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.