How to work out the "free area" of a vent hole with a mesh cover?

DIYNotIan · 29 Jan 2021

Hi

A question for the clever folk and mathematician types

We are trying to calculate what size vent we need for our wood stove. It's a 6kW in an old house, so we only have to allow for the 1kW above 5, which amounts to a 550mm2 area, which = a 26mm diameter hole.

We also want to put a mesh on the front for mice. The one we're looking at has 0.6mm wire with 6.35mm square holes. To satisfy HETAS / regs, we need to calculate the "free area", so we need to deduct the space taken away from the hole by the presence of the mesh.

Trying to work it out myself has left me confused, so I wondered if anyone here has done it before or is just clever enough to calculate it?

Many thanks

freddiemercurystwin · 29 Jan 2021

Any respectable mesh supplier will know the free area if you contact them.

endecotp · 29 Jan 2021

If it were just the obvious maths then:

0.6mm is about 10% of 6.35mm.
So increase the diameter of your hole by about 10%.
(Or equivalently, increase the area by about 20%.)

BUT, I think it’s more subtle than that; air flow is complicated. If you buy a complete vent it will generally tell you its effective equivalent free area. (Often it’s embossed on it somewhere. Don’t be caught out by imagining that the model number tells you the area - some do, some don’t.) But if you’re planning to modify a vent you don’t get that worked out for you.

bernardgreen · 29 Jan 2021

Count the number of square holes and multiply by 40 ( 40 being the area of one hole ) for a rough idea of the area available for slow moving air.

Fast air flow will create turbulence close to the wire of the mesh which means the area for free air flow will be reduced Hence multiply by 30 instead of 40.

As mentioned the supplier of the mesh may be able to give you a better idea of the effect the mesh will have on air flow

Tigercubrider · 29 Jan 2021

deleted

lostinthelight · 29 Jan 2021

That size mesh is far too large if you want to stop flies and insects coming in.

DIYNotIan · 29 Jan 2021

lostinthelight said:
That size mesh is far too large if you want to stop flies and insects coming in.

Yes, but regs don't allow anything smaller than 5mm, so I can't see a way around that.

We spend most of the warmer months with windows open all day, and they aren't about int he winter. So hopefully we'll be ok on the insect...

Thanks for the heads up though

foxhole · 29 Jan 2021

Put in 100mm dia hole.Simple.

DIYNotIan · 29 Jan 2021

foxhole said:
Put in 100mm dia hole.Simple.

In a lot of ways I'm reluctant to have the vent at all. At least until I see how well the stove performs with / without one. The stove is rated at 5.8kW so it would have only need be a mere 0.8kW less and no vent would have been needed at all. Bearing in mind that some stoves with a nominal output of 5kW have a maximum output of 9kW, it all seems a bit arbitrary and lot like a numbers game. But regs are regs and we need it signed off. But with all that in mind, I don't want a whopping great big hole if I can get away with a much smaller one.

foxhole · 29 Jan 2021

DIYNotIan said:
In a lot of ways I'm reluctant to have the vent at all. At least until I see how well the stove performs with / without one. The stove is rated at 5.8kW so it would have only need be a mere 0.8kW less and no vent would have been needed at all. Bearing in mind that some stoves with a nominal output of 5kW have a maximum output of 9kW, it all seems a bit arbitrary and lot like a numbers game. But regs are regs and we need it signed off. But with all that in mind, I don't want a whopping great big hole if I can get away with a much smaller one.[/QUOTE
My vent goes from back of stove straight thru wall , don't see anything. 100mm vent is small.

DIYNotIan · 29 Jan 2021

foxhole said:
My vent goes from back of stove straight thru wall , don't see anything. 100mm vent is small.

I don't have the option for that unfortunately. Ours' will be in a recess, which is on an internal wall. So the vent will need to be in a different wall entirely.

100mm may be small, but it's all relative I guess. And relative to 26mm it's big

Cheers

footprints · 29 Jan 2021

Unless the regs have changed vents for gas/solid fuel should not have any mesh, it is too easy to block with dust.
As I recall I think it was something like the holes should be between 10 and 5mm dia but do not have the specs to hand.
Any approved vent will state the free area on it.

DIYNotIan · 29 Jan 2021

Thanks @footprints The regs state mesh can be used so long as the hole size is not less than 5mm. We'll be doing our own vent as it seems all of those available are much bigger than we want/need. I'm trying to get hold of mesh manufacturers to ask them the free area, but can't get through at the moment. Will update here with what they say when I do.

Cheers for your help

lostinthelight · 29 Jan 2021

Have you considered or is it possible to have an air vent kit on the stove. Also If you go for the bare minimum you can get away with and then find lack of draw or smoke coming in the room when various extractors are on you'll have to start drilling through walls again.

DIYNotIan · 1 Feb 2021

lostinthelight said:
Have you considered or is it possible to have an air vent kit on the stove.

Sorry for the delay, I thought I had replied already! Unfortunately the set up of the room, the fire recess, and the fact the recess is on an internal wall, all mean the a direct air supply isn't possible; or if it were it would mean a lot of upheaval and possibly expense too. So we decided against it and have now ordered a wood stove that doesn't have a kit option.

lostinthelight said:
Have you considered or is it possible to have an air vent kit on the stove. Also If you go for the bare minimum you can get away with and then find lack of draw or smoke coming in the room when various extractors are on you'll have to start drilling through walls again.

Fair point, but tbh I think I'd rather have a hole that is too small and needs enlarging (or a second hole added), than one that is too big but cannot be reduced in size. Time will tell, but I don't think we'll even need the vent. So I think one done to the requirement of regs will likely be enough.

Many thanks for the help

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How to work out the "free area" of a vent hole with a mesh cover?

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