Size limits - ASHP

[gcm]

Looking ahead to next kerosene boiler replacement. Keeping track of what works and doesn't for retrofit to our site

May have another Grant like the current 36kw condenser (perhaps with the hybrid and the RHI (if can qualify), replace tank, upgrade pipe work, bundage to current spec).  I can't help but feel that these are like the car hybrids which are designed to the tax rather than to work as such.  The 36 is bigger than it's rather terrible utility predecessor that I inherited and may even be a little over what is needed.  I'll have the rads estimate shortly as I have been around measuring and thinking through how it could be increased x2 for t30

Question - on the Grant hybrid - how can it be long running burns and oil temp.  And ASHP temp. And work together. On the same size rads ?   If it only works in swing season for ASHP then it's a bit of a chocolate teapot with a potential subsidy attached reclaimed in pricing - may as well use our wood burner spring/autumn and buy the cheaper pure kerosene burner.  Am I missing something clever ? Surely it won't do much on existing (half sized) rads and with the upgraded ones the room controls will become key with all that entails.

In terms of alternatives - what is the maximum size for an ASHP solution in terms of annual kwh (or indeed peak days) for a single phase system with upgraded radiators to suit it (I am evaluating how close to 2x we can get on that - both per principal room to avoid colder areas getting worse and overall for the lower operating temperature. 
Don't have UFH and the retrofit is daunting to do properly the modern way , insulation, pad, excavation given wood and stone tile interior fit. So it's rads. 
Doubles become triples and/or bigger. And some new wall ones.
I appreciate that at the moment oil is materially cheaper than the extra electricity for full ASHP but that may not always be so.

I have eliminated skirting rads as the house is just all corners and I can't stomach so many o-rings/joints or indeed the cost per kw of dispersa being double rads.
GSHP unattractive. sloping site without room for coils. So pit or not possible.
ASHP or or oil hybrid can be sited easily in the existing input spot.
Plant room for pellet and store but I find the energy density and lack of supply diversity offputting

So is it ASHP + Willis for ultra cold days ?
Or just kerosene and hope the regulatory changes aren't too unkind for the next cycle

Air to Air units could be fitted instead/as well in a couple of places with dual outlets to major rooms but coverage would be incomplete. I worry these are an allergy monster as the filtration on the domestic units I have repaired (filters and remote batteries) for relatives was a bit risible.

My wife will not be at all happy if the heating is marginal and then just gets worse in the coldest of weather.

Any thoughts

[ProDave]

Why do you need to replace an oil boiler?  It can't be that old to be a condensing one so what is so wrong with it that it is end of life?

[A_L]

34 minutes ago, gcm said:

thinking through how it could be increased x2 for t30

if that is a dT of 30°C relative to a dT of 60°C (80°C/50°C) output is about 42%

 

38 minutes ago, gcm said:

And work together

It is one or the other, if they were in series the return from the radiators would be to high a temp for the heat pump to provide a useful COP. Goes for ASHP/Willis to.

 

40 minutes ago, gcm said:

what is the maximum size for an ASHP solution in terms of annual kwh (or indeed peak days) for a single phase system

the DNO will want to limit the size of the heat pump to maintain the supply, typically a thermal output of 14kW is the maximum HP size for a single phase install.

[Temp]

We have a 12/13 year old Grant Vortex. Originally 40kW but fitted smaller jets that reduce power slightly and improve efficiency a % or so.  Feeds a TS and UFH. Has performed faultlessly.

 

I'm happy to wait until the cost of oil makes it cheaper to switch to ASHP.  

 

 

[gcm]

Indeed a 2008 installation - not planning its imminent retirement.  Our Grant just had new expansion vessel this year so it's not dead yet.

However we are planning full redecoration - window reveal apertures were redone to fit the new windows better and the ceilings need doing including very high stuff I don't fancy myself.

So the pipe runs and rad changes for greening energy (and skirting where centres change, and spur for ASHP - belong upstream of paint along with electrical work for PHEV car charger etc. etc.   Thus trying to work out if an ASHP is even a possible future goal - as that drives the need for rad changes if I have understood it. 
I can just leave well alone if not or make a couple of insitu "upgrades" where the design was locally weak in a particular corner of the house.

I think i've got the basic idea  now - electric inline heaters (Willis) for higher system temp or ASHP - not both at once.  Diverter valves isolate.  Same with the oil boiler hybrid.

So I'll just work out if I can get to 2x size of rad panels via triples - size bumps and extra ones in not insane to plumb locations.  If not - then the idea is dead anyway short of excavating the floors.  But if I can I need to understand if this works as a set of preparatory changes while still on oil (doesn't break current solution adding the rads) and how to get a working control design appropriate for new TRV.  Our old TRVs were unimpressive and yet the house is not an easy one for all matters wifi.  We barely get a basic Siemens remote stat to work in a couple of spots and that has stronger specific RF than many a solution.  So manual (but quality) may still be best.

The Tank/bundage + soldered pipework is the "not refreshed as recently" stuff which will be needed sooner than the boiler - one summer - if there is to be a kerosene refresh in our future at all.