There's nothing to stop you having a mixture of radiators and UFH. Most likely, radiators in the existing bungalow and UFH in the extension. ASHP run most efficiently when their output is at a lower temperature than a gas boiler. That is perfect for UFH, but often needs larger radiators for an ASHP than for a gas boiler. They don't necessarily need to be physically larger; you could substitute double for single radiators for example. You need to calculate the heat output at the reduced temperature to determine the correct sizes.

Talk to whoever is designing your extension and retrofit about what you want to do and try to find a heating specialist in your area who understands ASHP and retrofitting them into a system such as yours. Between them they should be able to design a suitable upgrade.

If you're looking at rads then make sure you size them off delta T30 which allows for much lower flow/return than delta T50 - which is the normal figure manufacturers work off. Conversion is available via a quick search. You'll probably end up with roughly double the number of rads.

As an aside, how are you planning to get enough floor insulation in without digging out?

tony, where are you, to point out that with PH, or close-to with AECB/Enerphit, you hardly need a heating system, assuming you're harnessing solar gain in a controlled way and airtighting. Save all that money, use £49 Lidl on-peak electric convectors for the very coldest days, off-peak electric hot water storage.

Posted By: maxsmWe hope to insulate the property to a very high standard (somewhere between enerphit and aecb). We are currently leaning towards radiators due to budget and the disruption of digging out for UFH but would like to 'futureproof' our heating by installing a system that would be compatible with ASHP in future.

I assume, as you say "digging out" the floor, that you have solid uninsulated floors. If the floor is not insulated in some way it will be a large impediment to being able to reach Enerphit standards. Do you have sufficient room height to be able to insulate on top of the solid floor.

Posted By: PeterStarckIf the floor is not insulated in some way it will be a large impediment to being able to reach Enerphit standards

I'm a well known fan of extending EWI down, in a trench, to the bottom of the foundation, forming a 'coffer dam' of insulation around the perimeter of the ground floor slab (incl if it's a void under a suspended timber ground floor), as alternative to insulating the slab itself, the trench being backfilled as a french drain to guarantee that the downstand EWI stays dry, or at least not waterlogged. This makes a fair job of insulating the ground floor without disturbing it (and kitchens etc built on top of it). I don't imagine this could form part of a full PH but I wonder whether it could be good enough for an Enerphit or AECB retrofit.

"from the bottom of the foundations" up to unite seamless with the wall EWI.

If the underfloor void is thus kept warmer than outdoors (as intermediate between indoor temp and the subsoil floor of the void, and without uninsulated perimeter cooling), rather than maintained at outdoor temp by through-draught, then there's precious little to cause condensation in the void, so no need for through-draught.

The only half-likely condensation place being the subsoil surface as handy and harmless condensing plate for the whole house.
Any joist ends are maintained warmer than hitherto, and the french drain keeps the whole free of liquid groundwater and/or leakage from RW drain.

Posted By: djhASHP run most efficiently when their output is at a lower temperature than a gas boiler. That is perfect for UFH, but often needs larger radiators for an ASHP than for a gas boiler

Yes and no.

Posted By: jamesingramyou wont require much heating.

Posted By: fostertomyou hardly need a heating system,

I'm really anti this idea. There are far too many instances where you do need heating and often in the living area. It also depends how warm you like your house. My very airtight, close to PH insulation definitely needs heating in central belt of Scotland to maintain a 20.3°C temperature in the living area.

Yes ASHPs run at a lower temperature than boilers, but fitting them directly to UFH is a mistake IMHO. The OEM forum is currently filled with folk paying vast amounts on electricity because the HP is plumbed directly to the UFH and there is insufficient heat demand so the HP simply cycles. Also, you probably want to run the UFH colder than the HP will run at efficiently to prevent overheating. My UFH runs at just over 30°C and there is no way I have a DeltaT of 5°C when it reaches steady state. In the shoulder months, it cycles a bit, running for 30 mins and then off again - but as that is just a circulating pump, it isn't an issue.

I'm sure the right way to use a HP is to have a Thermal Store and then take off the UFH and use a blending manifold on the UFH. You can use that same TS to take the rads off as well, just at a higher temperature. A TS with a baffle/stratification so the top can be heated instead of the whole tank is even better as you then only need to heat to a higher temp for a while when you need DHW.

The important aspects or the renovation are airtightness and insulation. With airtightness there is then the need for a MVHR system (which again reduces the heat load).

Get a proper heat demand survey done to size the rads and UFH. If you run it cooler, you cannot have too many UFH pipes.

You can fit the tank to the existing boiler (turning it down to maximise efficiency) and when that turns up it's toes, replace it with a HP.

Posted By: borpin

Posted By: jamesingramyou wont require much heating.

Posted By: fostertomyou hardly need a heating system,

I'm really anti this idea. There are far too many instances where you do need heating and often in the living area. It also depends how warm you like your house. My very airtight, close to PH insulation definitely needs heating in central belt of Scotland to maintain a 20.3°C temperature in the living area.

Our house actually does have PH insulation and airtightness and yes it needs some heating overnight but then maintains or increases its temperature through the day as long as there is some sun. If there isn't going to be much sun, I put more heat in the night before. Where you live doesn't make any difference, since PH is performance based and the insulation needs to match the conditions. The same goes for EnerPHit.* Temperatures within the house can vary a degree or two from the north-facing to the south-facing rooms but nothing that bothers us.

Yes ASHPs run at a lower temperature than boilers, but fitting them directly to UFH is a mistake IMHO.

I didn't suggest connecting the HP directly to the UFH, so I agree with you.

As I said at the beginning, given the OP's state of knowledge they should be taking professional advice about the design of a PH/EnerPHit-level retrofit and also about installing an ASHP-based heating system.

Specifically, for an EnerPHit refit, you will typically need (25*floor area) kWh of space heating per year. See https://passipedia.org/_media/picopen/9f_160815_phi_building_criteria_en.pdf for more explanation.

Hi maxsm, to get to EnerPHit, you need about 100mm of kingspan-style floor insulation to get a good U-value around 0.15, backed up with a professional u-value calculation which you can feed into PHPP.

The coffer dam idea has been discussed a few times, it would certainly help a bit, but it won't get you those two things.

An Enerphit house of say 100m² needs 2500kWh/a of external heating. Using a fan heater would cost around £500 of electricity each year (depending what happens to electricity prices) and would need a fan heater around 1kW capacity.

It's hard to think of any heatpump or gas boiler system which could end up cheaper than that over their lifetime, they basically don't make them small enough, and they will cost multiple £k to install with the radiators or UFH.

Possibly a small air conditioner (a-a heatpump) could be worthwhile? Or a few m² of electric UFH mats.

If you went for a less demanding standard of insulation, then a heatpump would become worthwhile. Because of its efficiency, it might actually use only the same electricity as the example of a fan heater, but you'd need to do the sums whether to spend the capital on insulation or on a heatpump system.

1. Possibly depends where you are I suppose. IIRC would have to average below about 200g/kWh to beat a gas boiler. It's certainly been below that recently. Still can't get my head around the idea of direct electric heating being greener than gas though.
2. I'm fairly sure a well specified ASHP gets significantly better than that at those temps.
3. Surely the massive embedded carbon involved in installing the thing makes it not worthwhile from an environmental point of view?
What about multi split Aircon?

Posted By: GarethC
2. I'm fairly sure a well specified ASHP gets significantly better than that at those temps.

Even owners of well specified, well installed ASHPs are disappointed in their performance when it gets close to freezing due to the defrost cycles. A large fraction of my heat requirement is at those temperatures. Paying a lot of money for a non-ideal system does not look very attractive to me.

3. Surely the massive embedded carbon involved in installing the thing makes it not worthwhile from an environmental point of view?

An ASHP is a more complex device than a GSHP and therefore probably has a higher embedded CO2 footprint. Only direct electric is clearly better in this respect. GSHPs generally are said to have longer lifespans, too.

Posted By: borpinThe answer is to possibly use Glycol rather than water so no need for defrost cycles.

As far as I have been told, the problem is icing up of the evaporator ('radiator'). Humid air is sucked in at just above 0C. It is then blown past the evaporator, which sits well below 0C. Ice forms on the outside of the evaporator, blocking up the channels. The ASHP has to 'go in reverse gear' to heat up the evaporator, using an in-built electric heating element, to get rid of the ice, and go back to normal operation until it has detected it has iced up again. This is a fundamental aspect of ASHP operation and is independent of the refrigerant, sizing of the ASHP, or even quality of the installation (although this can make it worse).
For not so well insulated homes, the bulk of the heat is delivered when it is below 15 C but not close to freezing yet. At those temperatures it is not so much of a problem. Well insulated homes will require heat at lower outdoor temperatures, and then this issue becomes more important.

Posted By: bhommelsAn ASHP is a more complex device than a GSHP and therefore probably has a higher embedded CO2 footprint.

Only if you're considering just the heat pump box I suspect. Presumably you're referring to the evaporator and associated fans etc? But if you include the GSHP's equivalent long slinky pipe or boreholes then I suspect it may come out worse. There's a lot of embodied carbon in an HDPE pipe.

edit: crossed with Gareth. Yes, the installation work also has carbon emissions

Posted By: WillInAberdeenThe heatpumpmonitor.org that Borpin mentioned shows that poorly insulated pre-1900 homes are getting CoPs better than passivhauses, because it's mainly the indoor radiator/ufh temperature that determines the CoP, not the outdoor temperature or the insulation quality. Instead, those factors determine the total kW or kWh required.

I just had a quick look at the site and don't understand how you reach that conclusion? There's only one entry that claims to be a passivhaus and it uses a GSHP whereas all the others use ASHP so I'm not sure what conclusions can be drawn? And some heat DHW while others don't etc etc.

AIUI WiA's saying that mean CoP over the year is better for poorly insulated houses that PH ones. Though requiring much more energy, the whole-winter electric input to the HP is not pro rata, but more favourable. That is very interesting.