Old chimney - filling with concrete

[rjackson]

I have six fireplaces, each with its own flu and pot, in an 1890s house in the UK.

At least four of the flues will never be used again. One of the other two is now lined and I may line the other.

Is there any reason not to fill the redundant flues with concrete to the top of the chimney stack? There is a lot of talk on these forums about ventilating chimneys to allow condensation to dry etc but if the flues are completely filled with concrete then why is this a problem? I would fill one flu at a time in a series of small pours so that there is never too much hydrostatic pressure exerted anywhere on the chimney breast. There will be temporary damp patches as the concrete dries but experience with the lined chimney says these soon dry out.

Any thoughts?

 

[OwainDIYer]

how do you know the flues will never be needed again?

They're extrememly useful for running cables, pipes, whole house heat recovery ventilation, or even central vacuum cleaner system, through if they're not wanted for fires.

 

[rjackson]

how do you know the flues will never be needed again?

They're extrememly useful for running cables, pipes, whole house heat recovery ventilation, or even central vacuum cleaner system, through if they're not wanted for fires.

Good point, thanks. In theory this is true but in reality they are not conveniently located to facilitate alternative uses. The choices are either to leave them open and ventilate them or fill them completely.

 

[^woody^]

I can't see any problems in doing it, but it does seem a lot of unnecessary time, effort and money wasted

 

[rjackson]

I can't see any problems in doing it, but it does seem a lot of unnecessary time, effort and money wasted

Yes there is a cost but the building has to have scaffolding from front to rear and over the roof on either side of the stack to line the other two flues anyway so the incremental cost is the concrete to fill the four flues and some labour. Once filled they can no longer be a source of damp, dry rot fungus or bird's nests.

 

[jabuzzard]

I would be concerned about the extra weight all that concrete would have on the building. Are the foundations up to taking all that extra weight. My back of the envelope calculations suggest that it would be in the order of four to five tonnes of concrete.

An idea I have had to "fill up" an unused chimney to eliminate the noise and the heat loss is to fill it with expanding foam and then fit a concrete plug at the top to stop water ingress, possibly with a bitumen or similar coating over the top of the foam first.

You can get DIY kits that do large amounts of foam, £300 for 400 litres of expanded foam, or you could try a cavity wall insulation company. With the latter the polystyrene beads that set with a binder would also be an option.

I was considering these options before I decided to go for removal of mine from my bedroom, which if possible is the best option. The advantage of the foam option is that the weight added to the building structure is minimal.

 

[dann09]

Come house selling time the surveyors and potential buyers will most probably take a dim view of the bizzare notion proposed above.

There's no guarantee that all the pour will reach all the flue space, ( a problem with liners and vermiculite insulation ), hence fragile cold spots will remain.

There is also the possibility of the pour bursting some weak brickwork and lurching into wherever, notwithstanding doing it "in a series of small pours".
Do you actually intend to carry wet concrete up a pole ladder so many times over whatever period?
You will have to hod it, except for a massive cost, no pump will remain on site.

Is the house a semi?


If the operation is successful there is still the possibility ( unless chain cleaning has taken place ) of unventilated soot and tar "eating" it's way thro the c/breast.

And the stack and flashings will still have to be maintained.

 

[dann09]

jabuzzard's foam,concrete and bitumen notion is almost as bizzare as the OP's idea.

As for the heat loss, perhaps you've noticed the air bricks in most traditional houses - convection to the outside is good for the house and the people in the house.

Wind noise can be controlled with a hit and miss vent, and an appropriate cowl.

Any other noise then fetch ghost busters.

 

[jabuzzard]

Wind noise can only be eliminated by removing the chimney or blocking it completely. Any suggestion otherwise contravenes the known laws of physics. If the chimney is in a bedroom this is to me a very good reason for wanting to fill it completely. A good nights sleep is worth a lot and if the OP intends to stay in the house for any extended period of time they are entitled to consider it money well spent.

The air permeability of older buildings is often well in excess of what is required. Much of it was needed because all the rooms had open fires. Now the rooms don't have open fires the required air permeability of the building is drastically reduced. Consequently want to actually do that and cut their heating bill is entirely rational.

The problem with just blocking the top and bottom is that you get condensation and thus damp in the enclosed space. However if you completely fill the space then you remove the option of condensation forming and a damp problem developing.

Countless unused chimneys are removed in the U.K. each year, are the people doing it wasting there time? Perhaps removal is not an option here so filling is the next best thing.

 

[rjackson]

I would be concerned about the extra weight all that concrete would have on the building. Are the foundations up to taking all that extra weight. My back of the envelope calculations suggest that it would be in the order of four to five tonnes of concrete.

An idea I have had to "fill up" an unused chimney to eliminate the noise and the heat loss is to fill it with expanding foam and then fit a concrete plug at the top to stop water ingress, possibly with a bitumen or similar coating over the top of the foam first.

You can get DIY kits that do large amounts of foam, £300 for 400 litres of expanded foam, or you could try a cavity wall insulation company. With the latter the polystyrene beads that set with a binder would also be an option.

I was considering these options before I decided to go for removal of mine from my bedroom, which if possible is the best option. The advantage of the foam option is that the weight added to the building structure is minimal.

Good point, thanks. I will look at those ideas. I spent the afternoon measuring the chimney heights and flu areas to calculate the volumes. The total extra weight using a density of 1.75 for the lined chimneys and 2.4 for the unlined chimneys is 8.7 tonnes. The load bearing horizontal area of the stack is either 2.2 sq m or 1.7 sq m, depending how you allow for the flues fanning out sideways, which is roughly 4 to 5 tonnes/sq m or 5.6 to 7.3 psi. This location is not known for subsidence or sloppy foundations but it does occur to me that this is like having a 3m to 4m high fish tank that is 0.5m wide and 3.5m to 4.5m long.

 

[dann09]

1. The OP makes no mention of noise - you introduced that subject.
Far more noise enters thro windows - do you recommend blocking up all suspect windows? What about doors?

2. I dont think that using a hit and miss vent in a "controlled" manner
will "contravene the known laws of physics".

3. The argument now switches to the bedroom - again, no mention of bedrooms by the OP.

4. Air permeability is constantly seen to be lacking in buildings, hence the Blg. Regs. requirements for extractors and extra ventilation in modern or modernised properties. Condensation is rife out there, and in postings on here.

5. Where and by whom is it recommended to "block the top and the bottom" of a flue?

6. If you imagine that people remove full or partial chimney breasts and stacks to eliminate noise and heat loss issues, then i can only say that you are simply wrong. Well wrong. Research costs and issues ref. this on here.

AAMOI: i loathe the loss of any traditional features and so, in future years, will those householders who have lost them. But this is not my argument.

 

[rjackson]

Come house selling time the surveyors and potential buyers will most probably take a dim view of the bizzare notion proposed above.

There's no guarantee that all the pour will reach all the flue space, ( a problem with liners and vermiculite insulation ), hence fragile cold spots will remain.

There is also the possibility of the pour bursting some weak brickwork and lurching into wherever, notwithstanding doing it "in a series of small pours".
Do you actually intend to carry wet concrete up a pole ladder so many times over whatever period?
You will have to hod it, except for a massive cost, no pump will remain on site.

Is the house a semi?


If the operation is successful there is still the possibility ( unless chain cleaning has taken place ) of unventilated soot and tar "eating" it's way thro the c/breast.

And the stack and flashings will still have to be maintained.

Not sure why they would take a dim view provided there are no structural issues. The days of employing young maids to lug coal up to the third floor are long gone. Seriously, who is going to light coal fires on the top floors of old houses these days?

All concrete should ideally be vibrated at pouring to get the voids out. We can't do that here but having a few spaces here is not a major issue.

We already lined one chimney by going up in lifts where we let it set before we pour again. We had no issues with chimney failures or leaks. The concept of spreading the work over a few days and moving up in lifts works fine, done it before in different applications.

The pump cost is a non-issue because our contractor has his own pump but if he did not I'd just put the mixer on the scaffolding and do it that way. It really is not that complicated.

I like the point about unventilated tar and soot chewing its way through the chimney. Is it more corrosive for being sealed in concrete and less corrosive for being ventilated? Doesn't the alkaline cement neutralize the acid tar?

The house is a large Victorian semi and I own both sides.

 

[jabuzzard]

1. The OP makes no mention of noise - you introduced that subject.
Far more noise enters thro windows - do you recommend blocking up all suspect windows? What about doors?

I was trying to provide rational explanations as to why the OP might want to do this. As such noise in the chimney is one, and I was explicitly thinking of wind generated noise.

2. I dont think that using a hit and miss vent in a "controlled" manner
will "contravene the known laws of physics".

It won't eliminate all the wind generated noise in the chimney. To do so would indeed contravene the known laws of physics.

3. The argument now switches to the bedroom - again, no mention of bedrooms by the OP.

True, but with six flues in the property and wanting to block four then it is a certainty that some of them are in bedrooms.

4. Air permeability is constantly seen to be lacking in buildings, hence the Blg. Regs. requirements for extractors and extra ventilation in modern or modernised properties. Condensation is rife out there, and in postings on here.

Possibly but an unmodified building of the age given will have an air permeability hugely in excess of what is required. On a personal level I have massively reduced the permeability of my house from when I purchased it, and I don't suffer from condensation problems, and I dry clothes on my radiators. On the otherhand I can now easily heat my home to a comfortable level.

5. Where and by whom is it recommended to "block the top and the bottom" of a flue?

It is not, but if you want to eliminate wind borne noise from the flue you either remove it, plug it top and bottom, or fill it up.

6. If you imagine that people remove full or partial chimney breasts and stacks to eliminate noise and heat loss issues, then i can only say that you are simply wrong. Well wrong. Research costs and issues ref. this on here.

I don't imagine at all, I know for an absolute certainty that they do. I removed my unused chimney and fire place in the main bedroom of my house for three reasons. The first and by far the most important was to get rid of the wind born noise. This alone would have been enough. The second was extra floor space which was then used in the construction of some custom built in wardrobes. The third was improving the air tightness of the room and not having my heat disappear up the chimney.

AAMOI: i loathe the loss of any traditional features and so, in future years, will those householders who have lost them. But this is not my argument.

A fireplace in an upstairs room is never going to be used by anyone in the 21st century, who can afford to have a central heating system installed. Talking with my mother who grew up in a house with fireplaces in bedrooms she tells me that they hardly ever got used, and judging from the one I removed that would appear to be very true. Nobody in their right mind is going to shed any tears that they are gone.

 

[dann09]

It's interesting, that when these odd characters, who occasionally throw up on here, begin to lose control of their "arguments" that they will turn to moving the goal posts, or claiming justification in terms of vast physical laws, or weird attempts at philosophising.

Their final lurch of self justification is to argue against the man instead of the facts on the table, often bringing in singular examples that run counter to best building practice and the evidence of professional posters on these forums. Be cautious of playing with fire.

I am sad that you think that way, but like village idiots everywhere, the Higher Power calls on you to be content with your half baked self and your semi-literate lot; it's no disgrace, just a fact.

 

[jabuzzard]

It's interesting, that when these odd characters, who occasionally throw up on here, begin to lose control of their "arguments" that they will turn to moving the goal posts, or claiming justification in terms of vast physical laws, or weird attempts at philosophising.

The OP wanted to do "plug" a flue. The suggested method does have some issues as using plain concrete will add several tonnes to the building structure which it may not be able to take. I suggested another option of using foam which adds minimal mass and would not have these problems.

You then claimed that wanting to do either was "bizzare". Not me not anyone else. I made some suggestions as why a perfectly rational person may wish to "plug" a flue.

You then proceeded to rudely dismiss them, while making claims that contravene the known laws of physics. In particular even if one does fit a hit and miss vent and one remembers to close it when winds are forecast you can still get standing waves in the flue that will be sufficient to wake you. I know this on two accounts, first I have a masters degree in physics and second I have direct personal experience.

As I said contrary to your assertion that nobody removes a unused flue from a room for noise reasons, I provided a specific example that disproves your assertion. You are just dead wrong on that front and should have the good grace to accept it.

Their final lurch of self justification is to argue against the man instead of the facts on the table, often bringing in singular examples that run counter to best building practice and the evidence of professional posters on these forums. Be cautious of playing with fire.

Filling a flue with plain concrete to the top is probably not a good idea due to the additional mass added to the structure. I would want to get a structural engineer to give the OK first. However your assertion that a surveyor would take a dim view of it is without any evidence whatsoever, as is the claim that it goes against best building practice. Provided it is structurally sound it is perfectly reasonable thing to do.

Further your claim that soot/tar would eat through the bricks magically because it has been blocked, and would not do so anyway is again without evidence. I can see no reason whatsoever why this would be the case, though of course it would be prudent to get the flue swept before plugging it.

While plugging a flue might seem a bizarre idea to you, it is a perfectly reasonable solution to dealing with an unwanted flue in an older property where complete removal might not be an option. Just because someone else has the imagination to think outside the box is only a reflection on your inability to do the same.