Back to heat pumps. I posted this in the Telegraph in response to a turning down the radiators article.
It really is time that the Government took some advice from Engineers. All of these "everything electric" plans are completely unworkable without the replacement of the entire electrical distribution system and 10 new nuclear power stations. The cost £3 trillion. By 2030, you have to be having another laugh at the public. Air source heat pumps sound ok on paper until you understand the specification. With air at -10C (not unusual in Britain) and 50C outlet temperature they perhaps give 2 times the heat available from electricity directly. The electricity costs 4 times as much as gas, guess what, you pay twice as much money to run a complex system which is expensive to maintain! The article above is Green rubbish, no truth whatsoever. As for Hydrogen, forget that, it needs twice the electricity to make it as the heat from burning it. Simple chemistry, not understood by the new "Green" Government.
David CEng etc (Thats a real Engineer by the way!)
If you want to get decent efficiency out of a heat pump, then 50C output temperature is far too high. Try fitting bigger radiators, and turning it down to the mid 30's.
So, assuming normal UFH type temps of 40/30C and room temp of 20C, dT=15C. That means my (t)rusty steel panel radiator will chuck out about 20% of its dT50C rated output. So for a typical room of about 1kW heat loss I now need a radiator which is rated at 5kW! Or looking in my Stelrad book I go from a 500mm long rad to a 2.5m long rad. More's the problem, how the hell do I fit that in the room????
As it happens, I've been working on alterations to a very large domestic property which has UFH from an ASHP. The new owner is planning to build a major extension, just about doubling the floor area, and this was designed with UFH throughout. He later added a requirement for air conditioning to several existing rooms plus several more rooms in the new extension. I am now thinking, why bother with UFH in the new build, just install a/c throughout. No need to heat an intermediate liquid (water), just use the a/c units for heating (and cooling) plus the added bonus of being able to transfer heat extracted from the rooms in summer into the hot water system.
Which gets me back to the o/p, why are we going on about rads? If we want decent levels of heating in the existing building why not go down the a/c route? Okay, if you already have a wet heating system then you'd have to rip it out but since you'd have to do most of that anyway to fit the mega-size rads needed you are already halfway there! The downside is such systems will be more expensive to install, plus you still have to get the juice from somewhere (perhaps the hot air from parliament could be harnessed to solve the issue!).
Finally, one more sting in the tail for ASHP use: check the rated outputs at your design outside temps. I had a community centre with UFH fed from a ASHP, total heat requirement about 70kW for -4C outside design temp. Manufacturers came up with what they said was a suitable unit but when I checked I found the figures were quoted for 7C outside temp. When this was pointed out to them they upped the size to a 100kW unit which also wiped out most of the spare capacity on the new transformer which was going into the site!
Better designs of heat pump exist for use with a wider range of operating temps however, the offerings currently available commercially are all single stage and quite basic by comparison. There is the complication of different optimum refrigerant in the cascade design as well.. Confusingly some are described as dual mode, where the single compressor has switched bypass ports in the middle of the scrolls that allow better optimised operation at two different pump rates (as opposed to variable speed drive - compressors get inefficient at low rates, as the refrigerant has time to sneak back round the outside of the moving bits.).
diagrams below indicate some possible futures that may make it just about possible have an ASP with gas boiler exit temperatures and sensible COP without too much upheaval. Just have to wait for them to make it out of the lab into production. (Evaporator is cold side, condenser is hot side...,)
