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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.
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Yes, but then you've got the cost of installation that's gonna take a huge time to pay if the running cost is just similar to gas.
I'd posted a HeatGeek quote I recently got in the heat pump thread. It was 20k once the grant is taken off, and would save me an approximate 8 quid a month. The main reason for this is it being an old house with solid brick walls, being relatively large, and some of the existing pipework being microbore.
In situations like this, heat pumps still don't make sense IMO. In a newer place with cavity walls and insulation, absolutely it'll make sense.
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).