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Author Topic: Woodburning stove and ASHP to heat thermal store?  (Read 2258 times)
JamieWakeham
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« Reply #15 on: August 29, 2019, 11:22:25 PM »

So, partly in case it's a useful case study, here's all my thinking.

It's a three storey house, kitchen and storage space on the ground floor, living room and study on the second, bedrooms on the third. Insulation has been upgraded to sensible levels throughout. Current demand is about 7kWh DHW per day (2500kWh pa costing around 125 at about 5p/kWh) and 6000kWh pa for heating - the peak load is about 28kWh per day in the depths of winter - costing around 300. A woodburner on the middle floor is used for about six months of the year and I would guess probably delivers about 3000kWh of heat.

We have a large (5.7kWp) PV array, and around 75% of its energy is currently exported, despite my EV! Critically we are on metered export so any increase in PV self consumption actually costs us the export payment (4.7p/kWh).

Because our consumption is already quite low, saving money is actually quite hard. We are refitting the kitchen and are having wet UFH fitted regardless. The first thing we considered was installing an ASHP with an unvented cylinder for DHW and direct connection into the UFH. This was costed at 11600. Projected payments from the RHI and MMSP came to around 9000. Given that we need very soon to replace the old gas boiler (let's say 1000 to do that), it only represents a long term cost of 1600.

What would the effect of this change be, financially and environmentally? From a financial point of view, the fact that gas is three times to buy cheaper than electricity and that the heat pump has an average CoP of around 3 means there's no net change (the official calculations suggest annual savings of 116, but they are based on the EPC's heat requirements, which are greatly in excess of our actual usage). But what they do not take into account is that for much of the year, the system will run on excess PV - so the cost to me is the 4.7p export cost, not the 15p cost of imported grid electricity. Looking at the crossover of my heat requirements and my excess PV, and allowing for the fact that not all of my excess PV can be used for the ASHP (because when operational its draw is likely to be higher than my PV output, and when I run it first thing in the morning for heating it's dark) I think I will make fuel savings of around 100/year, suggesting a 16 year payback time. That calculation is working on the theory that I will self supply 60% of the electricity used for DHW (so 0.60 x 2500kWh/CoP3 x 10p/kWh savings = 48) and 25% of heating requirements (0.25 x 6000kWh/CoP3 x 10p/kWH = 50). There is an annual service cost for the UVC that's probably comparable to the service cost to my old gas boiler, so that's cost neutral.

As for carbon savings: it is generally the case that the increased demand on the grid will translate to a CCGT somewhere having to spin up. In the summer the marginal load may well be renewable; in the winter it might be coal or diesel, but generally it'll be gas, and will continue to be for some time to come. So if I am displacing three units of gas in my boiler with one unit of gas at a power station, it's a clear carbon saving. Gradual decarbonisation of the grid only helps further.

Then, we considered switching the unvented cylinder for a large thermal store. This would have two major advantages - it wouldn't need the annual service that the UVC would (saving perhaps 60pa) and it would mean that the ASHP could be used for charging the TS in the daytime on PV, rather than having to run at night for the UFH. Again I had to look at spreadsheets of heating demand through the year and PV availability, but I guesstimate a further 50pa savings on grid electricity by boosting the self-supply figure for space heating to 50%. If the increased cost of the TS over the UVC was 1000, it would have a 9 year payback. There's no carbon saving.

Finally, having put in a TS, we looked at our woodburner. I would guess the cost of removing the old one and replacing with a plumbed in boiler stove would be around 2500, of which we might offset 500 by selling the old stove. If it had a conservative 4kW to water and ran for four hours average for half the year, it would displace around 3000kWh from the ASHP. At CoP 2.5 (I am being pessimistic about that CoP because it's the colder half of the year) that's 1200kW electricity saved, and at an average price of 10p (halfway between grid cost and export cost) that's a saving of 120pa and a payback time of 16 years. A cheaper stove, or a more powerful boiler, bring that down quite dramatically. Carbon saving is arguable depending on your stance on wood versus gas - I source all my wood from local felling that would typically be chipped and left to rot. We find this option really attractive primarily because we don't like our current WBS and would like to change it for something that doesn't dramatically overheat the middle and top floors.

The other option would be to forget all about the ASHP, and go directly to a thermal store for the woodburner, supplemented by the gas boiler. This would not attract any RHI payment. If I had the time and the skills to do it myself it might be attractive, but sadly I have neither! I don't have a firm quote for this but I cannot imagine I'd get it done for less than 5000, and I would still need to have the gas boiler replaced as well, so a capital cost of 6000. My estimate of the WBS displacing 3000kWh would save 150pa, giving a payback time of 40 years.

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Countrypaul
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« Reply #16 on: August 30, 2019, 12:12:12 AM »

The pipes to and from a WBSwith BB should be copper not plastic. Because a WBS is an uncontrolled source of heat (you can't just hit and switch and expect it to stop instantly) should anything go wrong you could end up with water at over 100C going through the pipes - not good for plastic at all. There should also be a heat dump facility should the whole thermal store get too hot from the WBS, the heat dump could be a suitably large radiator or a quench coil etc. Between the WBS and TS you need copper pipe including to/from the loading valve. For most WBS the water cannot be presurised so it is an open vented system/ The header tanks for the TS also should not be plastic as water can exceed 100C, and any pipes from the header tank also need to be copper if they could have water over 100C in them. The overflow from the header tank needs to go through a tun dish. You can get polypropylene waste pipes (not solvent weldable) for disposing of the hot overflow.
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brackwell
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« Reply #17 on: August 30, 2019, 07:21:35 AM »

Countrypaul, i agree with what you say about copper pipes on the boiler i was referring to pipes from tank to outlet.Should have made myself clearer.
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Countrypaul
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« Reply #18 on: August 30, 2019, 11:58:34 AM »

Countrypaul, i agree with what you say about copper pipes on the boiler i was referring to pipes from tank to outlet.Should have made myself clearer.

Sorry Ken, I think it is me who should have been clearer, I was commenting on Jamie's post where he says "I do take the points about water running through a stove causing it to run colder, and making sure the pipework is run straight and in plastic wherever possible" as the way I read it implied that plastic pipes might be used connecting the boiler.
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JamieWakeham
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« Reply #19 on: August 30, 2019, 01:02:33 PM »

Oh, no - I totally get that those need to be in copper! Was responding to two separate points in one sentence...
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