Posted by: @thebarn

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The calcs predict a demand of 13.22kw, so an ASHP of 20kw has been proposed

A 20kW ASHP to supply a 13.22kW demand seems a bit on the high side, even allowing for the fact most heat pump suppliers inflate their rated outputs by quoting outputs in conditions that show the heat pump in its best light (and in conditions when you won't need that output anyway, as the weather is mild). You need to get the output (from the manufacturers data) at lower outside air temperatures, particularly at around 0-5 degrees C, when the dreaded defrost cycles will further reduce your heat pump's real world output, and find a model that can supply the demand at those temps.

Very roughly, 1 litre of oil of CH oil (regular stuff) contains about 10kWh of heat energy. Take off a bit for efficiency losses, and lets say 1 litre delivers 8 kWh. If you currently use 3200 litres per annum, that means you are delivering about 25600 kWh to you home to heat it (and the hot water). This is considerably less than your quoted 54,146 kWh (it's energy, not power, so kWh not kW) per annum. Furthermore, almost 55,000 kWh is a LOT of energy. I suspect the quoted figure is suspect (happens all the time) - go back to the source and see how they got to that quoted figure. If the source is an EPC, it isn't worth the paper it is printed on.  

If your actual annual energy use is around 25,000 kWh, and your ASHP has a COP of around 3 on average (it varies a lot day to day, but that is a not unreasonable average) then you need around 8333 kWh energy in, and at unit cost of £0.34, that comes out at around £2833 per annum. The fact is that the high price of electricity per kWh compared to oil per kWh means that heat pump central heating is generally more expensive to run than oil central heating, all other things being equal (but see next paragraph for a gotcha).

There is also another consideration, which I learnt the hard way (I went from oil to an ASHP). If you run your oil CH on a timer, as many do, much of the time it is off. An ASHP on the other hand is typically on 24 hours a day in the heating season. The overall effect is the house stays at a warmer average indoor air temperature (doesn't cool a bit overnight/during the middle of the day), meaning you end up using more energy overall. The problem is the heat pump is a steady Eddie, it struggles to recover from setbacks (heating off periods), where as oil is fast and furious, and so quickly recovers from setbacks, meaning the home is at the temperature you want it to be at the times when it matters. It is quite difficult to quantify this extra energy use in advance, but it is real, and does increase total running costs, sometimes considerably.

Hopefully others with larger heat pumps will be able to recount their experiences of running costs. You can't beat real world data.    

Thanks @cathodeRay - that's really helpful. I was expecting the cost to be slightly higher, but not what I calculated! Your calcs are more along of what I would hope for cost wise. The constant cost is also helpful thing to flag

Thanks again

25,000kWh over 6 months, at 23hrs/day of heat input (allowing 1hr for dhw) gives you a continuous heat input of just over 6kW.

I bet you could get away with a 10kW Aerotherm plus….

What is your oil boiler rated at?

@hughf thanks. I’ve currently got a 32kw Worcester Danesmoor Oil Boiler.

@thebarn massively oversized for the heat demand then…

Have you seen the heat loss calcs the quoting company have produced?

@hughf Not the actual calculation, just the end result figure of 13.22. It was done by a heating company. They were here all afternoon measuring room sizes, window sizes, insulation etc. They originally quoted a 11kw pump then upped this to 18kw after they did the measurements and now have gone awol about giving me a price. The new quote was from a new company using the data from the 1st.

Posted by: @thebarn

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They originally quoted a 11kw pump then upped this to 18kw after they did the measurements and now have gone awol about giving me a price. The new quote was from a new company using the data from the 1st.

Sadly, this sort of nonsense is all too common. At one point I very nearly issued a decree that if any employee of a certain heating company were seen within a ten mile radius of my house, they should be shot on sight, and the body incinerated in the nearest available biomass furnace.

I am sure @hughf will agree, it was/is very common for fossil fuel boilers to be hugely over-size for the demand, so the fossil fuel boiler's nominal output tell you very little about the building's actual demand. What is useful is looking at what the fossil fuel boiler actually uses to keep the building warm, as you have done above, because that is an empirical estimate of the actual heat loss, but it is only an estimate, and other things will come into play eg always on vs timed running.  

Posted by: @thebarn

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@hughf Not the actual calculation, just the end result figure of 13.22. It was done by a heating company. They were here all afternoon measuring room sizes, window sizes, insulation etc. They originally quoted a 11kw pump then upped this to 18kw after they did the measurements and now have gone awol about giving me a price. The new quote was from a new company using the data from the 1st.

The '20kW' Warmflow (AS03-R32) is 20kW at 7C, falling to 14kW at -7.  Not sure what your design temp is but it does sound rather oversized, and like others here I wouldn't buy based on the behaviours of the company quoting.  You very likely wont be happy and it will very likely cost a lot to run because it will constantly be short cycling. 

As @hughf says, if your annual consumption is 25MWh, you heat most of the time (or turn off only overnight) and are happy with the temperature, then a 10kW Arotherm plus or 11.2kW Ecodan should do the job.  If you can invoke a bit of supplementary heating at the rare extremes when its below design temperature for several days in a row you should definitely be fine.

You need another heat loss survey and they need to take into account any fabric upgrades done and also think a bit about what air change value to use, it appears that this may over be over-egged.  Ask for the calculations then sense check it against your actual consumption.

I had two full surveys (3hrs each) (only one of which I paid for, the other was free). Both ignored the fabric upgrades that they couldn't see (even though I told them about them repeatedly) and came up with 16kW.  One double counted about 1kW, by including room to room losses but not room to room gains!

If I do the calculation myself with the MCS 'recommended' air change value for my age of house (and include the fabric upgrades) I get to 10.5kW.  The actual measured consumption at the design temp of -2 is 7.5kW, gleaned over two years now, by eyeballing the smart meter frequently, doing a plot of consumption vs degree days from downloaded smart meter data, and from annual).  That means that the surveys were worse than a factor of 2 out which matters.  To prove the latter I down rated my boiler as low as it will go (8.5kW) and it is just fine. 

I can get a survey type calculation to match the measured 7.5kW, but I have to assume ACH = 0.5 per hour.  

With an ASHP minimum output matters almost as much as maximum.  Most of the time your demand will be around half the calculated value (my 7.5kW house has a measured demand of around 4kW on the majority of days) and its important that your ASHP can modulate down to that figure.  If its oversized it wont, which means that it will spend most of its life cycling, which will reduce efficiency.

(As an aside I recently saw figures which suggests gas boilers also suffer from significantly reduced efficiency when short cycling.  Basically most of the heat goes straight up the flue.)

@cathoderay Yes, it is very confusing to have been quoted 11kw, (16kw when I looked into it a few years ago), then 18kw and then 20kw! Go in too low and be cold and too high and be wasting electricity and money. 🤔

@jamespa Yes, I think another survey might be called for - but the guys that did the last one were here for about 7hours so I assume they did a thorough job. And now I'm even more confused as someone in my village I suggested I should get a R290 and not an R32!?

Posted by: @thebarn

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@jamespa Yes, I think another survey might be called for - but the guys that did the last one were here for about 7hours so I assume they did a thorough job. And now I'm even more confused as someone in my village I suggested I should get a R290 and not an R32!?

In fairness there are unknowns with a retrofit, air changes and often fabric construction where upgrades/extensions have been made, and these unknowns make a big difference.

Many designer err on the high side because they won't get call outs if the system is too big, but they will if the occupants are cold.  However oversizing leads to its own problems.  Arguably most, and certainly many, of the reported problems with heat pumps have their roots in oversizing.  Unfortunately you do need to check the assumptions surveyors make or sense check them against actual consumption.  The latter is fact, spreadsheets suffer from gigo and, since there unknowns, an element of gigo is almost inevitable.

If you would care to tell us more about your property construction, what you know about consumption and current use patterns, I am sure several people will make some helpful suggestions.

Re R290 Vs R32 I wouldn't be too concerned unless there is a specific reason to go for one over the other.  R290 has a lower gwp, works at higher temp, and is the more recent, but it does constrain the location of the outdoor unit a bit.  However the principal concern should be the quality of the system design and installation not the outdoor unit itself.  System design has the biggest impact on performance, much more than R290 Vs R32.