Air Source Heat Pump for an Older Alloa Home

The Brief

The owners of a 1930s semi-detached in Alloa had a gas boiler reaching the end of its working life and wanted to move to a heat pump for both running-cost and decarbonisation reasons. Like a lot of older Scottish stone-built homes, the property had a few unknowns (microbore pipework in places, original radiator sizes, a tired hot water cylinder), and the owners were understandably worried that an air source pump would simply not be able to keep the house warm. They wanted a clear, honest answer rather than a brochure pitch, and a heating engineer who would design the system properly rather than bolt a pump onto whatever was already there.

What We Did

Our work included:

• Full heat-loss survey: A whole-house and room-by-room heat-loss calculation against the actual property, walls, windows, insulation and air changes, so the pump capacity and radiator scheme were sized off real numbers rather than a kW figure copied from the old boiler.
• Pump specification (Vaillant aroTHERM plus 14 kW): A 14 kW Vaillant aroTHERM plus was specified to match the calculated heat demand, with headroom for the coldest days of a Forth Valley winter. The unit was sited carefully to keep noise away from bedrooms and the neighbouring boundary, and to comply with the planning noise rules.
• Radiator scheme designed and three radiators upsized: Three radiators (master bedroom, living room and dining room) were upsized to run at the lower flow temperatures the heat pump prefers. The remaining radiators came back as correctly sized on the survey, so they were retained, no replacement work that was not actually needed.
• New 250 L unvented cylinder: The old cylinder was replaced with a new 250 L unvented unit, sized for the family's hot water demand and matched to the heat pump's flow temperature for fast, efficient reheats.
• New flow and return pipework where microbore was undersized: Sections of original microbore pipework were replaced with properly sized flow and return runs so the pump could move the volume of water it needs without restriction. This is the kind of detail that decides whether a heat pump runs efficiently or struggles.
• Commissioning and tuning: Full commissioning on handover day, plus a follow-up tune of the heating curve once the system had been running through a real week of weather, generic factory settings always under-perform a properly tuned curve.
• Boiler Upgrade Scheme grant administered: We applied for and received the £7,500 Boiler Upgrade Scheme grant on the customer's behalf, so the price quoted was the price actually paid.

The Result

Across the first month of operation the system measured a SCOP of 4.1, that is 4.1 units of heat delivered into the home for every unit of electricity drawn at the pump. Running costs came in roughly 35% lower than the old gas boiler year-on-year, and the house heats evenly across every room, including the upstairs bedrooms that used to be the cold spots. The owners were happy enough with the result that they have already referred two neighbouring properties on the same street.

Job Duration: Survey through to commissioning and grant payment completed across the project window, with the household on hot water and heating the same evening as cylinder swap-over.

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