PhilT

Agreed where we are talking about replacing a perfectly good boiler system. But if it's 30+ years old like mine was, and needs a major upgrade anyway, the net cost to me of a new ASHP, DHW tank and bigger rads was no different to a modern upgraded boiler system. That financial equation of course varies enormously from the cheapest (possibly one of the utility companies like Octopus) to the most expensive (premium expert installers such as Heat Geek)

and an oversize heatpump like mine working 10% more efficiently than a smaller one running flat out/less efficiently on freezing cold days will mean 10% less electricity use at the time when the grid may be well over-stretched. I don't understand why people would burden themselves with additional expense and complexity of adding heating elements, boiler hybrid setups etc. when all you need is a bigger heat pump. Imagine a family during the recent freezing cold spell, they all come down with flu or lurgy of some kind (very likely this season). The house temp needs to be increased a couple of degrees all day and night to give them all the chance to get well again. In those circumstances you need a level of headroom similar to what you got from a boiler. This is just one of the many advantages of an oversized heat pump - extension, faster DHW reheat, less defrost recovery issues etc. etc.

Modern scroll compressor / clever software controlled heat pumps just don't suffer the cycling inefficiences of the bad old days. For me there was no difference in installation price between an 8.5 and an 11 so I went for the 11 and glad I did. It is outperforming spec on cold days by a significant margin. Compare 10% inefficiency on a mild day of say 10kWh consumption compared to 10% better efficiency on a cold day of 100kWh consumption. It's unlikely to be worse than break even over the course of the season. That's definitely what the evidence is saying for mine.

why exactly?

why not just get a bigger heat pump

what's the problem with it being a little louder than whisper quiet for relatively tiny periods of time. The compressor on mine after defrost is barely audible, and I'm sure it's no quieter than the Vaillant

To an extent, yes, but maybe not as much as people believe Green levies are 6% of a typical gas bill, 16% of an electricity bill, and 11% of the average dual fuel bill. Climate change levy rate for gas is 0.672 pence per kWh, while the rate for electricity is 0.775 pence per kWh.

I got my info from here https://www.theecoexperts.co.uk/news/green-levy#:~:text=The green levy is a,or should it be scrapped? but as you imply other sources have a different take, whatever, the point is it's on gas

Not sure that's right Green levy = A tax on dual gas and electricity bills that funds environmental and social programs. Climate Change Levy (CCL) = a tax on business energy (gas, oil and electricity) that encourages energy efficiency and reduces gas emissions. Energy (Oil and Gas) Profits Levy (EPL). A 35% tax on the extraordinary profits of oil and gas companies operating in the UK.

out of interest, how do you achieve that?

The speeded up fan whooshing caught me by surprise during the first freezing night after installation. But I can barely hear that inside the house. What kind of walls and windows are those?

It's about right for a commercial installation - mine is the same but it includes a 24 help line and call out, which I have used a few times.

All true and you're preaching to the converted here, but amongst the general public, I worry about the ever increasing and ill informed adverse publicity especially in national media such as The Daily Mail, This Is Money etc. by people like Jeff Prestridge, who should know better. I wrote him a stinking email recently in direct response to one of his most recent articles - he writes such utter rubbish, but there is a serious concern that perception has gone too far already in the wrong direction

Ofgem web site says about the BUS - "the new heat pump or biomass boiler will not have been used prior to commissioning, except in limited circumstances, eg to dry out plasterwork during construction or renovation (for shared ground loops" maybe if it's second hand but hasn't been used🙂 https://www.ofgem.gov.uk/environmental-and-social-schemes/boiler-upgrade-scheme-bus/installers#:~:text=the new heat pump or biomass boiler will not have,system after it is installed

This is how mine works - completely separate from heat pump and controls. I never use it though😬