Homeowners Professionals
Air and geothermal

Top of the COPS

Detached Edwardian home consistently near the top the COPS with Air Source Heat Pump


A desire to regain control of spiralling energy bills and move away from gas motivated semi-retired home-owner Rob Ritchie to investigate heat pumps and solar panels. Six months post-installation, his energy costs have turned into profits, his 1928 detached property is more comfortable than it’s ever been, and his system’s performance ranks among the best in the country.

Topping the chart

Having an outstanding heat and power system is undoubtedly a satisfying feeling. Chemistry textbook author Rob spends a lot of time these days looking at the heatpumpmonitor.org website which displays the performance data of all UK heat pump installations that use the open source OpenEnergyMonitor performance tracking system in addition to manufacturers’ own monitoring systems. “It's the best kind of timewaster,” he enthuses. “You can look in detail at how things are working.”  

And things seem to be working very well indeed. Since it was commissioned in August 2023, Rob’s air-to-water air source heat pump has consistently been in the top few positions. “Through December, my average Coefficient of Performance was 5.4, which is outstanding,” he says with pride. Air source heat pumps are expected to have a minimum CoP of 2.5-2.8, although when installed to   Viessmann’s requirements the Vitocals will score above 4. The gas combi boiler that the heat pump replaced could only achieve a maximum CoP of 0.94, or 94% efficiency, even when running at the lowest possible flow temperature to maximise efficiency.  

Rob’s two-year journey to energy excellence has not been entirely straightforward, however. “I’ve learned so much,” he asserts. “I wish I’d known at the outset what I know now!”  

Starting with solar

The energy price-hike that followed the start of the Ukraine war in 2022 was what originally prompted Rob to think about better ways to heat and power his three-bedroom house. He recalls, “The immediate thing I could do to gain more control was get solar panels.”  

These were installed in August 2022, together with a battery to store the excess power generated for use at night. “I liked the idea of using them for free electricity,” continues Rob. “But I also wanted to use the solar for heating water. The problem with doing that with the solar panels using my existing combi boiler at the time was that I didn’t have a cylinder.”

Extensive research

Researching water cylinders eventually led him to renewables enthusiast Mick Wall of energy-stats.uk. “It was actually Mick that got me thinking about heat pumps,” says Rob. “On his brilliant blog I came across a piece about water cylinders and plant rooms and found it very interesting. Mick had named Damon Blakemore as the installer behind his cylinder and heat pump, so I contacted him, knowing that the success of any heating system is dependent on the efficiency of the product itself, as well as an installer who knows it inside out!

“I initially asked Damon about getting a solar diverter put in, but we couldn’t get hold of one. Then, last December, I started looking at mixed heat pump systems. I asked Mick and Damon for their advice, and in March 2023, Damon did a heat loss calculation for me to get the ball rolling.”

Damon takes up the story: “As Rob had already installed PVs and a battery, an air source heat pump was the final piece of the jigsaw to really bring down his bills and carbon emissions. A £5,000 grant from the Boiler Upgrade Scheme (which the government has increased to £7,500 at the time of writing) helped to cover the cost.”


The new installation consists of   a single 10 kW Vitocal 150-A heat pump with weather compensation, built-in buffer for defrost and built-in instantaneous water heater. This is combined with a 200-litre OSO Geocoil water cylinder.  

For Damon, specifying the Viessmann Vitocal 150-A was easy. “Viessmann has the best controls on the market in my opinion,” he says. “I’ve installed Viessmann boilers for the past five years and don’t touch anything else. I’d been wanting to install the Vitocal 150-A for a while and this seemed to be the perfect project for it; there was space for the indoor and outdoor units, and I could get the flow temperature down to 40oC.” A low flow temperature maximises the efficiency of the heat pump.

Damon also proposed incorporating OpenEnergyMonitor to track performance, an unusual move as the Vitocal comes with its own integrated energy management and monitoring system. He explains, “OpenEnergyMonitor is popular among homeowners and installers who want an independent tracker to complement the manufacturer’s onboard monitoring. Rob really liked the idea of Viessmann’s ViGuide remote app for the engineer and the built-in ViCare app for himself. And it’s given us a real insight into how the system runs.”

The Vitocal’s outdoor module (which Rob says takes up less space than his council wheelie bins) has been installed at the side of the house while the indoor unit and water cylinder are in a previously unused area under the stairs. Says Damon, “It was a void before and now it’s a plant room - no space has been lost in the property at all.”  

According to Rob, the system generates virtually no noise, despite noise being a common concern about heat pumps. “You can hardly hear a thing!” he declares.  

The Vitocal runs direct via an open circuit, which means the entire property is heated to the same temperature, instead of having individual heating zones. Thermostatic radiator valves (TRVs) are all open, with radiator temperatures controlled by weather compensation. Damon set the radiator sizes based on his heat loss calculation. Rob explains, “I have my system running at 20 degrees in the day, then a setback temperature of 17oC at night –   but it never drops to that - it’s more often 19oC through the night. Damon has set up the system so that I’m heating every radiator in the house, and it’s all being done solely on weather compensation. Two years ago, with the combi boiler, I was only heating the rooms I use. The trouble with that is, heat is lost from warm rooms to cold rooms. Now, my whole house is more comfortable than ever before!”

Says Damon, “It’s a simple but very, very effective system, as can be seen from the monitoring results we’re getting.”


The Vitocal 15X-A heat pump series is the only heat pump available in the UK to have a built-in buffer to manage the defrost cycle. The outdoor unit will defrost regularly when frost conditions occur which drains energy from the heating system to complete this action. This means during the winter months a homeowner may notice a change in comfort within the property as the whole house temperature drops. The process then requires additional energy as the heat pump scrambles to get the property back up to temp which impacts efficiency.    

The Vitocal 15X-A has been designed with a 16l buffer in the indoor unit to manage the defrost cycle which means that the heat pump does not borrow energy from the home to defrost the outdoor unit. The heat pump preloads the buffer and delivers water   to the outdoor unit   heated up to fifty-five degrees (depending on what temperature is required) meaning the whole house comfort and energy efficiency is not affected. The evidence of this is in the higher COPs the Viessmann units can achieve compared to the rest of the heat pumps available in the market (see heatpumpmonitor.org)

From paying bills to earning income

For the new system to generate profit from selling excess solar electricity to the grid instead of costing money to run, the heat pump must be operating with a CoP of at least 3.5. In fact, it has been averaging almost double that at 4.7 (i.e., 470% efficiency). “Last June I made £300 profit in a single month from my solar,” says Rob. “Obviously in winter I’m paying for electricity, but it’s five times cheaper than it was because of the Vitocal’s CoP multiplication factor. As at 4th January 2024, I’m still £400 up on the year. And bear in mind this is for electricity, where I was previously paying for gas.”

Using the heat pump to heat his domestic hot water instead of a solar diverter as previously planned was a very good decision, too, he feels. “It now costs me around 6p to heat the water tank every two days. When I had the gas combi boiler, a typical shower would use something like ten times the amount of energy it does now. To say I’m pleased would be an understatement!”

System spec:

10kw Viessmann Vitocal 150-A air source heat pump with weather compensation  

OSO 200litre Geocoil water cylinder  

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