Air Source Heat Pump? (Hybrid)

KerrenLeach

Hi all! 

I’m in the midst of renovating a larger property which currently gets its hot water from an Aga and radiator heat from a Heat Only Boiler (38kw). 

I’m going to be adding an indirect cylinder for the hot water and getting rid of the Aga, however I wondered if there are medium term cost efficiencies in also installing an air source heat pump and running a hybrid system?

Ignoring the initial capital outlay, with gas prices as they are I wondered if it’s cheaper for the heat pump to bring the water temp up to a medium temp then the boiler to finish it off? Thoughts? 

The heating (when finished) will be a blend of traditional radiators and UFH. 

Interested to hear thoughts of people in the know!

Doozergirl

What's the insulation situation?  It's absolutely key, above anything else. 

Fabric First.

KerrenLeach

In short… crap! It’s an 1800’s detached Georgian house, lots of rooms, very high ceilings (3.5m). The only thing going for it is the walls are very thick! 

I’ll be lagging pipes as I come across them but the ceiling cavities between floors are uninsulated and the roof has moderate insulation. 

All windows are being gradually replaced with double glazed but that’ll take time.

gm0

1. Insulation (reduced heat loss all available methods incrementally - windows, doors, draught proofing, revisit insulation)

2. Ensure emitters are capable of lower temperature operation ~30-40C and change - so perhaps some UFH if redoing any floors or perhaps bigger triple radiatiors e.g. Stelrad K3 - as room sizes and shapes and plumbing permits.  This is not a nice exercise but your rooms will dictate what is possible and whether this will work.

There are niche solutions like skirting rads and wall emitters that can be added depending upon the renovations being undertaken if alcoves/wall gaps between doors etc. make a bigger rad solution difficult.  The more doors and corners and alcoves you have the less attractive skirting rads become - all those joints. 

From my reading UFH is preferred all day long so if you can dig floors up, proper depth insulated pad etc go for it but that's a nasty retrofit for many of us.

With low temperature emitters sorted you may be able to get heatlosses down via insulation to all year ASHP. If so great.  If the emitters are problematic or some of the renovations are planning/spoiling the historic building or cost prohibitive then you may need to look elsewhere

One electric (so renewable as UK supply moves across) wet solution which attempts to leverage time of day tariffs is the heat store e.g. Caldera.  Essentially you dump a fraction of 80kWh into hot ceramics in an insulated vacuum walled flask from your single phase 100A supply over a few hours - likely overnight.  Then heat exchange it back out at normal fossil boiler operating temperatures (existing rads) or a bit less based on what else you have done. 

Downside vs ASHP is COP=1 (actually more like 0.9) so completely dependent on a good gap in tariff unit prices by time of day to achieve parity even with direct electric heating.  It's a modern form of wet night storage with an extra step or two but for some buildings which can house it but can't easily change the emitters without ripping up attractive or listed floors or walls then it has a place to get those buildings off fossil fuel.  Sadly there is no certainty on the availability of sensible prices and tariff designs to make such an investment a confident bet.