Heat pumps

Specifying radiators with air source heat pumps

There are significant additional costs and practicalities to take into consideration when retrofitting air source heat pumps and underfloor heating. So what about fitting heat pumps with radiators? Steve Alldritt, technical director for energy efficiency at City Plumbing, looks at the considerations.

Homeowners across the UK are increasingly turning to more efficient heating solutions. With government policies and incentives set to create higher demand for heat pumps, the number of retrofit projects is likely to increase. This will no doubt be compounded by the impact of rising gas prices.

Unlike gas boilers, heat pumps work at lower flow temperatures, typically ranging from 40°C flow up to 50°C, depending on the application. This temperature makes it the optimal partner for underfloor heating, which works best at flow temperatures of 35°C-50°C.

But not every home is suitable for underfloor heating, and not every homeowner will have the budget to incorporate it into the project. Laying screed floors can be time-consuming and disruptive, and although there are overlay options that are quicker to install, sticking with radiators can still be preferable.

Considering heat loss

Specifying radiators for use with heat pumps is practically the same as specifying them with gas boilers but it’s crucial to carefully consider heat loss.

First, consider the position of existing radiators and whether there is space to replace them with larger units if necessary. Also examine the size of the wall cavities, the level of insulation in them, and ensure the loft insulation is 200mm thick or more as standard.

Pipework will be a key consideration: if microbore pipework drops are present in the existing system, radiators will not be suitable.

The quality of the building’s glazing and window profiles will make a huge difference to whether radiators can be used with the system. Single glazing is a definite no-no and, in some cases, upgrades to the existing double glazing may also be required.

Radiator sizing

Once the building has been deemed suitable for radiators, it’s time to look at replacing the existing ones with options that will be suited to the lower flow temperatures associated with heat pumps.

In some cases, the property’s thermal performance may mean that the existing radiators are suitable. In these instances, it is advisable to powerflush the system to remove any sludge or debris that might impact the performance of the new system.

Where radiators do need replacing, it’s important to apply the manufacturer’s Delta T correction factors when sizing.

For example: • For a 75°C flow temperature, a typical 600mm x 700mm single panel single convector radiator will give approximately 0.656kW of output. • If we put 45°C flow through the same radiator, a correction factor of 0.365 needs to be used. • 0.656 x 0.365 = 0.251kW. • To achieve the same output, the radiator would need to be replaced with a bigger size.

In this instance, a 600mm x 1100mm double panel double convector radiator would need to be installed, providing approximately 0.696kW.

The options

Where existing radiators aren’t suitable, you might choose to install fan-assisted emitters, although the radiator will need an electrical connection. Fanassisted radiators are a modest way to blow warm air around the room and distribute heat more evenly while keeping energy consumption low.

For radiators with no electrical connection nearby, you can choose deeper, triple-panel radiators, which have a high heat dispersion but take up less wall space. Vertical feature radiators can also be a stylish way to balance aesthetics with higher output.

Other considerations

Heat pump installations, even with the help of government subsidies, are a heavy investment for many. And sometimes customers may insist on keeping their existing radiators despite recommendations for a resize. At this point, it may be worth exploring high-temperature heat pumps that can run up to 75°C. These heat pumps run at similar temperatures to boilers and so there would be less need to upsize radiators.

As with all heat pump installations, the pump unit needs to be located as close to the building as possible and with 1 metre clearance on all sides. It’s also crucial to avoid mounting the unit on north-facing walls. These considerations help to put less strain on the system and ensure it operates more efficiently.

While it won’t affect the performance, it’s also important that the installation isn’t close to bedroom windows because the unit can produce a little noise that may disturb at night.

Space for a heatpump-ready cylinder, buffer tank (usually between 20L and 50L), a volumiser and the relevant control equipment needs to be considered. ■

www.eecityplumbing.co.uk