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Extra heat from woodburner


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My 5kw woodburner is 3 years old. My house has no chimneys, it was built on a housing estate in the 1990s.The living room flue is partly insulated. The insulated flue through the first floor passes thro’ a now little used much smaller wardrobe, there are no bends in the flue. The flue in the roofspace is within a metal cage The only evidence of the flue is the 1.5m of flue above the roof tiles. The flue in wardrobe is enclosed in a fireproof plasterboard duct with metal framing with at least a 50mm air space between the metal face of the insulated flue and anything combustible. A access panel was installed in the duct for maintenance purposes. The flue casing  temperature is too hot to touch with the palm of my hand for more than a second or two.

My question is could I isolate say one metre of flue with fireproof insulation and, using an inline fan, blow forced air into the ducting around the flue and out again as a heat source for the bedrooms. I would take advice as to whether I used flexible 75mm aluminium wrapped tightly around the flue or just blow the air into space around the duct. Would the cooling effect on the gasses in the flue have any detrimental effect?

 

Any advice or comments would be welcome. Photos can be provided

 

 

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Hello and welcome.

 

If you interfere with any part of the flue then you become legally liable for it as you are the last person to have worked on it.

 

To remove insulation as I think you are proposing would almost certianly invalidate your household insurance should you ever have a fire.

 

I suggest you get the company who installed it back to advise you,    if you are unsure who this was you can check on the Hetas web site ( go into buy a new certificate)   or if nothing there check with your local building control office.   It sounds like the install is a good job.     I found one a few weeks ago with exposed pipe in a wardrobe,  and clothes in the wardrobe touching the flue pipe !!.

 

A

Edited by Alycidon
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We had a woodburner installed in our old house and there was no room for an external flue (It was detached by about 1m from next door).

The twinwall that ran up through our bedroom proved to be a very effective vertical rad. Almost too effective.

It was eventually boxed in with fire board for a fitted wardrobe, but a vent still provided decent heat.

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1 hour ago, Rob_the_Sparky said:

I was wondering if all you need to do it put in a vent top and bottom of this enclosed box containing the flue and the heat will make its own way out into the room.

Yes it would but  not a lot , I have twin wall flues running up through 3 bedrooms and the amount of warm up in each are quite different dependant on size, exposure etc which is as expected .

Be aware that some  flues have different distances to combustibles depending on whether inside boxing or not so wise to ask retailer for the two distances before purchase if you want to keep the box size as small as possible as diy installers are not always aware of this.

Edit: before anyone jumps in about signing off etc, people do diy it

Edited by slim reaper
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1 hour ago, Rob_the_Sparky said:

I was wondering if all you need to do it put in a vent top and bottom of this enclosed box containing the flue and the heat will make its own way out into the room.

What worries me about all these ideas for extracting extra heat from the flue gases is, assuming a modern stove designed properly rather than just a steel box with a door and flue outlet,  that either the stove is being over driven and the flue gases should not be that hot or you will cool the flue sufficiently to cause condensation which will run back down and end up as tar deposits, plus stainless steel loses its corrosion resistance in saturated acidic conditions.

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48 minutes ago, openspaceman said:

What worries me about all these ideas for extracting extra heat from the flue gases is, assuming a modern stove designed properly rather than just a steel box with a door and flue outlet,  that either the stove is being over driven and the flue gases should not be that hot or you will cool the flue sufficiently to cause condensation which will run back down and end up as tar deposits, plus stainless steel loses its corrosion resistance in saturated acidic conditions.

Having re read first post I see op is not talking about the heat from the outer surface of twin wall so please disregard my previous post.

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7 hours ago, slim reaper said:

Having re read first post I see op is not talking about the heat from the outer surface of twin wall so please disregard my previous post.

 I read it that it was heat from the outer surface of the twin wall within the plaster board duct - which is fair game IMHO, providing some thought and precautions go into it.

 

On 24/05/2022 at 16:28, Clogs&logs said:

The flue in wardrobe is enclosed in a fireproof plasterboard duct with metal framing with at least a 50mm air space.....

 

Opening up the twin wall obviously not a good plan...  no point discussing ....

 

Can OP clarify intentions ?

 

 

Edited by bmp01
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I'd go with Openspaceman, if you are cooling the flue with a forced air system rather than natural ventilation you will have to consider whether this will cool the flue too much, the gasses condense and you'll get creosote in the chimney above what is normal. You might not know if you do until you put in a fan and the sweep tells you it is, get the chimney swept more often (once more each year?).

 

As for blowing air around the flue... we all have stove top fans which is pretty much the same thing, though these start when the stove top is up to temperature. Might be you reconfigure one of these to provide the air supply, it starts when it is warm, might not cool it too much.

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