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Green Building Bible, Fourth Edition
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Heating and cooling: What size of ASHP would be needed

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    Posted By: WillInAberdeenThe compressors in the outdoors units are nearly all inverter driven these days so don't have huge startup surges. You can spread this current across 3 phases, your example load would be 6A per phase.

    6A is the sort of power demand that would be manageable (anything up to 10 would probably be OK at a pinch)


    Posted By: owlmanI'm a bit sceptical Peter, my nature I guess, but;-
    You're; ( mid-winter, I presume ), currently heating 2 tons of water to 90-95C on a daily basis.
    You're throttling that down to 60C to supply on demand, to two dwellings, for both CH and DHW.
    You want to leave the current rads. and supply network in place.
    Even accounting for supply and storage losses, some of which are NOT going to disappear with a HP. I can see a HP struggling to replace all that "on demand comfort", even with a lower temperature, longer running regime; call me old fashioned.
    I have visions of you constantly having to boost the store temp with immersion heating in order to end up with anything other than lukewarm rads.

    Plan A would be to dump the TS and use the ASHP as an on demand heat source. This would save the TS losses.

    If push comes to shove the DHW could be left as it is now in the summer i.e. on over night electricity. This comes out of a different power budget (separate meter etc.) but if possible the COP of 3 or so would be useful but not the end of the world if DHW stayed on resistance immersion.

    needing to replace the rads would add significantly to the overall cost of the project, pushing at the feasibility of it all.
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    Have you looked at the costs of replacement rads?

    I'm looking at replacing some single panel radiators with modern double-panel-plus-fins rads that will be the same width and height, so no changes to the pipework. Just uncouple the old rad and couple up the new ones, to give 2.5x the capacity, which is happily what is needed to reduce flow temperature to suit the ASHP. Expecting to do this myself. The purchase cost of the new rads, should be well under £1k total, so not a humongous addition to the cost of the ASHP.

    If the existing rads are decades old, seems to me that when I revamp the CH is the right time to replace them anyway, probably rusty and full of gunge inside.

    Compare capacity here:

    http://mcssco.wpengine.com/wp-content/uploads/2019/08/Supplementary-tables-of-heat-emitter-outputs-1.pdf
  3.  
    The problem with upgrading the rads is that all the rads are of the double finned type plumbed in with top in, bottom opposite out with iron pipe (all welded joints) so like for like would be an easy change but change of dimension - a real pain (in the wallet)
    Even going to triple panel type would not be easy as the distance from the wall to the connection point is further out from the wall and iron pipe doesn't bend to pull the connection further out.

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