@guydeb Yes. No problem running just UFH from an ashp. The issue I think would be the very low heat loss in a 40m2 new extension. How sure are you that you would need two heat pumps for the whole house? Have you had more than one heat loss calculation? It would be much better to try and improve insulation levels so that you could just use one heat pump. 

My house is the same era with extentions about 120 m2 and gets by happily with a 5Kw heat pump. 

hi @bontwoody - waiting on another quote to confirm the original 2 ashp configuration. We burn c £2.5K of heating oil each year, both work from home, 2 teens, 2 bathrooms, 4 beds, a mixture of old stone floors and new insulated wooden ones, but we are double glazed throughout. 

Are you thinking 4kwh might be overspec'd?

Love to hear from anyone who has done just an ASHP for their UFH

@guydeb - a few extra pointers to consider, which I'll make by way of questions:

1: Is the UFH for the extension already part of your plans?

2: Does that mean you are also considering upgrading the rest of the house to UFH in phases?

3: How much of the extension and the UFH work is hands-on DIY?
Or are you only working with contractors?

4: Is that extension floor suspended, or a concrete pad?
These have different heat characteristics, which might be relevant to your future plans.

I have UFH which presently operates from a thermal store.
There's no heat-pump here at the moment.
The Thermal store runs from a roof-mounted solar-thermal array, and/or a gas boiler.

I have mixed together three different heating outputs from my thermal store's UFH port

a: Pipes with aluminium-heat-spreader plates in suspended flooring (engineered oak or tiles above)

b: Pipe embedded in a concrete slab in one large extension (2-storey-high)

c: small 'radiator' in the fresh-air output of my MVHR (heat-exchanger) unit in the attic. It adds heat to the circulating air.

No, I don't know anyone else who's ever attempted such a mix...
... but I've learned a lot from doing it, and can offer photos as required.

At this stage, feel free to think widely and let's see where that takes us!

Ours is just UFH with ASHP. UFH can perform very different depending how it's assembled.

You need to heat loss calculation for the extension to see what size heat pump you need. You will.most likely need a small one. 

@guydeb I'm just in the middle of constructing a 40m2 extention myself and looking at the amount of insulation required now I expect the heat loss to be well under 1kW.

By way of comparison my last 130m2 bungalow which was a self build to the older regulations so had less insulation had a5kW heat pump and UFH and was never put under any strain although I didn't have good monitoring then like I do now so I can't give you exact figures. 

Hi @transparent - yes UFH already part of plans

yes considering upgrading over time, if budgets permit

Only working with contractors but keen to do my own research

Normal pipes, screed and insulation - Nu Heat suggesting 14mm Fastflo^® heating tube (shouldn't this be 16mm??)

Heat Loss Estimate: ~45 W/m²

Have had the following ASHP recs: 

Mitsubishi Ecodan 2.5kW or 5kW
Samsung EHS R32 Monobloc 5KW

Both internal circulation pumps can apparently handle the pumping needed without an additional pump 

Just add extension to existing heating system, why spend multiple thousands to achieve the same this? Heating 40m² with a stand alone heat pump will be problematic as the load required will be tiny. 

If you plan to later add UFH to the rest of the house, you really need to get the downward heat losses similar or running that could also be problematic.

Posted by: @guydeb

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Nu Heat suggesting 14mm Fastflo^® heating tube (shouldn't this be 16mm??)

Move to a different contractor, 16mm pipe every day nothing smaller.