Hi Mars,

Like many other pieces of equipment, the efficiency of ASHP's is dynamic, in that the efficiency varies with operating conditions.

A good example would be a car. If a car is driven at constant speed on a flat straight road, it will achieve a much higher mpg than if it is driven up and down hills or even just around bends. The car is therefore working more efficiently under constant operating conditions. Another factor which affects the mpg of your car is the loading, a car containing just the driver will use less fuel than one containing 5 adults, under the same operating conditions. You can correct me if I am wrong, but I believe in the automobile industry, quoted mpg figures are measured at a constant 56 mph.

The data label on your ASHP, is specifying it's expected performance at different ambient air temperatures, when heating the water to a temperature within a specified range, 30C to 35C in your case. Both the ambient air temperature and the desired water temperature affect the overall efficiency of an ASHP, so to make meaningful comparisons, it is necessary to keep one of the variables as constant as possible. Obviously, it is much easier to control the water temperature rather than the ambient air temperature. The efficiency of an ASHP will reduce as the ambient air temperature falls, because it is having to work harder to extract the same amount, or even more, heat energy from colder air. The efficiency will also reduce, if the ASHP is required to produce warmer water, as the heat loss of a property increases as the ambient air temperature falls.

This is the major drawback with ASHP's, in that they are less efficient at the precise time when they need to be more efficient.

Looking at the specified data for your ASHP, if the overnight ambient air temperature was to fall to -7C, and the heating load of your property required a water flow temperature of 45C, Then the actual dynamic CoP under those operating conditions would probably be below the 2.68 quoted on the data plate. This is probably why your data plate specifies a water temperature of 30/35, which of course produces a higher CoP value.

If during the afternoon, the ambient air temperature were to increase to 7C, and the heating load were to reduce to a level that only required a water flow temperature of 35C, then the dynamic CoP would probably be in the region of 4.76.

During the same day, your ASHP operating efficiency can go from one extreme to the other. To get a meaningful CoP value for any particular ASHP, it is therefore necessary to record the heat energy output and electrical energy input over given time periods. It would then be possible to compare day to day, week to week, month to month and even year to year. Such records may also be useful in identifying any reduction in efficiency over time, which could be indicative of a fault developing. This technique is used extensively in industry and is called 'condition monitoring', which allows planned maintenance to be performed only when necessary, rather than on a regular basis.

Mars, when your say that your ASHP is running at a water flow temperature of 45C, is this a fixed temperature setting or is this a fixed upper limit?

Personally, for best efficiency, I would have expected the control system to vary the water flow temperature to match the requirements of the heating load of the property, such that as required by the above example, it would increase the desired water flow temperature as the ambient air temperature falls, but then reduce the water flow temperature as the ambient air temperature rises.

I hope this makes sense, but please feel free to ask any further questions.

Mars, when your say that your ASHP is running at a water flow temperature of 45C, is this a fixed temperature setting or is this a fixed upper limit?

@derek-m, this is the upper limit as far as I know. I've not really looked at the weather compensation feature and whether it kicks in when ambient temperatures outside drop. Global Energy Systems have informed me that this feature exists on the ASHP and that it is enabled. I will watch our flow temperatures, when we start to edge up to 10C, which will hopefully be in the weeks ahead. 

CoP is tricky - I think we have to read it with a "pinch of salt" 

To my knowledge; CoP is calculated based on the temperature at the condenser unit and not your radiator or Hot water tank. So if your ASHP is sitting far away from your end radiators or Hot water Tank - then you may want to consider the heat loss till the heated water gets there, which is further reduction in the "Target CoP" - We are learnt from the automobile industry - milage is never accurate and tools to measure is discouraged. 

Nevertheless- there is no question, ASHP is drastically economical compared to any oil/gas based boilers.

Hi Mars,

Our weather forecast is showing 9C at the moment and for much of today, so I would expect it to be somewhat similar in your area.

I would be interested to know what values your system is indicating for water flow temperature, heat energy generated and electrical energy input. If you system is operating efficiently your CoP should be well above 4.

I have my solar and battery storage installed and I'm still swithering over my ASHP. I have read a lot and watched an great deal of videos but I still have lingering doubts. If I change my gas boiler for the ASHP it will be my only heat source. Is this wise? 

I am being recommended the Vaillant aeroTherm plus since it uses R290 coolant and heats to a higher temperature removing the need for a secondary heater to guard against legionella. I hear all your comments about COP but Vaillant quote a SCOP (seasonal COP) which, or so I read, should be better as it gives a better indication of annual performance.

any thought?

@duncan-mac - I think you will benefit no matter what by moving away from that gas boiler, even if the ASHP performs at its worst SCoP - its better than the gas boiler but you have raised a very valid question, your battery storage may not be able to cope with the energy needs of ASHP. and yes, you will be drifting away from redundant energy source. If you keep the gas boiler as a backup then still you will have to regularly maintain that darn thing. 

I opted to completely decommission my gas supply, moved to ASHP, induction cooker etc. with the option of having my fireplace as my backup in case the ASHP goes south or for those rare power disruptions.

I think as Derek says, the manuafacturers' numbers are largely theoretical.  Our ASHP (to be installed) has -3 Air, 35/30 water out/return, COP 2.99.    There is nothing to say what the water being heated is doing, just that it leaves at 35 and comes back at 30.  It could be going to radiators or running in and out of a bucket. That's like saying a car will do 50mpg at 50mph on a rolling road.  Our calculated SCOP from our quote is 3.2. I'll be surprised if it's that high in practice but the higher the calculated number the higher the RHI payments (that's my undderstandiung anyway).  If I get the 2.7 SCOP Mars does I'll be happy enough as that will save me significant money.

@neilsondhi, if you're saying an ASHP uses less kW than a gas boiler then that's true most of the time.  But if you're saying it uses less £, then at current prices that's not the case. Electricity is 4-5 x gas in price per kWh and that's never going to be made up by SCOP and boiler (in)efficiency.

@kev-m That is what I mean - measuring the kWh and not the cost. Since electricity cost is variable per kWh, gas kWh is measured in joule BTU in ideal gas quality but we all know gas supply quality in joule is very hard to measure. Energy content of natural gas varies because natural gas has minor variations in the amount and types of energy gases (methane, ethane, propane, butane) it contains: the more non-combustible gases in the natural gas, the lower the gigajoule value. In addition, how much of any energy gas that is present in a natural gas accumulation — the mix of combustible gases — also influences the gigajoule value of natural gas. The more carbon atoms in a hydrocarbon gas, the higher its gigajoule value. 

Now think about electric - 1 kWh is 1 kWh - consistent all the time sadly that is not the case when it comes to Gas. 

The table attached is what I have for the aeroTherm. Are you saying I have to take this with a pinch of salt? SCOP of near 4?

Hi Everyone,

As it has been correctly pointed out, CoP is an instantaneous measurement of the efficiency of an ASHP (or other source of heat energy).

It is obviously used by heat pump manufacturers, since it is possible for them to quote the highest value obtained under ideal operating conditions.

Heat pumps do not operate under ideal conditions, for their full operating cycle, in the real world, so the preferred method of assessing efficiency is to use a Seasonal Co-efficient of Performance (SCoP) calculation. This calculation takes account of the anticipated number of hours throughout the year, that an ASHP will operate under defined conditions and therefore specified efficiencies. There are even different operating conditions specified for different climatic regions, from colder, average, to warmer.

Whilst I am a firm believer in the harnessing and use of renewable energy, I am not yet at the stage where I am willing to put 'all my eggs in one basket'. My intention for the future is therefore to keep my gas boiler, install an air to air ASHP that will run when there is available electrical energy from my solar panels. This will reduce our gas consumption even more than the 20% reduction we have already made by using direct electrical heating from our solar panels. The gas boiler may run during the night to maintain the desired indoor temperatures.

Any additional solar energy not utilised by the ASHP would be used to produce hot water, and I am also contemplating adding an electric storage radiator or two that can be powered solely by solar electric, in this way there should be occasions when the gas boiler is not required at all during the night.

The fact that our gas boiler also has a fire, which can be operated without the need for an electrical supply, provides us with an available heat source in case there is a power cut.

There are proposals, for the present natural gas supply to be at least supplemented, if not totally replaced with hydrogen. This could potentially provide a renewable gas system, which would dramatically help with reducing CO2 emissions.

At the moment I do believe that gas is still the cheapest form of heating in the UK.

@duncan-mac HAHA !! you got me on that. I have learnt not to get carried away by those advertisements and fancy numbers. Those numbers are based on lab tests under ideal conditions. Not your house and the energy efficiency of your house 😀 - Yeah - take that Pinch of salt - it will help keep your expectations realistic. 

Honestly thinking - better performing ASHP under lab conditions has better chances to perform in real world, so it's not a parameter to ignore. also remember there is always loss of energy when converted from one state to another. CoP is measured at the condenser and if you have heat exchangers down the line - your effective CoP could be a matter of thought.

I personally have reduced my heating expense to up to ~60% by moving to ASHP.