@holts, I realise we're not talking about a direct comparison oil --> ASHP given a larger house but, to give you some kind of idea, our ASHP installation was to replace an oil boiler (Worcester Bosch @ approx 85% efficiency) and our house was using pretty much bang on 2,000 litres a year (one delivery lasting 5 months during the winter months and 7 months for the rest of the year). That then translated into a sizing estimate that allowed the installers we ultimately went with to specify a 7kW Vaillant although supply problems ended up changing that to an 8.5 kW Mitsubishi. I'd be really interested to see your installer's calculations that justify two big Mitsi units for a well insulated house.

Hello @holts, a weather compensated flow temp of max 50degC via an ASHP is going to have quite a poor COP and be costly to run. If you can limit the LWT to 45degC, by using larger emitters, that will make a big difference. Every 1degC reduction in design max LWT saves around 2.5% on primary energy consumption & running costs. My system was designed for LWT 50degC, but I've now adjusted that to 45degC wit no adverse impact. It is keeping the house warm to 21degC average. That provides a reasonable estimated SCOP of 3.5 for the year including the DHW. Not class leading, but not cripplingly expensive. I've enabled the 3kW direct electric back-up heater on my 10kW ASHP low loss header, it wasn't wired up by the installer. It has made quite a difference to the performance when the unit is regularly defrosting. The 3kW heater comes on only during defrost cycles, so the additional energy consumption isn't that high, I haven't noticed any difference in energy consumption to be honest. It stops the LWT dropping too low during defrost and helps it get back to target LWT much quicker. That in turn helps the house to warm-up quicker in the morning. It was very sluggish on those cold damp winter mornings when the ASHP coil ices up quickly. We switch our ASHP off overnight between 9pm-4am. (By order of my wife who likes it cooler overnight) Currently the BUS scheme does not cover hybrid solutions else that would seem ideal in your circumstances.

@toodles 

thanks for reply .

The Scop was initially shown as 3.66 , that was before we opted for triple glaze and I suspect allowing for standard building regs minimums for extensions on insulation.

The 150mm in the rafters was the Celatex block type , were you thinking mineral wool type , I don’t think we could get too much deeper in ,  the TLX gold insulating membrane is there to add to that but keep the roof breathing .

I wondered about zoning differently but the I see heat geek proclaiming stop zoning , I see what they are getting at but needs must , they were aiming for a 23 degree temp .

I hope to be able to drop the flow temperature to 45 to get to SCOP of near 4 at least and up to your peak if I can.

A simple way to look at this is that a house needs X watts of energy to stay warm

An oil boiler is probably 80-95% efficient?

An ASHP is somewhere between 100-400% efficient depending upon flow temps etc etc

So for each unit of heat that you need, ASHP will always be more efficient. Whether it’s cheaper or not (as oil is cheaper per kWh than electric) depends if you’re at the lower or higher end of the ASHP efficiency

its wrong to say an ASHP will cost more to heat a house. I would focus on insulation and draughproofing like you are then get a few quotes to ensure the ASHP is the correct size

as long as you get a decent installer you should be quids in. Ours runs cheaper than oil did, except for about a week a year when the temp goes below zero  

Also if you do need two units remember only one will be running most of the time. Say you need (theoretically) 12kW for your house. If you have 2x8 installed then 1x8 will be fine until you hit say 2c outside. 

@majordennisbloodnok 

thanks for reply , this is what I have so far , it’s difficult to nail down until a completed house calc can be done .

Energy required to heat property 45,492 kWh Energy required for hot water 6,124 kWh
New Renewable System Information
Type of System* Manufacturer Name Manufacturer Model MCS Certification Number*
Flow Temperature* 50
MCS SCOP Heating* 3.66 MCS SCOP Hot Water* 1.75

Air Source Heat Pump
Mitsubishi Electric
PUZ-WM112VAA
037-0034-20-01
°C
Renewable System Provides
Hot Water Immersion Use* Once per week
Size of Hot Water Cylinder 400 ltr 52
Existing Heating System
Existing heating system fuel* LPG
Hot Water heated by* LPG
Age of existing system Post-2007
Efficiency of existing system 92 %
Estimated System Performance / Comparison Energy Requirement for the building
Heating and Hot Water Input % of heating requirements covered by heat pump Input efficiency (%) of new boiler
* based on 50C up to 60C, 3kW
243
Total
51,616 56,844
16,172

That’s all I have to date. , because it’s a complete refurb we are going underfloor downstairs and radiators upstairs where we will be able to install oversized Stelrads , those because I have discovered that their btu/wattage outputs are done to a British standard test whereas many other manufacturers do not , they are not obliged to which  to show ratings which is a bit odd .

So what I anticipate is to be able to come up with enough efficiencies to lower the required pumps , but that’s to be seen .

@holts

Posted by: @holts

↑

because it’s a complete refurb we are going underfloor downstairs and radiators upstairs where we will be able to install oversized Stelrads

Ah, now that's revealing! UFH downstairs + 50degc flow temperature must mean the installers are planning to using mixing valves on the UFH manifolds. Tell them to ditch that idea completely. It's woefully inefficient to raise water up to 50degC via an ASHP only to then blend to a cooler temperature by mixing. Thermodynamic lunacy [and you may quote me ;)]. This is old school and was the way with fired boilers that struggled to supply consistent low temperatures. So kick that into touch, tell them to redesign the UFH downstairs for 35degC flow rate max, directly from the ASHP with no mixing or zone controls, none are needed, you can control temperatures by simple heat mass balance, and size the upstairs rads upstairs accordingly. If your installers start arguing the toss tell them to get themselves on some Heat Geek training courses. It's your money and your energy bill you'll the wasting on an inefficient and expensive design. Don't worry too much if upstairs has a slight shortfall in heating capacity. Heat rises, and the upstairs of homes are usually a degree or two warmer due to natural physical forces. Heat load calcs don't account for natural infiltration of air movement from warmer to cooler places due to air density difference.

PS: you're existing LPG heating efficiency won't be 92%, probably nearer 82% in a real world application.

Posted by: @holts

↑

Reading the experiences on here it would seem the consumption will be vast , previously oil boiler did the job and required 500 litres every two months for six months and 500 for remaining 6 months , of course the house is now bigger so not a direct comparison. 2500 litres current cost approx £1700.

Posted by: @holts

↑

Existing Heating System
Existing heating system fuel* LPG
Hot Water heated by* LPG
Age of existing system Post-2007
Efficiency of existing system 92 %

That's a bit confusing. Is the current boiler running on oil or LPG?

@allyfish has already highlighted something I never would have done so your posting the extra details has already been of use. I expect others will chime in further with pertinent comments. However, I still can't see in all those figures you provided any mention of the heat loss figure; the maximum kWh figure needed to maintain the house at a constant temperature when the air temperature outside is -3 (or whatever design temperature they're using). You said it's difficult without a complete house calculation but then again I don't know how your installer could come up with an appropriate ASHP spec without that figure so either they must have that figure to hand somewhere or they are sizing the ASHP(s) via a wetted finger in the air which is absolutely not appropriate.

@majordennisbloodnok 

thanks , a bit more info.

The original house ran on oil , when they came round it had long gone and he just input lpg .

You are correct that there has not been a thorough room by room heat loss calc and that will be done when they can check all the elements . What he has done I believe is use info from the builder and his own experience to estimate the need , he did suggest that he thought we were on the cusp of 9kw pumps .

There was an original whole of house report but so much has changed since ,  I have noted a précis of it below, I have cut out the breakdown on air changes window proportion insulation etc , at that point he was assuming standard double glaze and minimum building regs insulation requirements.

Indoor - Outdoor delta T (oC)
23.5
Peak heat loss at design temperature (kW)
20.60
Space heating energy requirement (kWh/pa) 45492
Space heating electrical energy consumption (kWh/pa)
12430

However even with two 9kw units I wonder at the running costs if I can’t get that flow down a lot .

@allyfish 

Thank you for reply , I see your point and I am sending them the query . I’m not sure if this is largely my fault with original brief of ‘this house can not be cold for one of the occupants ‘ and hence the flow temperature chosen regardless of running cost , but that thought may be complete nonsense on my part and it was chosen to deal with the space . I did suggest two sets of flow and return pipes as I has seen installed by Urban Plumbers on YouTube .

The proposal sent to me recently was as below which doesn’t give enough detail to answer your query but from discussions we have I think it is not designed as you suggest .

1 SUPPLY OF AIR SOURCE HEAT PUMP SYSTEM
2 MITSUBISHI PUZ - WM112VAA (BS)
Defect:5 year(s) Performance:5 year(s)
1 NEWARK COPPER CYLINDER CO HEAT GEEK 450L TANK - HG450A
Defect:7 year(s) Performance:7 year(s)
1 JOULE UK 100L BUFFER TANK
Defect:25 year(s) Performance:25 year(s)
1 MITSUBISHI FTC6
Defect:3 year(s) Performance:3 year(s)
1 HONEYWELL DIVERTER VALVE
Defect:2 year(s) Performance:2 year(s)
1 HEATMISER COMPLETE UNDERFLOOR HEATING SYSTEM INCLUDING: MANIFOLD, PUMPS, ACTUATORS, STATS ETC...
Defect:1 year(s) Performance:1 year(s)
1 ADAPTAFLEX ELECTRICAL FITTINGS, CABLES AND CLIPS
Defect:1 year(s) Performance:1 year(s)
1 YORKSHIRE COPPER FILTERS, EXPANSION VESSELS, CIRCULATION PUMPS, ZONE VALVES, THREADED FITTINGS, AUTOMATIC AIR VENTS ETC... Defect:3 year(s) Performance:3 year(s)
1 CALEFFI HEAT PUMP ANTI-FREEZE VALVE
Defect:1 year(s) Performance:1 year(s)
1 SUPPLY AND INSTALLATION OF ASHP SYSTEM
1 5 Year Insurance Backed Warranty and Deposit Protection Scheme
1 Design, Installation and Servicing Covered By 5m Publ ic Liability and 250k Public Indemnity

Another factor that can have an impact on running costs for different heat sources is usage pattern, specifically the use of timers. Many fossil fuel based heating systems use timers, meaning the heat is not on 24 hours a day, whereas an ASHP based system is often on 24 hours a day, possibly with a night time setback. Fast and furious vs steady Eddie, because of the innate characteristics of each type of heat source. Over a 24 hour period, the ASHP based system will on average be a little warmer, and this in turn can increase running costs, sometimes considerably, as I found out, when I moved from oil based heating to an ASHP.

@allyfish 

In fairness installer has replied straight away , he advises .

on mixing valves

We never use mixer valves on ufh, we set a weather compensation curve and size the radiators to the designed flow temperature on that weather compensation curve. 

As an aside I asked about designing the whole system at 35 using the radiators to balance .

We can design at 35 degrees but the rads would be very large, we'd probably design at 45 or 40 but that's at -3 and so would go down to 35 degrees when it was warmer. 

However they do ultimately state , and this has not realistically been possible up until now , I think also they were worried about an entrance atrium which is very open , my silly idea from heating view point , effectively an open space but think ground , Middle and top floor open and this I am sure will have cause caution in the initial design .

 once we conduct a full room by room it will be balancing radiator sizing with flow temperature design. 

This is what is so good here from a layman’s viewpoint, you have experience to impart and it helps us all along . Thanks .