wood burner flue

When my mother's kitchen chimney was re-pointed a couple of years ago the builder caught his 'helper' dropping bricks down it. After giving him a telling off he took the flue above the wood burning stove apart
to remove the bricks. He then re-sealed the joins with fire cement. This has now cracked up and largely fallen away.
Ever since the chimney was worked on my mother swears that the stove hasn't been as good, she says it burns okay for an hour or two then goes out. It has a metal disk with a small hole in that pivots at the top of the stove to control airflow into a large flue. That flue goes up then bends 45 degrees to the side then 45 degrees upright again. The flue then disappears into brick where it undergoes two 90 degree turns (accessible from the outside through a metal hatch) then goes straight up the chimney.
Any suggestions as to a sealer for the joins in the flue? I see Toolstation do a high temperature sealant on pg 202 (no. 66124) but the description says it's only rated to 300 degrees C. Would that be good enough?
Any suggestions as to why it's okay for a couple of hours only? It doesn't seem to depend on external wind speed or direction. Could it be the effect of a brick somewhere?
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snipped-for-privacy@ymail.com wrote:

The HETAS installer I had used something like a silicone sealant tube - but it was rated for loony temperatures. Sorry - I don't know its name.
Re going out - the first worthwhile test is to get some smoke matches (B&Q sometimes have them with the gas related bits). Light a small fire and see if the smoke from the match is sucked swiftly up the chimney.
Repeat with a normal fire at aorund the time it starts to fail.
I would have thought if you had a blocked flue, you would have known about it (ie smoke everywhere).
Does the rest of the stove check out, particularly with respect to the inlet air vents?
--
Tim Watts

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On Thu, 20 Oct 2011 13:59:41 -0700, combi6793 wrote:

There must be something on the 'inlet' side of things, so it can draw air in - are those vents / valves / associated pathways clear? (does it source combustion air from within the room or from outside?)
cheers
Jules
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On Oct 21, 1:46 pm, Jules Richardson

It sources air from inside the kitchen, even before the chimney was re- pointed we knew that using the kitchen extractor fan subdued the fire. The inlet takes two forms, there are holes in the front of the stove that are covered or exposed by moving cast iron discs or the stove door can be opened and a fire excluder type mesh can be clipped over the doorway. However we try to regulate the draft the fire still dies away after about an hour or two.
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snipped-for-privacy@ymail.com wrote:

I hesitate to ask such an obvious question, but how does air get into the kitchen?
Chris
--
Chris J Dixon Nottingham UK
snipped-for-privacy@cdixon.me.uk
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Chris J Dixon wrote:

Yes this all sounds a bit worrying. The extractor fan should not be run when the stove is lit, indeed they shouldn't be in the same room.
What's being burnt and are there any smells? Smokeless fuels ( and charcoal) can be deadly because there is no acrid smoke to warn.
Does the fire die down after this time because the room has become warm?
Is there an air circulation path whereby the warm are in the room vents through the house or up another flue that exits higher than the one the stove uses?
AJH
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andrew wrote:

I've got couple of 6" pipes going to the underfloor cavity wherever there is an fuel appliance or open fire - or in the bedroom into the vented loft cavity.
If the OP has a suspended floor, Id drop a vent into it and put a grille over the top.
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We no-longer run the stove and the extractor at the same time.

Seasoned wood.

The extractor fan has a flap to prevent drafts when the fan is not running. There are no other vents than the stove's own flue. Air could move through one of three doors but they are only briefly open when the stove is lit.
In other words, I can not think of how warm air would vent higher than the stove. Plus no vents have been changed since the problem developed.
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I can't give a more specific answer than drafts.
I've now got hold of some smoke matches and will test the draft as per Tim Watts' suggestion. I'll also try opening a window to see if that helps.
Also, I've been up on a ladder and lowered a torch down the chimney on a string. I can't see any tar or blockages. The external hatch, about seven feet up, turns out to be something of a sloppy fit.
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On 23/10/11 21:25, snipped-for-privacy@ymail.com wrote:

and if you've got modern plastic windows theres no air coming in there, so you have a problem of not enough incoming air... - get a carbon monoxide alarm tomorrow!
[g]
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Looks interesting. The metal that makes the sections is pretty thin, any idea if it would come apart again intact if we used that stuff?

It doesn't have a thermostat, just more or less draft/fuel.
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The odd thing is the fire burns when the chimney is cold, but well after the chimney is hot the fire dies. I've seen spinning cowls on chimneys that are supposed to increase draft, but I don't know if it works.
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Because its used most of the oxygen in the room?
Get a CO alarm tomorrow!
[g]
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