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Author Topic: Retro fit back boiler  (Read 4958 times)
iann41
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« on: October 30, 2009, 08:23:03 AM »

Hello all,

Had a Dunsley Yorkshire for some time now and am well happy. Apart from a broken glass, its fine.

I'm thinking about retro fitting a boiler to it of some kind. What i want is to hook it up to the unused shower coil on the hot water tank. This will provide hot water to the house during winter when the solar is of little use due to rubbish weather.

Have you any ideas about suitable heat extraction from the stove?

how about the plumbing side of things? I am thinking of just having a vented pipe run to the shower coil and fitting a CH pump. I'll stick in a NRV to stop thermo syphoning and a large dip in the vent.

Sound feasible?
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KenB
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« Reply #1 on: October 30, 2009, 01:07:20 PM »

Iann,

Do Dunsley offer a boiler conversion - that would be the first thought?  As the Yorkshire is a clean burn stove it might not be too wise to mess with the combustion zone.

How about a coil fitted into an oversize flue section?  Get a couple of 6" to 7" adaptors and a short length of 7" flue pipe.  Wind the coil inside the 7" section and set in place of the first 600mm of fluepipe on the top of the stove.

The back boiler in a traditional woodstove is usually nothing more than a welded steel tank fitted with 1" BSP pipe unions.  It is sized and shaped to replace the firebricks that line the back of the stove.  Some boilers are shaped so that they slant forwards to capture more of the heat that goes up wards towards the flue.

In my opinion this is remarkable low tech, and anyone who could cut  2-3mm sheel sheet and bend it up into a welded tank could have a go at making one of these.  The fittings on the back are just 1" black iron femal  threaded pipe couplings set in opposite corners of the tank.  I watched as my stove supplier set about the back of my stove with a hole-saw and cut two holes through the cast iron to accept the pipe fittings - make these good with firecement.

However the standard back boiler is not very efficient and presents a huge mass of fairly cold water to the flames - which invariably causes problems with combustion.  A better approach would be to use a salvaged cast iron boiler from a 20 year old gas boiler, or wind yourself a monotube heat exchanger from stainless steel tube.  This would present good surface to the flames, but without the huge volume of cold water. Either a would coil or a frame of welded  tubes - like a gate, with 1" risers and 1/2" horizontal bars.

How about a 600 x 400 steel panel radiator?  Depending on the size of the stove - you might just shoehorn that into the back of the yourkshire.  You might even find a very small towel-rail radiator perhaps 400mm square that is constructed like the above frame - that might be a good starting point and save some fabrication work.

Possibly cut one of these up

http://www.diy.com/diy/jsp/bq/nav.jsp?action=detail&fh_secondid=9271324&fh_view_size=10&fh_start_index=10&fh_eds=%3f&fh_location=%2f%2fcatalog01%2fen_GB%2fcategories%3C{9372015}%2fcategories%3C{9372045}%2fcategories%3C{9372198}%2fspecificationsProductType%3dtowel_radiators%2f_wide_price_range%3d0-50&fh_refview=lister&ts=1256907355014&isSearch=false

Sourcing the boiler is the first problem.  The second is tying it legally into your DHW system.   You will need a header /expansion tank rated for multifuel stoves and some 28mm flow and return pipes. See elesewhere on this forum for what others have done.



Ken
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dhaslam
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« Reply #2 on: October 30, 2009, 03:26:40 PM »

There is a boiler version of the stove but may not be easy to buy and fit the boiler.   Also the boiler model would have thermostatic control which the other version may not have.     I think the problem with boiler stoves does largely relate to the surface area of the boiler.   When the boiler was designed for my last house the height of the flueways was quite critical because the smoke cools  as it rises and transfers heat.  The  boiler had three vertical routes for the smoke to pass but the distance was about 15 inches.  The person who made it had just invented the triple pass back boiler so the dimensions were based on previous experimental models.   The present stove I have seems to have some basic design fault.   It has three flueways as well but is  transferring  less than  one quarter of the heat to water compared to the other stove,  even though it is three quarters of the width.   
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Woodrascal
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« Reply #3 on: October 30, 2009, 10:04:18 PM »

Iann - another option if you only need a smallish btu output may be to try one of Charnwoods flue boilers

http://www.charnwood.com/charnwood-flue-boiler.asp

Because your Dunsleys a quite efficient stove, the cooling effect of the boiler on the flue gas temp may not be as much of a problem as it would be with a poorer stove. It would also mean that no modifications would have to be made to the Yorkshire.
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iann41
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« Reply #4 on: October 31, 2009, 08:14:58 PM »

Thx for the replies,
the flue boiler looked quite good.

Just what i was thinking of.

I don't think it would be too much of an inconvenience to retro fit.
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greg
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« Reply #5 on: January 06, 2010, 02:18:29 PM »

Hi,

Did you progress this idea;

I like the look of those, but it is a bit tall for my install (woodburner inside a fireplace) - I do not have the vertical height on my flue.

Another idea I had was to wrap some pipework around 6" of flue.  A similar install is described here;

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3788

I do not need huge amounts of BTU - just want to do some background water heating whilst the woodburner is on and spread some heat into the Hotwater.  This would compliment a solar thermal install during the winter months.

Only taking a little heat out of the flue should not pose any significant deposit problems.
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iann41
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« Reply #6 on: January 13, 2010, 01:56:10 PM »

not made any progress with this idea but i'm sure it would work.
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Eddawson
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« Reply #7 on: February 16, 2010, 06:08:39 PM »

speak to Goddards, http://www.goddardengineering.com/  
they made a one off back boiler for me to fit an unknown log burner and they were very helpful.
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