@mattc unless you are very set on the mixergy way of doing things, a good conventional unvented cylinder with a high flow rate large coil, will get the job done . Thats what a large % have. And is unarguably cheaper . the efficiency is a swings and roundabouts debate can go either way thing, I'm sure it has its uses in the right application and there are some here who make good use of them.

you won't be left with something that won't work if you pick a good UVC. Coil size, flow rate supported by that coil (at sensible head loss) are the key numbers.

@derek-m i have phe in my daikin. It seems to be  always 0.1deg higher then lwt. Is that the efficiency issue? I'd imagine  most daikin Have them as its readings are on the unit interface.

If the PHE to which you refer is actually inside the Daikin equipment then it will be the Condenser, which is an integral part of any A2W heat pump, and should not be confused with externally installed PHE's, which would be additional to the internal one.

The internal PHE has the hot refrigerant gas on one side and the water on the other side, and I would hope has been designed for optimal thermal energy transfer in the most efficient manner.

@derek-m ahh OK. Yeah it's the condenser so luckily by the sounds of things.

Posted by: @mattc

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so it looks like we're good to go.

I would not recommend you go with Greener Living. They do a lot of installs and are keen to get new business, but they are harder to chase up once you have paid up and are not seemingly interested in getting a system that works optimally. Several members have posted similar issues with them.

Posted by: @mike-h

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I would not recommend you go with Greener Living. They do a lot of installs and are keen to get new business, but they are harder to chase up once you have paid up and are not seemingly interested in getting a system that works optimally. Several members have posted similar issues with them.

Thanks. I had pretty much decided to avoid them anyway, simply based on their sales tactics.  "We've got some units from a large job that got cancelled at the last minute. Let me phone the warehouse and see if we could let you have one at a discount, but they'll all be gone by Monday so you need to decide today."  I'm old enough to have heard this a few times before from everyone from double-glazing to used car salesmen, so I wasn't tempted.

Hi all

It's taken a while but I've now had 3 surveys (only two of which I had to pay for) and here are the results in brief:

Supplier 1
Total heat loss: 10350 W
ASHP: 12kW Midea
Recommended upgrading 4 radiators and all the 10mm microbore (present on 5 rads) to 15mm, though the total cost below excludes this.
Cost: £10400 after BUS grant (up from £6675 pre-survey)

Supplier 2
Total heat loss: 8150 W
ASHP: 11.2kW Mitsubishi
Recommended upgrading 9 radiators
Cost: £7000 after BUS grant (as quoted pre-survey)

Supplier 3
Total heat loss: 13550 W
ASHP: 16kW LG
Recommended upgrading 6 radiators
Cost: £9100 after BUS grant (up from £5500 pre-survey, which excluded the HW cylinder)

The final prices above all include the recommended radiator upgrades and a 200+ ltr HP cylinder (I've moved away from the Mixergy option).  All have said they are open to reducing the number of radiators to reduce the costs a little.  Suppliers 1 and 3 are both wanting a deposit and most of the rest up front.  Supplier 2 is payment in full on day 2 of installation.

I think the 16kW option is likely oversized and I'm not sure why their heat loss calculations are so much higher than the other two.  Most days, our existing gas boiler tends to use around 10kWh in the first hour after it comes on in the morning and then drops down to less than half this through the rest of the day, depending on the weather.  This is with a 50C flow temp and the main stat set at 20.

I'm sure either of the 12kW units would be plenty and my only concern now is how they will perform in milder weather or when they are only being used for HW in the summer.  We're on a very cheap rate for 5 hours at night, which we use to charge the EV, so I'm assuming we'll use this to heat a tank full of water every night, which should easily last us for the day most of the time.

Just to correct an error in an earlier post, the floor area of the property (upstairs and down) is about 170 sq-m.

Any thoughts or suggestions welcome.     

@mattc

 Has the price of a heat pump system increased dramatically recently?

If you find the spec sheet of a 12kW Midea it’s only about 10kW at -2C (design temp for a lot of south England) whereas the 11kW Mitsubishi could actually be 11kW (someone on here may have both). Saving money on not replacing a couple of radiators isn’t a good saving since you’ll be either cold in those rooms or paying more on each and every bill.

Are you in a colder part of the country?

@derek-m I don't know. I've never bought one before.

Posted by: @judith

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Are you in a colder part of the country?

Not particularly. Cambridgeshire

@judith 

The Midea spec sheet quotes 11kW output at -2/50

The Mitsubishi claims 11.2kW down to -7/45