Highly Commended

Please tell us about a project you are particularly proud of – what was the problem and how did you solve it for the customer?

Over the last two years, I’ve been installing heat pumps into old houses with great results, but when a job came up in a 300-year-old property in the South Downs, I knew I had a challenge on my hands.

The customers are a family; retired parents with two daughters at university. They needed a hot water system able to cope with a high demand, and they needed to heat their home efficiently at a comfortable temperature, all day.

The property walls were wattle and daub construction, with alterations inside, making a heat loss calculation a theoretical test; there are no published U-values for these wall types. I found a study by Cardiff University where they published U-values for old buildings. Using this,

I recalculated with the alterations to modify the U-value. This was done for several fabric types because the property had been altered throughout the years. It was a real challenge to come to these U-values with confidence, but fundamental to making the project work.

With the help of Heat Geek’s new room scan technology, the heat loss calculation was 12.9 kW; right at the limit of a domestic heat pump’s output. The stakes were high. The numbers had to be correct. A full design ensued.

First, we sized the radiators for a flow temperature of 45 deg C, then at the request of the customer, we optimised this to 43C. 11 of the 13 radiators were replaced, and existing UFH left connected, all as a single open loop zone with no buffer.

The pipework was another challenge. Running large pipes through the property’s timber structure wasn’t possible, so I relocated the DHW cylinder from the middle of the house to the rear utility room and used a method I call ‘split the house’ to upsize the heating pipework. This is where I find a ‘centre-point’ of the heating system pipework, split the system into two halves, and install a new circuit to supply half the load. This was crucial because the only route for our pipes was through a gap between a steel and a joist, and 32mm MLCP was the max that would fit.

A noise calculation determined that the heat pump had to be 3.1m away from the property, and for the heat pump base, I used a white concrete border stone with mitred cuts to frame the heat pump. I added plum slate chippings as the soakaway substrate to match the rest of the garden, and overall, it was very easy on the eye.

I’m proud of this project because none of it came easy. My creative thinking, problem solving and heating design skills were pushed all of the way. After installation, January brought a very cold snap, with several days of sub-zero temperatures. The customer reported flawless performance; perfectly stable indoor temperatures at a January efficiency of 550%. They are confident their energy consumption backs this up. It is probably the biggest success I have achieved with a heating system installation.

Which products did you select for the job and why?

Having had success in the past with a 12 kW Vaillant Arotherm Plus air source heat pump, this is the heat source I opted for here. It is a R290 unit which has the lowest global warming potential of the refrigerants available, as well as providing great performance at higher temperatures if needed. The manufacturer, Vaillant, is an extremely established company in the UK which hopefully helps to keep the carbon footprint of the materials supply down.

This was fitted with a Vaillant Unistor 300 litre DHW cylinder, and a Vaillant Sensocomfort controller to operate the whole house on a single zone and control the heat pump at maximum efficiency. The controller is user friendly on its own, but to make it even better for the customer, we installed a Vaillant Sensonet internet gateway to provide internet control, as well as remote access if they ever had an issue that required us to access the unit settings and data.

The radiators are made by UK manufacturer Stelrad, and I specifically chose their Compact range for their 500mm size option and high outputs. Windowsill heights throughout meant that the tallest radiators we could fit in several rooms would be 500mm, so in the interests of pursuing lower flow temperatures, Stelrad was my choice.

I chose IMI Calypso valves for both style and practicality. Setting the balancing via the app means that any overflow to any radiators can be eliminated quickly, ensuring perfect balance of flow throughout.

Towel rails were the stylish, and remarkably high output Milano Bow D Bar anthracite range. The diverter valve is high quality, made by Esbe, and I chose the 15 sec, fast operating actuator to ensure quick changeover between heating and hot water mode. I chose Primary Pro for the external pipework insulation because it looks great and creates a proper weather seal with the seal and bond sealant.

Walraven Rapid Rail was the bracketry choice for pipework from the external unit to, and throughout, the DHW cylinder and plant area. This product allows for a fast installation and a high-quality finish.

Multipipe provided the flexible MLCP for the additional heating flow and return that needed to be installed through the house. This has a metal oxygen barrier in the pipe to ensure the long-term water quality remains high, while making the tricky route something that we could overcome. I tried to use products as locally as possible that would stand the test of time due to their exceptional quality, and I think that I struck a good balance.

Tell us what was different or unique about this job? Why does it stand out?

The project took two weeks, but it was important to the customer for the switch from LPG to a heat pump be done quickly because the work was carried out in November. Therefore, we made sure that the radiators and external pipework and heat pump were all complete before disconnecting the boiler. We then had a 2-day turnaround on getting the system back on to minimise disruption.

The building was extremely unique; a 300-year-old wattle and daub house with internal wall alterations, and two extensions added at different times. Wattle and daub by its nature of construction is impossible to calculate a U-value for with the typical method of using material thermal conductivity values. This necessitated a search for in-vitro testing of this wall construction, which was thankfully found in a Cardiff University research paper.

Modifying this U-value by adding the internal wall alterations added another layer of complexity to the heat loss calculation, and there was no guarantee that these numbers would be correct due to the variability of this wall type, but the results of the installation confirmed my calculations were accurate enough which was a relief.

The design certainly took some courage because if the heat pump was undersized, it was already the biggest single unit available, and we would have nowhere to go with it other than by supplying a cascade at our cost. This kind of pressure to get a unique design correct is one of the things that makes this project stand out.

Another design challenge was getting the heating flow temperature as low as possible. There was some iteration here. We designed to 45C first, then the Boiler Upgrade Scheme increase was announced which gave us an additional £2500 to invest, but due to the wall space available in 2 of the rooms, the lowest we could get the flow temperature was 43C. We really eked out every last bit of efficiency.

The cylinder was relocated to the utility room due to a difficult primary pipework route. In relocating the cylinder and diverter valve to the utility room we removed the need to install 35mm through the house. We could re-use some 22mm barrier pipe from the old boiler for the heating circuits, but this wouldn’t supply the full system flow rate required, so an additional 28mm pair were planned for.

However, on attempting to run this, we found a steel beam installed for the extension blocking our route. I had to change tact and order flexible 32mm MLCP which we managed to squeeze through a gap at the end of the steel.

At this point I sat with the homeowner David in the landing as we picked apart his pipework routes. He loved it and had a good idea of routes which was helpful. I simplified the house into two halves, a 7.5kW half, and a 5.4kW half, then did some fresh pipe sizing calculations. I split the house at this point, joined the existing 22 mm barrier pipe to the smaller section, and our new 32mm to the larger section. The heating was now converted to support heat pump flow rates.

I also had some fun with the outdoor unit base, making it a feature of the garden with a white concrete border stone, snowcrete and plum slate design to blend in with the existing.

And finally, tell us what the end result was for your customer?

Being owners of solar panels, and a home battery, meant replacing the LPG boiler with an electric heat pump was the final step in the customer’s eco home journey. The potential for harnessing these technologies in tandem with one another for maximum benefit was enormous. David took a keen interest here because of his dual electricity tariff rates were 30p/kWh peak and 7.5p/kWh off-peak.

In the winter, when his solar panels aren’t producing enough to charge the battery, he has been using his off-peak rate to heat the hot water cylinder for the day and then charge the battery. He also puts some additional heat into the property toward the end of the off-peak period. When the property cools, the heating then comes on powered by the battery.

His COP as measured by the Vaillant controller was over 4 for Dec and 5.5 for Jan. This meant that David could heat his house from as little as £1.36/kWh (approx.) throughout off-peak periods, and during peak periods until his battery is depleted.

This cost will come down further in summer months and will be zero when the solar panels are contributing. It’s an astonishing saving compared to their old fossil fuel system, and in David’s words, they are ‘very warm’. The heat throughout the property is consistent too.

The end result of this project is that amazing results in both comfort and efficiency can be delivered by heat pumps to old houses provided the design is correct. The best design is achieved with consideration given at all steps of the project, from heat loss calculation (e.g. custom U-values) through to challenges on-site (e.g. pipe sizing calcs to split the house), and stakeholders should be included in this process (e.g. optimising design flow temp with the customer’s investment).