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I'm in London but my boiler, which is ~ 60 degrees, is on for a fair chunk of time in the winter.
Given that the heating is only on (set at 18) for 6-8 hours a day those chunks of 25-30% boiler on is pretty much representing the boiler being full on when we're home with the heating on (I don't have it on when I wfh during the day).
It might be that the heat pump can do something clever with efficiency at lower temps during the day so there is less of a jump necessary but I'm very much struggling to find much about using heat pumps in houses with smaller radiators and poor insulation.
Of course improving insulation and replacing radiators would be the ideal but then it becomes a massive job requiring pretty much the whole house to have work (and the biggest heat sink of big end of terrace wall is very difficult to insulate).
aggi-
Ideally the heat pump would run continuously 24/7 and modulate its flow temperature, thus heating output, in response to outdoor temperature.
Providing the system is specified, installed and programmed correctly, with no buffers/low-loss headers/zoning etc and run in weather compensation mode, then it won’t be hard to achieve similar/lower running costs to gas, even with existing undersized rads.
ectoplasmosis
Look at it this way: unless you're in the Highlands, in the UK we only really get a week or so of truly cold weather a year, and this is the only time your heat pump will approach its max design flow temp with lower efficiency. The rest of the time, it'll be ticking over at a much lower flow temp, achieving a CoP and running cost that will be comparable to gas even without using a tracker/time-of-use tariff, whilst keeping your house at a constant, comfy temp all year round.
If/when you eventually begin upgrading radiators, the efficiency will only increase.
There's also the inevitable re-balancing of the 'green tax' from leccy to gas to consider, which will make running heat pumps even cheaper.