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

Fitting the GBS Performance side door

The side door of the house was inward opening, clashing with the cupboard-under-the-stairs access door as well as the bathroom entrance, and sweeping across the tiny corridor. An outward opening door would solve these problems so this was included with the order for the extension door set and windows. The door is from the Green Building Store Performance range, with triple glazed panel and a U value of 0.8 W/m2K.

This fine door has been sitting around in our house for more than a year, and finally I got fed up with it being in the way, and me thinking the fitting through over and over again so that on 10 November I decided to finally just do it. It was going to be a nice day, and if winter decided to come knocking it could take until March to get this job done.

The old door was fitted with steel lugs sunk in the frame so unfortunately the saw and angle grinder had to come out to get it removed.

The door is projected out into the EWI layer with the inside of the door flush with the pre-EWI outside. It is resting on 100mm of CompacFoam200, bolted to the outer leaf with 3 stainless M8 studs with counterplates and lock nuts on the inside, and recessed EWI fixings heads, washers & locknuts on the outside. The CompacFoam was buttered with Parabond adhesive before mounting and looking at the adhesive being squeezed out when tightening the nuts was very reassuring.

The door was fitted with a 18mm marine ply head plate and straps at the sides. these were all screwed to the masonry on the inside. Although an all around plywood/OSB box makes achieving airtightness easier, it would have reduced the door size even further as the structural opening size is of course fixed.

Bricks of the outer leaf are folded back and where they meet the inner leaf it is all less than perfect and very uneven, making an airtight seal to the door a challenge. In the end, Siga Majpell 5 airtightness membrane was taped to the door and folded over the brickwork, then  covered with 6mm OSB, shaped to level out the wall. Insulated plasterboard was screwed to the OSB, leaving the inner reveal smooth and hopefully well insulated.

The door closes with a reassuring thud and the inside stays free of condensation and I feel no drafts so I am happy. The airtightness foil is taped to the masonry, and where possible buried in render or plastered over.

I have not got much of an idea yet how to make it all look nice in the end, but with the MVHR taking most of the door head out of sight anyway it is not time to call in the plasterers just yet.

Compression strength of (floor) insulation

I was not sure what insulation to select for under the screed floor in the living room. The thickness needed to be somewhere between 250 and 300mm, and I had lots of 200mm white EPS70 board I did not know what to do with. For the flooring in the old kitchen and extension, Floormate XPS300 was used, which is not cheap, not off the shelf for most building merchants and probably OTT unless I want to turn the house into a storage facility and drive around with a forklift.
Anyway, a quick collate of information from some specification sheets I arrive at:

	lambda	σ(inst)	σ(long term)	remarks
	W/mK	kPa	kPa
EPS70	0.031	70	30	guesstimated σ(long term) from other EPS products
XPS300	0.033	300	130
Compacfoam200	0.087	2560	1910
Celotex XR4000	0.022	140	60	guesstimated σ(long term)

Of course this has to take the weight of the screed as well. Assuming a density of 2.5(x10^3 kg/m3), 100mm screed weighs 250 kg/m2 or exerts a pressure of 2.5 kPa. So despite me worrying, EPS70 is absolutely fine for normal floors, which should have a 500 kg/m2 rating. Although EPS70 is strong, it is not very good with point loads, you can make permanent dents by poking it with your finger. Although the top screed should spread the load evenly, 80mm Celotex was used for the top layer. It is stronger, has foil backings on top and bottom which acts as a vapour barrier and is good for clipping on the UFH pipes, and the low lambda brings down the U value even further. The final lay-up has a U of  0.1 W/m2K, which should be good enough for (almost) anybody.

BTW: I am not sure about the relevance, but the whole lay-up of floor and screed should just about float.

First Post

Hopefully this will be the start of documenting the things that were done to the house we live in to make it bigger, more comfortable and more energy efficient.

I tend to think the largest part of the work has been completed by now, and some of it probably deserves documenting if only for me to look back on.