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New Furnace - Electricity Savings?

Discussion in 'Environmental Discussion' started by boulder_bum, Apr 24, 2010.

  1. boulder_bum

    boulder_bum Senior Member

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    So I want to get a new furnace/air conditioner. Mine are about 30 years old and I figure the time is right with stimulus funds and tax credits.

    I'm curious, however, if my electricity usage will be drastically cut. I expect that a more efficient air conditioner will certainly use less, but how much should a two-stage gas furnace with a variable speed fan help?

    My old furnace is single stage with a one-speed fan, so I'm hoping that the variable speed fan will save a good amount of electricity, in addition to gas.
     
  2. icarus

    icarus Senior Member

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    Given your climate, (My guess is that you have some use for A/C. consider either ground source heat pump, or ideally a hot water recapture heat pump. Hot water recapture heat pump has great advantage in that it has the side benefit of giving you hot water, AND it allows the A/C to run more efficiently.

    Alternatively, consider solar hot water which in Boulder is pretty easy to do. Then, use the highest efficient gas furnace you can find. (I personally prefer to use zone heat from Rinnai gas fired vented heaters but many don't like having multiple units.

    Another option is the newest generation of Mini-splt A/C systems like those from Sanyo. The A/C is very efficient, although the heat is probably not enough in the Boulder climate. (They also are very suited for off grid solar systems, as they use a "soft start" compressor so that they don't have a huge starting current draw.

    Icarus
     
  3. richard schumacher

    richard schumacher shortbus driver

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  4. chogan2

    chogan2 Senior Member

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    DOE lists the typical furnace fan at about 500 KWH/year, here:

    U.S. Household Electricity Data: A/C, Heating, Appliances

    So that sets an upper bound on your probable savings.

    The Lawrence Berkeley National Lab fills in the savings estimate, here:

    http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1774-08.pdf

    If I read that right, you'd save half to three-quarters of the electricity of a single-speed blower. The higher the duct pressure, the lower the savings.

    So, something like $30/year or so? Not much.

    I put a variable-speed system in at my last house. Variable-speed systems are just way slicker than old-style single-speed systems. Here in the humid South, an advantage of variable-speed AC systems is that they can adjust themselves to react to humidity as well as temperature. As I understand it, running the AC colder and the fan slower gets more condensation out of the air. The one I installed at my old house had its own special thermostat with integrated hygrometer.

    One other thing that my installer sold me on: Puron instead of Freon. (Puron's the brand name, but what he meant was the new Freon 410A, not the old Freon R22.) At the time I put mine in, you could still get new R22 units. Probably still can. But R22 is on the way out. His argument was that if you're thinking in terms of 20-30 year lifespan, you might get some sticker shock down the road if you need to top up your R22. You'll still be able to get it, sure, recovered out of old systems, but at what price? Plus, it's just not smart to invest in guaranteed obsolete tech. So I went with Puron.

    Here's the timetable for R22 phaseout.

    What You Should Know about Refrigerants When Purchasing or Repairing a Residential A/C System or Heat Pump | Ozone Depletion - Regulatory Programs | U.S. EPA


    The house I'm currently came with a ground-source heat pump. I can't image putting one in as a retrofit -- I have just under 5000 feet of plastic pipe buried in as set of parallel trenches in my back yard. That's a whole different ballgame from replacing the condensing unit.

    But they work as advertised. They are cheap to run.

    Some considerations if you go for ground source. We've had three HVAC guys in at various times in my current house, and the first thing they say when they see the ground source heat pump is "We don't work on those." Commercial buildings have them, but they remain rare in houses. Second, for some reason, they installed the compressor units inside the house, for mine. (Which you can do, since you're exhausting the heat into the ground via pipes not into the air.) But when they run, ... it's like having an air conditioner compressor unit running inside your house. I'd build a shed for the compressor unit outside, if it were my call.

    Third, I've been told (though I'm not sure I believe it) that you get better efficiency if you use the ground source for heat and cooling roughly equally, so that the heat injected into the ground during the summer roughly matches the heat withdrawn during the winter. Otherwise, basically, you heat up or cool down your pipe field. At first, I thought that was kind of a joke, but I see by-gosh serious sources who ought to know what they are talking about say exactly that: If you only use it for heating, or only for cooling, your efficiency deteriorates over time as you cool (heating) or warm (cooling) your ground.

    In practice, well, that adds a complication to the install. We've got about 4500 heating degree days and about 1500 cooling degree days here. That means the ideal install has to have a second heating source. Ours was set up with the existing gas-fired baseboard as the secondary heat. We've since switched to a wood stove. Either way, in this climate, ground-source heat pump will work OK by itself, but will show greater efficiency if paired with another heating source. Near as I can tell, the ground-source heat pump and the gas furnace cost just about the same to run as heaters, while wood is somewhat cheaper. So it's not a cost issue, really, it's mostly that the system should be set up so that you do, in fact, use your secondary heat a lot.
     
  5. Zythryn

    Zythryn Senior Member

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    This probably depends largely on the local region.
    Here in the Minnesota it is getting uncommon to find an HVAC company that doesn't work with ground source heat pumps.
    There are also a number that advertise retro-systems, however I would check carefully in your own area for availability.
     
  6. boulder_bum

    boulder_bum Senior Member

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    Well, but my calculations, that's up to 31 kWh per month, which I would actually be excited about!

    My last months energy usage was about 450 kWh and I'm about to install solar panels that produce just under 400 kWh a month, so I'm a big step closer to being zero impact with electricity.

    Here's hoping.

    Thanks for the answers, all!
     
  7. drees

    drees Senior Member

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    Yeah, any electricity savings is going to vary a LOT depending on use.

    Mostly depending on how much your furnace fan runs. Personally, I wouldn't expect a huge difference.
     
  8. chimo

    chimo Junior Member

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    I did some research on that myself a few months ago.

    It's possible that your electricity consumption will go up a small amount but your overall energy consumption will go down.

    It depends on how you run your fan. If it comes on only during heating/cooling demand (auto), you will likely use a little more electricity. If you run your fan continuously now, you will use less electricity with a variable speed brushless DC motor. The cause of this is that the variable speed DC motor runs continuously to keep the air circulating.

    In either case, however, your air quality and comfort should improve.
     
  9. Pizza_Daddy

    Pizza_Daddy Junior Member

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    When my parents updated their furnace from a one stage to a two stage I not sure what the savings was, but my dad was saying he wished he would have gotten the two stage long before he did.

    My parents unit was about 20yrs old when they replaced it. And with the two stage, the low speed fan runs to maintain the temp. and the 2nd stage will kick in if you want the temp to be a 4+ degree difference.

    Also their old single stage unit basically blew the same as the 2nd stage of the new. And for the first stage, it doesn't even really have enough power to move their thin see through curtains in the kitchen. but does a really nice job of circulating the air/heat
     
  10. chogan2

    chogan2 Senior Member

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    I stand corrected. I guess in MN, if you're going to have a heat pump, it pretty much as to be ground-source. In Virginia, not so.
     
  11. jayman

    jayman Senior Member

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    I have a Bryant Evolution two stage with variable speed ECM fan and am quite pleased with the performance. At the lowest fan speed, which is recommended for constant operation (If you also have a Media filter or an electronic air cleaner, in addition to a Heat Recovery Ventilator) this keeps the indoor air quality quite good

    My ECM motor draws about 60 watts at the lowest speed. IN contrast, a conventional HVAC blower motor will draw around 550 watts. Not too many folks let such a motor run constantly, so actual power savings may be minor to insignificant

    Geothermal exchange is probably the way to go, but they are still quite steep to install. Quality of installation really effects system performance and efficiency