Accuracy of our air source heat pump proposal

by Mars

This post is in response to an interesting question that was posed by Mark Crooks on one of our YouTube videos regarding our air source heat pump proposal and it’s accuracy.

Mark commented: “Personally I’m dubious about the figures that are provided by the system retailers for the obvious reason that it’s in their best interest to give you the best/cheapest possible figures. It would be interesting to know what running cost figures you were quoted before installation.”

So we’re going to pull the curtain back on our proposal and see how close it was. I have no idea what the answer is as I start to work on this post, but I am very intrigued.

As a starting point, air source heat pump performance is typically averaged over one year (encompassing all four seasons), so we will see how accurate the estimates were for our first year, which in our case ran from April 2019-May 1, 2020. Luckily, we’ve documented our entire process, so I have the data.

Let’s get stuck in.

Proposal recommendations and assumptions

So, let’s start at the beginning. As part of the proposal, Global Energy Systems recommended the 18kW Caernarfon air source heat pump for our property that would be integrated with our existing oil boiler to create a bivalent system.

This was done in accordance with guidelines set out by the Microgeneration Certification Scheme (MCS) that specified that the combined system must be capable of providing sufficient heating for our property down to our local area’s design temperature (for us that is -3.3°C).

In light of this, our air source heat pump would supply all the heating and hot water needs for our property down to -0.5°C; beyond this temperature the oil boiler would provide supplementary heating if required, and the heat pump would continue to provide heat down to -20°C (albeit it very inefficiently).

Estimated running costs and breakdown

As part of the estimate, Global Energy Systems estimated that our property would require 15.7kW of heat to keep it warm at -3.3°C. This, according to them, suggests an annual running cost of £1,670, offering a potential saving of £547 against oil.

Estimated Property’s Annual Energy Use (based on EPC)37,782 kWh
Current Annual Running Costs based on Oil at 52p/litre£2,218
Air Source Heat Pump Annual Running Costs based on electric at 14.5p/kWh£1,665
Annual Running Cost with an Air Source Heat Pump + Supplementary£1,670
Annual RHI (Renewable Heat Incentive) Payment based on 10.49p/kWh*£1,459
Annual Savings£2,006
Full breakdown of costs

In addition to this, they provided us with heating fuel comparison graphs to see how we’d perform against other fuel sources.

Heating fuel comparison graphs

Our real world data versus the numbers in the air source heat pump proposal

Looking back at the proposal, the outline of base costs was very accurate. We were paying 14.5p/kWh at the time and the price of oil was around 48-50p per liter.

What I’ve always found odd is the ‘savings’ derived from the RHI that have been factored into the calculations – this simply isn’t a saving. The RHI is repaying us for the investment of the air source heat pump and related equipment, so I really don’t take that into account. This is definitely an exercise in loading the numbers.

So what were our numbers?

Energy consumed for heating12,890kWh
Running cost at 14.5p£1,869
One year of oil£2,400
Saving in year one£531

Essentially, we saved around £530 in the first year on heating. Yes, the outlay for the air source heat pump was significant, but using renewable, clean heating is a massive step for us on the path towards sustainability.

Another massive difference between the ASHP and the oil boiler is that with the ASHP our heating in the cold months runs 24/7 versus limited timings on oil. We’ve covered this aspect before.

And don’t forget that our solar PV array and iBoost also offset our daytime heating when the sun is shining, especially from spring to mid-autumn.

So the proposal numbers weren’t terribly off the mark, and the proposal was professional and detailed. While we shopped around for air source heat pumps, we also received quotations and proposals from other providers, and I’ve summarised those as a point of reference below.

Exclusive Offer

If you’re interested in an air source heat pump from Global Energy Systems, you can use this code when you contact them and you’ll get £200 off your installation (and we’ll get some Amazon vouchers): GESRFAF000160

What other providers claimed

Provider 1

We have calculated that your building will require 28,909 kWh of heat energy each year and that 28,909 kWh of this will be provided by the heat pump, leaving 11,263 kWh to be supplied by a backup boiler. The space heating seasonal performance factor (based upon the heat emitter guide) will be 3.3. This means that the electricity required to run the heating will be 8,895 kWh.

We have calculated that the annual hot water requirement will be 2,780 kWh, and 2,432 kWh of this will be provided by the heat pump with the rest coming from the immersion (for legionella cycle) or backup boiler. The hot water seasonal performance factor will be 3.1, meaning that the hot water will use 895 kWh of energy to run the heat pump. The maximum time to fully recharge this cylinder will be 8,895 mins.

The seasonal coefficient of performance (SCOP) factor for the combined heating and hot water will be 3.23, meaning the total electrical energy to run the system, including all the circulation pumps, will be 4,020 kWh.

Recommended unit: Hitachi S80 6.0, 25Kw (equiv) high temp high output 80 deg C split heat pump.

Provider 2

This company never bothered to give us a prediction estimate or breakdown but wanted to installed two 11.2kW Ecodan Monobloc Mitsubishi heat pumps.

Provider 3

Also never gave a detailed breakdown. They stated: “We’ve quoted for a hybrid installation including a 14kW Mitsubishi Ecodan to provide the majority of your space heating, with your oil boiler providing hot water and assistance during very cold weather. Your domestic hot water will continue to run from the existing oil boiler and DHW cylinder. The EPC for your property estimates 34,821kWh of space heating is required.”

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