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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorsmudger
    • CommentTimeMar 16th 2016
    We have a large (26kw - 16kw to water) boiler stove, yet to be installed. We also use PV and off peak (economy 10) elec for both space and dhw heating. Currently its seperate and not sure if we should combine for flexibility or install two cylinders (or small TS) with controls to switch between them. Our objectives:

    1. Off peak / PV hw in summer and largely in spring/autumn when boiler stove not being run (or run as long)
    2. Some storage of hw to heat the wet rad system in the morning for a few hours and avoid using elec boiler (6kw) -we do have storage heaters to give background heat all day
    3. Mains pressure HW

    We are at 1000ft so 'summers' are short, so we can live with boiler stove being lit for 9 months of the year, but not in the mornings to get any heat. Water demand varies but typically 4 showers per day but could be more with visitors. Limited by space and access so decided 500l TS was optimal, but alternative is 2 x300l HW Cylinders.

    If TS is only option for mains pressure hw, what are the key specs to ensure we have proper stratifixation and dont end up heating 500l of hw when we dont need it.

    Thoughts / advice welcome, along with recomendations for manufacturers.
    • CommentTimeMar 16th 2016
    It sounds like a big stove. What is your space heating load? How much heat energy will you need to store to 'heat the wet rad system in the morning for a few hours'?
    • CommentTimeMar 16th 2016
    I have had bad experiences with the TS and a 16KW Dunsley. A friend running the same stove, direct to radiators and a couple of heat exchangers for DHW has had a better experience!

    You need big thermal store to heat your radiators without a heat source topping it up at the same time in my experience.
    • CommentAuthorsmudger
    • CommentTimeMar 16th 2016
    djh -A Heating engineer had quoted for a 26kw oil boiler and 22kw pellet stove (proper quote -measured rooms)- so there or there abouts. Large 5 bed farmhouse with annexe. Uninsulated (for the moment) and exposed. We dont use a lot of dhw in the morning (except for those pesky visitors....) prefering to shower/bath in evening, so potentially all is available for heating (hence single store?) and TS 'emptied' for PV heating of water (mostly spring and autumn). I did a fag packet calc and said 40kw heat for 2 hours heating / heat capacity of water (3.6...?) = 160 l, so 200l or thereabouts to allow for losses but doesnt sound a lot so probably 'wrong calculation'. We only burn logs (mixed quality).

    richy - anyone and everyone I have spoken to has said it needs 'storage' / buffer (not necessarily a TS though). Apart from safety, not sure exactly why. We would install a laddomat / return valve thermostat and possibly a H2 panel as well so would hope this would address 'operational / commissioning issues'.

    In the end we cant get anything bigger than 500l upstairs (and that might need a few door linings removed) but option for 2x300l also there. Its the stratification (or lack of) of the TS, especially when stove not used that is my main concern. Some manufacturers have baffles or seperate cylinders on top of each other (ie 2 in 1). Are these a better bet?
    • CommentTimeMar 17th 2016
    Posted By: smudgerI did a fag packet calc and said 40kw heat for 2 hours heating / heat capacity of water (3.6...?) = 160 l, so 200l or thereabouts to allow for losses but doesnt sound a lot so probably 'wrong calculation'.

    Well without looking anything up, I would do it this way:

    40 kW for two hours = 80 kWh
    specific heat of water 1.16 kWh/m³K
    so 80 kWh = 69 m³K of water
    if we allow a delta T of 50 K
    69 m³K => 1.38 m³

    i.e. 1380 litres

    Now I may have misunderstood your 40 kW for two hours and I may have misremembered the properties of water or got my sums wrong, but I expect Ed will gently correct me if I have. :bigsmile:

    In reality, you could make the water a bit hotter to start with but you wouldn't be able to cool it all the way back to room temperature easily, so the numbers could vary quite a bit.
    • CommentTimeMar 17th 2016
    As I understand thermal stores, they all have a natural tendency to stratify, especially if they are tall and thin. What messes up the stratification is injecting water into the store at the wrong height (where the temperature of the incoming water is different to the temperature of the water in the store at that height). Since the temperature at a particular height varies depending on how much heat the store is holding, it's a tricky problem. So the returns from central heating and from external PHEs for DHW are the kind of things that cause problems by stirring the tank. As does water from solar thermal at varying temperatures. Immersions at the bottom of the tank must stir the tank too, but they seem to effectively heat it from the top somehow.

    Some manufacturers have shapes or baffles or pipes or whatever inside the tank to try to reduce the stirring. I think most of them don't work as well as advertised. I believe the best is the system used by the German company Solvis, but I've never been able to even discover whether they sell it in the UK, let alone discuss it with anyone.
    • CommentAuthorEd Davies
    • CommentTimeMar 17th 2016
    DJH's volume calculation looks right to me. Remembering 1.16 kWh/m³·K is useful though I only remember the SI versions: volumetric heat capacity of water is 4.2 MJ/m³·K and an hour is 3600 seconds. (I wish we used MJ instead of kWh which confuse people inordinately).

    Posted By: djhmmersions at the bottom of the tank must stir the tank too, but they seem to effectively heat it from the top somehow.
    Dunno but I visualize a column of hot water going up from the immersion and spreading out and down from the top. With a concentrated heat source like that it ought to be a fairly well organized circulation.
    • CommentAuthorsmudger
    • CommentTimeMar 17th 2016
    Thanks both.

    The 40kw was 'based' on 20kw heat demand for 2 hours, so if its half djh's calc then 500l wont be bad but nothing for hw potentially. However, in reality we currently use a 6kw elec amptec and it 'copes' with heating load- buts its peak electricity after 6am in winter. If we have a 'controller' of some kind eg H2 panel then we can use the elec boiler to supplement if need be but in the real world we use 2 storage heaters to maintain a background heat of 16-17 deg in occupied rooms to avoid mould on walls).

    We also have ex 160l Range cylinder which pushes me to retaining this as a 'safeguarded' dhw supply. however. So that's one decision made.

    Viz single TS v twin hc, we would be using twin immersions, one in the bottom as well. Reading other posts, we should heat top as a priority then bottom unless PV generates enough for both. The range cylinder only heats at the top (solar thermal not working) so we only get about 80l of hot water at any one time and thus we are not utilising all the available PV generation which I want to avoid on any new install (water doesnt circualte around the whole tank). The designer of the H2 panel was the first to question a thermal store and suggest seperating the DHW and heating, using a hwc as a 'buffer' tank. He said it would be cheaper but not sure it actualy would be if comparable specs but its more an issue of optimal solution.
    I too live in a large 5 bed uninsulated house (well the roof is insulated) with a granny annex/appt (3 story, 450m2 in all). My heating load at say, zero degs outside, is about 15-20kW (incl DHW). I have a 40kW WB and a 2000 litre TS (which is a bit too small). So something is very strange here: your 26 kW WBS I get, or your 20kW pellet I get, your 20kW load I get but for just 2 hours i do not get! 6kW heating system being adequate I do not get! 500 ltr TS I do not get!

    What is the heating load on your house? (not I need 20kW for 2 hours). What is the calculated steady state heat load for a known low outside temp and a known inside temp?
    • CommentAuthorsmudger
    • CommentTimeMar 22nd 2016

    Our house is c. 200sqm incl annexe. So half your own property.

    It also has low ceiling heights - 17th farmhouse with extensions (not a mansion....), so lower volume as well. South facing 'living rooms' so supplementary solar gain / radiant heat on some days too. North facing rooms are nippy I'll admit.

    Heat load, not just about heat loss? We 'maintain' temperture and do not allow house to swing from cool to hot - something we have learnt by living here over a few winters. 3 storage heaters use off peak elec to maintain this during the day so demand in the evenings is less (and infrared heaters in bathrooms for a bit of luxury). We wear jumpers. And toughened up (leccy blanket helps mind...). It would be nice however, not to use the elec boiler on peak in the mornings and have some hot water for visitors who insist on showering in the morning. That's all.

    I work from home. In colder weather, stove is lit daily in the early afternoon, if very cold, in the morning and when its warmer not till the evening. Its a very low tech but surprisingly effective control system. And highly responsive to changing weather patterns.:wink::bigsmile:

    Are you an engineer per chance?

    Most response to 26kw boiler stove is :shocked: ie its huge. We think running a stove is an art not a science and with the supplementary heaters we'll be fine. Just wanted some steerage on thermal store / buffer size (which will be loaded on a daily basis) and how we ring fence our dhw.
    • CommentAuthorGotanewlife
    • CommentTimeMar 23rd 2016 edited
    Ahh yes not quite as big ass mine and mine has 3m high floors (2.70m living hieght) as well. I have massive solar gain when it shines (and it does here a fair bit!!!). I heat 24/7 pretty much, with TRVs on all almost all rads. Lots of mass so even though I might be pumping around 40 degree water for several hours in my heating cycle the house stays pretty much bang on the required temp - which is 21 deg btw (80 year old in-laws in apartment and 18 year old kid living in coldest room in the house). House is always occupied too.

    Not an engineer but it does feel like i have had a thoroughly good education on this site. Ref heating my house - I too think it is an art form!!! Just putting that extra log or 2 on (or not) when I shut up for the night takes a lot of experience.

    Anyway, I am sure you will enjoy using your little baby boiler ;) In my experience there are few downsides to having more TS (other than cost and space that is). If I could change anything it would be to have a bigger TS but I already had to dig out a hole in the floor to fit my 2500 TS in.

    As to any pointers - I am not sure I can help. My solution to the space heating DHW issue is to have a tank-in-tank (potable water tank 350 litres) inside the top of my TS - surrounded by my solar coil. Pretty much instant re-heat! I have a choice of 2 tapping heights towards the top of my TS from where I can extract my space heating water (I can imagine using the bottom of the 2 only if the house was FULL. I have a sensor in the top of the tank and when that drops below my chosen temp it switches off the heating pump protecting my DHW.

    The general consensus on here seems to be to separate the DHW where possible I wonder if (given your specific limitations) there might be a one-off hybrid solution using the 2x300 tanks and some clever connections/motorised valves and control gear solution where you used only one tank usually but had the option to use the other (primarily DHW) tank.

    My overriding thought though is that you should not design a system based on being cold, or rather that doesn't let you be warm if you want to be without punishing you financially. With a 26kW WB there must be some way - what about storage heater being heated by hot water - sort of artificially extending size of your TS.

    btw - did you notice that you didn't answer my question re how many kW you reckon you need to maintain your house's temp (ie on a moment to moment basis)?
    Although I can't comment on the spec and calcs I can vouch for having a separate DHW cylinder to the main buffer or a TS setup. For our setup with a pellet boiler it has proved to be valuable on many occasions.

    Firstly summer months we can shut down the boiler/buffer tank and use the DHW as a standalone from immersion (solar coil ready for future).

    Even more helpful has been that we don't loose our DHW when we have either been low on pellets or as is the case at the moment and the boiler is playing up! We can get by for a day or two with our wood stove and the immersion.
    Posted By: restorationcoupleI can vouch for having a separate DHW cylinder to the main buffer or a TS setup.

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