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Heating and cooling: Soft starter for ASHP

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    In my seemingly eternal search for an air to water heat pump to satisfy our fairly low heating and hot water demand, I have come across a company here in Sweden that sell a split system which has a Mitsubishi outside unit and a hydro box of their own design and manufacture.

    The unit ticks most of the boxes for me, including being from a big name manufacturer so hopefully will be reliable in the future.

    The only problem is that the smallest unit they do is 7.5kW, it's an invertor so goes down to a minimum output of 2kW so whilst there shouldn't be a problem with oversizing it's a single phase unit with a rated maximum start current of 19A and a maximum running current of 13A and we are on a 3 phase supply which is fused at 16A on each phase.

    I've asked them if they can supply the unit with a 5kW outside unit which would have a start current below 16A but just got a flippant reply saying that last winter was very mild! *rolls eyes*

    If the 7.5kW unit is my only option does anyone know if it is possible to use a soft starter to bring the start current down from 19A to under 16A or will this conflict with any of the electronics in the heat pump? (I have of course asked the company but have so far not had a response.)

    I can move to a 25A supply but there is a substantial increase in our standing charge so I would obviously rather not.
    • CommentAuthormike7
    • CommentTimeNov 21st 2015 edited
     
    I suspect that the 16A rating of the fuses on your supply relates to a continuous current, and that they will not will not trip at this modest overload for a time easily long enough to get the HP up to speed.
    • CommentAuthorsnyggapa
    • CommentTimeNov 21st 2015
     
    also what type of fuse is it - a modern "breaker" or an old school wire that melts?

    The breakers I believe you can get different types (at least in the UK but assume Sweden is the same) - types C and D blow slower than types B to allow startup currents etc

    http://www.neweysonline.co.uk/MCBs/Static.raction
  4.  
    Posted By: Chris P BaconIf the 7.5kW unit is my only option does anyone know if it is possible to use a soft starter to bring the start current down from 19A to under 16A or will this conflict with any of the electronics in the heat pump? (I have of course asked the company but have so far not had a response.)


    I doubt soft-start capacitors work for inverter-driven compressors. I have a regular AC induction-driven compressor - the run current is around 14A but the start current (also known as the "locked rotor amps") is around 90A. It is on a 30A breaker, but this doesn't trip as the inrush peak is very short, though the lights do flicker. For what it's worth, we're on a 200A supply at 2x120V, but the size of the supply doesn't make a difference to the flicker, unfortunately.

    Paul in Montreal.
  5.  
    Posted By: snyggapaalso what type of fuse is it - a modern "breaker" or an old school wire that melts?
    The fuses are the old school wire type even though they are in a modern smart meter box.

    It's encouraging to hear that they may well sustain the start current of 19A.

    Will just have to wait and see what sort of response I get from the company now.

    Thanks for all the input.
    • CommentAuthorslidersx200
    • CommentTimeNov 22nd 2015 edited
     
    NIE (Northern Ireland Electricity) were demanding that I pay to upgrade a local transformer as part of our new connection fee. This was because I told them we were having an ASHP and they claimed this would affect neighbours on the same line.

    I challenged them and sent them all the documentation for the Samsung monobloc unit I was looking at and they then wrote to say that as long as I used that heat pump the transformer would be fine as it was.

    I'd need to dig out the emails to find the specific criteria they were interested in, but if all you want is a unit with a good soft start I'd recommend looking at the Samsung range.
    • CommentAuthorDantenz
    • CommentTimeNov 23rd 2015
     
    Yes DNO's are very sensitive to the electrical demands a heat pump places on the network as they have a duty to prevent electrical disturbance & flicker voltage to neighbouring properties.
  8.  
    Thanks. I'm reluctant to install a mono block as I am concerned about it freezing up.

    If I take today as an example. It has been around -3ºC for the last 24hrs. The UFH was on when I came downstairs at 5.15am but then didn't run again all day as it was a bright clear day and solar gain lifted the indoor temp. from 20.5ºC to peak at 22.4ºC around 2pm and the house is still at 21.9ºC now at 7.30pm. So there will probably be a gap of 24 hrs or more before the UFH runs again. If we were away from home and not using hot water then a heat pump wouldn't start in that time and even today when we are home there would have been a period of about 11 hours when no hot water was used but it was below freezing all the time.

    If I were to have a monoblock then it would mean running a glycol mix and a plate heat exchanger.

    Although I do remember reading of a monoblock which I think was the Samsung which doesn't leave any water in the outdoor unit when it is not running so eliminating the freeze damage risk do you know anything about that?

    But probably the best option for me is a split system where there is absolutely no freeze risk.

    There is no problem with the grid here in regards to heat pumps all my neighbours have heat pumps and we never have any flicker. All houses here have a 3 phase supply.

    The issue for me is that the standing charge increases with the fuse size so moving from 16A to 20A increases the standing charge by 1335kr ~ £100 a year or 16A to 25A increases by 2940kr ~ £220 a year and as the heat pump will only save me about 6000kr. a year then I want to avoid increasing the fuse size if at all possible.
    •  
      CommentAuthordjh
    • CommentTimeNov 23rd 2015
     
    If you know how long the starting current of your preferred ASHP lasts, you could simulate it with a switch and a power resistor easily enough. I take it you have spare fuses?
    • CommentAuthorringi
    • CommentTimeNov 23rd 2015 edited
     
    The issue will be the main fuse, that you can not replace yourself if it blows. Also if you overload a fuse once for a short time, it will be OK, but overload it often for the same time, and it may blow.

    So trying to simulate it with a switch etc may not tell you that much.
    • CommentAuthorcjard
    • CommentTimeNov 23rd 2015
     
    Is there a 3 phase motor option?
  12.  
    Posted By: cjardIs there a 3 phase motor option?
    None of the big brand names do a small capacity 3 phase heat pump at a reasonable price unfortunately, the cheapest is about double what I want to pay unless I go for a Chinese one.
  13.  
    Posted By: cjardIs there a 3 phase motor option?


    If it's inverter driven, then the starting current is much less than a single-phase AC induction motor, where the locked rotor current can be up to 10x the run current. The OP said the starting current is only 19A - this will not be a problem for a circuit breaker. I would imagine the supply's main fuse could be changed to a slow-blo one as 16A is really not a lot. As I said earlier, the start current for my GSHP is around 90A - and this doesn't trip a 30A breaker (though the main supply's fuse is 200A).

    Paul in Montreal.
    • CommentAuthorCWatters
    • CommentTimeNov 25th 2015
     
    Do Mitsubishi make a three phase version? Perhaps not enough demand?
  15.  
    Posted By: CWattersDo Mitsubishi make a three phase version? Perhaps not enough demand?
    What Mitsubishi sell here is single phase for the outdoor unit and 3 phase for the hydrobox. I still haven't had a response from this other company who make their own hydrobox to pair with the Mitsubishi outdoor unit so I'm still none the wiser as to its spec or whether they say it can be used with a 16A fused supply.

    To be honest if they are this bad at answering a simple pre-sale question it doesn't bode well for what they may be like if there are guarantee issues after purchase.

    I will send them another mail in the morning and if I don't get a prompt reply I will forget about them.

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