Air-Source Heat Pumps (ASHP) and radiator replacement: How to work out the optimum radiator size for maximum efficiency?

We're only 2 weeks in but so far the answer is yes. Rather than the control system making the predictions, you do need to think differently about how you use your heating. As you say, you can't just put the heating on and expect it to bring the house up to temperature quickly (although we have fan assisted radiators so our rooms do heat up quicker than with standard radiators). You have to programme the temperature changes in earlier. For example, set it to come on at 6am instead of 7am to allow time for the rooms to warm up. I’ve never lived in a house with UFH but I guess that's probably the same, requiring a longer period to heat up.

A programmable thermostat (we have a Tado) is, I think, very important to getting the best out of a heat pump system. It allows you full control over the temperature throughout the day so you don't ever let the house get so cold that it's going to take hours to warm up. 

That's very interesting to read, thanks @M.isterW.

In the meantime my old boiler with its timed on/off programmer has gone, replaced by a more efficient boiler, wireless remote controlled TRVs and a fancy time+ temperature+more zoned controller (evohome).

I mention this because, if it's allowed to, the evohome will demand heat from the boiler ahead of time, in order that zones reach set temperatures on time.

It needed a period in which to "learn" how quickly and how strongly zones respond to changes in heat input, but it seems to do a reasonable job. Mostly, it is very good at maintaining set temperatures, even as set values change over the course of a day (the system can schedule up to 6 changes in the set value per day, independently across all the zones).

If I were deliberately to impose (in software, somehow) an artificially low limit on boiler output, I suspect that the evohome would have learned to demand heat even further ahead of schedule, and zones would still have been brought to temperature on time.. I will do this, eventually, because I want to know whether a heat pump could do the job

The reason I'm bothering to describe all this relates to modulation.

Most of the time, the heating power needed is a great deal less than the 32kW that the boiler can deliver. No problem, I thought, the new boiler can modulate down to 10%, i.e. 3.2kW so, much of the time, it should be able to come on and stay on.

It's a good job I was interested in this, and enough to want to observe "with my own eyes" sustained slow burning of gas at a rate of the order 3 or 4 kW…

Because every time I watched, laboriously recording gas meter readings and times, and correlating them with when the boiler fired up and shut down, I worked out that it wouldn't go below 11kW.

Eventually, in a revealing conversation with the manufacturer's technical support, it emerged that the boiler was an old model, and 11kW was indeed the minimum output.

(No, that error would never have come to light without my obsessive reading of smart meters. The wrong boiler will soon be replaced by the right boiler, and at the manufacturer's expense.)

Back to the controller.

Its link to the boiler is crude. The evohome just demands heat or not: on/off. It could say how much (via OpenTherm), but doesn't. Whatever the boiler manufacturer might claim about OpenTherm compatibility, in practice, the installer doesn't try to make it work that way (not with evohome, anyway).

Receiving only simple on/off commands, the boiler is left to work out what output to deliver, what modulation to run at. Maybe it will do a good enough job, I'll have to wait and see. All my experience with control systems (including designing and implementing some) tells me that evohome could do a better job - it has "learned" how zone temperatures respond to heating. The boiler hasn't got that information, and I don't see how it can do as effective a job all on its own.

I have a plan though. Over the years, I've sometimes seen "micromanagement", but fortunately never applied to me. If need be, I'll micromanage the evohome, and do everything I can to help the boiler to modulate its output the way I expect it will need to.

With apologies for straying far from the thread topic.