What makes a hot water cylinder right for a heat pump?

Heat flows through the coil wall at a rate set by three things: how well the wall passes heat, its area, and the temperature difference across it (heat flow = U × A × ΔT). Cut the temperature difference and you have to add area to move the same heat.

Heating a cylinder to 50 °C with a boiler flow at 75 °C leaves a 25 °C difference at the end of the heat-up. With a heat pump flow at 55 °C, it’s about 5 °C. That’s why heat pump coils are so much bigger. Building Regulations (Approved Document L, 2021) already ask for new heating systems to work at a flow temperature of 55 °C or lower (what Part L says, and what changes in 2027).

An undersized coil means the heat pump has to raise its flow temperature to push its heat in. Each degree costs efficiency: measured heat pump data shows the efficiency falling by around 2% for every degree hotter it runs. The rest of the time, the heat pump cycles on and off, the reheat takes longer, or the immersion heater finishes the job.

Our coils, from our product data: our boiler cylinders (Stainless SuperFlow indirect) have a 1.1 m² coil; our Climacyl heat pump cylinders have 2.29 m² (90 to 180 litres) or 3.31 m² (210 to 300 litres). Our heat pump cylinder sizing tool uses a rule of about 0.25 m² of coil for every kW of heat pump output.

The flow a heat pump needs is its output divided by the temperature drop across the system: flow (litres a second) = kW ÷ (4.18 × temperature drop). An 8 kW heat pump at a 5 °C drop needs about 23 litres a minute. A boiler giving the same heat at a 20 °C drop needs about 6.

In standard copper tube, 22 mm pipe has a bore of about 20 mm and 28 mm pipe about 26 mm: roughly 70% more space for water. At 23 litres a minute, water moves at about 1.2 metres a second through 22 mm and about 0.7 through 28 mm. Resistance to flow rises steeply as the bore narrows: at the same flow, a length of 22 mm pipe has roughly three to four times the pressure drop of the same length of 28 mm.

Coil tube sizes vary between makers, and a small heat pump may be within its limits on a 22 mm coil. That’s why the question to ask is whether the coil has been checked against your heat pump’s flow rate. Our pre-plumbed heat pump cylinders have 28 mm primary pipework and components throughout, and their datasheets give the pressure drop (16 kPa at 17 litres a minute).

Heat leaks through foam insulation roughly in proportion to 1 ÷ its thickness. Halve the thickness and you roughly double the heat lost through it. On a 210 litre cylinder at heat pump temperatures, going from 50 mm to 25 mm of polyurethane foam adds something like 30 to 40 watts, all day, every day: around 300 kWh of heat a year. (An estimate for the foam alone; connections and fittings add their own losses.)

The Energy Saving Trust suggests adding an 80 mm jacket to a tank that has only 25 mm of foam, and estimates that saves around £45 a year. A cylinder’s standing heat loss (in kWh a day, or watts) and its energy label (ErP class) are on its datasheet. Compare them at the same size.

Ours, from our datasheets: 50 mm of factory-injected polyurethane foam, made with a CO₂ blowing agent. Every cylinder’s standing heat loss and ErP class is published on its datasheet.