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ASHP choice help - 55C flow temperature required

Wintergreen · 2023-04-07T08:05:58Z

Hello all. We live in a large Victorian house in North London. I am looking to get off gas and stabilise our energy costs more generally. ASHPs seem the best option. I am about to put in 10MW of solar PV (expected annual yield 7MW). There is a large main space, double-height, running the full length of the house. This was thermally upgraded 10 years ago, and fitted with UFH under an insulated floating engineered wood floor. The emitter value of the floor is on the low side, and it was specified at 55 degrees (although the flow rate indicates a speed of around 0.5 m/s). Heat loss for the room is probably 7kW and -2 degrees ambient, and the UFH at 55 degrees produces around 6kW. There’s also a wood burning stove on the mezzanine living room. We tend only to heat the room to 18 degrees, and burn some wood in the evenings to keep it warm in the living room. The rest of the house is in practice heated to a very modest degree, but the total heat loss is 43kW. Most of the rooms have large cast iron radiators. We have some towel rails. I have two questions that I would really appreciate help with. First, the 55 degree specification is quite important. If we go much lower than that the emitter value of the UFH drops significantly, so it seems critical to identify the ASHP with the best COP at this flow temperature, for various ambient temperatures. I have found this information very hard to come by. Samsung give excellent technical specs, but most of the rest seem content only to offer broad SCOPs, or figures which don’t take account of the defrosting cycle. At 55 degrees flow, the 14kW Samsung EHS Mono HT has an integrated COP of 2.16, 2.45 and 3.35 for ambient -2, 2 and 7 respectively. I found those figures are a bit disappointing, but perhaps I’ve just got used to reading overblown sales pitch. Are there any ASHPs which do materially better than this? The second question relates to DHW. Do I upsize an external ASHP to heat up a cylinder, or do I use a dedicated internal heat pump cylinder? There is an uninsulated loft on the top floor which will rarely get colder than, say, 14 degrees. I could put something like the Vaillant Isostor in there. It would take a bit of heat from the building, but produce 270L of 50 degree water for around 2.2kWh per day. My instinct is that that will be more economical than using an external heat pump working at ambient temps, but I’ve struggled to find figures. Depending on the best DWH solution it might be best to get a pair of smaller pumps, one attached only to the UFH, the other doing DHW and the radiators. Any thoughts regarding the above would be hugely appreciated. I am in awe of the rigour and expertise with which issues like these have been discussed on these forums.

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Air Source Heat Pumps (ASHPs)

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Filipe

(@filipe)

Trusted Member Member

Joined: 3 years ago

Posts: 65

10/04/2023 8:59 pm

Posted by: @wintergreen

↑

I will need a device that can output 55 degrees at a market-leading COP.

You could add additional emitters to that area so that a lower flow temperature would work.

Phil

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Kev M

(@kev-m)

Noble Member Member

Joined: 5 years ago

Posts: 1276

11/04/2023 7:41 am

@Wintergreen,

apologies if others have covered...

is your total heat loss really 43kW? That's huge; the biggest domestic ASHP I've seen is 16kW.

55 degrees is high for any ASHP; most will do it but the COP isn't going to be a lot better than 2, even with the ones touted as HT. I think (don't know for sure) that while these units can do high temperatures, the COP is not necessarily better than 'normal' units. My ASHP (14kW Ecodan) will output 55 degrees all day at -2 but the COP will be around 2. One thing you rightly pointed out is to be careful with defrosts, which will lose you at least 10-15% of capacity below 2-3 degrees ambient. Also be careful with the rated outputs; they are often lower than the badged value at low ambients.

Remember an ASHP can only output at one LWT at a time. If the upstairs doesn't need 55 degrees LWT then you might be better with one for the downstairs and another for everything else. If your heat loss really is 43kW you're going to need multiple (or commercial grade) ASHPs anyway.

Finally, I don't know much about ufh but you do have to make sure it can emit enough heat at the flow rates an ASHP needs, which are normally higher than gas/oil systems.

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Marzipan71

(@marzipan71)

Estimable Member Member

Joined: 3 years ago

Posts: 120

14/04/2023 11:51 am

Hi – I have a 16kW Daikin HT ASHP – system details below – which we’ve been running since August 2020. We have a combination of UFH and radiators. The system we have can only be used with a basic Daikin controller which gives no information on consumption or COP. I’ve managed by a process of elimination to work out consumption data and to date the heating consumption (not incl. DHW) is as below. Heating has been off since the start of April, and I’ve used this past winter’s figures for this next winter:

During 2021 and early 2022 we were using the system as if it was a gas boiler, doing everything wrong – room stats and zoning, running at high temperatures (there’s a blending valve on the UFH), no weather comp. It coped and was happy producing LWTs up to 70 or so for the radiators (!) but was obviously very thirsty in terms of consumption. This past winter with the help of the folks on this forum I’ve corrected these errors and obviously seen a big difference in consumption. I’m not actually running the HT capabilities of the system now but the house is quite comfortable; we possibly didn’t need a HT system but that’s what was recommended and we are just trying to run it as efficiently as possible.

I realise this doesn’t help much with the topic specifically but just wanted to add some insight into my Daikin HT journey particularly in respect of consumption. I’m more than happy to answer any queries about our experiences with the Daikin HT system.

System details:

An outside unit - Daikin ERRQ016AAV1
An inside unit - Daikin EKHBRD016ADV17
A 500L thermal store - Daikin EKHWP500PB (no immersion heater installed)
A very basic Daikin controller EKRUAHTB
Manufacturer stated COP’s are 2.11 at -7, 3.01 at 2, 4.26 at 7, and 4.67 at 12C
We have a 6.1kWp PV installation with 20 SW-facing panels and a SolarEdge SE6000H 6kw inverter, which produces around 8MWh pa, 3MWh of which goes back to the grid

👍

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Wintergreen

(@wintergreen)

Active Member Member

Joined: 3 years ago

Posts: 11

Topic starter 22/04/2023 10:29 am

Many thanks for the helpful responses.

@marzipan71 I'm still struggling to get figures out of Daikan. It's mystifying to me that suppliers aren't more transparant as to the key performance parameters of their products. Do you know what the LWT is for those COP figures?

@kev-m apologies - that heat loss figure was not quite right. The total heat loss in the rooms we use (!) is 18,000. But 5 of those are bedrooms, which are in practice scarcely heated, and even then only for a short time in the morning and evening.

For anyone interested, this is the result of my researches to date:

I am slightly surprised that there isn't a closer relationship between these COPs and the MCS SCOP figures, but of course they only cover a small part of the device characterisitics.

My current plan (it changes daily) is to go with a pair of Arotherm Plus 7kW pumps - one dedicated to the UFH and the other for rads around the house. I will post separately about my slightly eccentric DHW plans, because it would be great to get some input on those.

Stiebel Eltron WPLA07 7kW ASHP. 26 x 400W PV. 10kW Sunsynk 3-phase inverter. 6 x 4.8kWh Pylontech US5000C batteries.

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Marzipan71

(@marzipan71)

Estimable Member Member

Joined: 3 years ago

Posts: 120

22/04/2023 10:47 am

@wintergreen here's what I have - apologies if it's too small to read. I was left two sheets by the installers - with completely different COP figures for the same pieces of kit! First file has the figures I quoted above. Agree about difficulty getting data - I have no way of monitoring COP via my controller as other users are able to do with their kit.