Sizing and sourcing air duct for an open hearth

[Steve Firth]

I need to make provision for a supply of air for an open hearth. This should be ducted through a concrete floor which will have underfloor heating installed after the air duct is in place. The idea is to duct the air to directly under the hearth where the fireplace design allows the air to flow around the ashpan and enter the hearth via grating. I need to estimate the cross section of the duct and also to source appropriate ducting and an external vent which I would prefer to have a fine mesh screen to prevent scorpions entering via the duct.

Has anyone sourced this sort of thing in the past and can they provide a pointer to suitable standards for construction? The architect is being totally bloody useless at present and the only sensible advice I can get relates to construction of the chimney (25cm diameter and must exceed the height of the roof ridge by 0.5 metres minimum, cannot be located within 8 metres of trees or other buildings.)

[jkn]

Hi Steve I can only be a little help ... I have heard of someone doing this (in a large timberframed house) and the impression I got was that they used poly water drain type diameter ... say 30mm. They used copper pipe for the last few ... inches? so you might imagine 28mm for this.

I'd be interested to learn if you find a better source of information.

HTH jon

[Ed Sirett]

I have no idea what sort of fire you intend to use. The reckoning for a solid fuel fire in a typical trapezium shaped hearth of 400-450mm on the front side would be about 14kW.

A display type gas fire (7-14kW) would require 100 cm². If you can't get chapter and verse fit 100cm².

BS 5440 (which is for gas appliances) would say 4.5 cm²/kW but allows the first 7kW free.

HTH but this advice is worth what you paid for it.

[Steve Firth]

Olive wood.

Hmm, no idea. It will be a hole, we will burn logs in it.

Errm any idea where I would get a 100cm2 duct?

Thanks.

[Alex]

I assume you are either in southern France or Spain,lived there for a couple of years and never had a scorpion inside seen plenty climbing the building walls opposite that took quite a bit of getting used to especially when in bed, they tend to shy away from man,are nocturnal and not interested in warm spots

As to the ducting a piece of stainless steel flue lining would suffice,as it is coming up under the hearth,the flue vent can be plastic which normally comes with insect screen they are called hit and miss vents available from Travis and Perkins