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High efficiency furnaces

Bubba Shirk | Posted in General Questions on

Besides being the right thing to do is there another argument I can give my sister in Missoula, MT to spend twice as much to go from an 85% efficient furnace to 95%? Unfortunately she should have replaced the furnace over the summer when it was probably cheaper.

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Replies

  1. jtruog | | #1

    It is easy: With an 85% efficient furnace, with every $1 she spends on heat she is getting $0.85 in heat and throwing $0.15 out the chimney. With a 95% furnace she will be getting $0.95 worth of heat with only $0.05 going up the chimney. Any good HVAC guy will demonstrate her payback.

  2. John Brooks | | #2

    Not so Easy
    Money might be better spent on Envelope improvements

  3. John Brooks | | #3

    Er .. Excuse me Dr Joe
    Enclosure improvements

  4. GBA Editor
    Martin Holladay | | #4

    Bubba,
    It depends.

    If she is spending $900 per year now for heat with an 85% efficient furnace, then a 95% efficient furnace will mean that she spends $804 for heat -- a savings of $96 per year.

    How much do these two furnaces cost? If the cheap furnace is $5,000, and the expensive furnace is $10,000, then it will take her 52 years to justify the investment, assuming that energy prices don't change.

    Your mileage may vary. The more fuel you burn, and the more expensive the fuel, the faster the payback.

    As John pointed out, it may make more sense to do air sealing work in the attic, and deepen the cellulose.

  5. Ernest G. Mueller | | #5

    It was not stated what kind of heating we are talking about. Is it a hot air system or base board heating or what? Also what is the fuel? And what is the age of the existing boiler? I just revamped a hydronic heating system and installed a Weil –McLane Ultra series 3 high efficiency boiler. For highest efficiency, the boiler has to run in condensing mode to recover the latent heat from water vapors in the exhaust gas. This requires a good match between the boiler heat output and the radiator heat dissipation capacity to operate at a relatively low water temperature. Fortunately, modern boilers can modulate between about 20-100% heat output which makes zoning more flexible. I would suggest that it is not simply a matter of replacing an older boiler with another one. The entire system should be evaluated from an engineering point of view, starting with a heat loss calculation of the building, selecting the right size boiler for the heat total load as well as the capacity of individual zones. In conclusion: A “leaky” building should be fixed as a first step and then the heating system should be upgraded after a thorough evaluation. Even if the payback may not be very attractive today, the energy prices will go up for sure in the future. The questions is only how soon and how much.

  6. Riversong | | #6

    Is it a hot air system or base board heating or what?

    Assuming the OP understands the meaning of "furnace", then we know it's a hot air system.

  7. Ernest G. Mueller | | #7

    To Robert Riversong
    Thanks for the sarcastic remark. I thought this was a forum to offer help.

  8. Bill . | | #8

    A furnace with variable speed circulation fan that is 95% efficient is will reduce fuel use and also makes for much more comfort in multiple ways. For one... they run most of the time very slow.... which makes them very quiet. Variable speed costs more but it is worth much more than a granite countertop to me.

    I would suggest she get supplier prices for each furnace and let the installer know she is willing to pay that difference only. They should bite in this economy.

    There is I think still a good tax rebate to do the work too if done this year?

  9. Riversong | | #9

    To Robert Riversong
    Thanks for the sarcastic remark. I thought this was a forum to offer help.

    There was nothing sarcastic about it. It is a straightforward statement of fact.

    But answering a question that was not asked is not in any way helpful.

  10. James Morgan | | #10

    Robert: "Assuming the OP understands the meaning of "furnace", then we know it's a hot air system."

    FWIW, I don't think we know any such thing. "Furnace: an appliance fired by gas, oil, or wood in which air or water is heated to be circulated throughout a building in a heating system." Industry-specific usage aside, I believe this is a generally understood definition.

  11. Riversong | | #11

    Industry-specific usage aside, I believe this is a generally understood definition.

    I disagree, and the distinction between furnace and boiler is specific to the "industry" we're discussing: residential housing.

    US DOE:
    "Most U.S. homes are heated with either furnaces or boilers. Furnaces heat air and distribute the
    heated air through the house using ducts; boilers heat water, providing either hot water or steam for heating."

  12. David Meiland | | #12

    If she is spending $900 per year now for heat with an 85% efficient furnace, then a 95% efficient furnace will mean that she spends $804 for heat -- a savings of $96 per year.

    Bubba, you need to get the specific numbers that apply to the house and determine how long it will take to pay off the added cost of the more expensive unit. Then, you need to factor in which (if any) rebate/incentive/tax credit scenarios apply to the job, from the Federal and state government, the local utility company, etc. Then, you need to add to that whatever manufacturer rebates are available. It can take some study to get down to the actual cost of changing the equipment.

    You might want to get an energy auditor to inspect the house. They should be familiar with the above details, they can test air leakage and duct leakage for you, etc. I bet there are plenty of good folks available in MT, and most houses have areas that can be improved fairly cheaply.

  13. James Morgan | | #13

    Robert, the DOE's industry-standard definition is not the issue: my point was that we can make no assumption that this particular question is posted by someone who is aware of it, as there is also a general usage with a broader definition. That said I imagine that there is a very good chance that this is a warmed air system if only because these are more common than not, especially where a/c is also used. But how these guesses contribute to answering Bubba's question I have no idea.

    BTW I have always thought 'boiler' was a misnomer for those systems which employ hot water rather than steam.

  14. Riversong | | #14

    BTW I have always thought 'boiler' was a misnomer for those systems which employ hot water rather than steam.

    Of course, the term came into common parlance when heaters and engines ran on steam. After we turned down the aquastats to less than 212°, we kept the common name. Now, rather than using the heat of vaporization in the radiators, we capture it in at the heat exchanger in condensing boilers.

    The lower temperature radiators might have been the result of "boilerplate" legal warnings about the danger of burns. ;-)

  15. James Morgan | | #15

    Ha!

  16. Amy (Homeowner) | | #16

    I had the ROI discussion with a HVAC contractor. Using Martin's example, it will take 52 years to pay back. For my case, I only use $100 a year for space heating and it does not make economical sense to use high efficiency furnace.

    However, my HVAC contractor indicates 2 stage high efficiency furnace can have the following additional benefits:

    1. more comfortable and quieter. It can use constant low speed blower to maintain the desired temp rather than the high speed louder blower with frequent on and off cycles which also wears out the system more.

    2. During the allergy season, he uses the low speed blower and high efficient filter to get rid of the pollens and dusts. He is basically using the furnace as a whole house air filtering device without heat.

    I don't know if his points are true or not - just want to share what I heard.

  17. David Meiland | | #17

    You're only using $100 for space heating per year? Is this the 3000 foot house with a 5-month heating season? More details please.

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