@Katcesca,

Hello and welcome.  I think you've summarised the issue very well.  You're comparing heating some of the house some of the time with all of the house all of the time. Although the former doesn't work very well with a heat pump in the way you are currently running your system, you can heat different rooms to lower temps (or not at all) and run with some moderate setback at night or when you are out.  You can also supplement with a wood burner, lots (including me) do.

You have a relatively large and poorly insulated house with high ceilings and that's going to be expensive to heat however you do it.  It's really hard to say what will happen with fuel prices but oil is relatively cheap at the moment. 

Do you have a heat loss in kW at design temperature and the room temperatures you describe? It should be a number a bit less than 14! 

The COP your supplier is assuming is a little more than 3.  That's achievable with radiators but there are plenty of systems out there that don't get that high.  You will need these big radiators and low flow temps to achieve that and it will be a lot lower when it's properly cold outside.  Your house will take a long time to heat up with an ASHP compared with your oil.

Having said all that there are plenty ASHPs working happily in houses like yours.  It won't be cheaper than oil - at the moment - but that's the price of going green.  

Thanks Kev, very helpful. 

From the design summary: Average design temp 18.71, design outdoor temp -1, total heat loss 14.47kw (is that the number you were after?), heat loss per m2: 60w

Digging into the room by room calcs, one of the issues is that the it's designed to heat the very very large living room (which is 44m2 has 11ft ceilings and a very leaky/badly insulated protruding bit at one end) to 21 degrees, which we never do. We only use that room in the evening and we'll add a fire usually. 

Yes I was also concerned about the COP - it's based on system efficiency of 368% and I'm very aware that many systems don't reach anything near that, and I'd need to adjust my electricity projections quite a lot upwards, potentially. But the quote is also proposing we change 9 radiators for K2s at an additional cost of £9k (and the rest if I want anything beyond basic radiators). 

Given the size/age/construction of the house it is surprisingly not as expensive to heat as you'd guess on paper. It is rarely properly cold here in Cornwall - currently 9 degrees outside and very comfortable indoors with 6 hrs of central heating running a day plus the Aga heat. It only drops down anywhere near freezing for a week or two a year tops. Even then, we don't have any need for the central heating to be on overnight. 

The fact that the AGA heat (which is in the middle of the house and sends a plume of warm air up the central staircase) does make the whole house so much more comfortable (this year we turned it on mid-december for the cold snap) does make me wonder whether low-and-long heating from ASHP would actually be more pleasant for us than the fiery bursts the our current boiler throws out? 

I think the next step for us might be to upgrade some of the radiators, add some TRVS to give us some more control, and look at whether we can turn the flow temp down on the boiler and have it on for longer, to see what effect all that has. 

Posted by: @katcesca

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Thanks Kev, very helpful. 

From the design summary: Average design temp 18.71, design outdoor temp -1, total heat loss 14.47kw (is that the number you were after?), heat loss per m2: 60w

Digging into the room by room calcs, one of the issues is that the it's designed to heat the very very large living room (which is 44m2 has 11ft ceilings and a very leaky/badly insulated protruding bit at one end) to 21 degrees, which we never do. We only use that room in the evening and we'll add a fire usually. 

Yes I was also concerned about the COP - it's based on system efficiency of 368% and I'm very aware that many systems don't reach anything near that, and I'd need to adjust my electricity projections quite a lot upwards, potentially. But the quote is also proposing we change 9 radiators for K2s at an additional cost of £9k (and the rest if I want anything beyond basic radiators). 

Given the size/age/construction of the house it is surprisingly not as expensive to heat as you'd guess on paper. It is rarely properly cold here in Cornwall - currently 9 degrees outside and very comfortable indoors with 6 hrs of central heating running a day plus the Aga heat. It only drops down anywhere near freezing for a week or two a year tops. Even then, we don't have any need for the central heating to be on overnight. 

The fact that the AGA heat (which is in the middle of the house and sends a plume of warm air up the central staircase) does make the whole house so much more comfortable (this year we turned it on mid-december for the cold snap) does make me wonder whether low-and-long heating from ASHP would actually be more pleasant for us than the fiery bursts the our current boiler throws out? 

I think the next step for us might be to upgrade some of the radiators, add some TRVS to give us some more control, and look at whether we can turn the flow temp down on the boiler and have it on for longer, to see what effect all that has. 

The 9 radiators that your installer is suggesting, are they gold plated by any chance? 🙄 

I would suggest that you have a look at the range of radiators being offered on the Screwfix website, which should give you some idea of more realistic prices. Size up your present radiators and then look for a K2 or even K3 of comparable size, which you could then purchase and have a local plumber install, or maybe even install yourself.

If your proposed installer is trying to overcharge for radiators, then I would suspect the rest of any quote they provide. Try to get more quotes if possible.

The size of your radiators (heat emitters) is quite important, since they are the means of transferring the heat energy from the heat pump into your home. The larger the heating area, K2 or K3, the lower the water temperature to provide the same quantity of heat energy. An ASHP operates more efficiently when it heats the water to lower temperatures, although it still needs to provide sufficient heat energy to meet the present demand.

I presume that you have looked into the cost of internal or external insulation for your walls. I always recommend starting with energy efficiency improvements, since reducing heat loss has a knock on effect to all the other factors.

I missed that about the new radiators. I had all new rads and pipework fitted (I had no central heating before) and it was about £4.5k extra for everything. That was 12 K2 radiators and 2 towel rails.  That's East Midlands though, probably cheaper here!

Stick with oil, it currently is and likely will continue to be the cheapest way to heat a property for a good while yet. Leave your current rads, install a new oil boiler with modulating controls (very important), and you’ll probably find you can knock 1/4 off your oil consumption, and have a more thermally comfortable property.

The other options would be to ditch the wet central heating and go to air-air if you wanted a low carbon solution. 

And just because your oil boiler is 20yrs old doesn’t mean it’s due for replacement, my boss still has the danesmore that was installed in the early 80’s, still works perfectly.

@katcesca what flow temperature has your MCS estimate based and sized the design on? This is THE most fundamental element of an ASHP efficiency and therefore running costs. The lower it is, the more efficient and cheaper it will be to run. I would advocate forgetting K2s rads and go to K3s throughout, to try to get the flow temperature down. You can't really oversize heat emitters with ASHP. I realise there may be aesthetic considerations with a period house, and standard Stelrads are not particularly appealing, especially the K3s.

Your house has a huge thermal mass with 2ft thick stone walls, so heat low and slow and forget timed boosting on/off for periods each day. The thermal lag of the building fabric is probably measured in days not hours. So it will keep a fairly stable temperature once it finally gets there! Climatic compensation on ASHPs is instantaneous and real-time, I've yet to see a manufacturer that allows a programmable offset to lag the climatic control response to synchronise with the building thermal lag. Your house would be an ideal candidate.

Keep the secondary heating, the wood stove. I would not be without mine. You can close these off when not in use so you don't get the huge infiltration air-change that open fireplaces and chimneys create.

The MCS estimate will assume all rooms heated with no diversity. They are usually unrealistic and pessimistic. You can un-heat or set back rooms not in use, or rooms that are impossible to heat cost effectively, but do consider that the energy demand of a heated room next to an unheated room will be higher as heat energy will flow from high to low. That's another reason to oversize the rads, as the radiator sizing estimate will not have considered the additional heat losses of a heated room adjacent an unheated. 

However you heat your house, it's going to require the same energy, just a question of whether you want to do that using renewable heating. Long-term, electricity per kWh will get better parity with gas and oil, but we're years and years away.

@katcesca

If you do decide to go with an ASHP, I would suggest that you perform a room by room heat loss calculation. It will then be possible to size the radiators to the heat loss of the room, since it is rather pointless installing a K3, which then has to be throttled back because it is too large, when a K2 would have been perfectly adequate.

Hi all

Wow, such useful considerations here! 

Allyfish "Your house has a huge thermal mass with 2ft thick stone walls, so heat low and slow and forget timed boosting on/off for periods each day. The thermal lag of the building fabric is probably measured in days not hours." - this is really significant so thank you for articulating it for me. This is why I think the AGA actually works quite well weirdly, and why we don't really need heating on overnight - it doesn't cool down that fast. 

Designed flow temp is 45 degrees. 

Annoyingly, we don't have the capacity with the current boiler set up to heat 'low and slow' given we have no thermostat, no TRVs and no capacity to adjust the flow temp ourselves - my plan is to work with a heating engineer to see if we can lower the flow temp as I do think there are major efficiency gains we can make with our current system. 

Derek, re the gold plated rads (!) in the quote the actual radiators aren't the bulk of the cost, they are itemised at about £290 each for large Stelrad K2s which is about what I can find them for on eg Plumbnation, but the pipework and fittings add thousands on to the overall quote. Because some of our radiators are outdated and undersized anyway, I'm looking at one option being to start upsizing them piecemeal now, as and when we can afford, with TRVs fitted. 

The quote does do a room by room heat loss calc and radiators specified are sized to a designed room temp for each space according to heat loss. 

Hugh F re: sticking with oil - annoyingly we can only replace the oil boiler if we relocate our oil tank because the tank doesn't comply with current regs where it is now, and relocating the tank is not a simple problem to solve! I'm going to take your optimism about it running well for a good few years yet (which actually corresponds with our heating engineer's assessment - it's a healthy beast and actually running pretty efficiently).

Long term we would prefer to heat using renewables not oil but we can't do that if the tech/costs aren't there yet. 

So I guess the plan needs to be to see what efficiency gains we can make with our oil boiler in the immediate term, and hope it holds our a few more years until renewable tech is more affordably in reach. Maybe right now we reduce our carbon footprint by switching the diesel car for an EV which we can run off our panels! 

The other option that is on the table for us now is biomass as we do have the space to accommodate it, but it feels like that's a bad compromise? 

Posted by: @katcesca

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Hi there knowledgeable people

So we are in the process of slowly upgrading/retrofitting a large, unusual Victorian house. We are trying to work out if an ASHP is a viable proposal in terms of long term running costs.

TLDR: current MSC calcs put ASHP running costs at 50% more than we currently spend on oil. 

It's the central portion of a large country house, but in terms of construction/build if you imagine a very large 3 storey 5 bed Victorian terraced house with 2ft thick granite walls you are pretty much there. 240 sq m of heated living space, some with very high ceilings so a large volume of air to heat. 

We've done the basic structural stuff, done heat loss survey, insulated roof spaces and basement floor, all windows double glazed, chasing down drafts, thermal curtains etc. There's not much more we can do in terms of insulating without massive cost. We've installed solar and a 10.5 kw battery, which at current rates will cut our elec bill by about 60%.

Currently we have a large and old oil fired boiler which heats the house well but has very basic controls - on or off on a timer, no thermostat, no TRVs etc. We have to make a plan for its replacement as it's at least 20 yrs old. We run our heating in 2 or three blasts, maybe 8 hrs a day tops if it's really cold but otherwise more like 6 hrs. We also run an oil fired AGA for a couple of months in winter which helps maintain the base temp of the house but drinks oil like a fish. Currently we use about 3000l of oil a year, supplemented with a woodburner. The house isn't heated to a uniform temperature of eg 20 degrees - we migrate to the warm bits and add jumpers in winter. We are in a mild climate (Cornwall) and the house seems to maintain its ambient temperature pretty well due to thick walls. It's mostly between 16-19.5 degrees in winter. We don't tend to need any heating between May - late September when the house will be 18-22 degrees. 

Here's my Q: we've had an MSC survey by a company done, which calculated that with some pretty extensive radiator upgrades/additions (9) we could make it work with a 14kw Mitsubishi pump. However, the running costs they have suggested would see us spending quite a bit more than we currently do on oil. I guess it's because they are designing towards a temperature that we don't currently heat the entire house to all the time. (Design temps in calc are 18 degrees in some rooms and 21 degrees in main downstairs spaces.)

They've calculated annual energy heat loss of 39495Kwh, plus hot water need 4900, total energy demand 44386. 

Currently, with our oil usage (at 10kw per l of oil) we are using about 30000Kwh for heating and hot water, so their calculation is 50% above our real-life usage.

Their calc for annual Kwh heating consumption at 13155 kwh - at the price cap of 34p/Kwh that's £4472/yr which is £1770 more than we spend on oil currently. 

I guess the short answer is that we are currently under-heating the property and their proposal is costed for a more regular temperature all round. It would be lovely to have more constant and comfortable daytime heat if that's an affordable prospect. 

We are really keen to make the switch, and could find the renovation budget - but we can't risk being stuck with very high bills long-term if a new system required us to heat the house more uniformly and for longer resulting in high electricity usage. Given the large space heating demands, the margin for error here could be quite expensive!

Currently we are thinking it would only make sense for us to switch if a) electricity prices for renewables were guaranteed to be lower than they are now and b) there was a more generous subsidy for switching (we'd be spending north of £15k even with the £5k grant)

Can anyone bring any other perspectives or experience to the table? 

 

 

 

do you have any capacity in the project to add solar and a battery? This is how I have made my ASHP add up economy wise.

ASHP installation including radiators total cost is £12k. This is covered by a £9k ScotGov grant and £3k interest free loan. 

Then I am adding a 5.5KW solar array and 10KW battery. This is costing £13k. £2500 ScotGov grant and the rest is interest free loan.

I sold my boiler and oil tank for £1100. 

My ASHP will use circa 5000KWh in the year. At 34p that’s £1700. We were previously spending circa £2k on oil. I do believe that electricity will come down late 2023 and then on into 2024 and I also intend to change to better tarriffs with off peak rates etc so the battery can charge overnight and the ASHP can run more cheaply overnight, so still advantages to be gained. 

My solar array will give us circa 5500KW per annum and can be managed using the battery, so offsetting the electricity the ASHP will actually use. 

So that £1700 per year to power the ASHP is offset and means that the solar and battery pays for itself in 6.5 years. By which time the interest free loans are well on their way to being paid and I would probably just look to clear the balance at that point, creating effectively free heating and hot water. 

@Marvinator80 we fitted 4Kw solar array and 10.5kw battery in August - just under £10k all in. We did this phase first to see what real-world impact it had on our electricity profile. Initially we were optimistic we could make something like your plan work, but it doesn't pan out like that actually. Broadly, for us an ASHP would use around 13000kwh/yr and the solar panels will generate maybe 3000. 

Annoyingly our roof is very steep and the angles mean that we don't get amazing solar gains from it, but nonetheless the system is on track currently to reduce our electricity usage by at least 50-60% (was 5000kwh annually). We have just started charging the battery at night on Octopus Agile tariff to take advantage of cheap rates and so are using zero grid electricity in daytime hours - early signs are encouraging about how much this will knock off the remaining bills but obviously the energy price volatility make that saving hard to calculate long term. There aren't any other cheap tariffs currently that work with a battery if you don't have an EV. 

However, having it up and running, I don't really think the solar/battery would make a huge dent in ASHP running costs for us - in cold weather a 14Kw heat pump is going to be using far more a day than the 10kw we can download cheaply onto the battery overnight, and on a December day our panels will generate between 0 and maybe 3/4Kwh. And we need heating in the daytime, not at night. 

Yes, we'll be generating a lot of energy in the summer months, but we don't need to heat our house in summer anyway. The solar panels do cover our electric showers, so we have very low hot water demand from the oil system. 

If I could access a £9k grant for the ASHP the sums would stack up differently, but I can't!

Posted by: @katcesca

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Hi all

Wow, such useful considerations here! 

Allyfish "Your house has a huge thermal mass with 2ft thick stone walls, so heat low and slow and forget timed boosting on/off for periods each day. The thermal lag of the building fabric is probably measured in days not hours." - this is really significant so thank you for articulating it for me. This is why I think the AGA actually works quite well weirdly, and why we don't really need heating on overnight - it doesn't cool down that fast. 

Designed flow temp is 45 degrees. 

Annoyingly, we don't have the capacity with the current boiler set up to heat 'low and slow' given we have no thermostat, no TRVs and no capacity to adjust the flow temp ourselves - my plan is to work with a heating engineer to see if we can lower the flow temp as I do think there are major efficiency gains we can make with our current system. 

Derek, re the gold plated rads (!) in the quote the actual radiators aren't the bulk of the cost, they are itemised at about £290 each for large Stelrad K2s which is about what I can find them for on eg Plumbnation, but the pipework and fittings add thousands on to the overall quote. Because some of our radiators are outdated and undersized anyway, I'm looking at one option being to start upsizing them piecemeal now, as and when we can afford, with TRVs fitted. 

The quote does do a room by room heat loss calc and radiators specified are sized to a designed room temp for each space according to heat loss. 

Hugh F re: sticking with oil - annoyingly we can only replace the oil boiler if we relocate our oil tank because the tank doesn't comply with current regs where it is now, and relocating the tank is not a simple problem to solve! I'm going to take your optimism about it running well for a good few years yet (which actually corresponds with our heating engineer's assessment - it's a healthy beast and actually running pretty efficiently).

Long term we would prefer to heat using renewables not oil but we can't do that if the tech/costs aren't there yet. 

So I guess the plan needs to be to see what efficiency gains we can make with our oil boiler in the immediate term, and hope it holds our a few more years until renewable tech is more affordably in reach. Maybe right now we reduce our carbon footprint by switching the diesel car for an EV which we can run off our panels! 

The other option that is on the table for us now is biomass as we do have the space to accommodate it, but it feels like that's a bad compromise? 

 

What you could consider, for the time being, is an Air to Air (A2A) heat pump, with several indoor units. It could be much cheaper, and possibly easier to install, than an Air to Water (A2W) heat pump. Such a system could be used to supplement your oil boiler, particularly during the Spring and Autumn periods and could also provide Air Conditioning (AC) during the hotter days. The other benefit is that it provides a backup heating system, so if your oil boiler should fail, you at least have some heating whilst you get the problem resolved.

We have an A2A heat pump which is running at the moment, powered free of charge from our solar PV system, and in doing so is helping to reduce our gas consumption and reduce our carbon footprint.

Since you have solar PV, do you also have a power diverter to help produce hot water?