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Do GSHP break even compared to AWHP?

  • 02-08-2020 2:25pm
    #1
    Registered Users Posts: 152 ✭✭ matrat


    Hi all,

    Undergoing a costing exercise for an upcoming project and would be interested in peoples opinions/inputs on the overall costing of GSHP v ASHP.

    For purposes of the exercise i will standardize as many of the variables as possible. This may not be fully accurate but i dont have any costings within the last 4 years to look at for actual data. If you have priced GSHP and ASHP for the same project and have the costings, then that would be great info to have.

    For now, assume the cost to install a GSHP v ASHP for a given house are identical with the exception of the ground works. My findings from 4 years ago were that the installation of the ASHP was actually very slightly cheaper, i.e. cost of machine and plumbing, but wont assume that here unless others can confirm this is still true.

    So far, the main cost difference would the ground works, and for the given site, a borehole is the only option. The cost of this borehole is e3750.

    The expected annual cost for the GSHP is e600.
    Assume equal maintenance costs (is this fair?)
    Assume life expectancy of GSHP to be 20 years (seems generous).

    e3750 / 20 = e187.50

    187.50 / 600 = 31.25% increase in annual cost.

    What this means is that if we expect the heat pumps to last about the same time, cost the same to install and cost the same to maintain, then if an ASHP can cost less than e787.50 annually to run then it should be the natural choice. I cannot find any hard figures but it appears than allowing an ASHP to cost an additional 15% over the cost of GSHP is generous.

    What am i missing here or is it this simple?

    From the quotes i have received in the past, it appears that ASHP are generally cheaper to purchase, from my research, it appears to ASHP are generally no more (if not alot less) to maintain, and there are some reviews stating that they will not last as long as GSHP but they would need to fail alot sooner than GSHP to make up for the savings, and this only widens with the purchase cost difference and assumes that GSHP will last, which is not guaranteed.

    Any other thoughts on other factors etc?


Comments

  • Registered Users Posts: 11,183 ✭✭✭✭ KCross


    matrat wrote: »
    The expected annual cost for the GSHP is e600.

    What are yor expected costs for the A2W? All things being equal the GSHP will be cheaper to run as it will have higher efficiency.

    matrat wrote: »
    Assume equal maintenance costs (is this fair?)

    Cant see that being the case.
    Again, all things being equal, the GSHP will be running less to achieve the same result therefore it should have less maintenance but, of course, alot of that is down to the units you pick and the quality of the components they use and crucially the installer you use to ensure they have spec'd it right and installed it right.
    matrat wrote: »
    Assume life expectancy of GSHP to be 20 years (seems generous).

    The GSHP will/should last longer than ASHP because its running less and not exposed to the weather.

    Its hard to put an exact figure on it as it all depends on the units you compare and an element of luck (like in cars).

    General wisdon is 10-15yrs for an ASHP and 20+ for GSHP but its very much a case of YMMV.


  • Registered Users Posts: 152 ✭✭ matrat


    KCross wrote: »
    What are yor expected costs for the A2W? All things being equal the GSHP will be cheaper to run as it will have higher efficiency.


    Cant see that being the case.
    Again, all things being equal, the GSHP will be running less to achieve the same result therefore it should have less maintenance but, of course, alot of that is down to the units you pick and the quality of the components they use and crucially the installer you use to ensure they have spec'd it right and installed it right.

    The GSHP will/should last longer than ASHP because its running less and not exposed to the weather.

    Its hard to put an exact figure on it as it all depends on the units you compare and an element of luck (like in cars).

    General wisdon is 10-15yrs for an ASHP and 20+ for GSHP but its very much a case of YMMV.

    Yeah the COP will be higher for GSHP v ASHP, but do you have any idea by how much on average? And how much of that will be negated by installing a slightly over sized ASHP? Assuming a 10% drop in COP should give an assumed ASHP cost of e660 annually?


    Yeah again this is due to better COP and less cycling, but these can be overcome by using a buffer tank and a slightly over sized unit. Per other forums i have reviewed however, seems to be more of a consensus that GSHP have a higher maintenance cost than ASHP, primarily due to the changing of the brine after 8-10 years.


    Yeah that makes sense and thats what i am trying to get some more idea on, as getting 15 years out of an ASHP may be cheaper in the long run as opposed to getting 20 years out of a GSHP.


  • Registered Users Posts: 11,183 ✭✭✭✭ KCross


    matrat wrote: »
    Yeah the COP will be higher for GSHP v ASHP, but do you have any idea by how much on average?

    I dont and I dont think anyone will be able to definitively tell you either. It will depend on the unit, the installer, your house and your lifestyle... lots of variables there that will make an average pointless to you.


    But look at it this way.... technologically, the GSHP will run less to achieve the same result therefore, all things being equal, will last longer and cost less to run. The decision you need to make is, is the difference in upfront cost for GSHP worth it in the long term. Thats a really tough decision. What is the cutoff point.... is it €3k, €5k, €8k....?

    I'd say get multiple quotes from multiple suppliers for A2W and GSHP and contrast and compare and make an informed decision.... which I guess is what you are trying to do with this thread but real quotes for your house is what you need really.

    Also, If you are anywhere near the sea you also need to ensure that any A2W system you buy can handle the sea air on that external unit... GSHP wont have that problem, of course.

    Also factor in the noise of the A2W system. If you are rural and its very quiet at night and the external unit is cycling you will likely hear it. Maybe get yourself in front of one of them actually running to see/hear for yourself.
    matrat wrote: »
    And how much of that will be negated by installing a slightly over sized ASHP? Assuming a 10% drop in COP should give an assumed ASHP cost of e660 annually?

    Where is the 10% coming from?
    The drop in efficiency of an ASHP during the coldest spells, when you need it most, will be huge relative to a GSHP.... alot more than 10%.

    During the summer when the two systems are only heating hot water it will be fine but the cost of heating hot water is very small relative to the overall cost of running the system.... space heating during the winter is what counts when you are comparing COP's.
    matrat wrote: »
    Yeah again this is due to better COP and less cycling, but these can be overcome by using a buffer tank and a slightly over sized unit.

    Adding a buffer tank will reduce cycling but it wont improve the COP. An ASHP still has to overcome the outside air temperature and defrost its coils etc to "fill" that buffer tank. That buffer tank and larger A2W system increase upfront cost and maintenance and running costs also.

    matrat wrote: »
    Per other forums i have reviewed however, seems to be more of a consensus that GSHP have a higher maintenance cost than ASHP, primarily due to the changing of the brine after 8-10 years.

    Im not aware that the brine needs to be changed after 8-10yrs.
    Maybe the forums you read were users who had leaks and it required a replacement but AFAIK the brine does not need to be changed every 10 years.

    FYI: I have a GSHP (horizontal collector). Its in nearly 10yrs. No maintenance or issues except one... I had to replace a "soft starter" on it. Thats a €150 electrical part, not really related to the fact that it is ground or air source based. Its running fine. So, I am biased here! :)
    matrat wrote: »
    Yeah that makes sense and thats what i am trying to get some more idea on, as getting 15 years out of an ASHP may be cheaper in the long run as opposed to getting 20 years out of a GSHP.

    If that specific example were your experience then yes, I'd agree, but you could also be looking at replacing the entire A2W system in, say, 10 years and the GSHP lasting 25 with only a new compressor being required... the wide variation that is possible is just too wide to make any sweeping statements.


    NOTE: You will get plenty folks who got GSHP and had bad experiences because the systems were installed by cowboys who have since gone bust. Whoever you use and regardless of whether its A2W or GSHP get a recommended installer with track record to limit your exposure to that.

    You will also hear from people who got A2W and are not happy with their running costs.

    Get quotes with the system specs included... only then will you really have any data/figures to work from. Anecdotal evidence from individual users wont tell you anything really.


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