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I'm installing a flueless gas fire to an old chimney breast in a ground-floor living room and the fireplace in that chimney is being completely sealed off as a result.

I'll be capping the chimney pots off with a spigot that allows ventilation from above and will stop water ingress from rain. However, I'm concerned about having a column of stagnant air and the possibility of damp.

I know (I think!) I need to ventilate the sealed-up chimney (and it has to be to the outside, for reasons I'll cover below), but this is well outside my limited experience and it's hard to get a straight answer from anyone as to what exactly needs to be done.

My questions are:

  1. How much air flow in do I need to allow for? Or, more practically, how big a vent do I need to put in?

Will a 100 mm diameter vent be large enough? What am I best off doing?

  1. What height should I place the vent at?

I'd presume that nearer the ground is better for proper air circulation and that venting into the old fireplace rather than the chimney flue would be best. However, rats and mice are a big concern locally (I live near a rocky shore), and I don't want to put it too close the ground. Slightly higher would be better, maybe 0.8m or off the ground so would keep the vent going into the old fireplace. Is this an issue?

(The flueless gas fire itself requires a vent to be put into the room and I considered killing two birds with one stone and venting the room into the chimney but the fire manufacturer says that's a no-go and the vent it requires must go outside, and that's what Building Control will go by. Similarly, ventilating the chimney into the room won't work as the gas fire has to be a certain distance from sources of air flow, and I can't make that work, even going under the floorboards.)

Please do put me right on anything that seems wrong. And thanks in advance!

Background on the house

My house was built in the late 1930s, in the north of Ireland and it has cavity walls, which are not insulated. The chimneys are unlined brick flues. The house itself is in a very exposed, very windy and rainy area and is a couple of hundred meters from the sea (hence no cavity wall insulation).

The chimney itself is internal - it protrudes into the room rather than outside the walls of the house. It's around ten meters from the hearth on the ground floor to the top of the chimney stack, and the flue is straight up, with no bends. The room in question is 2.7 meters high, give or take, and the flue is approximately 500mm x 300mm internally; the flue proper begins about a meter above the hearth.

  • I've never heard of anyone ventilating a retired flue. I'd think that the masonry will be damp no matter what if it's exposed to the weather. – isherwood Jan 15 at 14:40
  • @isherwood Thanks for the fast reply. So you'd think venting would harm things rather than helping? – tmgr Jan 15 at 14:54
  • Didn't say that. I'm just not sure what the benefit would be. Are you concerned about mold? – isherwood Jan 15 at 14:54
  • @isherwood That's it, in a nutshell! – tmgr Jan 15 at 14:56
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    Frankly, I'd think that circulating air will serve mostly to bring in more mold spores. I'd seek to close and seal the flue from the weather and then wait to see if there's actually a problem that needs a solution. – isherwood Jan 15 at 14:57

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