Air Source Heat Pumps

[Skyedriver]

Equus said:

Jambo85 said:

Aye but there is still a muliplier eg. something like 3 kWh of heat energy from 1 kWh of electrical energy. Storage heaters are 1:1.

But of course storage heaters use cheap, off-peak electricity.

ASHP's use electricity whenever they happen to be running, which includes peak rate.

There is an argument - particularly with increased use of renewables like wind and solar, which can be producing electricity whether we need it or not - that intelligent use of systems like storage heaters or heat stores that 'soak up' excess electricity that would otherwise go to waste can be more sensible than systems that increase overall demand at peak times (when we're having to burn gas to generate the electricity, anyway).

The other big unknown in McBook's calculation is the number for that multiplier... he's assumed a COP of 3.0, but this can vary according to numerous factors - I've seen figures ranging from manufacturer's fantasies of anything up to 6.0 (which I've never seen achieved in practice), down to barely better than 1.0

Storage heaters use economy electric but you are tied to certain suppliers and often the non economy supply is highly priced.
We were tied to SSE as they were the only ones doing THTC (Total Heat Total Control) by going ASHP we dropped SSE and moved to Bulb at a much lower tarrif

[caziques]

A heat pump, any heat pump, is not a substitute for a boiler as they work in a fundamentally different way.

Take radiators. A boiler is quite happy with 80 degrees out, 70 return. A heat pump isn't.

Bigger radiators can mean 50 out, 40 return, which a heat pump can do - but not very efficiently.

Now consider underfloor heating, 35 out, 30 return. A boiler, (gas, oil, wood, dog turd), is still OK, but now a heat pump can be 4:1 efficient (depending on the outside temperature if air sourced).

(Conventionally the coefficient of performance of air sourced hot water heat pumps is shown at 7/35. 7 degrees outside temp, 35 degree water, - virtually all units made now are around 4:1. Smaller gap, higher efficiency)

The closer the temperatures a heat pump has to pump through, the higher the efficiency. (conversely the bigger the gap, the lower the efficiency, plus air source has to defrost below 7c).

A boiler is a lazy way of providing a heat source. A heat pump CAN be far superior.

An air sourced HWHP is best when there is thermal mass, (such as a 100mm concrete slab). Heat pump run during the day, energy stored overnight.

An alternative is to use cheap night rate power, which more than offsets the lower efficiency.

I've done a few hundred underfloor systems with air sourced units in NZ, and the biggest idiots in this trade are invariably "heating engineers" from Europe. I reckon I could have a business here fixing other people's fk ups - such as the £5000 control panel...that didn't work.

Always do underfloor heating if you have a choice.

[Equus]

caziques said:

The closer the temperatures a heat pump has to pump through, the higher the efficiency.

^^^ This bit is important.

If you're cynical by nature, what you'll have already spotted is that it means heat pumps work least efficiently when you need them most (ie. when it's cold outside and you want it to be warm inside... imposing a big temperature difference).

If you get anywhere near an average COP of 4.0 in the UK, you're doing very well indeed.