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Author Topic: sizing wood burner with back boiler  (Read 13786 times)
bar
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« on: March 10, 2009, 08:02:00 PM »

we are looking to install a wood burner with backboiler for domestic hot water and central heating as well as space heating our main room.

advice requested on sizing wood burner.

it appears most burners are solid fuel and the power outputs seem to be quoted for coal. It is difficult to get specific data on units for wood only.

we have a good mix of wood including oak ash and some pine and would like the wood burner to be capable so we do not have to burn oil.

currently we have a kerosene 460 rayburn which does ok. we have 12 rads with around 50,000 btu capability.

currently thinking about stratford 70 - not sure a 90 would be better?

any thoughts?

thanks
Bar
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Brandon
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« Reply #1 on: March 10, 2009, 09:33:11 PM »

you could put it through a thermal store via a laddomat or laddomat-u-like, then heat from the store.

how much DHW do you use?
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« Reply #2 on: March 11, 2009, 11:42:14 AM »

Hi Bar,

I'm in the middle of doing what you're looking at and will be hooking the WBS into my existing oil fired system via a Dunsley-Baker Neutraliser.  I did look at the Thermal Store route but that would have put an extra 1k minimum onto the cost....

As for sizing up the WBS, my only advice here is to check the 'nominal values' not the max values and most importantly, the heat output to room.  Our living room isn't very big and most of the stoves that could give a good amount of hot water also gave out in the region of 5kW or more where our room only needs around 2 so we were at risk of over-heating the living room just to get enough heat into the rest of the house.

In the end I've compromised by going for a Hunter Herald 8 which is sized just about right for us.  2kW into the room and 6kW into the water which will give me plenty of hot water for showers and baths and will keep the rest of the house just about comfortable. 

Hope that helps Smiley

Ian.
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daftlad
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« Reply #3 on: March 11, 2009, 12:07:06 PM »

As for sizing up the WBS, my only advice here is to check the 'nominal values' not the max values and most importantly, the heat output to room.  Our living room isn't very big and most of the stoves that could give a good amount of hot water also gave out in the region of 5kW or more where our room only needs around 2 so we were at risk of over-heating the living room just to get enough heat into the rest of the house.
I agree, if the room output is too much and the boiler is too little then there is a problem.
We fitted a morso dove with a retro fit boiler and have that problem, if you go with one with an integral boiler (hunter, welded steel ones etc), the water will get more heat. In the end if there is too much hot water and the room is still cool then put a radiator in the room with the wood burner.
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« Reply #4 on: March 11, 2009, 03:09:43 PM »

I have a Mulberry Beckett stove.  It has nominal output of 12.2 KW to water and 5.5Kw to the room.    The documentation with the stove said 7.5 Kw with wood,  I think.  However the actual output  to water using coal is less than 7.5KW.  My buffer tank is fairly close to 860 litres so it should gain one degree per Kwh i.e 12.2degrees with coal per hour  but it was closer to 3 degrees  with coal  and about 2 degrees with timber.  I have just fitted a V4043B valve on the return which brings the return temperature up to a range of about 55-65C. It   doesn't  seem to change the output by much but can produce higher temperatures in the tank.  Increasing the return closes the thermostat which reduces output.   Fuel usage is about 20Kgs of coal or 30Kgs wood per day.   That should be the equivalent of about 130 kWH gross so an output of about 30Kw to water and 10Kw to  the room makes an efficiency rate of about 30%.  Using coal is more expensive than full rate electricity and timber cost about half way between off peak and full rate electricity.     
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bar
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« Reply #5 on: March 11, 2009, 07:43:55 PM »

Thanks for the comments

i think i might have to put a bit more analysis into this project

certainly need to get balance between room heat and hot water duties balanced. althoug we are planning a conservatory off the main room which will gladly take any excess room heat.

Cheers

Bar

 
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Cliff top
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« Reply #6 on: March 26, 2009, 01:15:57 PM »

We have a tF50  and wished we had gone bigger.
The balance of heat to the water or room is good with the Aarow. Our 14x11 ft room doesnt get too hot.

Troll my other posts for more on the Aarow WBS and our heating system as it sounds similar to yours.
Most wood you get will have some dampness still left in there...this makes it run the rads quite a bit cooler than coal or dry glue packed chipboard etc.
For the cold 4 months of winter you need to run it quite hard with wood Sad  especially when you've been out at work all day and the hot water store is cool too.

Cliff
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PhatBob
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« Reply #7 on: March 26, 2009, 03:38:41 PM »

Our Firebelly2 puts 4KW to the room and 8KW into the back boiler, I don't ever think we've had too much heat from the stove in the room so in hindsight 6KW to the room and 6KW to the back boiler might have been a better mix.  But we've had the radiators in the room with the stove turned off for all except the coldest days and the oil consumption has dropped massively - we were cold last winter and used two tanks of oil, this year we've been warm and used 1 tank.  Balancing the costs of extra wood and less oil I'd say its a 300quid saving. 
So that extra 1k for putting a heatstore in makes sense, as if the oil prices stay static (which they won't) and the wood price remains as it is (which it won't) you'd see an ROI after 3 winters.
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RichieC
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« Reply #8 on: March 27, 2009, 08:50:21 AM »

Hi All

 I too have a Firebelly FB2 with back boiler. I found room output to be too much for our living room on all but the coldest of days. You can always open the living room door which helps  Grin

 This is the second wood burner I have installed and ran, the first being a Stovax Stockton 8. My advice would be get a wood burner where most of the heat goes to the boiler (if Poss) as once the heat is in the water you can control where it goes via central heating and thermostatic radiator valves. I have a Dunsley Baker Neutralizer with a Worcester Bosch 12kw boiler running in to it. Apart from initial output queries (see other posts) I have had no problem with my set up.

 Phatbob how have you found your Firebelly? I've been burning for nearly a season and both fire bricks on the sides have crack and will need replacing soon also I've changed a door glass  Shocked

Rich
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PhatBob
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« Reply #9 on: March 27, 2009, 09:13:44 AM »

Phatbob how have you found your Firebelly? I've been burning for nearly a season and both fire bricks on the sides have crack and will need replacing soon also I've changed a door glass  Shocked

Now its in we've had no problems with it.  But the installation was delayed because our Heatas engineer refused to fit the first one as it was distorted and the fit and finish was so bad (door didn't open and close properly, vents didn't move properly, paint was marked).  Customer service was on a par with the quality of the stove, and with complete disinterest from the directors it was down to the one member of staff who actually seemed to care to sort out a replacement.  So instead of an install in August we had the joy of the heating system being installed over the three days in October that it decided tp snow...
The new one is okay, apart from the fact that the paint goes soft when the stove gets hot, so the top of the stove is marked...

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30 x 47mm Navitron Tubes.
250l McDonald Engineers DHW Heat store.
Firebelly woodburning stove.
davec
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« Reply #10 on: March 28, 2009, 07:43:13 AM »

A couple of points:-

An inset stove (i.e. fitted within an existing fireplace) gives less output to the room than a floor standing model.

If the stove has a thermostatic air control, it will self-regulate to the boiler water temperature so as long as it's big enough to drive the full load of radiators + DHW.

Here's a radiator sizing guide: http://www.inspiredheating.co.uk/radiators.htm. Caveat: according to this I needed 65K BTU; I fitted 45 and we're fine and comfy.

I find that a bucket of anthracite will stay in all day; with wood I'm reloading 2-3 times to get the same steady heat. I.e. wood's fine when we're in the house but anthracite's better for work days.

Ours is a Charnwood multifuel LA45iB... not the cheapest nor the 'lookiest' but it's a very good machine.

Hope that is useful. Dave.


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RichieC
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« Reply #11 on: March 28, 2009, 07:33:37 PM »

Bob

 I'm glad i'm not the only person who has resprayed my stove to get rid of marks Undecided

Rich

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PhatBob
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« Reply #12 on: March 30, 2009, 09:01:07 PM »

If I'd bought a cheap stove then I'd not be too bothered, but they charge a healthy price.  Now we have it all plumbed in and its adding heat to the system I'm happy with its function and can put up with the downsides - hut there are stoves that you don't have to have things that you put up with...
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30 x 47mm Navitron Tubes.
250l McDonald Engineers DHW Heat store.
Firebelly woodburning stove.
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