Air Source Heat Pumps

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Comments

  • lomb
    lomb Posts: 46 Forumite
    My split system uses about 8-15watts when they are turned off.

    That is for a large 9kw outdoor unit. Obviously in summer when ambient temps are higher the crankcase heater doesnt need to operate so this will drop to about 6watts.

    Cheers,its not as bad as I thought, it varies wildly between manufactuers, in Australia from April the crankcase heater consumption will be combined with COP to give an annual figure. They want to ban or reduce crankcase heater consumption.
  • albyota wrote: »
    the defrost cycle will pull max power for 2 minutes (Ecodan) no heating being supplied for this short time..... never noticed with UFH...even with air to air not an issue, as I'm sure richard would agree....

    I would agree defrosts are not an issue for me its about 5 minutes every few hours or so.

    My unit turned on at 7am this morning it was -2C here and 17C inside and my unit didnt go into a defrost till 8:20am and this lasted about 4 minutes. So during this 1hr 20minutes the unit was running flat out to achieve target temperature. After the first defrost the unit was ticking over maintaining set point and didnt go into defrost for nearlly 4 hours during which outdoor ambients had risen to about 3C.

    During which time the indoor units wernt blowing warm air but the room was upto 22C which is what the set point was, the units if slightly oversized will go into defrost less often as they wont be working flat out.
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  • lomb wrote: »
    Cheers,its not as bad as I thought, it varies wildly between manufactuers, in Australia from April the crankcase heater consumption will be combined with COP to give an annual figure. They want to ban or reduce crankcase heater consumption.

    If your using the system daily then the crankcase heater only comes on when the compressor core drops to about 5C. Its not going to be in operation when the unit is running.

    So when you turn your unit off at night, the compressor which is very hot and insulated in a jacket (like a hot water tank) is going to take a long time to drop to below 5C. So it might not even come on during the night at which point the unit will have probably turned back on for the day.

    I cant say I have ever seen consumption in standby go above 15watts. And thats at subzero temps so it must be very brief and at a time when I am in bed.

    Most people leave their heat pumps running at a lower temp when its very cold anyway to maintain internal temps.

    The heater is a strip heater and is very low wattage, your only increasing the core temp of the compressor to about 5C its not very hard to do.
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  • On the Danfoss GSHP units each meter shows 200w when the pump is running - I take this out for the calculation when working out the COP.
  • Cardew
    Cardew Posts: 29,036
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    albyota wrote: »
    cardew, when I first had my system installed, I had to wait 24 hours for the crankcase heater to get the compressor up to temp, I would not think this has needed to operate very often as the thermostat will control its operation only when the system has been in an off state for a while, (not sure how often I'll check this though) as for defrost cycles and whether it is factored in to the COP's or not, 2 minutes once in an hour only if conditions require i.e. 0 - 3 degrees foggy/moisture laden air at low ambient. yes you could probably factor in a loss....as for the manufacturers accounting for this, I wouldn't have thought it would impact the COP much, as they would have to say xxx unit has a COP of 3.59993 as opposed to 3.6.


    Albyota,

    I hope the 3.59993 comment is TIC!!

    The difficulty is getting any objective evidence of the effect of defrosting on overall COP, much of the 'evidence' is anecdotal from owners or 'blurb' from manufacturers who are not going to point out any pitfalls.

    Ironically much of the 'evidence' pointing out the defrosting disadvantages of ASHPs come from the advocates of GSHPs!!

    This article is interesting:

    http://www.bsdlive.co.uk/story.asp?storycode=3142370

    A professor(with presumably no axe to grind!) points out disadvantages of ASHPs and his arguments disputed by a product manager of Mitsubishi heating.

    With regard to defrosting the professor states:
    Clearly, whatever type of de-icing system is used, the energy needed will markedly reduce the seasonal performance of the system. The energy used has a big impact on economic performance too. There are three other serious concerns:
    • noise from the fan and compressor can be intrusive in domestic/urban locations;
    • snow could block the airflow around an ASHP, preventing the system operating unless it is manually removed, which would be unacceptable to many householders (particularly the elderly or infirm); and
    • the heat output from an ASHP reduces markedly as outdoor air temperatures fall, so a system may have difficulty meeting demand when it is most needed. Most manufacturers include an additional heating system to avoid this problem, usually a direct-acting electric flow boiler or a bivalent system which includes a gas- or oil-fired boiler, with all the attendant additional costs and complexity.

    The Mitsubishi man incidentally states:
    Inverter-driven compressor technology means Ecodan has a typical defrost cycle of three minutes, keeping energy use to a minimum, and this is likely to occur about once an hour in exceptional conditions.

  • "noise from the fan and compressor can be intrusive in domestic/urban locations; "

    I dont agree with this:

    http://www.youtube.com/watch?v=6JZvqFWpA7U

    Thats my unit in defrost, the fan doesnt run below 0C in defrost and you can clearly hear traffic over the unit.
    If you found my post helpful, please remember to press the THANKS button! --->
  • paceinternet
    paceinternet Posts: 355 Forumite
    edited 15 February 2010 at 11:11AM
    'evidence'
    Yes it would be really useful if albyota and others could post real data for defrosting.
    2 or 3 mins?
    Can you confirm that the system can be heating on full load, go into a defrost cycle for 2 mins, and is then immediately running on full heat output again?
    If so, it may be reasonable to say:
    At 0 deg C, a 3kw in / 6kw out unit would cost what?
    (I am assuming an 8-10 kw heat pump would be outputting 6 kw at 0 deg C.)

    2 mins x 3kw of electricity to run the defrost + another 2 mins x 3kw to recover the lost heat in the defrost. Costs about 4 pence an hour.

    But I doubt it is that quick, so some real numbers would be good.
  • Yes it would be really useful if albyota and others could post real data for defrosting.
    2 or 3 mins?
    Can you confirm that the system can be heating on full load, go into a defrost cycle for 2 mins, and is then immediately running on full heat output again?
    If so, it may be reasonable to say:
    At 0 deg C, a 3kw in / 6kw out unit would cost what?
    (I am assuming an 8-10 kw heat pump would be outputting 6 kw at 0 deg C.)

    2 mins x 3kw of electricity to run the defrost + another 2 mins x 3kw to recover the lost heat in the defrost. Costs about 4 pence an hour.

    But I doubt it is that quick, so some real numbers would be good.

    Ok here it goes:

    It takes about 5minutes after the defrost before it is running at full out put again.

    Heres capacity tables at different outdoor temps for my multisplit unit:

    Page 63 of the pdf reader or page 268 if looking at the bottom of the pages like a book. Use combination of 4 units as then the outdoor will be providing full output with 4 units connected. Numbers in the column that are bold are the design ratings.

    http://mylg.co.uk/data%20for%20site/multi%20f%202008%20outdoor.pdf

    You can see the unit is rated at 8.5kw output with 1.96kw of input based on ambient temp of 6C and internal temp of 20C.

    Follow this across to the left and you can see that at 0C it drops to 7.66kw output with 1.96kw input.

    Then -5 is 6.86kw output with 1.97kw input.

    Loss is just short of 1kw of output every time the external temp drops 5C.

    At -15C the output drops to 4.84kw with 1.97kw input.

    Of course 20C isnt that warm to keep a house but even so looking at the internal temp figures of 22C it isnt much different.
    If you found my post helpful, please remember to press the THANKS button! --->
  • lomb
    lomb Posts: 46 Forumite
    edited 15 February 2010 at 11:50AM
    i wouldnt worry about defrost on air source or fall off in heating output at low temperatures. Just size for an outside of 2 degrees cent rather than 7 . Many modern ASHP give 70% output down to -15 degrees. COPs dont suffer to much at low temperatures either.Ill see if i can dig up some cop versus ambient for some ASHPs.

    Do a google search for H2 cop mitsubishi electric. This is the COP at 2 degrees ambient in heat mode. For the one i looked at it was 3.79 at 7 degrees at 3.59 at 2 degrees. Something that wont affect the figures at all.
  • lomb
    lomb Posts: 46 Forumite
    I have not put in gas in a new commercial building with small offices that would approximate domestic room sizes even though gas was available and am putting only heat pump splits in. My big preference to this was I needed heating and cooling as needed and wanted a set and forget digital temperatrure for comfort,i also didnt want space robbing radiators and there was no room for underfloor. I wanted something competitive with gas but not necesarily cheaper as I wanted the above also. I will let know how they feel in due course in terms of comfort. The units are slightly oversized to take account of low ambients and powerful cooling when needed. There is also redundancy with multiple outer units so if one fails no big deal.This is also cheaper to buy and fit.They are MHI units.
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