Ah... so based on that response from @jamespa I'm assuming the ACH which @matwin was enquiring about is Air Changes per Hour.

Perhaps I was thinking too widely.
In my mind I was also considering

• anti-condensation heater
• auxiliary central heating (for when the heat-pump is turned off)
• alternating current harmonics (the filter which is sometimes inserted into a heat-pump supply to reduce losses on the grid)

@editor thanks Mars - as a newbie I am just not familiar enough with the existing topis and whether my questions would fit into them...

1. what is the real world experience with Dr Modesta's Gas kWh / Heating Degree Days formula? Mars, what do you think?
2. I checked, my annual gas consumption for the last three years is actually 20MWh for heating and water. If an ASHP were to deliver that power with a SCOP of 4 surely we only need a 5kW ASHP....?
3. is there a topic discussing the risks of undersizing and disadvantages of oversizing?

cheers

Posted by: @matwin

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my annual gas consumption for the last three years is actually 20MWh for heating and water. If an ASHP were to deliver that power with a SCOP of 4 surely we only need a 5kW ASHP....?

No. But keep asking this question until we answer it satisfactorily!
That's what a forum is for!

On this occasion you've used two different units.

MWh is MegaWatt-hours which is the total amount of energy consumed over a period of time (presumably a year?)

kW is kilowatts is the instantaneous amount of power being delivered. There is no time-frame inferred here.

I have a 3kW kettle, but if it boils in just 2-minutes (one-thirtieth of an hour) then it's only consumed 0.1kWh of energy.

@matwin Hello, I’m not Mars but nevertheless, this never has stopped me chipping in before!

I can’t help with Q1 but, Q2, yes.

Your annual energy consumption is anything but equal throughout the year; during the winter months, your heating requirement is likely to be double or more compared to a summer month and also higher than spring or autumn times. You may find that your consumption from late November to the end of the following February accounts for the majority of that 20 MWh’s.

Were you working with a constant temperature throughout the 365 days with a similar energy requirement each day, then with a COP of 4, an ASHP with a 5 kWh rating might be somewhere in the right ball park. However, with winter loads being higher than the rest of the year, this approach does not compute!

A heat loss survey may reveal a more accurate idea of your winter requirements and I suspect it might be higher than a loss than a 5 kWh heat pump might cope with.

As to Q3, there are numerous discussions on undersizing or oversizing heat pumps - just do a search and I suspect you will find some to start you off. If you do not find anything that quite hits your target, you could always start a new topic in ASHP’s.

Regards, Toodles.

@matwin  I believe the degree days calculation is an accurate method of assessing heat pump size. Im not sure how much detail the YouTube video goes into but you can read the four full articles here https://protonsforbreakfast.wordpress.com/2022/03/15/heating-degree-days1-a-brilliant-idea/  

Personally I would trust it over the room by room assessments which vary widely between installers. 

A bigger issue you might have is convincing an installer to fit the size of heat pump you want. 

If the Degree Days calculation is suggesting 5kw and the installer 12kw I would be very wary of going with the big size. 

@matwin - one of the more confusing until you have got your head round it yet important things is the distinction between power and energy. For whatever reason, the scientists (physicists) who dreamt up the units used didn't do a great job on the terminology, largely down to overlap, eg power is measured in (kilo)watts, and energy in (kilo)watt hours (the kilo prefix just means 1000 times the base unit eg 1 kilowatt = 100 watts), and the definitions, which can be a bit abstract. Being a bit dim myself, I like to use words rather than formulae to understand things...

Take a car for example. Some cars are more powerful than others. The way we measure a car's power is usually in horsepower, but that is just another unit of power - how powerful something is. If you look in a modern car's handbook, or V5C (registration certificate or logbook), you will also see the power of the car given in kW (kilowatts), eg 95kW. If my car is rated at 95kW, and my neighbours car is rated at 80kW, then my car is more powerful than than neighbour's car. It has more power. 

But so far neither car has actually done anything. They are both parked in the garage, doing nothing. Power then is a static rating, a number that tells us the potential power of whatever it is that we are looking at. A 3kW electric heater is more powerful than a 2kW electric heat, but until we plug them in and turn them on, they do nothing.

The moment we start using the item (car, heater) to do something, we start using energy. The more powerful something is, the more energy it will use, and the more useful work it can do. A more powerful car/heater will use more energy, but will also achieve more. Because it is used for a time, energy use is dynamic - it changes over time. Staying with the electric heaters, a heater left on for two hours will use more energy that a heater left on for one hour. In fact the former will use twice as much energy.

Taking this one step further, we can use a combination of power (kW) and time (hours) to say how much energy we have used, and this is how we get the kilowatt hour (kWh). We multiply (this is the only bit of maths involved) the power by the time it is used for to get the amount of energy we used. Thus a 1kW heater on for 1 hour uses 1 x 1 = 1kWh of energy. The same heater left on for 2 hours would use 1 x 2 = 2kWh, as would a 2kW heater left on for 1 hour (2 x 1 = 2kWh).

Now, the conventional electric heater is 100% efficient. You get back 100% of what you put in. Feed it 1kWh of electric energy, and you get 1kWh of heat (which is also a form of energy - that's how steam engines work - the heat produces steam which drives the engine, though not very efficiently, a lot of the heat energy goes up literally in smoke). Heat pumps are a bit different, in that they give back more energy than we put into them from our mains electric supply. It looks like we might even be getting something for nothing, but we are not, that extra heat comes from the surrounding air, hence air source heat pump (and ground source when the heat source is the ground etc). Thus there are two energy figures: the electric energy we put in, and the heat energy that comes out. My heat pump may use 1kWh of electric energy in and put out 3kWh of heat energy. We can use the ratio between the two as a measure of how well our heat pumps perform. 3kWh out for 1kWh in is a ratio of 3, and we call this the coefficient of performance, or COP for short. A COP (which is not fixed, it varies as the performance of the heat pump varies) can be given for any period, including a whole heating season, when it becomes a seasonal coefficient of performance, or SCOP for short.

Thus for our heat pumps we have four numbers. The first is the kW power output rating - that just tells us how potentially powerful it is when it is running flat out, which in practice it hardly ever or never does. This is the number we aim to match to the building's heat loss - if out heat loss at design conditions is 10kW, we aim for a heat pump that has a power output 10kW (and a bit to cover domestic hot water). Next we have the electrical energy input it actually uses, over time, in kWh, measured as the sum of the actual power level it runs at, times how long it runs for. This is the number, in kWh, that we see on our electricity bills. Because of the way heat pumps work, they put out more heat energy than the electrical energy we put in, and so we also have an energy output, also in kWh. Some heat pumps measure and report this, others have external monitoring to get this number. Finally we have the COP, which is the ratio of the energy in to the energy out.

Unfortunately many people get these ideas mixed up, use the wrong terms (kW when they mean kWh or vice versa), and generally cause confusion. When this happens, remembers the basics:

Power in kW is a measure of power - how (potentially) powerful (power-full) something is.

Energy in kWh (a certain amount of power for a certain time) is what actually does the work (heating your home). 

Posted by: @matwin

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I checked, my annual gas consumption for the last three years is actually 20MWh for heating and water. If an ASHP were to deliver that power with a SCOP of 4 surely we only need a 5kW ASHP

My gas consumption was 18-20MW/yr.  It was 10% higher before I turned down the flow temp to 50 (from 75 which is where the maintainer left it) and started operating as near to ashp mode as the boiler would permit.

My 7kW Vaillant (which can do 8.5 at the ft I'm operating at, but somewhat less once defrost is accounted for is a good fit

I sized it by plotting consumption Vs degree days using smart meter readings. I also plotted max consumption averaged over 3,6,12 hrs.  All of these told me 7-7.5kW was the right size.  This was consistent with calculated loss is I assumed ach=0.5.  the had boiler was being run in a mode as close to how as ashp would be operated as possible.

@cathoderay thanks, I and probably, many others appreciate the clear explanation

I was confused by the rules of thumb (eg Modesta's total gas MWh with Heating Degree Days or / 2000 to give a clue to ASHP kW)

and I see you have a 14kW Midea and I wonder why you add "for now" as Im being offered a similar Clivet!

@bontwoody thank you - I am wary! The Youtube channel gives warnings about being oversized for the convenience of the MCS (via crazy ACH assumptions) and the installer who thinks like boilers, bigger is less trouble for them later.

Posted by: @matwin

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@editor thanks Mars - as a newbie I am just not familiar enough with the existing topis and whether my questions would fit into them...

1. what is the real world experience with Dr Modesta's Gas kWh / Heating Degree Days formula? Mars, what do you think?
2. I checked, my annual gas consumption for the last three years is actually 20MWh for heating and water. If an ASHP were to deliver that power with a SCOP of 4 surely we only need a 5kW ASHP....?
3. is there a topic discussing the risks of undersizing and disadvantages of oversizing?

cheers

Good questions.

1. Dr Modesta’s gas kWh/HDD formula is high-level estimator when combined with known internal set points and occupancy habits, but it shouldn’t be used as a substitute for a proper room-by-room heat loss calculation if you’re moving toward a heat pump. 
2. If your property uses 20MWh annually and an ASHP achieves a SCOP of 4, then yes, your electrical input theoretically should be around 5MWh. But a 5kW-rated ASHP doesn’t mean it can deliver 20MWh over the entire heating season. The sizing needs to match the peak heat loss of the home on the coldest days, not the average. That’s where loads of people get caught out (including installers, worryingly).
3. We’ve had loads of great discussions about sizing across these forums. I’d recommend using the search function and typing in “sizing.” You’ll find plenty of threads that go into detail.

Also, definitely take a look at this article which highlights how misleading the kW ratings of heat pumps can be in real-world performance: https://renewableheatinghub.co.uk/is-your-6kw-air-source-heat-pump-really-a-6kw/

The key takeaway is that heat pump sizing isn’t just about annual energy totals. It’s about matching the output capability to your home’s peak heat loss at design temperature, while keeping efficiency and modulation in check. It’s one of the most critical parts of the process, and sadly where a lot of poorly designed systems go wrong.

@matwin

This post is a real world example of the effects of oversizing and buffers.  It's worth reading it and the subsequent discussions.

https://renewableheatinghub.co.uk/forums/postid/45575/

Posted by: @matwin

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I see you have a 14kW Midea and I wonder why you add "for now" as Im being offered a similar Clivet!

It's there for historical reasons. In the early days after I had my heat pump installed I suspected it may be under-powered because a 14kW Midea heat pump is really an 11kW heat pump when it has to work hard. Many but not all manufacturers badge their heat pumps based on what it will put out on a sunny day in spring, and bury the fact they will do far less when it is colder, when you need the full output, in obscure tables of engineering data. At that time I believed my heat loss at design temps was 12.3kW, meaning my 11kW Midea output would be insufficient. It then transpired that the Midea 12 14 and 16kW heat pumps are all the same internally (same hardware), the badge rating is set in software using dip switches, and for a while I dithered about whether I should set mine to 16kW - hence the 'for now...'. Subsequently, using the methods discussed above, using actual energy use and output, I determined my actual heat loss is nearer 9kW, meaning the 11kW output was sufficient. Along the way there was another cautionary tale (about throttling a system by over-zealous radiator buffering, meaning I didn't even get 11kW), but that is another tale covered in other posts. The key points are:

(1) most manufacturers over rate their heat pump output. It's a legal scam, because they are allowed to quote output in undemanding conditions. It's the heat pump equivalent of quoting a car's performance when it is going down hill on a clear road with a following wind. To counter this, you need to dig out the performance data for more demanding conditions, including the design conditions. It is almost always available, but many manufacturers do their best to hide it.

(2) the familiar point about spreadsheet based heat loss calculations tending to over-estimate heat loss. Lazy installers like this - it gives them a reason to over-size the heat pump, just as they used to over-size boilers, to avoid customers moaning about inadequate heating. But with heat pumps size does matter, significant over-sizing incurs extra costs and will also usually mean a higher minimum output, meaning the heat pump can't modulate down enough to match lower heat losses on warmer days, and will end up cycling. Whether this matters or not is a moot point. If the cycling frequency is once or twice an hour, that is probably OK.

Sometimes we are lucky: (1) and (2) cancel out. The heat pump puts out less than it should according to its badge rating, but at the same time the spreadsheet based heat loss was an over-estimate of the actual heat loss. My real world heat pump output of ~11kW is an OK match for my actual heat loss of ~9kW.