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  1. #1
    Join Date
    Jan 2009
    Posts
    56

    Is the cost of running a Geothermal heat pump less than an air-based heat pump?

    Hi,

    I live in Montreal, Canada and have an oil furnace and standard heat pump. My heat pump runs until -12C at which point the oil furnace takes over full-time. I have a dual-energy electric rate, so the cost of my electricity is about 1c/kWh cheaper when the temp is above -12C, but shoots up to 3x the cost when it is colder than -12C. Although it gets more expensive on the really cold days/nights, it ends up being more profitable the other 95% of the year. Generally speaking, this solution works well. None the less, I go through about 2000-3500L of oil per year (depending on how cold the winters are). At an average cost of about 1$/L, oil alone costs about 3K/year. When I look at my electric bills, they average about 100$/mth, slightly higher in the heat of summer, and in the winter months.

    I am now looking at adding floor to my house, and will likely have to do work on the HVAC system. So the obvious question is whether it is worth the $$$ to move to geothermal.

    I've already had a quote which seem incredibly expensive, so I have to really sit down and decide if it is worth the investment. Obviously, I realize that I would save the cost of oil every year. However, what can I expect to see in electricity costs? Does a geothermal heat pump require less electricity than a conventional heat-pump? I presume that both use a compressor, so I can't imagine the consumption being that much less significant.

    Additionally, do any geothermal systems use fosil-fuel as suppplemental (emergency) heat supply? Or do they all use electric coil only? Does the electric coil get used that often, or is it there truly for emergency purposes?

    As you can see, I am very much in the preliminary stages of evaluation (I don't even have plans for the extension drawn up yet), but just wanted to start investigating my options.

    Any advice, or good resources that you can offer would be greatly appreciated!

    Thanks,

    Eric

  2. #2
    Join Date
    Dec 2011
    Location
    New Zealand
    Posts
    1,972
    Not as easy a question to answer as one would think.
    A heat pump, is not like an element, the amount of heat it produces and the amount of power it use (when running) changes all the time. So a simple comparison can not be made.
    So lets assume for ease that the hot side of the heat pump is the same for geo and air source. (a constant). It could be said with small system that the power used by each would be somewhat similar at this constant.

    Now we need to look at how much heat energy is produced.
    This is simply a function of the energy absorbed from the source plus the energy required to drive the comp.
    What determines the efficiency/energy absorbed from the source is the temperature at which the refrigerant boils. The higher the temperature the higher the efficiency, low temp lower efficiency.
    So as a simple example.
    Geo
    The ground is 10C, the fluid circulating is 5C and the refirgerant boils at 0C
    Air
    Air is to 10C, refrigerant boils at 5C

    In this example the air source would be better.

    But your ambient is much lower
    Air is -12C, refrigerants boils at -17C
    So the geo is much more efficent plus the geo does not need to defrost, the air does. (even more efficiency)

    If the ambient only goes below 10C for 5 days a year, then you would go air source, but if it much colder for longer periods then geo maybe a better option.

    Also remember that the colder the ambient more heat you require so the heat pump (either one will be running long per day)

  3. #3
    Join Date
    Apr 2012
    Location
    Georgetown Delaware
    Posts
    197
    When you find the correct installer in your area they will be able to provide you with the anticipated operating cost per year based on the results of the manual J. It will then be an easy comparison between what you have now and what you would pay going forward.

    Geo is NOT for everyone. there are just to many factors to consider in different places in the states, let alone the world. You allready identified one, the cost per kw.

    Geo can be backed up with fossil, electric, or be designed for 100% coverage. Get on the net and find another installer to get a quote from or at least do a design for you.

    Eric
    Eric Sackett
    weberwelldrilling.com
    Delta P= 8 ATA
    www.youtube.com/weberwelldrilling

  4. #4
    Join Date
    Apr 2006
    Location
    Barrie Ontario
    Posts
    312
    I would highly recommend looking into a Mitsubishi Zuba system.
    For some reason I learn more with my mouth shut

  5. #5
    Join Date
    Jan 2009
    Posts
    56
    Quote Originally Posted by turkey View Post
    I would highly recommend looking into a Mitsubishi Zuba system.
    Thanks for the tip. I presume you have experience with these systems? I did take a quick look at their site and it seems too good to be true. Whereas a conventional HP ends up being inefficient around -10C, this one is listed to be as efficient at -30C as +30C.

    I guess the big question then is if it is significantly more expensive than traditional ones? Otherwise, I would expect these to be used all over the place here, and yet, I don't think I have ever seen one outside anyone's house.

    Do they still require a defrost cycle like a conventional unit? It would appear that one no longer needs supplemental heat source except in the case of emergencies using something like this. Is the power draw consistent with a conventional HP, or lower?

    If it is as efficient as it claims, does this not make it as efficient as geo-thermal, yet at a fraction of cost since there is no need for the drilled heat-exchanger?

    Thanks for the info!

  6. #6
    Join Date
    Dec 2010
    Location
    Toronto
    Posts
    1,253
    You do have an opportunity to reduce the heat load of your building first. Look up "passivehaus" and see if you can apply some of the ideas to your house. If you are putting another floor on the house, it is possible, if done right, to have a bigger house with the same or lesser heat loss than the original house.

    If you can do this, and use radiant floors, then your options are even better. An ASHP (possibly with solar) might work well with a simple element in the tank for extra boost but this depends on your cost to drill wells for Geo.

  7. #7
    Join Date
    Apr 2006
    Location
    Barrie Ontario
    Posts
    312
    Yes it is as good as they say, yes it out performs geo in many applications, yes it needs a defrost which it does in under two minutes. Find a mits dealer in your area to come out and go over the system with you all my customers some former geo owners are extremely happy with this system
    For some reason I learn more with my mouth shut

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