Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to determine efficiencies of electric heating systems?

  1. Aug 12, 2017 #1
    I am building a house and would like to understand how to perform an energy balance/efficiency calculation on various electrical heating systems
  2. jcsd
  3. Aug 13, 2017 #2


    User Avatar
    Science Advisor

    They all convert 100% of input energy to thermal energy. If they are forced air there is a tiny amount used for air circulation. But even the waste heat of the blower motor adds to the heating.

    If you are in a mild to moderate climate, radiant heating, is adequate. This is often done with relatively low temperature baseboard heaters with a thermostat in each room. In harsher climates (think snow country with many feet of snow per season), radiant heating bothers some people because they are warmed but are often breathing somewhat colder air; at least in older houses. A new build with lots of insulation and double- or triple-pane windows would make this less of an issue.

    In the harsher climates, forced air heating (usually central) may be more comfortable than radiant.

    Electric heaters that operate at higher temperatures (i.e. portable electric heaters) can be useful in harsher climates because their higher temperature moves hot air around by convection.

    Be aware that electric heat is often the second most expensive option, with propane costing even more. Also consider the reliability of the electric supply.

    Oh, I just looked at your profile page. Looks like you are in one of those "harsher climates."
  4. Aug 13, 2017 #3


    User Avatar
    Science Advisor
    Gold Member

  5. Aug 13, 2017 #4


    User Avatar

    Staff: Mentor

    While all electric resistance heaters are 100% efficient, heat pumps are in effect much more than 100% efficient (up to 4x)...though the efficiency depend on the temperature of the source reservoir (air or water).
  6. Aug 13, 2017 #5
    Thanks for your reply - it is most helpful in determining the appropriate heating technology for my geography.
    I assume that there is a formula (s) to determine efficiency of the system, based on input energy vs output heat?[/QUOTE]
    Last edited by a moderator: Aug 13, 2017
  7. Aug 13, 2017 #6
  8. Aug 13, 2017 #7


    User Avatar
    Science Advisor

  9. Aug 15, 2017 #8


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Presumably that's Propane in cylinders? Here in the UK you can get LPG (bulk Propane) delivered by tanker and it works out at around 6.13p/kWH compared to Electricity at around 18p/kWH and heating oil/kerosene at 4.11p/kWH. By the same measure electric heat pumps deliver heat at around 5-7p/kWH.

    So "raw" electricity is around three times more expensive than most alternatives for us unless you use a heat pump.

    See "Pence per kWh (after boiler efficiency)"
  10. Aug 16, 2017 #9


    User Avatar
    Science Advisor

    A heat pump will give a factor of three advantage over a resistive heater. The problem is finding dry air or liquid ground water to extract heat from. When the cooling air is near freezing point the heat exchanger will ice up and force an inefficient defrost cycle.
  11. Aug 16, 2017 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Fortunately here in the UK the air is usually drier in winter than summer.

    Most modern GSHP are also closed loop/sealed and don't rely on ground water. This also has the advantage of reducing corrosion as the loop can be dosed with corrosion inhibitor.
  12. Aug 16, 2017 #11


    User Avatar
    Science Advisor
    Gold Member

    Any evaluation of heating methods and efficiencies really needs to take into account the thermal properties of the building that is being heated .

    In an old style house design with poor insulation the efficiency of heating devices matters a lot because you need a lot of heating .

    In more modern houses which are much better insulated the efficiency of heating devices matters less because you need much less heating .

    In the limit some houses which have been built in the uk in recent times are so well insulated that very little heat is needed at all and efficiency of heating devices no longer matters .

    Any evaluation of heating costs really needs to be done on a long term basis and take into account the trade off between cost of running heating devices and the cost of making the house (on initial build or by retrofit work) need less heating .
  13. Dec 29, 2017 #12
  14. Dec 30, 2017 #13


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hope the moderators won't mind but there is a good forum for self builders in the UK. Google Buildhub.
  15. Dec 30, 2017 #14


    User Avatar
    Gold Member

    Given your proximity, is it safe to assume you've looked into the "Drake Landing" community system?

    per Wikipedia; "In 2012 the installation achieved a world record solar fraction of 97%; that is, providing that amount of the community's heating requirements with solar energy over a one-year time span."

    I checked it out this summer. Here's something I sent my brother in June, via an email:

    [Drake Landing]
    Total system cost: $7,000,000
    Number of homes: 52
    $/home: $135,000


    What I spent for heating in the last 12 months: $1040
    [baseboard heat]

    How long I would have to live at that rate to get my money back: 130 years

    But I have a feeling I could do it cheaper. A “do it yourself” 4000 watt solar collector cost me $100.
    I get 3 hours of sunlight on a good day. (Stinkin’ trees!)
    Maybe 90 days of sunny weather in the summer.
    3 hours/day * 90 days * 4 kw = 1080 kwh
    kwh I used for heating last year: 8600 kwh
    Number of panels I would need: 8
    Cost of panels: $800
    Well cost: $5000 (I googled)
    Hardware and crap: $1000
    Total system cost: $6800
    Time to payoff: 6.5 years

    Sometimes it pays NOT to let the government get involved.
    But prototypes are expensive, and proof of concept is paramount, so I’d say the Knuck Bucks were well spent.
    It works! I would have guessed such a system was impossible.

    These are of course, just back of napkin type numbers.

    I have a "sand and gravel" company about a mile from my house. Their pit is about 35 meters deep, and shows no sign of hitting bedrock. I'm not familiar with the dirt depths in Edmonton. Might be something to look into. I would imagine digging a hole in dirt is a lot cheaper than digging one through bedrock.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted