Fitting a flue liner

Flue-liner fitting is a bit of a game and it's not entirely obvious how to do some of the fiddlier bits, so here's some info on fitting mine.

Our chimney started leaking in the loft so needed lining. We have one central chimney containing two flues. One come from the woodburner in the front room (A Charnwood country 4, which is an excellent little 5kW stove). The other comes from the kitchen and is disused - there presumably used to be some kind of heater or cooker in there.

Twin-wall liner is a requirement for solid-fuel appliances. Even though 316 stainless is deemed adequate for woodburners we decided to get the more expensive 904/904 liner (20 yr guarantee) so it wouldn't need fixing for some time.

Working out exactly which parts you need from one of the many online flue-parts suppliers is tricky, especially if you've not done this before. We have no existing chimneypot, and there seemed little reason to buy one so I opted for a suspending cowl, as recommended by the supplier. By using this you can just not bother with the conventional clamp, top-plate and sleeve used with a pot. I bought a nice big cowl with a rain cover and bird-guard, as we get a lot of pigeons and I've laready fished one out of the chimney (alive!) when we originally opened up the fireplace (who knows how long it had been in there - a very lucky bird).

At the bottom you can either use an adaptor+clamp or an adaptor with integral plate. I opted for the latter, largely because it wasn't at all clear how to seal the clamp+adaptor to the closure plate with good airtightness.

We are trying to maximise airtightness so having gaps around the closure plate up the chimney would be bad.

It turned out that the hardest part is fitting the closure plate (which took about 1.5 days of faffage). The enormous-snake-flue-down-the chimney bit took about 10 mins.

One thing I forgot to work out beforehand was the implications of the minimum bend radius for 6" pipe. My flue angles off sideways immediately at top of fireplace and with a minimum bend radius of 380mm, given that the flue needs to be central in the fireplace (a non-centred stove looks silly), the closure plate had to move down about 100mm from where it was previously with the open flue. That put it at the same level as the stone/concrete lintel above the fireplace, which it turned out was impossible to drill mounting holes in with a drill that would fit in the (small) fireplace, so I made an angle-iron frame mounted at the sides, and holding the front piece up.

The adaptor plate was slightly bigger than the depth of the fireplace so I hacked 10mm off the back and 30mm off the front. The supplied spigot was quite long (200mm) and would stick out a long way below the fireplace lintel so I cut it down to about 90mm so only black enamel stove pipe will be visible when it's finished. There was nothing like enough room for the adaptor plate to mount above the cloure plate/frame, so it was fixed from below with 12 self-tapping screws. This gave much more room for fitting the flue into the adaptor, and generally made the fitting much easier, although drilling all the holes in the right places took a while.

All the rest of the closure plate work was done before putting the flue liner down, to give more room. The gaps to either side of the adaptor plate were filled with galv plate sat on top of the angle-iron frame. The frame was cemented and screwed/nailed to the wall. The plates were siliconed to the frame and fire-cement filleted on the back. All this was done one day and allowed to set before moving on to the adaptor plate and flue fitting the next day. The adaptor plate was siliconed (and ceramixed for good measure as it will get quite hot). The flue was ceramixed and fire-roped into the adaptor plate. Holding the plate in place against flex of flue whilst trying to get first couple of screws in was a right game involving amusing gynastics in the fireplace.

Filling the chimney with leca was easy enough except that it was rather windy so kept trying to blow away. It would be best to do this with two people - one to hold the flue up and central, and another to pour. I chose leca because unlike vermiculite it does not sag if it gets damp, and it's cheaper. It is harder to source, especially in the smaller sizes best for chimney backfilling. I used 5-8mm leca, which is ideal (supplied by Mike Wye, as all local suppliers could not do less that a palletfull of 21 bags). My simple calcs of a 6" pipe in a 9" flue suggested that 250l would be needed (5 bags). This didn't allow for the extra space where the flue widens for the fireplace or the corners, and in fact it took almost exactly 6 bags (300l).

I wondered how much flex there would be to help with fitting the cowl snugly, and the answer is none, once the backfill is in. The cowl has straps to go outside the flue and take a jubilee strap, but these are about 150mm down and there is no way I could get the jubilee on that low (I have no idea how you do it with a chimney pot either, short of drilling a hole in the pot about 150mm down). It could probably be done _before_ insulation fill because there is lots more movement of the liner possible, but once the cap is fitted there is no way to get he insulation in, so that's not much use. So I cut those straps off and put the cowl and straps down inside the liner, then used the other set of straps to screw the cowl to the chimney-top. Finally I topped up the leca that had blown away, put down a bed of cement around the flue, put the cowl in place and screwed up the jubilee. Then screwed it down to the concrete cap and flaunched all round.

The one bit I failed to work out in advance is that the 5" spigot on the adaptor plate does not fit inside a 5" flue pipe - it's exactly the same dia, so you need a stove pipe with a socket on top. That ruled out the one I already had so had to go shopping again - grumble.

One other tip is that an angle grinder is a very easy way to cut the liner off nice and square.