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Heating and cooling: ASHP electric supply

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  1.  
  1.  
    What power supply do I need to provide to my kingspan aero cylinder 3kw immersion and ecodan 8kw ASHP?
    • CommentAuthortony
    • CommentTimeJan 19th 2015
     
    Keep them separate, 16A for the immersion and 40A for the pump, they have high start up current so special slow blow trip may be needed.
  3.  
    I'm sure I read 30a?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 19th 2015
     
    What does the ASHP say
    RTFM
  5.  
    W=VxA.

    8000W = 230V x 34.78A, therefore more than 30A
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 19th 2015
     
    Isn't it 8000 W thermal input, so with a CoP of 3.5 that you be 2285 W running load
    2285 = 230V x 10A

    The starting load is to do with the 'stickiness' of the compressor than the size of the unit. I have seen my fridge pull 2kW for a few seconds.
  7.  
    Hmmm... Yes, ST, I think you must be right, but I seem to remember that our 6kW GSHP at the Energy Centre needed a 40A motor-rated CB. It must just be the starting current.
  8.  
    For reference, our 11kW GSHP has 40A breaker on a 240V supply, despite only running with a continuous load of around 3kW. It's the starting current of a single-phase AC induction motor that's the killer (around 96A if I recall correctly for ours). Most breakers will not react fast enough to trip, but if you use a "regular" fuse, then you would need a slow-blow type. Even with a 200A mains "entry", the lights still dim for a fraction of a second when the compressor starts. 3-phase motors will start without any assistance from a capacitor and so do not dim the lights. Inverter-driven DC compressors are similarly "friendly" to your power supply - particularly important if you're trying to run from PV plus an inverter.

    Paul in Montreal.
  9.  
    Having spoken to mitsubishi direct they have said:

    25amp mcb for the outdoor unit
    13amp mcb for the immersion
    16amp mcb for the flow controller (not sure why more than the immersion)

    I just want to check that all of those can be run with twin and earth 2.5mm cable? My electrician who doesn't deal with ASHP said it would suffice as you divide the app by 10 to get rough cable size. Thoughts?
  10.  
    • CommentAuthorringi
    • CommentTimeFeb 2nd 2015
     
    Victorianeco

    Get a new electrician! That is not how you work out cable sizes.

    There are lots of calcs to to done, include taking into accounted how the cable is mounted. Just putting a cable under plaster instead of clipping it to a wall, reduces it's rating.
  12.  
    Posted By: ringiGet a new electrician!
    God yes! If a sparky doesn't want to do the sums at least they ought to oversize and then add some.
    • CommentAuthorCWatters
    • CommentTimeFeb 3rd 2015
     
    Very important NOT to undersize the wire to motors (heat pumps, pressure washers, woodworking tools etc). If you loose voltage in the wire the motor may draw extra current in order to maintain the same power output. That can significantly reduce the life of the motor. Might be worth oversizing the wire even if that's not strictly necessary to meet the regulations.
  14.  
    I intend to use 6mm cable to the heat pump, and then supply the indoor unit with 2.5mm cable

    I assume a 32A MCB along with a 32A Rotating Isolator switch is sufficient?

    Thanks
  15.  
    Can anyone advise? I seem to get conflicting info from mitsubishi
  16.  
    Mitsubishi have said I should use a 25a mcb, what would happen if I used 32a mcb but with a 25a isolation switch?

    For the indoor unit they have also said a 3 pole 16a isolator, can I use a 20a DP switch?
    • CommentAuthortony
    • CommentTimeFeb 24th 2015
     
    Might need a special type of mcb that is slow blow due to the high startup current, type ?

    20 A dp switch should be OK inside, I normally plug them in to a standard 13 A socket.
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 24th 2015
     
    Posted By: VictorianecoMitsubishi have said I should use a 25a mcb, what would happen if I used 32a mcb but with a 25a isolation switch?
    If the manufacturer says to use a 25A breaker then that is what you use, no advantage in using a larger one. Also it would depend where you put the 20A (or any size) isolator. There are specific rules laid down for wiring in the 17th Edition, don't try and think you can change them to suit what you have available or to save money, it don't work like that.
  19.  
    MCBs are size to protect the cable from over current not the appliance
    cable current carrying capacity depend on installation specifics
    Get an electrician to take a look at it.
  20.  
    I've asked two electricians in work, they differ to what mitsubishi have said. But two people within mitsubishi have said different things too.

    When you say you plug them in to a standard 13amp socket is that for the immersion heater or the flow control unit?

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