I read a lot about the need for larger radiators when switching to a heat pump, and feel inspired to share my own experiences of having a heat pump for radiators (and hot water).

I don’t think radiator size would be an issue in most houses in most parts of the Uk, if one uses the heat pump in the way one does here in Scandinavia.

This is because the water temperature to the radiators is determined by the outdoor temperature (sensor) and the type of building (these are parameters you program into the pump).

For example, today, where I live it’s around 3+ degrees outside, the temperature the pump is sending to our radiators cycles between 35 and 45 degrees and we enjoy 20.5 degrees indoor temp (at the temp sensor). Our house is in no way super insulated.

One difference to the Uk gas boiler approach, is that the pump runs 24/7, so lower radiator temperatures work with this approach.

Our pump is EAHP, and with outdoor temperatures around 0 degrees and higher, the electric “booster” element hardly ever runs for space heating.

So, I reckon, with the relatively high winter temperatures in the Uk, and the correct configuration, you could run an ASHP economically without changing the radiators.

"Weather compensation" has been a standard feature on heat pump controls for many years (at least since the one I bought in 2007). That heatpump ran at up to 70⁰C in midwinter, 40⁰ in spring, without replacing any radiators in an old stone house.

Weather compensation is also common for new gas boilers in the UK (and Sweden!) because of the ErP efficiency regs across the EU. Cool flow temperatures are good for condensing boilers.

A weather-compensating (only) controller is ErP Class 2. A more sophisticated "load-and-weather-compensating" controller works by looking at how far the room temperature is away from the intended setpoint, and also how cold the weather is, and chooses the best flow temperature - ErP Class 6.

Gas boilers are often wildly over-sized, for DHW and also just industry laziness. If it's a mild day and the house needs 2kW of heat, but the boiler is rated 24kW, there's no way the boiler can modulate down to 2kW, so it has to fire on-off intermittently. The ErP Class 4 controllers adjust the on-off cycle time (TPI) to give the right room temperature.

As minisaurus said, that on-off cycling is bad for heatpumps. It's important that the heatpump is not oversized in the first place.

However, in the early days there were complaints that customers had been sold heatpumps that were too small for their houses. The MCS set standards that all heatpumps must be plenty big enough. They require that the heatpump *and the radiators* must be oversized for keeping the living rooms 21⁰C on the 99.6% coldest day without any electric backup or solar gains, using a very generous heat loss survey. The flow temperature must be 55⁰C or less. (MIS-3005-D)

In practice this means that radiators often need to be made oversized to comply with MCS.

There are complaints that new heatpumps are too big, cycling on-off inefficiently, and the capital cost is expensive.

The actual cost of replacing our radiators is actually not too much, compared to the cost of the heatpump and the electricity. Many of our radiators were already oversized by the previous boiler installers, especially since we added insulation.