.5 = 1983.15
Total Living area: 3944 SqFt
2 Furnace: basement + attic
Last maintaince service: November
1st floor: 1bed+3rooms+kitchen+1bath
2nd floor: 5bed+3bath
Thanks.
Andrew.
abn31@hotmail.com
My community is about to pass a law requiring that 90% efficient furnaces be installed whenever a unit must be replaced. The requirements for draining away the condensate produced have raised some questions with me:
1. What's in the condensate? I have heard that it is highly acidic. Is that the case?
2. If the condensate is highly acidic, how is it disposed of without compromising ground or surface water?
=== author's response follows ===
All distilled water and combustion product water is mildly acidic. That includes the acid gases in you car exhaust. Not enough volume to worry about though.