How to vent a bricked up fireplace?

[Sajhed]

Hi grateful for any advice please
I am getting my fireplace bricked up,advised by one builder no vent will be required in the living room if i seal the chimney pot, another builder tells me a vent is required in the chimney pot and the living room for airflow
A third builder tells me to put an airbrick in the hearth and vent directly to the subfloor
Fourth person came last night and advised knocking out the hearth put a wooden floor and vent to the subfloor.
Which advice is correct please not wanting to end up with damp

 

[foxhole]

Chimney should be capped and vented or you may get damp , vent method can vary .

 

[ree]

Good practice is to sweep the flue before blocking off.
Redundant flues are best served with a vent at the bottom & a redundant flue cowl at the top.

 

[Sajhed]

Thanks
The chimney has been swept today
The chimney has a vented cowl on already
So the vent at the bottom should it be on a wall

 

[ree]

Use a hit & mis vent centred in the blocking material.

 

[mrrusty]

I can't be bothered to answer this question yet again....the search function is quite useful

 

[Sajhed]

I tried the search function before I posted difficult searching on my phone for me for my specific query

I have now edited my post for the fourth opinion but please don’t read it I wouldn't like to waste your time

 

[mrrusty]

OK. The sole purpose of ventilation is to evaporate any moisture that may be in the chimney - no moisture = no point in ventilating. Where could moisture come from? 1) down the chimney, 2) penetration through the wall, 3) attracted by hygroscopic salts and the chimney has enough air inlets in to it to allow moist room air to get in (including through vents) and get drawn in by the salts, which if they get enough moisture will liquify 4) condensation caused by moist room air condensing when it hits the cold bit of the chimney that sticks up through the loft onto the roof. Again, an open vent bottom to top can turn in to a continuous condenser.

Cutting to the chase - install a vent only if it serves a purpose, and be careful leaving it open when the room is nice and warm and humid and the chimney is colder than the dew point of that air, or that air WILL condense on the cold internal surfaces. Basically if you have a hit-miss vent at the bottom and the chimney has a vented cowl, then open it in summer and shut it in winter. But if there's no chance of moisture in the chimney anyway, just block it all up - the vent serves no purpose.

Apologies - I've written that so many times. There are many different opinions about venting redundant chimneys. That's my opinion

 

[Dereekoo]

OK. The sole purpose of ventilation is to evaporate any moisture that may be in the chimney - no moisture = no point in ventilating. Where could moisture come from? 1) down the chimney, 2) penetration through the wall, 3) attracted by hygroscopic salts and the chimney has enough air inlets in to it to allow moist room air to get in (including through vents) and get drawn in by the salts, which if they get enough moisture will liquify 4) condensation caused by moist room air condensing when it hits the cold bit of the chimney that sticks up through the loft onto the roof. Again, an open vent bottom to top can turn in to a continuous condenser.

Cutting to the chase - install a vent only if it serves a purpose, and be careful leaving it open when the room is nice and warm and humid and the chimney is colder than the dew point of that air, or that air WILL condense on the cold internal surfaces. Basically if you have a hit-miss vent at the bottom and the chimney has a vented cowl, then open it in summer and shut it in winter. But if there's no chance of moisture in the chimney anyway, just block it all up - the vent serves no purpose.

Apologies - I've written that so many times. There are many different opinions about venting redundant chimneys. That's my opinion

That has been my philosophy on the couple of chimney's I have capped, blocked off at the top, blocked off at the bottom, no moisture to get in from above or below and I'll treat it as if it is cavity wall space like what the rest of the house has

 

[mrrusty]

The point is air does contain moisture and the amount depends on temperature and relative humidity. Saturated, air at 20 degrees contains about 17g per m3 - about a tablespoon. So if you were able to fully condense all the water vapour to liquid, that's what you would get - a tablespoon per m3 of air. In reality the air won't be saturated, and you would never condense it all out because even air at zero degrees can hold about 5g of water per m3

If that air is trapped in and not replenished, the amount of moisture that could either condense or react with salts is limited and finite. The risk with ventilation is you introduce replenishment, which is why if there is no moisture coming in from the top or through the walls, there isn't much point ventilating.

 

[mrrusty]

Your other proposal will lead, whether you understand it or not, to a vacuum?

With regard to the building regs, no they haven't cocked it up, and ventilation if it is actually serving a purpose, is appropriate. What the building regs don't detail is that ventilation is only any good when the ventilating air is evaporating moisture. If the ventilating air is condensing, particularly now we insulate lofts and chimneys passing through them are much colder, then it's making the problem of damp worse through condensation, and you can't alter physics. Air at say 20 degrees and 60% RH will start condensing liquid water at about 12 degrees.

The information from Historic England below is much more nuanced, and concurs exactly with what I suggest. Note how the first para suggests the optimum ventilation is when the lower vent is to external. This is ideal because ventilating air entering from outside at ambient temperature will have no opportunity to cool further and hence condense (unless the brickwork is colder than the dew point).

There is one circumstance where even with external air, there could be condensation, and that is when there is a sudden change in the weather from cold and dry to warm and wet. It can happen overnight, and the thermal inertia of the brickwork takes time to catch up, so the warm wet air might condense on the still cold brick. Anyone with an outside wc will have seen this - sometimes the internal walls are wet with condensation even though the wc is outside and unheated. It's also the reason stuff goes rusty in an unheated garage, and is also in fact the reason we get fog and dew - the incoming air is warm and wet but the ground is still cold, so we get a condensing layer close to the cold ground

 

[Dereekoo]

I guess it all depends whether you have a soot covered flue when you decide to cap it off as to any ensuing problems, mine must have been OK???

 

[mrrusty]

I guess it all depends whether you have a soot covered flue when you decide to cap it off as to any ensuing problems, mine must have been OK???

IMHO it's only a problem if it presents as a problem. I'm sure many chimneys, including mine, have a degree of soot and salt contamination - some will show, some won't.

I have one particular chimney that is now lined for a gas stove that is very contaminated, and I still have problems with salts around the breast. One day I will strip it all back and deal with it - I've done one in another room with 100% success, but this one just gets sanded and painted regularly at the moment. This particular fire place had a coal fire for 100+ years, and the bricks are so salt contaminated. I didn't realise when we started this refurb, and partly by accident this 1902 house has become a test lab for dealing with condensation and salt damp; it's been interesting to see what has worked and what hasn't.

 

[stuart45]

The amount of moisture getting into a chimney flue varies in different situations.
External flues with tall, large stacks are more exposed to penetrating damp. Ones on the party wall with short ones much less.
When chimneys first started getting blocked up on a large scale in the early 70's, ventilation wasn't really considered. After a few years when the damp issues started to surface the idea of ventilation came in. Whenever I worked for firms as a jobbing bricklayer, I was often sent out to stick a vent in a blocked off chimney somewhere because of damp issues.
There's bound to be blocked off flues with no ventilation around with no problems, but there will be others that need venting.
One of the complaints about internal venting from people is that they either lose heat, or get a cold downdraught sometimes. With external venting it's when the cold air causes cold spots and condensation.

 

[foxhole]

Vents reduce air moisture , chimneys get damp from many sources , you don’t wait till you have a problem the vent is a preventive measure .