How many vents required in unused fireplace?

shesblas

I have an unused fireplace in a first floor bedroom but the previous owners cemented over the top of the chimney. The room smells musty and there is a damp area at the top of the chimney breast, which I assume is condensation. 

I plan to:

1) put a vent at the top of the chimney to allow stagnant, warm air to escape

2) block the fireplace internally with some insulation board but put a vent in that to allow cool air to be drawn from the room into the chimney, pushing the stagnant, warm air up and out through vent mentioned in point 1.

Would this be enough ventilation to allow the chimney to dry out and send the musty smell up and out? Or should I put another (3rd) vent on the chimney externally at a slightly lower height than the other, and at right angles to it to increase airflow and circulation?

tacpot12

I think your choice should be between one internal vent or one external vent at the bottom of the chimney, assuming you have access to install an air brick in the outside wall. 

Either would be ok, but with an external vent, obviously the air venting the chimney is going to be unheated so won't rise automatically. It might fall and I think that's fine as well as long as you get some air flow. Using heated air is definitely better (i.e.it's more likely to solve the problem), but comes at a cost to your heating bill.

I went with an internal vent, but probably one that was too small. I used a Soffit Vent in the underside of some plasterboard that I used to block of the chimney in the fireplace. (The plasterboard was pushed so that it was horizontal inside the chimney and fixed with a couple of battens underneath it. With hindsight, some rigid insulation would have been a much better idea, but the vent needs to be bigger as you aren't going to get any useful warmth coming through the insulation.

ThisIsWeird

shesblas said:

I have an unused fireplace in a first floor bedroom but the previous owners cemented over the top of the chimney. The room smells musty and there is a damp area at the top of the chimney breast, which I assume is condensation. 
I plan to:
1) put a vent at the top of the chimney to allow stagnant, warm air to escape
2) block the fireplace internally with some insulation board but put a vent in that to allow cool air to be drawn from the room into the chimney, pushing the stagnant, warm air up and out through vent mentioned in point 1.
Would this be enough ventilation to allow the chimney to dry out and send the musty smell up and out? Or should I put another (3rd) vent on the chimney externally at a slightly lower height than the other, and at right angles to it to increase airflow and circulation?

I understand that in most cases (and from personal experience), your two-vent proposal should do.
Ensure the chimney top vent prevents rain from coming in, and is of a good ventilating size - that should then do the bulk of the work.
Then fit a simple 'register' plate up inside the fireplace opening, using whatever material you prefer - you should get away with p'board, but rigid insulation would be better as it can be easilly trimmed to fit, squished up, is damp proof, and can be held using something very simple like frame sealant (or any other cheap sealing/adhesiving product) around its perimeter.
Check out Facebook Marketplace for insulation offcuts - it'll cost next to now't.
Then fit an adjustable vent grille in t'middle of this board. 
I don't see any advantage in fitting a second external vent, and it might even lower the overall effectiveness by reducing the whole-stack draw from the very top?
It shouldn't require much to keep the stack dry - just a trickle. So, in winter, only open the vent partially, and monitor things. Once the weather warms up, and your heating is being used less, you can always open it further. In Summer, open it fully, and get the chimney fabric bone dry.
Even now, if say your heating is off during the day when you are out, you can open it fully to give a blast, almost closing it fully on your return.
Your walls 'breathe', and that includes your chimney breast. The warm, moist indoor room air diffuses through your chimney breast surface, and ends up inside the stack where it condenses in the unventilated coolness. It is common for the resulting cond to trickle and drip downwards, often seeping through any tiny gaps in the blocks in the breast where it appears as dark stains - it often takes the sooty mess with it. The good news is, it doesn't take much to prevent that from happening - just a slow venting trickle should do, along with a nicely open top - coupled with a rain guard.
Is the fireplace currently open? I'm wondering where the musty smell is coming from if not?
The 'damp area' at the top of the breast - is it also stained? Is it localised spots, or a general wide patch?

shesblas

@ThisIsWeird thanks for the info. The fireplace is open so assume that's where the musty smell is coming from. The damp area spreads about half way across the chimney breast. It isn't stained.

ThisIsWeird

shesblas said:

@ThisIsWeird thanks for the info. The fireplace is open so assume that's where the musty smell is coming from. The damp area spreads about half way across the chimney breast. It isn't stained.

Sealed at the top but open in the fireplace is not good - all that warm, moist air circulating up there, and condensing out.
Needs a complete reversal - nicely open at the top, rain cowl of some sort (I used a pepperpot), and then a controllable trickle from t'bottom.

Any chance of a pic of the damp?

'Trickle' ventilation is usually all that's required, but you may wish to give it a thorough flushing as soon as you open the chimney top. Any day you can, have the fireplace vent removed or fully open - get the fabric dry.