Chimney Cowls And Caps

How to determine if you are experiencing downdraught in your chimney

Over the last 8 years or so we have firmly established ourselves as one of the top internet suppliers of  chimney cowls for all fuel types in the UK, whether you want a chimney cowl for simple reduction of rain or a more advanced cowl with anti-downdraught capabilities Hotline Chimneys is the company you can trust.

I'm Brian the proprietor of Hotline Chimneys, I have personally fitted most of the makes and models of chimney cowls on our site in almost ten years of working with chimneys. It's also me that answers the emails relating to your queries.

Below we have a short list (not so short anymore) of the various potential problems that exist with chimneys or flues that may possibly not be caused by the type of chimney cowl or terminal on the chimney. Note: It's very rare that simple bird guard or hat type of chimney cowls can cause problems and they are recommended for longer flue and chimney life as well as reduction in damp problems, caps for closed off flues are essential.

The above statement doesn't mean you shouldn't contact us, if you need to, then do so (see contact link above). But please give as much information, if photos will help we will request them.

Before you browse through the notes below I should give a special mention to 'air supply'. Many people contact us with symptoms that are immediately obvious  to us as a lack of air, a good comparison to make is the car engine, this needs fuel & air to produce exhaust, most of us have  probably experienced a faulty car with either an air intake or fuel problem. They also may have noticed that the exhaust is interrupted, spluttery, and possibly full of fuel. The chimney flue works in a very similar way. The first and most obvious symptom is the smoke just not leaving via the chimney, but partly into the room. Less obvious to the average person is when the fireplace seems to burn well and all looks fine, but with the starvation of air mentioned towards the top of this paragraph, it isn't working as well as one would think. Like the engine, if a fireplace or appliance hasn't enough air, it effects the exhaust, known in the trade as products of combustion) which in turn then re-effects the fire. Other problems caused by this include condensation and the damage it causes to the flue. Chimneys soot and tar up more quickly so need sweeping more often,  and especially with wood burning, a far higher risk of chimney fires from excessive build up of creosote exists. 

Apparent Downdraught  This is not where a chimney has no draw or a poor draw, but where everything seems to be ok, with no escape of fumes into the room, and a good draught but then, suddenly, smoke will bellow under the fireplace lintel or beam, or through stove or other appliance's air intakes.

Things to check for: The lack of air mentioned above can cause any of the following to have a sometimes far more dramatic effect on fires or appliances. It may be that you do have a downdraught problem, but it can often be made worse by any of these things below, so even if it is found that you need a cowl as well please bear them in mind first.

1, Poorly fitting loft hatch.

2, Poorly fitting upstairs windows.

3, Any extractor fans, bathroom, kitchen, cooker hood etc. Not just in the same room.

4, Another appliance, boiler, range or especially open fires, more especially large open fires.

5, Air vent fitted on the down-wind side of a property.

6, Irregular shaped stacks sometimes due to fancy traditional capping and corbelling.

7, Leaning stack or pot.

8, Broken or cracked pot, stack with holes, bricks missing or other irregularities. 

Explanation 1,2,Gaps near the top of a building such as loft hatches can cause problems simply because the house acts like a big chimney, with the place full of warm air which naturally rises and can do so with immense suction .

3,4, These all need air to work, if they have a stronger pull than anything else close by, they win and the weaker loses.

5, Less common, but if the wind hits a building from a certain direction  it causes negative pressure on the down side, which can literally suck the smoke back out of a chimney, I have even seen the smoke from a test pellet dragged half way across a room and out through the vent, and this was a fire that in all other weather conditions and wind directions worked very well. The vent is better on the primary wind side, but with a cowling over it to stop the wind from blowing straight though and causing other problems 

6, This fancy work can look nice but can also effect  the aerodynamics of the chimney, although a chimney cowl will often solve it.

7, These chimneys may be fine 99.9% of the time, but on the occasion that the wind blows from a unusual angle it can cause the smoke to be blown straight back down the flue instead of the wind moving smoothly across the top of the pot or stack.

8, As with No.7 anything that is not smooth and symmetrical in shape can effect the way the gasses leave the chimney in gusty weather . 

The Smoke Never Fully Draws Away (1)  Even with plenty of air or sometimes even a window fully open, the smoke just fills the room.

This one is for people that have either had the fire working successfully in the past or have evidence that it has worked ok, and no alterations have been carried out to fireplace, chimney stack or flue.

1, Bag or paper draught proofing up chimney (happens often believe it or not)

2, Nest or other obstruction in the flue.

3, Damage within flue.

4, Excessively dirty chimney.

5, Blockage of soot in chimney cowl or bird guard on top of chimney.

6, Change to anything in the house, such as numbers 1 to 5 in the problem above.

Explanation 1, Well, I think this one is fairly obvious.

2, Not always as easy as first thought this one, beware chimneys that appear to be clear and the problem then re-occurs, many old flues contain voids and bends that can harbor birds nests, the drying and shrinking of the nest with use of the fire can often cause the nest to drop off of a ledge or shelf  re-blocking the flue. (Re-lining should seriously be considered for flues like this)

3, Many things that interrupt the smooth passage of the flue gases can effect the working of the fire, an extremely common occurrence is when the bats or mid-feathers fall or erode away (these are the dividing bricks or stones that separate one flue from another), re-lining is the only option left.

4, Some chimneys can have problems with relatively little soot in the flue, this depends on how finely balanced they are, but it is usually evidence that the flue to fireplace size ratio is so accurate that the slight narrowing of the flue with soot is enough to tip the balance, if this occurs after 6 months, just have the chimney swept more often, on the other hand, if it happens monthly, probably a reduction in fireplace or increase in flue size is called for.

5, A chimney cowl or bird guard that looks clear from the ground can often have enough of a blockage to effect the fire, beware that other problems such as lack of air can cause them to block quicker, and ensure that chimney cowls or rain hats with the correct mesh for the fuel are being used. 

The Smoke Never Fully Draws Away (2)  Even with plenty of air or sometimes even a window fully open, the smoke just fills the room.

This time for those who have had alterations carried out to a chimney, fireplace or flue, or the property is new or new to you. Please Note: We have had many householders that say the previous owner told them the fire worked great, just for us to have to inform them that there is no way it ever worked properly. A smoking fireplace can be a depressing thing (although you may already know this) and some do lie to help the sale of the property.

1, No air supply, please see 'air supply' in the top section

2, Fireplace too big.

3, Flue too small.

4, Wrong shaped fire back  

5, Wrong shaped gather

6, Reductions in flue size

7, Large voids in flue

Explanation 2, This can be determined by fixing of a non-combustible sheet across the top of the fireplace, decreasing the opening size continually to see if it has any effect, or blocks or bricks could be placed in the opening  under the grate, one under each leg won't do the trick though, you need to fill this area, because you are looking for a reduction in the actual fireplace opening. 

3, The minimum diameter permitted for any open fire in the Uk is 200mm (8") or 175mm (7") for some gas fires (this is required for the very smallest open fires) if it is smaller than this it's illegal, If your fireplace is larger refer to No.2. 

4, So many people have contacted us saying that they have opened up their fireplace and now it smokes into the room, the reason it smokes is that they often haven't opened it up, but taken it away. When many of these large fireplaces were built (excluding some large inglenooks) they contained either a fireback, a cast inset fire or sometimes a range. This opening up as it is referred too, is in actual fact just revealing what we call the builders opening, this was, if you like, just the container for the fire, built at the time of the rest of the structure so that the chosen infill could be fitted after, it is rare that you will get a fire to work properly in this oblong hole in the wall, except with special designs such as Rumford fireplaces which where designed to work like this and are tapered towards the back, or closed stoves. 

5, The gather is the bit above the fireplace lintel that tapers the fireplace into the flue (hidden behind the mantelpiece area) If you look up and it's just a flat concrete slab with a hole in it to the flue, or you can see stepped bricks like a staircase up each side, it's wrong. It should look like an upside down funnel, smooth all the way from the fireplace to the flue.

6, Flues should remain close to the same size throughout, if the reduction is nearer the top of the stack (perhaps because the stack has been re-built and reduced in size) you may find that the fire works ok for up to ten minutes then suddenly smokes the room out, this is because the fire draws alright but just can't take the volume of smoke, a common example is when an inglenook is un-covered after years of being used at a reduced size, when the fireplace was half the size of the original that 8" diameter chimney pot wasn't a problem, but open up the fire place and suddenly  it needs a far larger outlet for the gasses to get away. Note: Many of these very large fireplace chimneys were never fitted with chimney pots when they were built but just had a lime mortar flaunching or corbelling around the top of the stack.

7, The most common occurrence of this is with closed stove and fireplace canopy installations. If a register plate has been fitted across a large fireplace and then a canopy or stove are connected via a pipe or duct, allowing this to just protrude the plate will cause these very hot gasses to cool as soon as they reach this large void, nothing slows the draught of a chimney like a cold block of air, and nothing causes wood burners to leave more tar deposits on a flue.

I hope this helps, If you have questions contact me here

Brian