Hooded Cowl on chimney. Why is this wrong?

Nebbit

Every year when I get chimney swept the chimney sweep gives me certificate saying he has done the work but marked 'Do not use with hooded cowl'. (Although he knows I am in fact using the chimney, burning wood and solid fuel.) Here is a picture of the cowl. It is only 20 years old, much newer than the house itself. Why is this supposed to be wrong?

stuart45

It's because that type of cowl is for ventilation only. It restricts the draw of the flue.

Nebbit

 It restricts the draw of the flue.

Our house is on top of a hill and the fire burns very well

ic

Do you have a carbon monoxide detector in the same room as the fireplace?

Nebbit

ic said:

Do you have a carbon monoxide detector in the same room as the fireplace?

no we have never had one

Grenage

You should definitely have a CO detector (I'm surprised that didn't come up in the sweep).  Regarding the cowl, In practice if your draw is good then you don't have a problem.

WIAWSNB

Flues are a bit of a blunt device. There are design requirements which should be followed, but every single one will then end up 'drawing' differently due to many different factors such as house location, surroundings, on which side it's located, prevailing winds and stuff like that. And they should be terminated with a suitable cowl, or simply left open.
One glance at the range of terminals subsequently used - needed - on many houses, including flashing spinny chrome ones, shows that one minimum design of flue does not fit all. Sometimes changing factors like the house next door having a loft conversion is enough to mess things up a bit.
Bottom line in your case is that the HETAS has determined that your cowl ain't the right one, and shouldn't be fitted to a working flue. He's done his job by pointing this out and covering his bottom. But, he's also obviously chatted to you about it, you have presumably told him it draws like Leonardo, and he isn't actually worried.
Your call, but a CO monitor should be sat on a nearby shelf, tobesuretobesure. For £10, why wouldn't you have that extra reassurance?

stuart45

Small amounts of carbon monoxide can do irreparable damage to people when it happens over a long period.