Air vent for wood stove - advice needed please.

[GavIsGone]

Hi all

I'll be having a 6kW wood stove installed soon. It's an old house, so we get the first 5kW for free and only need to provide air for the remaining 1kW. From what I've read, this needs to be 23mm x 23mm for square, or 26mm diameter if round. We don't want any large grill faces or anything like that. Would prefer it to be visually as unnoticeable as possible. Can't do anything with the window, as that's being replaced in the future.

So...

- Would it be best to do one hole to spec, or several little holes?
- Can anyone work out what size the little holes would need to be if doing a few? Or is that as simple as dividing 26 by the number of holes?
- My installer said it would be enough to just have the holes lined... Any ideas what's best to use?
- How to make the holes rodent mouse proof?

Also, I've read several times how random the nominal rating of stoves is... How 5kW stoves can be largely different in physical size, have different sized fire boxes and therefore different fuel loading capacities, have different ranges (the Arada Ecoburn is rated 1.5 - 9Kw, whereas the Clearview Pioneer is rated 1kW - 5kW)), and yet all be rated as nominal 5kW. And on top of that, how the testing of stoves is very artificial and easily manipulated to reach a desired rating. With all that in mind - and forgetting HETAS, regs, etc for a moment - how much real life need is there for an air vent with a 6kW stove installed? For the record, the stove is a Woodwarm Fireview 6kW (5.8kW nominal to be precise).

Many thanks

(PS - We'd love to have a direct air feed but for several reasons it's just not possible.)

[kdmnx]

As with all such questions the answer is “it depends”. As you’ve gathered stoves vary hugely and the nominal figures are useless at anything other than comparing small/medium/large of the same make/model. Many stoves are listed as 5kW even though they are more powerful because that skips the requirement for additional ventilation. A 5.8kW stove is therefore very rare (and possibly better avoided).

The truth is that available air for combustion varies dramatically from one property to another but that doesn’t make for simple regulations. So you must have a vent irrelevant of your actual need and it has to be one that cannot be closed. Siting of the vent is important. You don’t want to be sitting between the vent and the fire because you’ll get an icy breeze hitting you no-matter how roaring the fire.

[pantsmachine]

With absolutely no sarcasm, buy a 5 kW stove. Save yourself the cold air 24/7.

Ask or read to find which stove is on the nudge/wink upper limit of the 5kW rule. Check with installer for the ok, fit it, buy an ecofan air max and enjoy that breeze instead.

[kdmnx]

With absolutely no sarcasm, buy a 5 kW stove. Save yourself the cold air 24/7.

I was kinda hinting at that...

[titan]

Yes let it suck warm inside air up the flue    Outside air supply into the fire is the only sensible way whatever the fire size imo.

[Stig]

Yes let it suck warm inside air up the flue    Outside air supply into the fire is the only sensible way whatever the fire size imo.

I'm inclined to agree.  I have a room-fed gas fire in the living room which is very rarely used (but a good back-up for power cuts) and it's a massive cold sink.  The flue acts as a very effective source of cold air into the room so I've wrapped the thing in an old duvet during the winter as I can't close off the flue (for obvious safety reasons).  Even so, the bricks around it are still cold. 

[kdmnx]

Yes let it suck warm inside air up the flue    Outside air supply into the fire is the only sensible way whatever the fire size imo.

Sure, outside air supply is by far the best solution, but the guy already said that isn't possible (I guess the stove is in the middle of the house and he can't come up and over or down and under for whatever reason).

[chasfromnorfolk]

I seem to recall you can provide the airsource from an adjoining room as well as (ideally) the outside world. Any chance you can drill unobtrusively under the stove?
Yes, that means you may be losing some heated air from ‘next door’ but a) it’s from low, so not that heated and b) it satisfies Regs in the cheapest way...

Cheers, Chas

[marshman]

You say it is not possible to have a direct airfeed. Are you sure there is no route to get one in?  We were faced with a similar problem. Very old house, wood Stove fitted in inglenook fireplace literally in the middle of the house. Solid floors. HETAS approved installer said he could see no way of an external airfeed - we needed 75mm diameter to connect onto the Clearview air box. Knowing how much cold draught the old wood boiler caused (a lot!) I really wanted an external airfeed as I was in the process of "sealing" the house and wanted to install MHRV as well.

After a week or so I came up with a solution - squeeze the ducting under the floorboards upstairs.  I core drilled 100mm dia hole through the outside wall at first floor level checking that the hole would exit in line with the floor joists. I then also had to coredrill a 100mm diameter hole through the inglenook "hood" at first floor level. I then simply fed 100mm diameter aluminium flexible ducting under the floor boards and through the holes. Fitted a grill to the outside. Inside the ingle nook I fitted a reducer down to 75mm and then used 75mm flexible ducting to the airbox on the rear of the stove. I sprayed this matt black and shaped/bent/curved it to fit and run down neatly in the corner.  I did stuff insulation around the duct under the floorboards to hopefully stop too much condensation on it in cold weather.

Your situation will be a lot easier as you only need a small duct - I would be tempted to see if you can run a 40mm waste pipe from the outside at first floor level. Painted matt black where it exits and runs to the fire.

It really is worth trying to do - A stove may feel nice and warm when you are near to it but without an external airfeed there will be cold spots where it is dragging in cold air from the outside. 

Roger

[Pile-o-stone]

I am just finishing a small project where we have a large stove 9kw (I think) in the centre of a room with solid flagstone floors. The stove fitter put a 75mm hole in the external wall and put a plastic vent over it. Apart from the noise that echoed thru from outside, we had a reasonable draft flowing in that corner.

I ended up running ducting from the vent hole to the back of the stove (via a hole drilled through the inglenook wall. I am now in the process of boxing it all off - I decided to fit a radiator above the ducting and have all of it boxed into a radiator cover.

[billi]

I personally like the idea that the "fresh cold air" is  sucked in through the outer layer of the chimney  pipe and kind of is preewarmed from the inner hot flue gases ....

Sure in an old cottage there is probably enough fresh air  coming in , ....

Was  not there at the time , but the stove installer drilled a 2 foot long 4 inch diam  hole through that old irish cottage wall  to allow  to install the stove ,     on a groundfloor of the house that had an suspended  timberfloor with openings underneath through the wall     , what a clever idea that was to drill  towards  the West   facing  New York  and the wild wild Irish sea   some evenings i even expected that a wild salmon jumps through that hole into the livingroom , but we decided  to  plug that hole and get vegetarian 

[Pile-o-stone]

If your stove doesn't have the option to connect external air ducting, could you not install ducting to a vent inside the inglenook? At least then if there is a draft from your vent, it's right next to the stove where you wouldn't feel it. You could cut a decorative cover over the vent to make a feature of it, or fit something like this:

https://www.castironairbricks.co.uk/product/stove-and-fire-vent/

[brackwell]

A friend of mine as a standalone stove freestanding with a twin lined flue just like a gas boiler would have.

[desperate]

Don't just assume "it's under 5kW so I don't need a vent, or it is 6kW so the vent needs to be "X" cm2 " When the stove is installed any proper installer will start off with the nominal size recommended and then do a spill test with doors closed and extractor fans switched on, if it fails then in all likelyhood the vent is too small and will need to be larger irrespective of the stove output.
Any thing else is potentially very dodgy.

Desp