Setback Thermostat use?

Discussion in 'HVAC Heating & Cooling' started by Bosun, Oct 10, 2007.

  1. Bosun

    Bosun New Member

    Messages:
    89
    Location:
    Minneapolis
    Some heating guys seem to think that "setback" is over-hyped. i.e. that is doesn't provide much savings because of that amount of 'work' required to raise/lower the temp in your house kills any efficient operation of your furnace.

    Any thoughts or experience on this?

    I have a boiler from 1996. The CI radiators in my old house take a while to heat, and when they are cooking, it takes a while to relax.

    Also, any recommendations on thermostats for either programmable or non-p?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    My brother-in-law is a mechanical engineer. His rule of thumb is 1% saved for each degree set back for an 8-hour period. So, set back 10-degrees for 8-hours/day, save 10% over leaving it on.

    From a comfort standpoint, a long cold-soak can result in a lot of discomfort. In an 8-hour period, things won't cold-soak much, and while the air temp may be lower, the walls, floors, etc. may not drop that whole amount, so when you rewarm the air, it is pleasant fairly quickly. Conversely, if you set back for say 16-hours, it might take a long time for you to feel comforatable, since the furniture, walls, floors, might still be absorbing heat even when the air temp is 'normal'.

    Some of the thermostats (many of the Honeywell ones, and maybe others), have an adaptive logic control. They monitor how fast the temperature drops, and learn how fast the system can respond, and instead of setting it to turn on at say 6am so it is warm by 7am, you set the time you want it warm. Then, if it is say a mild night, it might not turn on until 6:50am or on a really cold nasty night, maybe at 5:30am.
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    The "smart" setback described by JAD....is a good feature. I think it does depend on what type of heat you have.....radiant electric, baseboard, steam, forced hot air, etc. I suspect that if you call your local utility company, they would have some data customized to your climate area, and the types of heat commonly used thereabouts.
  4. abikerboy

    abikerboy DIY Senior Member

    Messages:
    202
    Location:
    VA
    I believe Jad is right. I have forced air, and a well insulated house. With my ac use over the summer, a setback tstat has been good to me....(I must state Im away for 10-12 hours a day or longer even... With heating on an oil furnace, it does make a diff, but not enough that I can see to make it worth while. I have to set the cycles too close on heat to come home to a warm floor, and comfortable toilet seats. Lol!
  5. alleycat

    alleycat New Member

    Messages:
    72
    Location:
    Illinois
    I'm curious if people have opinions on a similar issue - is there a benefit to turning off the hot water heater during the day?
  6. Furd

    Furd Engineer

    Messages:
    446
    Location:
    Wet side of Washington State
    There is absolutely no question that temperature setback will save energy. This has been proven by countless experiments over the last seventy-five years.

    However, comfort is a different animal when it comes to temperature setback. Forced air systems are usually quite a bit larger (in BTU capacity) than is required except in the coldest of weather. Forced air systems respond quite rapidly to heat input (the furnace running) and therefore can use a much larger setback than slower responding systems.

    Radiant floor systems, especially when they employ a large thermal mass such as tubing embedded in concrete, respond quite slowly and cannot use a large temperature setback.

    Hot water baseboards are in-between these other two in response time.


    As for the domestic water heater issue...in most cases there is no economic payback for removing the power from an electric water heater as these are (usually) well-insulated and lose very little heat during the day. Gas water heaters may, under certain circumstances, have some savings but the means to remove and restore "power" is far more difficult.
  7. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    gas water heater setback

    We have a 70 gallon tank type gas water heater because of a Jacuzzi tub. Most of the time it wastes energy. We have it turned down but sometimes have lukewarm showers after periods of non-use. Is there a setback gas water heater scenario yet, or a pilotless ignition available that could be disabled by power interruption? I have been considering switching to a 30 gallon electric set low feeding a gas tankless inline with it, any thoughts?
  8. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Water heaters do not waste a huge amount of energy when in idle, but if you are talking about only using the jacuzzi once a month, there is something to talk about. Setting any water heater below 120 is dangerous, microbe-wise. Some states now require a minimum WH setting of 140º, and of course also require a tempering valve installed on the outlet.
  9. MarkHash

    MarkHash New Member

    Messages:
    60
    Location:
    Wisconsin
    Microbe issues, never thought of that. The tankless I am looking at only raises the water temp around 40 degrees. So that is why I haven't heard of that arrangement. Typically then it is a tankless heater feeding a storage tank acting like a capacitor in electronic terms? Would that be a solution or would the energy costs be similar?
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    Many tankless can raise the temperature much more than 40-degrees, but its ability to do that depends entirely on how much energy it uses. High flow rates can decrease the maximum gain radically, as will the incoming water temperature. Personally, I think that a well-designed storage tank will be cheaper to run than a tankless, especially when you take into account the installation, maintenance, and potential infrastructure upgrades that may be required to handle the significant power requirements.
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