Homeowners Professionals
Homes
Residential
Air and geothermal

Groundbreaking air source installation in Grade II listed building tops the efficiency charts

Having worked in sustainable building and heating systems for a long time, Alex Hilton was keen to dispense with gas altogether when renovating his large, timber-framed Grade II listed home in Buckinghamshire. Despite being told by several heating engineers that a heat pump wasn’t feasible in such an old building, Alex persisted. Now his Viessmann Vitocal 150-A is consistently near the top of the Open Energy Monitor heat pump monitoring data comparison website’s scoreboard with a Seasonal Coefficient of Performance (COP) of 4.8, while his project’s innovative approach to piping and water heating sets it apart from the crowd.

Viessmann Vitocal 150-A Grade II listed building

Environmental aspirations

As former Head of Sustainability for a large organisation, Alex Hilton is no stranger to environmentally friendly technologies. He spends a good deal of time thinking about them, both at work and at home. So, when he and his wife set about replacing the 1980s gas boiler in their large 17th Century house in Aylesbury, his heart was set on a heat pump from the outset.

The existing heating set-up consisted of a typical gas boiler and radiator network with microbore piping. There was also evidence of previous oil heating before the gas was installed.  

“The old boiler was sitting ‘pride of place’ in the kitchen fireplace,” Alex recalls. “It looked awful, but because of our listed status, we would have needed listed building consent and possibly planning permission to move it. So, given a like-for-like replacement would’ve been such a pain, and because I know what’s happening in terms of the government and how gas boilers will be phased out, I didn’t want to put a temporary fix in.

“We had experience of solar panels and batteries from our previous home, and we have an electric car,” he continues. “So, we were already down the line in terms of adopting renewables. We’re both very invested in the concept of being more sustainable. For us, a heat pump was the next obvious step.”

Breaking the mould

Although Alex knew going down this route in a building that was over four hundred years old would be flying in the face of conventional wisdom, he wasn’t fazed. “I know everyone says heat pumps don’t work in low-efficiency houses, but I also know gas boilers don’t either. It’s a mindset, not a real problem. I knew it was possible.”

Alex’s first challenge was to find a contractor that agreed with him. He came across Liam Barry, Managing Director and heat pump and low temperature heating system designer and installer of Custom Renewables, while reading about the technology on Heat Geek,  an online learning hub for heating installers and consumers  advancing system design and hydronics for heating systems.

Unlike the other installers he’d approached, Liam was very much open to the idea. “He seemed brilliant!” remarks Alex. So, after some preliminary discussions, Liam visited the property to carry out a heat loss survey.

Feasibility

The results were encouraging. Says Liam, “What we found was that the property wasn’t that far away from being able to run off one big 16 kW heat pump, because the heat loss was only around 12/13 kW, which is the output we’d expect for a 16 kW heat pump running at -3 degrees Celsius.

The fact that Alex and his wife had already decided on under-floor heating (UFH) throughout the ground floor regardless of the heating system they ultimately chose helped nudge the efficiency equation in the right direction, as it could run with lower flow temperatures. Also helpful, was the couple’s willingness to replace the building’s microbore piping. “It was easier to re-pipe the whole house and it didn’t take that long,” Alex remarks. “Probably just over a week.”

With some additional insulation in the loft and the making sure the secondary glazing was all in good condition, the stage was set for the creation of a cosy and energy-efficient home.

16 kW Viessmann Vitocal 150-A air source heat pump

Viessmann advantage

A 16 kW Viessmann Vitocal 150-A air source heat pump was selected on Liam’s advice. “I picked a Viessmann unit because their controls are top-tier, and they offer the best weather compensation on the market. Not only does this give higher efficiency, but the small internal buffer used during defrost on a four-way valve, means any defrosting during colder spells is minimal.

“I also recommended the unit because of its backup,” Liam explains. “So, if there were ever any issues with a higher heat loss, or if the homeowners ever wanted extra heat, they would have that option to pick it back up again.”

“I also rate Viessmann heat pumps because of their controls, especially when working alongside weather compensation.”

The heat pump, which provides heating and hot water, is powered by 18 PV panels with 10kWh battery storage, which together provide enough energy to cover most of the energy demands of the home, including an electric hob and an EV.  

Additional regulations

Because of the property’s listed status, there were a number of regulations and approval processes that made the installation more complicated and costly. For example, holes could only be drilled through certain external walls and only traditionally styled column radiators were allowed to be used. The building’s size and long, narrow shape – its three floors are each 3 rooms wide and one room deep - also added complexity.  

“It’s a large house and when you have heritage elements, you’re starting to pay premiums,” explains Alex. “It cost us an additional £7,000 for the radiators that the historic buildings people wanted us to use. In our previous house, a 1930s semi, the microbore pipework would’ve been fine. Not the most efficient, but fine. And it probably would’ve been a lot less costly to install the system.”

A £7,500 grant from the Boiler Upgrade Scheme (BUS) helped to offset some of the costs.

Fortunately, the downstairs flooring wasn’t original, and had a concrete screed, which meant UFH could be installed throughout the ground floor – 1,800ft2 in total. Says Liam, “We enlisted the help of JK Flooring, who did a great job.” Additional stone flagging was also added throughout, which created a bigger mass for the heat, as well as being aesthetically pleasing.

Innovative centralised piping layout

The visual appearance of the installation was also enhanced by running the pipework out of sight through the house’s eaves. “We used a store room space on each floor,” Alex explains. “For example, on the ground floor it was a wood store and on the first an airing cupboard. The system runs all the way up and then drops down from the attic into the first floor. It’s great from a visual perspective, minimally invasive from a heritage perspective, and means far less wastage from a heating perspective.”

The Hiltons opted for multilayer composite (MLCP) instead of copper piping in most of the building, aside from copper drop feeds to the new radiators and two towel rails. MLCP can be easily bent around corners with no joins or spring-back, making it ideal for use in refits with little space or where drilling needs to be kept to a minimum.  

The pipe connection to the outdoor unit (ODU) of the Vitocal 151-A air source heat pump is also unusual because it runs through a 5 m underground trench. “In normal properties, the external heat pump unit goes against the wall where you want it in the house, so very little needs to be weather proofed and insulated,” Alex explains. “Where we’ve put the external unit isn’t directly where it goes into the house, so we’ve used piping that’s fully insulated and not easily damaged.”

The aesthetics of the ODU are also very appealing to Alex. “I like the look of the Viessmann heat pump unit. So much so, I even questioned whether we needed the willow screen around it. And it’s very quiet. We don’t notice it’s on. We do hear a quiet hum from the indoor unit at times because of the vibration through our timber-framed walls, but we’re hoping filling in some of the gaps around the pipework will help.”

New type of HW cylinder

Another novel aspect of the system was the incorporation of a ‘super cylinder’ to improve hot water efficiency. “I used to find that many heat pumps on the market were struggling with this,” recalls Liam. “A normal cylinder only does 250-300% efficiency, pulling down the overall performance in winter.”

To solve this issue, he worked with Newark Cylinders and Heat Geek, to create a range of ultra-high efficiency cylinders known as the HG Series, launched in April 2024. “They contain 6 m2 of coil, as well as a few other special creations,” he enthuses. “This means a heat pump system can produce up to 500% efficiency in hot water mode.”

Alex confirms the effectiveness of this solution: “The water that comes from the shower is super-hot!”

No buffer

A further innovation Liam brought to the project was that the heat pump was installed in an open loop system without a buffer tank. “I was told this wouldn’t work,” recalls Liam. “Even the manual says that. But I re-calculated the maths, because the Vitocal actually has a 7.5m head pump inside. I realised that Viessmann’s product designers had calculated 40 m of pipe between the indoor and outdoor unit which I wasn’t going to use, so I knew I had a certain amount of pressure loss to play with. I believed it was doable but had to go ahead and install it to prove my theory. I put everything on the line.”

Fortunately for everyone involved, the calculation paid off. Liam describes the nerve-wracking commissioning moment: “When I turned it on, there was air in the system. It’s quite a big system to get right. But as soon as I sorted the air, it was running smoothly, and is currently operating at 520% efficiency.”

‘Magical’ results

Since the system went live in April 2024, it’s has stayed in around second place on the Open Energy Monitor scoreboard. “Not bad, when you consider how many people said it couldn’t be done!” quips Liam.  

Alex is delighted, too. “It’s brilliant!” he effervesces. “I still think it’s a bit magical in terms of how it works and how it keeps us warm. Before the building had all the hallmarks of an old house, but now it feels cosy. There are fewer drafts too; we’re not seeing the same shifts in temperature that we were before. My mum came to visit and said it’s really noticeable how cosy it feels.”

The new system did take a bit of getting used to though. “Because we’ve got the UFH, which is new to us, and the low temp of the system generally, it’s a slightly strange first experience. You don’t go up to a radiator and feel a burn like you’ve been out in the cold. It's a comfortable warm. It feels healthy. And it’s better for the house, too. With the gas, we were fairly frugal with our heating, which wasn’t good for it.”

Coming off gas and seeing utility bills fall dramatically is the icing on the cake for Alex. “In the last couple of weeks, we’ve been using barely any electricity from the grid!” he beams.  

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