rjb wrote: ↑25 Oct 2021, 11:30am
Why are heat pumps so expensive. After all as those in charge like to tell us they are only fridges in reverse. My last fridge was £200. So what's the mark up. Can I install my fridge in the shed as an experiment with the heating coil inside and the chilled side open outside. Anyone tried this
Yes, it'll work and I'd love to see it
but your shed won't be well-insulated and how big is the heated side of your fridge? Assume that the heating coil will heat a volume equivalent to that which it cools (which is wrong but will do to give the idea). Now scale that up to the volume of house you're wanting to heat. And I think it'll heat that volume up by roughly the difference between 4 degrees and air temperature, which might not be what you want. A freezer might work better.
So yes, a heat pump is essentially a fridge in reverse, but it's a blooming big fridge and capable of generating a bigger temperature difference. I think that's the source of most of the cost. And then it has a big silent fan to blow the cold air out of the cooled space.
The next biggest cost source is probably silencing and ruggedising the outdoor unit. Fridge coils are fragile and there are dire warnings about damaging them. Fridges live a sheltered life. A box in the backyard in all weathers not so much, even before you get to things like defrost cycles and so on. Edit to add: and fridges are relatively noisy. If they were scaled up to whole-house size, without other changes to reduce noise, you'd get something like those industrial or farm heat pumps which no-one would tolerate inside their house or outdoors in a residential area.
And things like defrost cycles and even the basics like being able to heat a hot water tank to one temperature and heat radiators to a different temperature and possibly heat floors to yet another require more complicated control electronics and pump switches and so on. I don't think many people would now tolerate a 1-10 dial on their home heating like you still have on most fridges, or having to manually switch between heating and hot water like some boilers still required even in the 1980s.
The control units on heat pumps are like the smartest boiler controls, with such features as "weather compensation" mode, where it calculates the most efficient home heating based on the outdoor temperature instead of the old way of pretty dumbly chasing room thermostat cutoff settings, running the radiators very hot yet always lagging slightly behind what's needed when the temperature changes... but that calculation alone probably doesn't add much cost when the control unit has to be "smart" to deal with managing an outdoor unit and all the switching and so on anyway. It's a pretty big user benefit IMO, although not unique to heat pumps.
And then there's the internet-connected smart reporting and so on which government requires to get the full payout, which is another box in the water tank room and must add cost.
Btw I've done all the simple stuff like low energy bulb, loft insulation, double glazing, draughtproofing. Next step is to insulate the concrete floor but as we have wooden boards and carpets it might not be very cost effective.
Yes, somewhere around here I have an assessment for floor insulation (we have concrete slab too) but the payback time is poor. I think solar photovoltaic is the next best option, despite only having a small south-facing roof.
Edit to add: there's more explanation of "weather compensation mode" (including why some heat pumps run radiators at 37°C by default despite that being less efficient) by an ex-Samsung engineer at
https://www.linkedin.com/pulse/weather- ... ham-hendra and there are links to more at the end. Even if people stick with gas, requiring that mode and lowest-possible radiator temperatures on new boilers would help to eke out the remaining supply and reduce costs.