80% afue versus 92% afue

lynchmob723August 13, 2008

I have a 80% AFUE Armstrong Ultra SX80 furance. I was thinking of upgrading to a newer furnace that is 92-95% AFUE.

What kind of savings would this give me on my bill percentage wise?

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depending on your location/climate and electric rate,I think an 80% AFUE eff furnace is a dinosaur unless paired with a HP.

with an 80%, you throw $20 away for every $100 spent on nat gas.

with a 92-95% eff furnace, you throw away $5-$8 for every $100 spent on nat gas.

you will have to figure your usage,potential savings and the cost difference between the two different eff furnaces.


    Bookmark   August 13, 2008 at 11:04AM
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Ya I dont have a HP. Im in MN so winters get expensive with the gas $200-280 a month

    Bookmark   August 13, 2008 at 11:39AM
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no 80% for your location. go for a 95% with var speed blower.


    Bookmark   August 13, 2008 at 12:00PM
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If your 80% furnace were running at it's peak (which it is not) the same heat that cost you $280 would cost you $230-$238 with a 90%+ you can expect to save $50-$70 bucks a month though because of price fluctuations it is hard to guage

Here is a link that might be useful: Air Conditioning Repair made easy

    Bookmark   August 13, 2008 at 5:07PM
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A quick calc from the energy star site for an average home in MN shows th payback on the upgrade to be 4.7 years. Even if you dont stay in the house 5 years, the remainder should be easily recouped in the sale of the house due to consumer perception that a better furnace is installed (which it is)

Go with Variable speed and you will appreciate considerable electric energy savings to boot

    Bookmark   August 13, 2008 at 10:09PM
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Thanks for the input guys...How does variable speed differ from a standard fan? Obv it changes speed but why is it so much more efficient?

    Bookmark   August 14, 2008 at 12:28AM
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The variable-speed fan is efficient from an energy consumption standpoint, but it also varies its speed to maintain proper airflow. It will ramp up to provide the correct airflow, which is beneficial for slightly undersized ductwork systems (in no way a cure), while the standard blowers will not vary their speed and can provide improper airflow.

    Bookmark   August 14, 2008 at 12:34AM
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really, interesting....I have a slightly undersize main branch....or anotherwords I have too many runs off of it, (1 or 2 too msny ivr been told) so a few runs closest to the furnace are a litle weak since the backpressure doesnt really build enough too push air through these runs.

Would my air flow to these vents possibly also benefit from a change to a circular 4" vent versus the current 4" square vent I have?

Thanks a bunch for your help guys. Now I can at least be educated when Im looking for a furnace

    Bookmark   August 14, 2008 at 1:46AM
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1. Why are you replacing your existing furnace? Is it age? Is it because you want higher efficiency?

2. Each manufacturer does it a little different, but generally a variable speed fan will run for 10 minutes at 80% speed, then increase to 100% if the thermostat is still calling for heat. At 80% speed, the motor will use about half of the energy as it does at 100% speed. The variable speed fan will also slowly ramp up when the furnace turns on and slowly ramp down when the furnace turns off. This makes for quieter starts and stops. Also, variable speed fans create more even temperatures between upstairs and downstairs because the fan is running mixing the air in the house for a longer time period.

3. 90%+ furnaces will cause some of the water in the exhaust to condense into a liquid. There is increased risk when dealing with this condensate because it may can cause problems if the condensate drain line plugs, or if the condensate, which absorbs some of the nitrogen in the exhaust gas to form nitric acid, causes corrosion in your sanitary sewer pipes. Normally, these things arent a problem, but do understand that 90%+ furnaces are more prone to problems. The 90%+ furnaces use stainless steel secondary heat exchangers to make sure the condensate doesnÂt corrode the material. Stainless steel is much more expensive than plain steel. This is one reason they cost more.

4. 90% furnaces have powered combustion fans and will exhaust through PVC pipe out of the site of the house. 80% furnaces can exhaust through your existing furnaces stack through the roof.

5. IÂm in MN too. I recently replaced an aging 80% furnace with another 80% furnace. I did my own payback calculation on 80% vs 90%. Here are my data for my 2,000 sf house:

Installed cost:
80% = $2650
90% = $3200

Yearly operating cost with $1/therm gas based on my actual meter gas usage:
80% = $606
90% = $539

This makes the payback 8.2 years. If you get the 90%+ furnace and ever need repairs due to items in #3, it will never pay for itself.

6. There is a difference between the peak efficiency that the manufacturers use to sell their products, and the actual operating and seasonally adjusted efficiencies that you will see. Payback analyses generally overstate the actual savings you will realize.

7. In a new house, the high efficiency furnaces make a lot of sense because you donÂt have to run a 4" double-wall stack up through the house and through the roof, which saves a lot of money and mostly negates the additional cost of the high efficiency furnace. However, in a retro-fit like we have, you already own the stack so you wouldnÂt realize these savings. In fact, there is additional cost for high efficiency because you have to run a new PVC exhaust pipe when you could have just reused your existing one.

8. If you post your monthly gas usage every month for the last year, I can tell you what it will cost you to operate both furnace options for each month. IÂve got my old spreadsheet already set up, I could just plug in your usage numbers.

9. If you want to go high efficiency because you care about the environment and want to use less energy that is fine. Just understand that financially, high efficiency doesnÂt make sense in every case.

IÂm a HVAC engineer in MN, if credentials matter.

    Bookmark   August 14, 2008 at 1:42PM
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Heating gas usage of $539 Mr. Minnesota?
You must have a small house, like it 55 degrees or are never home.


Try the calc below and see where you shake out
Personally I have never seen anyone over 7 years and that was before gas went up

    Bookmark   August 14, 2008 at 1:56PM
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Shoot, I didnt realize I still had a lower gas price in there from a few years ago. With $1 gas it would be:

Operating Cost:
80% = $823
90% = $734

Or a savings of $89/year. Clearly this reduces the payback, however, the first cost data is also too low in todays dollars due to inflation the last couple years.

The higher gas price doesnt change the fact that the higher efficiency unit costs 11% less to operate. If OP knew what he spend a year on gas, it is pretty easy to calculate what the payback is.

    Bookmark   August 14, 2008 at 5:00PM
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I've seen low gas prices like that here in MN. 2000 sq ft home with great insulation, set back during the day...

    Bookmark   August 14, 2008 at 9:13PM
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