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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorTOM W
    • CommentTimeOct 28th 2013
    Is there a reason for installing a 3000lt thermal store in our situation? We will be using a 130kw ETA woodchip boiler to heat (underfloor throughout) office & house plus hot water and a pool. I would have thought that once pool was up to temp it doesn’t need it and therefore 3000lt is way over spec? It just seems (to my simple mind) to be a waste of energy to keep that heated no matter how well insulated and that’s before you look at the price tag on the tank!
    Depends a bit on how auto is the chip-feed, presumably. If you wanted to operate it like a log batch boiler, burning hard and fast, then relying on the store till the next burn, then maybe 3000 ltr is not OTT. If an auger automatically feeds when temp drops, maybe significantly smaller is OK?
    • CommentAuthorTOM W
    • CommentTimeOct 28th 2013
    Fully automated. As long as nowt goes wrong I fill the hopper every 6 weeks and empty the ash can - that's the theory anyway!
    • CommentAuthorowlman
    • CommentTimeOct 28th 2013
    What Nick said, I don't know how responsive modern pellet boilers are to constant on/off demand but using it in that mode like a gas boiler may not be as fast as you would want therefore the batch burn and store route would obviously demand a buffer tank of some description, for e.g. DHW.
    I take it, Tom, that your query is, not whether to have a store or not, but over the *size* of any potential thermal store? I imagine you would have to have one of a reasonably significant size (a) to provide a buffer if needed, if, say, the auger stuck, and (b) to get the stratification to give you the sufficiently low flow temp for UFH?
    • CommentTimeOct 28th 2013
    Without working out the numbers, if the pool is acting as the thermal store (which it can), there are evaporation losses to take into account. This will cause the temperature to drop, causing the heater to be called on, which will overheat the pool a bit. Then more thermal and evaporation losses.

    Would it not be best to split the two systems, they have different heat demands, as does DHW and Space Heating.

    Tom, do you know the heat loads that are needed for the pool and the office.

    I suspect the 3000lt thermal store is more matched to the burner than the demand.
    • CommentAuthorTOM W
    • CommentTimeOct 28th 2013
    Thanks for all your answers. Don't know the actual heat loads I'm afraid and yes it was a question of the size of store required. It probably is to match the size of boiler but just seems awfully big when I can't see us having any sudden large heat demand. I suppose it won't do any harm having it as the heat loss is so low, apart from the initial cost.
    • CommentTimeOct 28th 2013
    Work out your heat loads, then work from that.
    • CommentAuthorTOM W
    • CommentTimeNov 1st 2013
    Not sure if this will help - 70kw for house, offices & hot water + pool needs 44kw to bring up to temp but only 27kw thereafter.

    As a wee aside, does anyone have experience of Heizomat or ETA woodchip boilers? Surprisingly everybody says the one they sell is the good one but the other isn't! I know I was shocked too.
    • CommentTimeNov 1st 2013
    That implies a heat load of 1.125 kW, split 60:40. Unless you mean kWh instead of kW a day, then it is 16 kWh a day for the house and 11 kWh for the pool. But that seems small to me.
    • CommentAuthorTOM W
    • CommentTimeNov 1st 2013
    Sorry, that's the figures I was told that we needed, i.e. the output from boiler(s) required to keep us warm.
    • CommentAuthorEd Davies
    • CommentTimeNov 2nd 2013
    Anybody else find this conversation weird on a number of levels?

    First off, the original post asks about “our situation” without giving much clue as to the situation in question - general construction and size of house would be useful to know, for example. 130 kW boiler, even with a pool and offices, makes one think rather big and leaky, though.

    Then Steamy pulls 1.125 kW out of somewhere. Where? All I can make match that is interpreting the 27 kW to keep the pool at temperature as 27 kWh/day then dividing by 24. But that's a) silly small (as he notes), b) ignoring the house and offices and c) not what TOM W said anyway (even where there's so much confusion, as between kW and kWh, it seems rude to re-interpret what somebody says without at a least querying it first).

    Anyway, the volumetric heat capacity of water is a bit more than 1 kWh/(m³·K) - so taking 3 kWh out of a 3000 litre store will reduce its temperature by one kelvin (i.e., one degree Celsius). With heat loads of the order of 90 kW when the boiler is not running the temperature of the thermal store will be dropping at about 30 °C/hour or 1 °C every two minutes. Sounds like 3000 litres is pretty minimal to me if the circumstances really are as said.
    I'm with Ed. I would normally try and add value to a thread on this subject but v busy, so as soon as I saw 130kW (or as quoted 'kw') and a 3000 litre TS being far too large I thought i'd leave it. So FWIW TOW W, following on from Ed's sums, 3000 ltrs is in fact far too small. But before moving on you need to describe what mammoth building needs 70kW to heat and why a pool needs 'only' 27kW to maintain its temp, bearing in mind 97kW would be enough to heat a good 10 basic spec new houses and that I have a 450m2 house with uninsulated solid stone walls and seriously leaky windows that has a heating load of 20kW.
    • CommentAuthorTOM W
    • CommentTimeNov 2nd 2013 edited
    Well I am very sorry to have troubled anyone. I was expecting the bit about kW but apart from that I'm none the wiser. Here was me thinking that growing our own timber to burn instead of oil was a good thing and that I would just ask for a bit of independent advice. Having built a house with double the insulation required and put up a turbine, I thought that after being a member on here for 4 years it was worth a go.
    TOM W, sorry - I have offended you. I was however just being honest - I read all the threads but very superficially, I make decisions about where I can add value and where I can garner info of help to me in just a few heat beats. I sometimes stick up a post that gets nothing of help and wonder why, I wish someone would tell me why too. I do want to help but I/we need to have something to go on. Size and structure of house most critically but also how long do you want the TS to heat your house and or pool in the event of a power cut or when you clean or otherwise maintain your burner?

    It remains hard to conceive of a domestic scenario where the standing heat load is 97 kW. If that really is true then look at Ed's figures - with a max TS temp not much above 80 deg and a minimum useful temp of 40 deg, then you have barely one hour of heat in your TS - I most certainly wouldn't accept that, even allowing the pool to cool you have less than 2 hour's 'heat' in the TS. Something is badly wrong here.......is it your numbers (factor of 10 out somewhere maybe) or the advice you have had.....
    Tom W

    I am also struggling to grasp the figures but again I have an old stone farmhouse on a 20KW system which replaced a 65KW oil fired system so insulation was a key element. Unfortunately there seems to be a breed of wood stove boiler salesman appearing whereby bigger is better especially in relation to RHI schemes even though they cannot be justified. No personal experience of Heizomat heaters but they do understand woodchip as I have a Heizohack chipper which consistently produces G30 spec chip unlike lesser quality chippers.
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