Blocking up a flue.

[pinheadplanet]

Hi

Still have an open fireplace in the Kitchen although it's very unused.

The fireplace in the room above is sealed over and plastered.

I've heard of something that blocks up a flue like a big balloon type thing but can't seem to see any on wickes etc websites.

Anyone got any ideas where I can get one or if it is in fact OK just to block up an open flue?

Cheers

 

[Joe Malone]

It's ok to block up an open flue so long as you have a room sealed boiler. If you have an old type open flued boiler then it needs ventilation, if you block up the flue and have an open flued boiler then make sure you have sufficient airbricks installed.

The sensible way to seal the flue would be simply to cap off the chimney but bear in mind that if you do this there needs to be airbricks installed, one at the top and one at the bottom. If you just seal it up then you'll start and have damp problems.

 

[samantha08]

Joe-
We've blocked our flue and didnt install airbricks- i think we now have the damp problems you're talking about around the fireplace. Please excuse my ignorance here- how would it cause the damp and what can we do about it now? Could we install the airbricks now?

 

[Joe Malone]

Hi Sam,

The soot generated from burning an open fire contains a lot of hygroscopic salts which over the years become deposited on the surface of the flue. The salts actually draw moisture into the brickwork from the atmosphere which is why it's important to keep the flue ventilated. You can install airbricks retrospectively, preferably one just below where the chimney is capped off and one at ground level preferably on an external wall if one is available, failing that I'd suggest you install an air vent on the internal wall at ground level. How did you block up the flue? If it's not capped off or you don't have a top hat fitted then this will also form a route for moisture ingress.

Joe

 

[samantha08]

Thanks for replying.
The flue was skillfully and expertly blocked using a paving slab. Not by me!
When i moved in here there was and still is a gas fire in place, although, i have never used it.
The walls to the left and right of the fireplace are showing signs of damp- there are dark patches showing through the wallpaper and there is what looks like salt there too. Would this be what you were talking about? Have been told by someone else that it is rising damp and am praying that it isnt. Didnt give the chimney a thought till i read this post.

 

[Joe Malone]

Don't worry about the rising damp issue at the moment, rising damp is actually extremely rare. The salts are more than likely by products of combustion. One of the tests I do for rising damp is to check for chloride and nitrate salts using a chemical test. They are present in groundwater and are leached up the wall in cases of rising damp. You absolutely must not sample for these salts on a chimney breast due to the fact that these salts can be present from the combustion process and so can give a false positive reading for rising damp. Unfortunately if you do have salt contaminated plaster then it's sensible to replaster for two reasons...

1. These salts block up the pores of the plaster and masonry so stop it from drying out.

2. They're hygroscopic and will continue to draw in moisture from the atmosphere leaving the wall continually damp.

As I mentioned earlier, theres a much bigger issue here due to the age of the property.

Joe

 

[bikerm4]

Chimney ballons here http://www.fluesystems.com/sundries/info/chimney_balloon.htm

 

[pinheadplanet]

Thanks for the chinmey balloon link. They look great and cheap too and I can remove it in the summer when it gets too hot in the room!

Now that I've removed the small front wall that was blocking up the fireplace (it was fully vented) and removed the sheet of decorative stainless steel that was in front of the brickwork at the rear of the fireplace I can see that on the left at the bottom the bricks and mortar are in really bad condition and are just crumbling away.

If you get the mortar in your fingers you can feel that it is damp.

What to do? get a dehumidifier and dry out as much as I can before the plasterer comes to plaster the fireplace or what?