Posted by: @transparent

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That's it!

Emitters on a roll.

Now why didn't I think of that? 😜 

Available (and indeed standard) in Italy I am told (by an Italian).  Radiators are assembled onsite from small, standard, sections that plug together (and presumably seal!) into the length required.

@jamespa It is all about design choices. In my view heating at extreme low end temperatures is a small minority of overall heating which might not even occur some years. I choice to match a more efficient, smaller heat pump to 15C OAT to eliminate cycling and provide a wood-burner in the lounge coupled with a thermostat. At low temperatures, the wood-burner would be activated and the large lounge load from HP would be switched off by the thermostat, allowing the HP to concentrate its heat on the rest of the house.

The major mistake I made is mixing radiators in bedrooms with UFH which complicates things enormously. In hindsight I should have implemented UFH throughout regardless, whereby WC would be a breeze. There is still the problem at low temperatures due to HP modulation range but I solved that one. I am a believer in designing from the 15C OAT end. I also believe in 2 heat pumps, one for UFH and one for DHW.

Posted by: @jamespa

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Im cant see why two compressors in the same heat pump falls foul of the rules.

Because there isn't one on the MCS Approved list.

This list can be bizarre and overly pedantic at times.
For example, installations of rooftop PV panels must use 65mm screws to attach the brackets to rafters.

I fell foul of that because there's 50mm of insulation and a 22mm counter-batten on top of my rafters!
When I handed the installer a box of 140mm stainless steel screws, he said that was 'non-compliant'.

We arrived at a compromise which allowed him to produce the MCS certification...
... and you'll appreciate why I won't explain more here on an open forum 😉 

I like the idea of a double-compressor heat pump.

But as I'm based not far from the workshops where Newcomen and Trevithick pioneered Cornish steam-engine designs, I favour a more innovative approach.

There should be two impellers on the same shaft, whereby the energy still remaining from the first stage can be fed to the second.

Ie, this mirrors the compound engine, whereby dual pressure expansion cylinders greatly increased the efficiency of early steam engines.

Posted by: @heatgeek

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@jamespa It is all about design choices. In my view heating at extreme low end temperatures is a small minority of overall heating which might not even occur some years. I choice to match a more efficient, smaller heat pump to 15C OAT to eliminate cycling and provide a wood-burner in the lounge coupled with a thermostat. At low temperatures, the wood-burner would be activated and the large lounge load from HP would be switched off by the thermostat, allowing the HP to concentrate its heat on the rest of the house.

As it happens I wouldn't disagree, but if it can in fact eliminate cycling then it wont meet the requirements of the BUS grant to be capable of heating the house 100%, at least not with a 3:1 modulation ratio.

@jamespa I agree. That is where the design choices come in. You have to design within the constraints imposed on you, realistic or not.

Posted by: @transparent

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Posted by: @jamespa

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Im cant see why two compressors in the same heat pump falls foul of the rules.

Because there isn't one on the MCS Approved list.

There is, the Mitsubishi R290 8kW has a 6kW inverter and a 2kW fixed in the outside unit. The 2kW is only used when more output than the 6kW can produce is needed. As a result it can modulate to less that 25% of its badged max output. 

As per Robs comment @robs

this is a R290 model which will operate down to 2kwh output….

ironically R290s are being hailed by some as the high temperature heat pump but here is one which modulates down to 2kw minimum output. 

@sunandair @robs 

Makes a lot of sense to me.  I wonder how much adding a second fixed output compressor adds to cost,  I would guess very little.

@jamespa

The 6kW is £4666 and 8kW £5295 on midsummer's, so no not much. 

@jamespa Quoted performance figures @35C flow, -7C OAT = 8.2kW and @25C flow, 15C OAT = 2.1kW. Quite something. Other output models as well.

Posted by: @heatgeek

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@jamespa Quoted performance figures @35C flow, -7C OAT = 8.2kW and @25C flow, 15C OAT = 2.1kW. Quite something. Other output models as well.

Nice!

@sunandair This new Ecodan validates your argument about following WC closely for maximum efficiency. You can now set your design flow and radiator size at 15C and turn down the top end accordingly. However, the WC facility is linear which is fine for UFH. It appears that there are no radiator compatible curves. There is a mid-point setting so you can produce a simple triangle. Nevertheless, it will do me fine as it meets the 25% criterion and all-UFH is the only way to go if you are rational. As @jamespa rightly said in my view, if you want air to air heating, use fan coils, not radiators.