Stelrad Case Study: South Downs 300 Year Old Home

Leading UK radiator manufacturer Stelrad Radiators supplied eleven radiators for a residential refurbishment project on the South Downs. Nothing particularly strange in that you might say until you find out a little bit more about the property and the challenges it posed for the company undertaking the refurbishment.

Enter Michael Paini and his company Option Energy Solutions, based in Steyning near Brighton. He was called out to look at a 300-year old property by soon to become customers – a retired couple with two daughters away at university. They needed a reliable hot water system, able to cope with a high hot water demand and they needed to be able to heat their home efficiently at a comfortable temperature for the whole day.

This is where the challenge gets interesting. The walls were of a wattle and daub construction with alterations made inside the property that made heat loss calculations a theoretical test – one might say, a lottery. Michael found a study by Cardiff University where they published U-Values for old buildings and using this he recalculated with the alterations to modify the U-Value of this specific property. He did this for a number of different fabric types because the property had been altered throughout the years. This was challenging to say the least, but doing so accurately was fundamental to making the project work.

With the help of Heat Geeks new room scan technology, the heat loss was calculated as 12.9kW – right at the limit of a domestic heat pump’s output. The numbers had to be correct and a full heating system design was begun. At the request of the customer the radiators were sized to be optimised at 43 degrees C. Eleven of the thirteen radiators were replaced and the existing underfloor heating was left connected all as a single open loop zone with no buffer. Ther radiators were a mix of K3 and K2 Stelrad Compact radiators, the K3’s designed specifically to work well with low temperature renewable heat pump driven heating systems and providing three panels and three sets of fins to ‘up’ the metal surface area and heat generated by around 50% for a similar radiator footprint as a standard K2. These radiators have proved popular up and down the country in heat pump installations – in new build homes and in retrofit heating systems where the original boiler driven system has been replaced by a heat pump solution.

Other challenges in this 300 year old home included the wall thicknesses for pipework runs. Running large pipes through the property’s timber structure wasn’t an option, so the domestic hot water tank was relocated from towards the centre of the property to the rear utility room and Michael used a system he called ‘split the house’ to upsize the heating pipework.  He located the centre point of the heating system pipework, split the system into two halves and installed a new circuit to supply half the load. This was crucial because the only route for the pipes was through a gap between a steel and a joist – and 32mmwas the maximum that would fit!

How did the new system perform? Well shortly after installation January brough a cold snap with several days of sub-zero temperatures. The customer reported flawless performance, perfectly stable indoor temperatures at a January efficiency level of 550%. The family is confident their energy consumption backs this up.

Over to Michael to explain some of the detail: “This is probably one of if not the biggest success story I’ve had in my career in the heating industry,” says a rightly proud Michael Paini. “We used a Vaillant Arotherm Plus 12 kW air source heat pump alongside a Vaillant Unistor DHW cylinder and Sensocomfort controller and Sensonet internet control. The radiators I chose were 500mm high Stelrad Compacts – chosen as the windowsill heights in the property limited us to anything higher – and they look very good in the property as well as perform well and provide the levels of comfort heating needed in an elderly property of this type. All in all we were delighted with the components of the system we chose and they operate excellently together. It’s been a major success story for us as a business and them as a family.

“The family being owners of solar panels, and a home battery, meant replacing the old 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 – the owner - 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 throughout off-peak periods, and during peak periods until his battery is depleted. This cost will come down further in future 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 courtesy of the Stelrad radiators 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 that is, custom U-Values - through to challenges on-site such as pipe sizing calculations to split the house, and stakeholders should be included in this process optimising design flow temp with the customer’s investment.

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