replace large outdo...
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replace large outdoor unit with smaller one

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Trusted Member Member
204 kWhs
Joined: 2 years ago
Posts: 24
Topic starter  

I have a complex issue with the heat pump that was fitted to my house - far too big and unaffordable to run. There are many reasons for this but the main one is that I was not given a choice. The six heat pump installers who have visited since disengaging from the cashmachine that did the intitial work, have all tried to make decisions for me - they know best and they quote £7k, £4k and £1.2k, without showing any proof that they know what they are talking about. Most do not seem to have any credentials regarding the environment etc. and two out the six were intent of selling biofuels or lpg systems.
I am reasonably convinced that a heat pump is the right solution for my house, especially as there is one fitted! I cannot solve all its problems in one post so I have reduced my question to this:
If I disconnect the 16KW Samsung out door unit and replace it with a 5Kw unit, will the rest of the system accept the new device without needing an expensive installer?

Illustrious Member Moderator
14001 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4203

Do you have any heat loss calculations for your home?


Trusted Member Member
204 kWhs
Joined: 2 years ago
Posts: 24
Topic starter  

I have many spreadsheets full of heat loss data, all diffferent, all dependent on the guess work of experts, none of which are trustworthy or beleaveable. I have 55sq mtrs of living space that I want to heat with ufh in most of the space that requires heat.  I have followed discussionsand the MCS spreadsheet is an excuse to oversize the hp and laugh all the way to the bank.

This one makes sense and suggests that 5kw would be sufficient.


Noble Member Contributor
3206 kWhs
Joined: 2 years ago
Posts: 383

55m2 is a very small heated area, a 5kW unit would provide 90W/m2, which is at the higher end of the 65-85W/m2 'typical installed heating load' for a home, especially underfloor. You have spent a lot more time researching your requirements than any of the professionals you've engaged with, and you know best how you want to heat the property, what heating profile (24/7, timed, etc.) and what works for you.

I asked my MCS installer to drop from their calculated 13kW ASHP (Heat Engineer software) to my calculated 10kW. My calculations took in some diversity - we don't heat all the rooms in the house all the time to 22 or 23degC. This winter has validated my 10kW ASHP as 'right sized' not oversized. The practical difference meaning the ASHP can modulate down to 20% without cycling on and off on low load.

It coped, just, during the very cold two weeks before Christmas, but that was due to spending so much time in defrost, which all ASHPs are similarly afflicted with. I would sooner have the system a little slow to warm up and maybe struggle a little for 3-4 weeks a year than be oversized for the remaining 18-20 weeks of a typical UK heating season. I can always put the log burner on if I need to boost the downstairs quickly, and we prefer cooler bedrooms upstairs.

In my mind, oversizing plant is just as bad as under-sizing. Others on here have a view which may differ. It certainly costs more to run oversized heating plant, whether gas, oil, ASHP or whatever. It's inefficient too. In my day job I calculate, size and specify custom build HVAC equipment for projects that cost several millions of pounds to purchase, so maybe I'm a little more sensitive to adding contingencies and factors of ignorance onto equipment capacities than most. 😉


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