Ac vs gshp

SolarLurkerSolarLurker Solar Expert Posts: 122 ✭✭
I am looking at replacing my 4 ton 11 seer ac with either a 4.5 cop ground source heat pump or a 24.5 seer central air unit. Which unit would use less electricity? I just cannot wrap my mind around the calculations.

Comments

  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Ac vs gshp

    If I remember right you can't directly compare COP to SEER, because COP is at a defined operating temperature where SEER is an average over the range of temps possible during operation. Also make sure the COP number is its COP for cooling and not for heating.

    Take a serious look at the new inverter-based mini-split air-sourced heat pumps, if you search here for "mini-split" you'll see several long discussions. Currently the highest-rated mini-splits (@ 26 SEER) operate at the same efficiency as ground-sourced heat pumps, without the huge expense of burying lines and adding air ducts within the house. I was ready to move towards a ground-sourced system to replace my aging whole-house systems until I learned about the mini-splits. Way cheaper, can be added piecemeal, and you get room-by-room zone control... so you can leave 3/4 of your rooms at 80 F and just the few rooms you usually use at 74 F. If you need to use one of the other rooms you can cool it down in a few minutes.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • BB.BB. Super Moderators, Administrators Posts: 31,053 admin
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SlimDieselSlimDiesel Registered Users Posts: 17
    Re: Ac vs gshp

    I doubt anyone would use a GSHP to replace just the A/C. I have a 4-ton GSHP and the A/C function is exceptionally oversized and very low cost to operate. This is because the system will be sized to replace your heat loss on the typical winter low. Around here (southern Ontario), that means the unit will be capable of maintaining at least a 70 degreeF differential to the outside. In A/C mode the worst differential it would have to maintain is about 1/3 of that. On the coldest nights the system will run continuously on high and just maintain. On the hottest days, it will only run intermittently on low with maybe a burst to high when coming off a large setback. Due to the short A/C run time we run a dehumidifier on the lower level.
  • SolarLurkerSolarLurker Solar Expert Posts: 122 ✭✭
    Re: Ac vs gshp

    The ground source unit would be used for heat in the winter if I installed it. However it costs nearly 30,000 and I am not really ready to make the plunge. If I knew that the AC would use less electric I would just replace that for the moment and hold off on the GSHP.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Ac vs gshp

    Slim's post reminded me of another advantage of the mini-splits. They have a "dry" mode which runs the unit as a dehumidifier. Basically it runs the interior fan on its lowest setting and the pump in cool mode so you get little temperature drop but a good reduction in moisture.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • LucManLucMan Solar Expert Posts: 223 ✭✭✭
    Re: Ac vs gshp
    I am looking at replacing my 4 ton 11 seer ac with either a 4.5 cop ground source heat pump or a 24.5 seer central air unit. Which unit would use less electricity? I just cannot wrap my mind around the calculations.

    The initial installation costs of either of these units will be quite high and may not offer you any payback on the upgrade.

    The COP and HSPF ratings are for the heating side of a heatpump / a/c unit.
    The SEER rating is for the a/c energy rating.
    The rating that will give you the most accurate energy usage is the EER rating (energy efficiency rating).
    Since you are looking for a ducted a/c only system anything over 14 SEER has a negative payback over the life of the system since you live in PA. ( not enough cooling days). If you lived in the south or southwest a higher efficiency unit would give you a better payback.

    Here is a link to calculate energy costs.

    www.eia.gov/neic/experts/heatcalc.xls
  • SolarLurkerSolarLurker Solar Expert Posts: 122 ✭✭
    Re: Ac vs gshp

    well I have learned a little more, I had an energy audit done and I only need 1 ton for cooling. My current unit is 2 ton, the gshp was sized at 4 ton to meet heating needs.

    It would be nice to know if a 4ton GShP would cool the House cheaper than a 2 ton or 1 ton AC unit. I kind of suspect that if the GSHP is oversized than it may not keep up with my humidity. At the moment my unit cools the house, however it still feels sticky.

    I think I am going to replace my 11 seer unit with a 24.5 seer ac unit. I am looking at the Maytag IQ drive.(I have not gotten any quotes yet) It uses inverter technology like most of the mini split units. It can run at 40% capacity so I should be able to cut electric usage a lot, more importantly this should be better at dehumidification.

    Who know if it will ever pay for itself, however a few online calculators suggest I will use 2600 KWH yearly. I would need 3kw of solar to produce the 2600, so if the ac unit is less that 15,000, there is some money savings.
  • SolarLurkerSolarLurker Solar Expert Posts: 122 ✭✭
    Re: Ac vs gshp

    Well i finally had an iq drive person give me a qoute, they want 15,000k to change the ac to the iq drive. They said I would need a 3 ton unit because of the amount of vents I have, plus according to them I will need to replace my furnace to the maytage furnace.

    I like the maytag furnace, however I have a lot of money tied up in the Freewatt futnace and really I am not prepared to scrap that system.

    So I think I am just going to tinker with the current unit and put the money saving into more PV.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Ac vs gshp

    Again, you can add the mini-splits piecemeal, so you could just add a few of them right now in your most-used rooms and then turn your current A/C up for the rest of the house. And you can keep your current furnace.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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