Hypermiling the furnace

Discussion in 'Environmental' started by warthog1984, Oct 11, 2009.

  1. warthog1984

    warthog1984 Well-Known Member

    OK CleanMPGers:

    I moved into a 1bdrm bungalow rental where I'm responsible for the utilities.

    House is semi-leaky (I've been caulking and weatherstripping everything).
    Insulation is minimal (R-13 to R-19 attic, R-13 or none in walls)
    Furnace is middle-aged 110k BTU natural gas unit. Guesstimate 80% efficient.

    What is the best "hypermiling" pattern for me? The house has to be 72 degrees during the late evening and at wake-up time. Every other time just has to keep the pipes intact.

    Also, an estimate of what I can save in terms of 1 degree-hour vs gas burned would be helpful.

    I have/will be sealing/shrinkwrapping doors and windows as well as adding loosefill insulation to bring the attic up to R-19.

  2. Taliesin

    Taliesin Well-Known Member

    The biggest savings I get are from keeping the thermostat at 58.

    That's right... 58...

    There are more people in the house, so we can't put a timer on it to change the settings throughout the day (there is always someone in and about the house), but you could get a thermostat with a timer and set it up to change the settings from 45 to 72 and back at the appropriate times:

    If you wake up at 7 am:
    6 am switch from 45 to 72
    7 am switch from 72 to 45 (assuming you leave for work at 7:30 or so)
    4 pm switch from 45 to 72 (assuming you get home about 4:30 or 5)
    9:30 pm switch from 72 to 45 (assuming a 10 pm bed time).
    Last edited: Oct 12, 2009
  3. TheForce

    TheForce He who posts articles

  4. msirach

    msirach Well-Known Member

    One thing I would recommend that hasn't been mentioned is a blanket for the water heater and insulation for the hot water lines also. This will help immensely if you go down to the frigid temps mentioned. Normally an advertised savings of around 10% of water heating cost can . Abe realizedlso if you set your water heater temp to no more than 115° or so, it can save another 10%.

    Why is 72° a necessary temp for heat? Why not 70° or 69°? It's about a 2% savings per degree of drop. I drop mine to 65° if I am the last one up and going to get up at 4:00a.m. I set it on 69° when I get up and the warm air feels great even at 65 or 66°. When I get home in the evening, I have to turn it down. They will have it on 72 to 75°.

    Don't forget to change your lights to CFL's.
  5. Harold

    Harold Well-Known Member

    Probably the best thing is for it to burn down! We have to many things that waste energy in our world. So do the owners and you a favor. Hal
  6. raveneon

    raveneon Well-Known Member

    Wouldn't this sort of huge temperature jump be really hard on the furnace? Or more specifically the heat exchanger?

    Also is it a myth about the objects in the house holding heat? Some people swear that you use less gas maintaining temp due to this phenomenon than huge temp swings suggested above.
  7. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Would you turn down the temperature if you were leaving on vacation for a month?
    Would you turn it down for a week?
    Why not for a few hours?

    Just maintaining temperature is a losing game, due to heat loss. You have to keep adding heat to keep up. If you drop the temperature and then bring it back up, you're only adding heat once and not all day long.

    The furnace will run at a set output level. The temperature requested controls how long it runs, not how hard.
  8. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Think of it as heat leaving the house. The bigger the difference in temperature between the outside and inside the greater the heat exchange rate OUT. During the day when there is no need for heat a cooler inside temperature reduces the heat exchange rate.

    As has been said the heater doesn't care if it has to run longer during reheat.

    A lot of furniture etc. may allow a cooling room to stay warmer longer but net the heat is leaving the house and there shouldn't be much benefit to a heat sink as it must take heat to warm up again.

    Now if you had the sun heating a sink that would be good free heat released after the sun goes down.
  9. rdprice64

    rdprice64 Still Learning

    In addition to the excellent suggestions mentioned above, we ensure that the south facing blinds/curtains are wide open from sun up to sun down, trapping as much solar heat as possible during the sunny days. Our south facing rooms can often run themselves up to 74-75 degrees by the end of the day, while the north facing rooms are sitting at 66-68. Luckily the thermostat is in one of the south facing rooms.
  10. warthog1984

    warthog1984 Well-Known Member

    Because my body is usually a blast furnace and I don't mind the cold unless I am:

    1) wet (getting out of the bath in 50 degree heat- brrr)
    2) waking up (No internal heat, and I am NOT a morning person)

    Those times I'll have it up at whatever temp I don't feel like I'm freezing. Otherwise, anything goes.


    Nope. I'd love to do it, but after trying 3 separate brands, I get migraines from 30 min exposures to CFLs after about a month. And its only CFLs- reg. florescents are fine, incandescents are fine.

    But considering my incandescent use is probably 80% efficient and my electric bill ranges from $10.71 ~ $26, I'm not too fused about it.
  11. Taliesin

    Taliesin Well-Known Member

    Well... You are the one that said you needed 72 degrees. :p

    And the thing about the house (and objects) holding heat is true to a point. That's why you can shut it back half an hour before you leave for work or go to bed.
  12. msirach

    msirach Well-Known Member

    Thermal mass is what can maintain temp for longer periods of time. If you have lots of soft stuff like stuffed furniture and carpet, it won't matter much. Tile floor, granite and glass furnishings, will hold more heat. It will also take longer to heat back up. There is a point where reducing the temp too low will expend more energy to heat it up than you save.
    Do what is comfortable for you.
  13. Taliesin

    Taliesin Well-Known Member

    I have had many complaints that I give off a lot of heat, especially when I sleep. It has to do with a lack of natural insulation (5'10", 130#).

    I just wear more clothes.

    It does help a lot in the summer. I'm perfectly comfortable in 100 degree temps, so AC use is minimal.
  14. TheForce

    TheForce He who posts articles

    Do you know that point or an optimum setting for maximum savings? Or can you point me to a site that has that information? I've been trying to figure that out since I got my programmable thermostat.
  15. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Way too many variables outside temp being the largest, my hunch is it unless you are extremely uninsulated you won't see that great a drop over the course of an 8 hour period during the day even in sub zero weather. There is just too much heat to move your house from 68 to even 50 in that amount of time, your safe bet is to go as low as you can and actually check on say a saturday when you are home to see how much you drop.

    It's been 25 years since my HVAC classes in college but a smaller delta T reduces heat transfer, I'd say it would be hard to go too low unless you freeze pipes.
  16. msirach

    msirach Well-Known Member

    I will have to do some digging to find it. I think it is saved as a favorite on my old desktop.

    There are several factors. Exterior temp, interior temp, insulation, wind, window space, personal comfort level, humidity, etc.

    If it is 45° outside, it is not bad to drop the thermostat to 55° at night for 5 or 6 hours from 69.

    If it is 5° outside, I usually drop it to no lower than 60°.
  17. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Here is what Energy Star has to say on the subject.

    "It is a common misconception that it takes more energy to heat up a cold house than it does to keep a house warm all the time. Turning down the thermostat will always save energy, as will turning up the air conditioner temperature setting. Heat moves from hot to cold, and the rate of heat transfer increases with greater temperature differences between inside and out. Smaller temperature differences between your house and outside generally means you’ll lose less heat (or air conditioning) from inside the house to the outside. Therefore, you will save energy by only heating or cooling as much as necessary, for the occupants and time of day."

  18. brick

    brick Answers to "that guy."

    The Energy Star advice most likely applies to gas, oil, and electric resistance heat. In more southern climates like mine you tend to find air-air heat pumps since up-front cost is low and heating demand is moderate. For these systems it can be a problem to set back the thermostat for a long stretch and then request a higher temperature later. The reason is that the outdoor coils may get too cold and ice up from a prolonged run, either causing many de-icing cycles or causing the system to give up and rely on the auxiliary electrical resistance coils. This is a much less efficient use of electricity, so one has to be careful. My experience is that a few degrees of set back won't hurt anything, especially when temperatures are mild. But as it approaches freezing I have learned to let the system cruise at the same temp (usually 66 or 67F which is comfortable for me).
  19. worthywads

    worthywads Don't Feel Like Satan, I am to AAA


    Thanks Brick, I had considered exempting that advice for heat pumps but th OP said natural gas. Quite a bit more complicated with heat pumps.
  20. TheForce

    TheForce He who posts articles

    So since I have a gas furnace I should be OK in setting WAY back? Well as far back as I can stand the cold. But in the summer time I should only set back a little?

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