woodburner with backboiler - worth it?

Discussion in 'Plumbing and Central Heating' started by crappy, 14 Oct 2021.

  1. crappy

    crappy

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    Hi folks,

    I'm about to move into a new house / converted barn which has heating (oil-fired) and an existing wood/coal burner. The entrance all is crying out for a statement fireplace which I want to add. The house is a large, open plan type of house and the fireplace will warm most of it.

    I've seen a number of posts here relating to log burners and back boilers but they're all from 2013ish - are they still relevant.

    Are they worth it? Any the "anti-boil" issues that just make it a faf?
    I was looking at a 30KW, 27KW to water, 3KW to room (Full details)
    • The oil boiler is not far from the intended location so plumbing won't be a hassle.
    • Property has ample free wood and I *want* a burner so not about cost / oil/gas.
    Thanks

    C
     
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  3. muggles

    muggles

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    Depends what you mean by "worth it". If you're asking whether it's going to pay for itself then the answer is almost certainly not, unless you have access to a limitless supply of seasoned hardwood logs, and even then the payback will be very very long if at all. To achieve the 30kW output you're going to have to feed a little over 7kg of wood into it every hour. 12 hours a day for a 4 month heating season is about 10 tonnes of wood....

    Do you actually need 30kW? Has a heat loss calculation been done on the property to determine the true heat requirement? 30kW is a lot of heat (incidentally the stove says it produces 23kW to water and 7kW to the room).

    You can't just join a couple of pipes from the stove into the heating system, it's much more complicated than that, especially if you want to keep the oil boiler as well.
     
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  4. Exedon

    Exedon

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    As has been said not simple install correctly sized buffer tank substantial heat leak radiator overheat stat to bring pump on low limit stat to provide back end protection the list goes on!
    We have been installing wood burning appliances since the 80s
    Would I own one NO muck dust ash plus the report written by the university of Sheffield re indoor particulates don't make happy reading.
     
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  5. crappy

    crappy

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    @muggles -
    I think your point made me aware of something; you need to fire to be roaring for it to realise the 30KW. I've currently got a burner and after the initial start, I choke it down to 1 (of 3) so it just glows. The neighbours overheat if I leave it on high for too long :D

    My ponder though is, at it's simmer, would it add any heat to the CH system? If it's adding a few KW then that would be worth it (would it?)
    Is it worth it? I am buying a burner - I have two options - with and without back boilers and prices don't vary much for the unit. Ancillaries add a bit more.

    @Exedon
    It's those ancillaries and protections which I need to consider. Stopped pump, simmering fire, I'd imagine the water jacket would turn to steam quite quickly. Poof.

    Wood is not a problem and basically free. As for the nasties - I drive and work in central London. I could get home, put my mouth over the flue and get less nasties from that than spending a day in the big smoke :ROFLMAO:


    Above all, I like the idea of it being "different" or "cool". But there is a price/practical limit to what I am willing to do.:unsure:
     
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  6. muggles

    muggles

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    The effect of turning them down is roughly linear, so if you turn it down to half power both the heat to the room and the heat to water will halve, thus your 7kW to the room and 23kW to the house becomes 3.5kW to the room and 11.5kW to the house. There is a limit though, turn down too much and the fire will burn less efficiently & start producing more soot and tar. This is especially true when you have a back boiler due to the cooling effect of the water jacket. Woodburners are much happier burning hard than slumbering.

    The cost of ancillaries is not insignificant. If the chimney needs lining or a factory twinwall flue needs building you're looking at £1500-2000, a thermal store will be another £1500-ish, plus pipe (28mm is currently nudging £10 a metre), pumps, labour, controls etc etc. You could easily sink £5-6k into this and end up with something you very quickly get fed up of having to feed wood into all the time. Even if you only need half your 30kW stove's output, that's still 1.25 tonnes of wood a month assuming it's hardwood - wood which should ideally be split and seasoned at least 12 months before burning.
     
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  7. crappy

    crappy

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    Thank you. Very helpful.
     
  8. Spursnwagos

    Spursnwagos

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    From experience of our multifuel stove and back boiler system. It is very hard to maintain sufficient temperature of the stove for a wet system to be effective. You become like a steam train driver constantly monitoring the stove temperature and loading with the correct fueld. As time progresses the ash stunts combustion and the temperature is almost impossible to restore without letting the stove cool, clean and refuel. We have a large stove, which makes the process slightly easier, but unless you want a part time job monitoring, would opt for another solution!
     
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  9. crappy

    crappy

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    Ha ha. I'm an IoT nut. I've got sensors measuring everything. Boiler cold water inlet, heated out, ch flow, return etc. I've already planned the sensors for this.

    I laughed at your comment though - be a steam train driver managing the fire.

    This, along with other research done, I'm inclined to think its not a great idea as a compliment to existing system.
     
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  11. Nige F

    Nige F

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    I don't think ASLEF would approve of your driver managing the fire - it's the fireman's job.;)
     
  12. cdbe

    cdbe

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    I've got one, not fitted yet. It has various duct/blanks that you can connect ducts to in order to circulate heat round the house, I believe you can also incorporate a fan to assist. Might be worth considering although I believe that if you have plenty of "mass" that will store any surplus heat so it's never wasted - that's my plan - it's going right in the middle of the house.
     
  13. oldbutnotdead

    oldbutnotdead

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    Thermal store is the only sensible way to harness the water heat from your woodburner, saves all that careful management of the fire (burn it hard for a few hours then either let it go out or bank it overnight).
    As noted above, there's a fair few things to install (loading valve, heatloss radiator, thermal store, lots of tube, metal F & e tank) and running it is labour intensive (even on my baby 13kw Villager, on timber at full chat you're stoking every 15-20 minutes, just as well you can pause live TV these days).
    Financially unlikely you'll break even but emotionally a real fire is lovely and you will spend less on oil if you burn timber instead to heat the place- if you can afford to write off the capex and focus on running costs then the picture looks better. If you've got kids it's a great way for them to earn pocket money (cutting, splitting, stacking, stoking) :)
     
  14. Marty79

    Marty79

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    I installed a boiler stove 2 years ago, so this will be our 3rd winter with it. I only burn anthracite coal and I get a steady 65-70 degrees for eight hours on only 8kg's of anthracite. We have a total of 13 rad's and a 120L cylinder. I love it personally. Sure, it would be far easier to push a button and get instant heat but I really love the whole process of building the fire e.t.c. It doesn't have a thermostatically controlled air intake but after some testing I worked out where the primary air control slider needs to be to sustain the necessary flow temp's, so I get the fire going, chuck the entire 8kg bucket on, let it got for about 30 min's then use I little spacer that I built to choke it off to where it needs to be and that's it for at least 8 hours. I got 12 hours once at a 55-60 degree flow temp. I get a ton of anthracite each year for winter and it cost us £350ish this year. Lasts us from November to March/April
     
  15. JulianM

    JulianM

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    It would appear your 'experts' may be experts in installing wood burning stoves, but may have no real experience with using boiler stoves that have been thoughtfully installed. I am a chartered mechanical engineer, and designed my own system to accommodate a wood boiler. It works extremely well as long as you accept the issues that are common with any stove, backboiler or not, and that is the mess they create both in terms of the logs themselves, and the ash clearing etc, and the feeding required. Admittedly these are magnified in a back boiler stove because you are using it so much more, but that's all. Oddly I had a similar experience to this negativity when I went into the local stove store asking about back boilers, mine is getting rather long in the tooth now and will need replaced sometime. The 'expert' was very negative about boiler stoves, saying no-one buys them, so I suspect the issue may be unfamiliarity as much as anything. However one other point that also put me off him, he stated categorically that after 2023 it will be illegal to burn anything other than kiln dried wood!!!! As far as I can see the legislation actually says it will be illegal to sell anything other than kiln dried wood, and that as long as the wood you use is less than 20% moisture content it can be burnt, which can be achieved by correct air drying and storage. If you are burning anything wetter you are daft anyway, as it generates far less heat.

    Anyway, to my personal experience and design. I live in central Scotland with my own source of wood. My back boiler was fitted about 25 years ago. It has outputs of 27kW to water and 3kW to the room, a much more effective boiler than is available today. It feeds into a 250L or so thermal store via a 28mm bore thermal siphon (no pump) which goes up to the floor above and across about 2m to the thermal store. If I have the stove absolutely flat out I have on rare occasions heard the stove boiling slightly, but all that happens is bubbles of steam will travel up the pipes, up the thermal store and out the vent, no harm done. The thermal store is also fed by an oil boiler that is controlled from the store temp, so if the wood burner keeps that temp high enough, the oil boiler doesn't cut in. In practice I run it such that the fire does all the work in the evenings, and the oil the rest of the time, this drops my oil consumption to around 40% of what it is without wood use (I have run with and without wood as a comparison) and should allow you an idea of payback time. With this design you have to make sure that the huge quantity of water in the heating circuit (it includes the 250L store) is all properly dosed to avoid corrosion. It will take a few bottles of inhibitor, not just one.

    As for wood, yes you need a LOT, but if as you say you have your own supply, like I do, then that isn't an issue other than having to generate it, which is in itself good exercise. One trick that not many people are aware of for creating firewood that doesn't require the wood to be stored for a year is as follows. Fell your tree in summer, preferably in one piece, leave the felled tree on the ground for 3-4 weeks until the leaves are all crispy, then chop/split it into your firewood lengths, if you test it at this point with a moisture meter you will probably find it is already below 20% and ready for the fire. If you chop branches off first, or before the leaves are crispy, the main trunk will not dry so thoroughly and will need additional air drying.

    I have more recently also added in a solar element into my system, I have 14 panels (nominally 3kW, bought cheap) directly connected into an immersion element in the thermal store (actually 2 of 3 x 1kW 3 phase elements which on sunny days runs inefficiently as it max's out at 2kW with just 2 elements connected). From early April until late Oct when the heating is off, I don't need any external energy input to get my hot water needs met, and I use it in my dishwasher (fed with hot water instead of cold) and my washing machine also (I have a dual feed washing machine). Anyone who says this is pointless because of the lag in getting the hot water to your machines is wrong, all you do is run the hot in the adjacent basin until it runs hot before you run your machine. Direct coupling solar panels however has to be done carefully as you are talking high DC voltages, and appropriate isolating switching, earthing etc is required, your standard 240Vac switches won't cut it. I have designed a control system to control the output using an Arduino and solid state relays so that I can connect all 3 elements and run the system in a pseudo MPPT form by balancing the applied load to the panels output, but as yet I haven't built this system, albeit I have tested it in a simulator so am confident it should work and when created will increase the solar input.

    I have like yourself an interest in IoT and my thermal store is kitted out with a wifi enabled thermostat that drives the oil boiler and allows me to monitor the thermal store temp and judge when more wood is needed to be put on the fire. The oil boiler 'set temp' varies during the day so that in the evening when I am running the stove, it is at the minimum temp needed to run the central heating effectively (35C), rather than the temp required for hot water (45C) which is the set point in the mornings. This ensures the oil boiler cuts in as little as possible when the stove is in use.
     
    Last edited: 20 Oct 2021
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  16. oldbutnotdead

    oldbutnotdead

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    As above I've got similar linking (if store temp is above 50-odd then inhibit the gas boiler). Top tip on timber drying, will give that a go next year :)
     
  17. JulianM

    JulianM

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    Hope it works as well for you, most people don't have the space to be able to do it. I trimmed some 4-6" dia limbs off a maple around 6 weeks ago that had just started to turn, left them with leaves for 4 weeks and even in the wet weather we have had it seems to have dried the timber to around 25%.
     
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