Air Source Heat Pumps

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  • stevehead
    stevehead Posts: 215 Forumite
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    Cardew wrote: »
    an overall COP of 2 to 3 is excellent.

    If I got an overall COP of 2 or 3 I'd be ..... well I'd be amazed at how well insulated my house is.
    As I've previously stated, I use ~10KWh a day to fully heat my home. So with a COP of 2, I could heat my home with just 20KWh of resistance heating.
    I should have just bought £19.99 B&Q oil rads, would have saved a fortune.

    And a point about seasonal ajustment; my 10KWh / Day is under the the most adverse contitions my ashp's will face, and for the 6 month heating season average that figure will be revised substantially downwards.

    BTW Still hasn't snowed in Plymouth
  • albyota
    albyota Posts: 1,106 Forumite
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    Hi all, if i may add to this discussion, i have an 8.5kW system and it has replaced an 18/22 oil boiler, this is because you need to calculate the heat load and design so that you seperate the hot water heating load from the space heating... i.e central htg 8 kW, HW 4 - 6 kW but timed before or after heating, rather than the 14 - 16 kW required from conventional heat load calcs of adding htg and Hw together plus 10%.
    There are three types of people in this world...those that can count ...and those that can't! ;)

    * The Bitterness of Low Quality is Long Remembered after the Sweetness of Low Price is Forgotten!
  • mech_2
    mech_2 Posts: 620 Forumite
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    stevehead wrote: »
    If I got an overall COP of 2 or 3 I'd be ..... well I'd be amazed at how well insulated my house is.
    As I've previously stated, I use ~10KWh a day to fully heat my home. So with a COP of 2, I could heat my home with just 20KWh of resistance heating.
    I should have just bought £19.99 B&Q oil rads, would have saved a fortune.
    Why assume Cardew would put yours at the bottom end of that range?

    Personally I would have said 2.5 to 3.5 for new units, but what do I know about ASHPs? Only what I've read online. Back of envelope sums suggest you might be getting a COP of up to 3.5 in January. I'm impressed.
  • stevehead
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    mech wrote: »
    Why assume Cardew would put yours at the bottom end of that range?

    Because at below optimum performance conditions (7C) the efficiency drops off badly to the point where the system spends most of it's time in deforst and the COP becomes rubbish it says so on the net.

    The 7C outdoor test temp is important to me because it is close to the conditions my ashp will be exposed to.
    TT003810_plymouth.gif

    Source: BBC Weather, Plymouth
    The average min will occur at 4am when the heat pump won't be working anyway.
  • richardc1983
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    stevehead wrote: »
    If I got an overall COP of 2 or 3 I'd be ..... well I'd be amazed at how well insulated my house is.
    As I've previously stated, I use ~10KWh a day to fully heat my home. So with a COP of 2, I could heat my home with just 20KWh of resistance heating.
    I should have just bought £19.99 B&Q oil rads, would have saved a fortune.

    And a point about seasonal ajustment; my 10KWh / Day is under the the most adverse contitions my ashp's will face, and for the 6 month heating season average that figure will be revised substantially downwards.

    BTW Still hasn't snowed in Plymouth

    Steve, can you tell me what your units are. i.e what the kw capacity of each one is for heating and cooling?

    Mine are 3.5kw coolikng and 4.2kw in heating.

    Thanks.
    If you found my post helpful, please remember to press the THANKS button! --->
  • stevehead
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    Steve, can you tell me what your units are. i.e what the kw capacity of each one is for heating and cooling?

    Mine are 3.5kw coolikng and 4.2kw in heating.

    Thanks.

    SRK Wall Mounted SRK20ZGX
    Power source 1 ph 240v
    Type Inverter High C.O.P
    COOLING (nominal) max. kW 2.0
    HEATING (nominal) max. kW 4.6
    UK Cooling (max) kW n/a 2.8
    UK Sensible Cooling kW 2.0
    E.E.R. 5
    Energy label in heating A
    Energy label in cooling A
    C.o.P 5.45
    ECA - Yes
  • richardc1983
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    stevehead wrote: »
    Because at below optimum performance conditions (7C) the efficiency drops off badly to the point where the system spends most of it's time in deforst and the COP becomes rubbish it says so on the net.
    .

    Steve I disagree especially in your case as your system is coping so well. My units are oversized for the flat and only if the house is very cold say 12C does the unit have a struggle to bring it upto set point, the outdoor unit then works flat out to bring temp up and has more regular defrosts, however it gets there after 2 or 3 hours and the unit then happily floats along at a nice pace using very little electric and going into defrost after about 90minutes. When its first started defrost cycle can happen every 40 minutes till its got hold of the indoor temps.

    Only if your equipment is undersized will it spend most its time in defrost. If its sized correectly and has a little extra capacity so it isnt struggling to heat then it will not spend its time in defrost when its cold as it has the extra power to cope with colder house temmps and the higher heat loss in the house.

    I quote here from the Mitsubishi Website so this is relevant to your equipment being Mitsi and all that. Check out the bit in red at the bottom!


    from Mitsubishi site:
    DEFROST PERFORMANCE

    Defrost performance and frequency is critical to the efficient operation of a heat pump.
    The period of time as frost builds on the outdoor coil, during the defrost period and after defrost as the
    pipe work and indoor coils heat up reduce the hourly heat transfer rate of heat pumps and this affects
    the units performance. While the unit is defrosting it is not heating.


    Undersized units will defrost at regular intervals as low as 30 minutes apart in low ambient conditions
    when they are struggling to reach set point.


    Correctly sized units will have shorter total compressor run times and lower frequency operation
    speeds. These conditions combine to reduce defrost frequency from the minimum of 30 minutes to up
    to 120 minutes or not at all depending on the load requirements.
    If you found my post helpful, please remember to press the THANKS button! --->
  • richardc1983
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    stevehead wrote: »
    SRK Wall Mounted SRK20ZGX

    COOLING (nominal) max. kW 2.0
    HEATING (nominal) max. kW 4.6

    Very good, you see your unit heat output is double the cooling out put!.

    My 3.5kw unit for cooling only puts upto 4.2kw of heat at max rating.

    So I guess a unit from LG that is 2.5 kw heating is only going to output approx 3.2kw of heat max rating

    mitsi has very good efficiency!
    If you found my post helpful, please remember to press the THANKS button! --->
  • Cardew
    Cardew Posts: 29,040 Forumite
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    I think you missed Steve's irony!!!

    and perhaps mine about a water to air ASHP - or should that be WSHP!
  • stevehead
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    Steve I disagree especially in your case as your system is coping so well.
    Tongue embedded firmly in cheek Rich
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