Sizing your ASHP

[ragg987]

1 hour ago, le-cerveau said:

Ok, to answer the question, why not just stick with gas:

... the breakeven point for an ASHP is 2.84

That might make sense if you look at fuel costs only, but if you factor in the additional running and captial cost of having BOTH technologies does it still stack up. Very simplified example.

 

100% gas:

boiler cost £600, installation £1,000

annual fuel cost £600

annual service £50

20 year total £14,600

 

100% ASHP:

pump £2,000, installation £2,000 (includes hard-standing and piping for external unit)

annual fuel cost £550

annual service £50

20 year total £16,000

 

Both gas and ASHP:

capital cost £5,600

annual fuel cost £575

annual service £100

20 year total £19,100

 

Above is very simplified and does not factor in inflation, nor the extra running costs or extra capital costs in integrating the gas and ASHP (e.g. larger TS, more controls, more pumps). Costs I use are for illustrative purposes, you need to plug your own scenario in.

 

I would suggest you go with gas only, put in a largish TS (e.g. 500l based on your concurrent bath / showers scenario), put in a gas boiler based on DHW recovery times you are willing to put up with (12kWh boiler to 500l TS is approx 2hrs recovery from cold), and use a standard S-plan configuration to run UFH direct from the boiler (no separate buffer). In 20 years time, if gas becomes prohibitively expensive, then revisit the solution based on latest technology. (In reality, our electricity base is derived by burinng gas so increase in gas price also impact electricity price).

[Nickfromwales]

The only thing I'd add to that would be ditch the S plan and feed the TS direct from the boiler. Feed the Ufh from the TS, it's a far better solution than running off the boiler imho, and go for a boiler which can match your max dhw flow ( kW that the dhw coil can provide ( 28mm coil and TMV is circa 46kw )). It'll not need to idle then as it won't be serving Ufh on demand, instead it'll be pulsing higher grade heat into the TS thus optimising the temp range and condensing efficiency will be maximised. 

Edited November 7, 2017 by Nickfromwales
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