Friday, December 14, 2018

What is the worst retrofit heating option?

So with the old GlowWorm Back Boiler Unit 45/4, including its swank Miami Fire Front with fake teak details now replaced by a modern boiler, it is time to ask myself whether I have made the right choice....? Not in time surely as it would have been better to think this through before getting a new heating system installed.

Retrofitting is of course different from building anew. Most existing houses have a wet central heating system fitted with a boiler of some kind: close to a mile of carefully folded & soldered pipework hidden in the ceiling void and a few radiators here and there. When building, say, a new passivhaus there are savings to be made by not having this. But not for retrofit, so let's use what is there already.

Making a few guesstimates, I think that our retrofit will end up somewhere between AECB and EnerPhit. Probably a smidge better insulated and airtight than AECB, but realistically the <1.0 AC/h airtightness required for EnerPhit will be a massive challenge requiring a lot of obsessing about.

Anyway, about heating costs. Let's consider the PassivHaus heating demand limit as well if only for fun. The house has 80 m2 of treated surface, and current Ecotricity pricing for a kWh of electricity is 17.66p, with 3.97p for a kWh of gas, with the standing charge for the latter being £ 89.72 per year. 

Taking the old back boiler out,  running new pipes, electrics and controls costs £ 2500 before anything new & shiny is even brought in.

So what are the options? Looking at the AECB heat demand, a modern gas boiler (90% efficient) would be a good choice. For EnerPhit or PassivHaus, a small, cheap electric only system would probably become competitive, so consider an electric boiler (100% eff) or MVHR electric inline heater (95%). Then there are the more efficient but capital intensive options of Air Source (220% eff.) or even Ground Source (350% eff.) heat pumps. Let's add the capital expenditure to the mix, with a write-off time of 20 years: £ 0 for the MVHR inline heater as it is there anyway, £ 1000 for the electric boiler, £ 3000 for the gas boiler and £11'000 and £ 19'500 for the ASHP, GSHP respectively.

Taking it all together, I arrive at the following annual cost:

kWh/m2a kWh/year gas boiler electric boiler ASHP GSHP MVHR inline
AECB 40 3200 230.87 565.12 256.87 161.46 594.86
EnerPhit 25 2000 177.94 353.20 160.55 100.91 371.79
PassivHaus 15 1200 142.65 211.92 96.33 60.55 223.07

captial writeoff 20 years 275.00 175.00 550.00 975.00 0

The capital costs are a large part of the annual total, and dwarf the consumption figures in the case of heat pumps. For PassivHaus standard the actual amounts are low anyway so any method is fine. Anything above that and direct electric is ruled out.
With the (unfair?) price difference between gas & electric, the gas boiler wins for anything not as good as PassivHaus. 
Subsidies could reduce the capital cost for heat pumps, and should I have had £ 20k in a sock somewhere it would have been nice to invest in a GSHP as it is more efficient and could be driven from renewables in principle. But if you have to take out extra mortgage to install one it is probably not worth it.

Considering the CO2 cost of a well insulated warm house, assuming a CO2 production of 220g for burning a kWh of natural gas, and 520g for extracting a kWh of electricity from the UK grid:

kg CO2 gas boiler electric boiler ASHP GSHP
AECB 782 1664 756 475
EnerPhit 489 1040 473 297
PassivHaus 293 624 284 178

CO2 wise, as well as annual consumption cost wise, the gas boiler is on par with the air-source heat pump in this calculation, where the ASHP has a much higher capital outlay. So not such a bad decision after all, at this point in time. Phew.

With the upgrade to a low temperature heating system it will be relatively easy to replace the new boiler with a heat pump once it has reached the end of its life, hopefully it will make more financial sense, and the UK grid will have decarbonised a bit more by then to drive down the CO2 footprint

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