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Real world advice on ASHP installation - 2 18kW ASHP's recommended

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Air Source Heat Pumps (ASHPs)

Last Post by Alan 2 years ago

27 Posts

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Derek M

(@derek-m)

Illustrious Member Member

Joined: 5 years ago

Posts: 4425

23/01/2024 7:33 pm

Posted by: @mrj

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Hi,

I would really appreciate the advice, if anyone has the time or inclination to humour an extreme novice in renewable energy.

We're about to undergo a full reno of a late 70's property, which is just shy of 300sqm in the South West.

As part of the works we're very seriously considering switching from gas to ASHP and have appointed a recommended engineer to design the system (and ultimately use an experienced installer too). The system would be for both heating and water.

My concern is less of whether we are considering enough capacity (from the looks of it, two 18kW units is on the very large side) but more of real world running costs.

The reno works will include insulating the ground floor and installation of a slab with wet UFH, replacement of all windows, doors and garage doors (currently visible air gaps through all, so extremely leaky) and upgradings the rads upstairs and UFh to the bathrooms. We're not installing cavity wall insulation as the property is prone to driving rain, so there is too much concern about water ingress, damp and cold spots.

I don't have the heat loss calcs to hand, (I have asked for them so hopefully I will receive in a few days), but recall 20kW as the output of his calcs.

Am I crazy for considering ASHP, is this far too little information for anyone to begin to pass judgment on? Any advice would be greatly appreciated.

Thanks,

J

I fully agree with all the previous poster's comments.

The starting point should be to insulate as much as possible to the highest standard, since it should be a win - win. No need for a second heat pump and much lower running costs.

From my limited knowledge of UFH, I would suggest keeping the pipe to pipe distance as small as possible, thereby providing greater heating surface, so able to run at lower Leaving Water Temperature (LWT), therefore higher efficiency and lower running costs.

Any radiators should have the highest output capacity possible, whilst maintaining the overall balance between rooms, so that they all require approximately the same LWT to keep the various rooms at their desired temperature. In rooms where space for radiators may be limited, there are fan assisted radiators available, which although more expensive, could mean the whole system could operate at a LWT several degrees lower.

If you already have a mains gas supply, you could consider a hybrid system, with the heat pump performing the bulk of the heating, with the gas boiler being a back-up for heat pump failure and/or extreme cold weather conditions.

There are also hot water tanks available which include a small ASHP. The larger heat pump would therefore benefit from not having to stop heating the home to spend time heating the DHW, and would also not need to run for much of the Summer when hot water production is the main job of any heating system.

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Alan

(@alan22)

Eminent Member Member

Joined: 2 years ago

Posts: 29

23/01/2024 7:55 pm

Posted by: @bontwoody

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@alan22 Just what I was thinking. I should get 100mm of kingspan over most of it and 50mm at the studs. Thanks

150mm + PIR will one day be considered standard, it's worth it, but it has to be installed carefully and thoughtfully, if you have window and door reveals consider Superquilt, the closer you can get to passive the less heat you need to generate to heat the space.

I have seen new builds by reputable construction companies that install the minimum they can get away with, and the installation can be shocking, people don't look at wall cavities and loft space when buying a house, it's a bit nuts really builders put a lot of effort into 'finish' over what is essentially out of date insulation for a non cheap gas future.

👍

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Alan

(@alan22)

Eminent Member Member

Joined: 2 years ago

Posts: 29

23/01/2024 8:04 pm

James, I'm pretty clueless with this stuff yet but I have 300sqm in freezing Scotland and the quote I got was for one 11kw ecodan, the quote came from a reputable company and the guy really knew his stuff, one difference perhaps was my openness to any size and number of radiators.

👍

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