Domestic air source heat pumps, UK

[Nutzman]

Good day all,
There has been a lot of hype in the media of late regarding the use of heat pumps versus gas fired boilers. There are heaps of articles giving journalist "level" information. However on trying to put some numbers against each "technology" - heat pumps vs gas boiler, I'm coming up short.
I have applied the concept of fenestration, in which one calculates the heat loss/gain of a house, taking the material/design of the walls, glazing (single/double/triple, type of glass low e, type of window frames (wood, metal, plastic) etc. Percentage glazing area vs floor area per room. Insulation in the roof space, orientation of the house in previous projects (but not in the UK). (I would love the UK values, to put into my spreadsheet). The fenestration determines (theoretically) how much heat your house "leaks".
Then we have the radiator calculations to apply. Thanks to 2013 EN-442 EEC, this standard furnishes a load of info. The aforementioned spec, calls for the calc's to be done applying a DeltaT of 50 deg C.
However the gas fired boiler typically feed "heated" water at 60-70 deg C into the radiator (and works on a short sharp temp rise into the room). Whereas a heat pump typically works at 55 deg C and will most probably reach the radiators at 45 deg C. The heat pump works on a longer cycle at lower temperature in order to maintain the overall "desired" room temperature? However to add to the mix, a heat pump typically requires a "storage tank" to (The literature is talking of volumes of 200-300 liter). The heat pump maintains the volume of water at ~55 deg C and when the radiator is required to heat the room, it draws off the storage tank (recirculates).
The one issue I cannot find info on, is the fact that the radiator is sized for a DeltaT of 50 deg, but the heat pump is supplying water at a temperature's to achieve DeltaT of 30 deg C. This may mean that the (existing) radiator is now undersized. One may find that the current radiator may never get the room up to temperature, due to the "leakage" out the room/building.

Any values, formula, guidance would be appreciated.

Regards,

 

[LittleInch]

Yes,
It's all shrouded din a bit of mystery really and I think it's been sold as a green solution to the housing heating issue. Any issues are brushed aside with " you might need some bigger radiator's" or just ignored.

As far as I can tell air source heat pumps work great if you can fit underfloor heating as the huge surface area allows the heat to enter the air space and keep your feet nice and warm.

But suddenly having to double the size of my radiators or at best install double or triple skin ones in place of singles is not easy.

So I would say you need to take the size or the power of the radiators as sized for 50C delta and then apply a 0.6 factor to it.

However there is no doubt that most radiators are sized with a higher DT in mind and rarely operate flat out for long periods without turning the room into a sauna. ASHPs will operate for much longer than the traditional boilers to work better and hence the increase may be as low as 0.8 factor.

But you're right - there isn't much out there on radiator sizing for lower delta temperatures.

This is quite interesting on why short cycling isn't a good ide hence the large buffer thank.

https://assets.publishing.service.g...389-effects-cycling-heat-pump-performance.pdf

and see page 23 of this - implies at 30C delta they need to be TWICE as big!

https://assets.publishing.service.g...iance_with_MCS_installation_standards_v32.pdf

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nutzman]

Hi LittleInch,

Many thanks for the links. There seems to be more info/links in the "references" at the end of the document than in the document?
Considering I'm sitting at home (thanks to Covid), perhaps I need to look at developing this as a means of income?

 

[LittleInch]

It's not a bad shout. There's so little intelligence in the industry it's frightening and an awful lot of cowboys.

I'll confess I didn't get to the end but as a govt document should be better than a lot of the vendors, who don't tell you that as it gets colder, the performance of the heat pumps drops off, just when you don't want it to.

But the gov't has set its heart on not installing any new gas boilers from 2025? or 2023? so there will be a lot of cold people otherwise and also suddenly getting big electricity bills for all this extra heating that is needed, including the hot water systems which need to get up to 60C.

So all of a sudden you need to find space for a big hot water cylinder which you ripped out when combi boilers came in or never installed in the case of new homes....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nutzman]

Hi Littleinch,

Here is a perfect example of the discrepancies one "will" encounter when applying factors.

https://www.tandfonline.com/doi/full/10.1080/09613218.2014.967977

 

[EdStainless]

These units tend to leave chilly houses, when it is very cold outside the 'heat' being supplied is just a few degrees hotter than the house. So while the total Q may be enough it doesn't feel like it is being heated.

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P.E. Metallurgy, consulting work welcomed