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Author Topic: After market back boiler for my wood stove?  (Read 15025 times)
Imintheshed
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« on: October 27, 2008, 02:18:18 PM »

Hi all.
With my Solar panel struggling to achieve much in the current gloom, I was wondering if any one knows of an after market 'bolt on' water boiler that I could mount behind/on top of or around my existing wood burner? I have no idea what model it is (it has French origins judging by the hilarious 'allo 'allo style handbook), but, being an 'insert' type its rather conveniently rectangular in shape (ie no pretty canopies) and it does belt out the heat. I couldnt bring myself to conceal the top of it (like I guess you are supposed to with this design) because it cooks our chestnuts in the winter. There is therefore a good sized 'hole' on top (60cm wide x 30 deep x 15cm high); alternatively I could risk a hernia  Lips Sealed and drag it out of the fireplace to free up some space behind.
I've read the various threads on where the water would go once heated......is nothing ever simple?? And will cross that bridge when I get to it!
Any ideas gratefully recieved!
Cheers
Rich
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lightfoot
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« Reply #1 on: October 31, 2008, 11:28:46 AM »

Hi Rich,

If you don't know anyone locally that could knock you one up, the likes of McDonalds Engineers sell back boilers....

http://www.mcdonald-engineers.com/products/backboilers.htm

You could also possibly install some kind of heat exchanger in the flue way to extract the heat from the flue gases, like a high efficiency log boiler/WBS, rather than a back boiler around (and cooling) the fire itself.  Obviously you would need access for cleaning and ensure that it doesn't adversely effect the passage of the flue gases etc, but also you need to ensure that you don't cool the flue gases too much, especially with over sized/poorly insulated or short chimneys, else you may not have sufficient draft and also cause condensation and tar to form in the flue etc.


Good luck,

Lightfoot.
« Last Edit: October 31, 2008, 11:41:31 AM by lightfoot » Logged

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Ivan
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« Reply #2 on: November 01, 2008, 10:06:00 PM »

How well would a heat exchanger from a gas boiler cope inside a woodburner? I know it's not ideal, or designed for the job, but there are very cheap units available off the shelf.
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dhaslam
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« Reply #3 on: November 01, 2008, 10:21:47 PM »

If you only need hot water then any small back boiler should do.  If you use stainless steel you could connect direct to the cylinder without a header tank.  Fitting will need two holes for the pipes which need to be well sealed.  If you can replace the baffle plate with a small boiler on top it would avoid reducing capacity.     Can you photograph the stove, someone may be able to identify it?  It would be easier to fit a boiler made to fit.

http://www.naturalheating.co.uk/pdfs/boilerguide.pdf
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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
firefletch
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« Reply #4 on: November 08, 2008, 04:51:04 PM »

Hi, I have a wood burning stove thats not got the option to have a clip in boiler, but i would like to put one in myself, if & when I get the stailess steel boiler made, as it came from Spain.There is plenty of room if I remove the baffle plate!

Can I just drill holes in the cast iron then seal them, if so what with!
I also have a thermal store, 1000ltrs that takes on the solar panels & oil burner, giving heat to underfloor heating & hot water.
Having just installed the stove myself I would like to transfer this heat into the store from the stove.
Do I need pumps(the stove & store are on the same level) if so, can they start automatically when the temp is correct.
Also am I right in saying that there will be minimal risk of too much heat with 1000ltrs.

Any advice would be grateful.

John
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lightfoot
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« Reply #5 on: November 08, 2008, 05:36:46 PM »

Hi John,

I don't know the size/output of your proposed WBS/back boiler, but the following may help answer some of your questions.....

http://www.navitron.org.uk/forum/index.php/topic,5077.0.html

http://www.termoventiler.se/default.asp?webb_ID=110&webbsida_ID=71

http://www.termoventiler.se/upload/dokument_PDF/lm2100_e.pdf

http://www.termoventiler.se/upload/dokument_PDF/Rrtermostat_E.pdf



Lightfoot.
« Last Edit: November 08, 2008, 06:16:47 PM by lightfoot » Logged

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Ivan
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« Reply #6 on: November 08, 2008, 10:56:09 PM »

I've seen production stoves predrilled for optional boiler - they seem to simply use firecement to seal a circular piece of steel into the hole. Easily knocked out if necessary
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firefletch
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« Reply #7 on: November 09, 2008, 11:38:19 AM »

Thanks for the reply Lightfoot.
Not to sure about this laddomat 21 !
The pipework from the wbs would come down about 1mtr, under the floor go along for about 10mtrs then rise 1.8mtr & t into the same connections as the solar panel flow & return, installing one way valves on both the solar & wbs.
Forgive me if im far out, but could i just put an inline pump with a flue thermostat & switch.
I think the big question would be ....it the pump fails! or would gravity do it work!
What size pipework, 28mm?

John
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lightfoot
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« Reply #8 on: November 09, 2008, 01:00:51 PM »

I may have misread your post - but if the flow from the WBS drops down before rising to the store, then it will no be able to circulate by gravity alone - no matter how big the pipework is.  Also, I'm not sure if I would share the same connections as the solar (which I assume is a sealed/pressurised circuit and heats the store via a coil in the bottom of the store).  As apart from potential pumping conflicts, I assume the store and WBS circuit will be open vented (unless it has the appropriate control/safety devices for a sealed system etc) and also it is often a good idea to connect the WBS/Boiler directly (no coil) to the store, so that it can be heated from the top down, promoting good stratification - and thus the use of a Laddomat 21 etc on a pumped circuit.  However, it maybe hard to justify the likes of a Laddomat 21 for a low output WBS and/or a small store and to some degree a simple pump in combination with a pipe & flue stat may do the job.  But I feel, some form of loading valve/charging unit is a more refined solution for many pumped WBS/Boiler primary circuits.


Lightfoot.

« Last Edit: November 09, 2008, 01:02:22 PM by lightfoot » Logged

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firefletch
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« Reply #9 on: November 10, 2008, 12:18:30 PM »

Thanks Lightfoot !
I think a simple pump in combination with a pipe & flue stat may just be the ticket (for my pocket).
Just need somebody to make me the boiler now as it is a triagular shape in the corner but with a massive space above the baffle.
Are these boilers just a empty box or do they have baffles & pipework?
John
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firefletch
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« Reply #10 on: November 10, 2008, 10:05:14 PM »

Thanks for the info Ivan
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Imintheshed
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« Reply #11 on: November 11, 2008, 08:10:57 AM »

Thanks folks. Really useful stuff and I think I'll follow Firefletch's thinking with a boiler to replace the baffle plate, and a pump controlled by a stat.
One question though, my wood burner's flue runs about three feet vertically (via a plate made of cement board) into the chimney void. Can I run the flow and return pipes from the boiler, up through the flue and then loop them 180 degrees back down through the plate so that they 're-appear' above my burner, or will the presence of cold pipes (I was going to run 15mm copper) impact on the flue gasses dramatically? If this will work, could I put the stat on the flow pipe in the flue/void or will the temperatures get too high for the cable?!!
Cheers
Rich
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Ivan
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« Reply #12 on: November 13, 2008, 09:52:07 PM »

I'd say that's bad practice, and probably against regs, though I don't know what the regs are, so check with someone who does. You'll need to make sure you don't have soldered joints - my flue pipe will easily hit 250C in normal operation - and can hit 350C - enough to melt solder.

I think the main concern will be the smoke attacking the copper, and eventually perforating it. If you did the pipes in high grade stainless steel you could get around this problem.

I wouldn't worry about the local cooling of flue gases - a boiler does that anyway! You'd need to consider how the pipes would affect chimneysweeping - you wouldn't want to be banging the brush into the pipes regularly in case they fracture.
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firefletch
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« Reply #13 on: November 19, 2008, 12:43:40 PM »

Hi Lightfoot,
Would it be possible to use a boiler unit from a gas boiler, rather than using a clip in or after market stainless steel  back boiler, surely it would absorb heat alot quicker than a 'box full of water' ?
John

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Amy
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« Reply #14 on: November 19, 2008, 01:48:44 PM »

Please check regs first as im pretty sure a back boiler must be at least prototype pressure tested.

The heat exchanger from a modern gas boiler will be made from an alloy and may not stand the temperatres. Also the finns will block with soot.
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