Rooftop ASHP with microbore tubing more than a pipe dream!
When Richard Smith’s combi boiler began to fail at his modern three-bedroom semi-detatched home near St Albans, he knew he wanted a more efficient and environmentally friendly system to replace it. Despite limited outside space and narrow pipework throughout the property, his dream of owning a heat pump was successfully realised with a roof installation.
A greener solution
Built in 2018, Richard Smith’s home in Bricket Wood, Hertfordshire, is a compact (108 m2), flat-roofed, two-storey, brick building in a contemporary Dutch style. However, despite being fitted with a modern gas combi-boiler and solar panels, it wasn’t as energy efficient as it could be.
“The boiler had been there from the start and wasn’t much good,” explains Richard. “When it started to malfunction, I saw no point in replacing it. I’d always been thinking about a heat pump but wasn’t too sure how to go about it. So, I did my research.”
He soon discovered that the size of the pipework throughout his property might present a problem for the flow rate of the potential new system. “To get the best efficiency out of a heat pump, you want the pipes to be as big as possible. That’s why you hear about them making pipes 22 mm wide. The pipes in most UK homes are 15 mm, but the ones in my property were 10 mm or even smaller.
Pipes under 15 mm in diameter are termed ‘microbore’. Changing them is a big job which would then require much of the house to be redecorated – an expensive disruption that Richard was keen to avoid.
He set about looking into the topic more deeply, finding lots of helpful information on the Heat Geek website, among other places. By the time he reached out to heat pump experts Tom Sullivan of Active Heating & Renewables and Andrew Millward of Home Comfort Assured, he had already carried out his own heat loss calculation using a free estimator tool from heat-engineer.com.
Existing infrastructure
Richard was delighted to discover that, with full insulation, it would be possible to install an air source heat pump and achieve an efficiency level several times that of a gas boiler, even with his home’s narrow pipes. “This is why I think my project is good for others to hear about when they’re doing their research,” he enthuses. “It proves heat pumps can work in modern houses with microbore tubing. So much so, in fact, that we only needed to change one radiator! And that was in a room that has always felt cold because it has two patio doors, and the previous radiator was too small.”
Tom and Andrew advised Richard to opt for a 4kW Viessmann Vitocal 151-A air source heat pump with weather compensation and integrated 190-litre DHW cylinder. Optimised for power consumption from photovoltaic systems, it offers both heating and cooling and is exceptionally quiet and highly efficient in operation.
Lack of space
The next issue to look at was where to site the heat pump itself. The compact indoor unit fitted snugly inside the property’s small boiler cupboard. But, with only a very limited external footprint to play with, the location of the outdoor unit was less clear cut. Says Richard, “As this is a modern development, there is no space at all, really. The roof is the only place to put things.”
Tom concurs: “The roof was definitely the most sensible place. We’ve done heat pump roof installations before, so we knew how straightforward the process could be, with the right crane driver. We used London Transport and Lifting Ltd. There was no need to close the road for the crane, as there was plenty of space for cars to go around it.”
Richard organised scaffolding to provide access to the roof, and the installation of the outdoor unit took around an hour in total.
“It’s a flat, new roof, with solar panels there already,” continues Tom. “So, we knew the structure would be sound. For anyone thinking of doing similar with a heat pump installation, if they’re unsure if their roof is capable of supporting the unit, they can get a structural survey to check.”
Comfort and economy
Richard was keen to add an OpenEnergyMonitor device to the system to share data about its performance online and help to dismantle some widely held myths about where heat pumps do and don’t work. Since going live in April 2024, the Coefficient of Performance (CoP) of the new heat pump has averaged 3.5, with a flow temperature of 45-50oC.
Richard switches the hot water on once or twice a day during off-peak periods. He uses the Octopus Cosy tariff for heat pumps. However, he is also making use of his scaffolding to add further solar panels – ten in total – as well as a battery to store generated power, to his roof. This means the heat pump will operate at virtually no cost at all. The cost of all these upgrades has been offset by a £7,500 grant from the Boiler Upgrade Scheme.
“Installations like this one showcase that heat pumps can go anywhere,” observes Tom. “There’s so much more flexibility than most people realise. I can definitely see installations like these becoming more common.”
Equipment
OpenEnergyMonitor device
