@sune is ideal to answer this, but I think he's on vacation at the moment.

Hi Topher,

Thanks for the tag Mars. I am.indeed on hols in Denmark, home of the wood stove perhaps!

A direct air supply is a good idea for reducing draughts, but you have to be aware of a few things. Resistance in the duct is one, how it terminates is another, and making sure the fabric of the house is protected if you get downdraught- as the duct will then get hot. 

I made a direct air guide - I can get it emailed to you if you like. I'm Sune@firepower.co.uk 

The other consideration is that when you open the stove door to refuel then the duct doesn't supply that air. That air has to come from the room, and is a LOT higher flow rate than is just needed to.keep the stove burning (ie 25lt/s+). You may have enough air leaking in to supply that, you may need a direct vent to the room (nor so great for air tightness), or I came up with Airsmart which automatically opens a sealed vent to the outside whenever the stove door opens.

Hetas have a helpful assessment procedure to work this out (TN0020), which is basically shutting all doors and windows, turning on all extractors, and then testing a warm stove and chimney with the door open for smoke leakage with a smoke pellet. You could initially try yourself. If it smokes into the room a bit then open a window - does that improve things? You probably need a vent or Airsmart if so. Bear in mind that if you try to test when it's hot weather then your chimney draw will be poor, ie wait till its cold weather. 

Good luck with it! 

Sune

Posted by: @sune

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I am indeed on hols in Denmark,

I love the new fully Viking look... best personal avatar on the site.

Posted by: @topher

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Hello all, first post here and a question 

Doing a full retrofit on new house. Getting ASHP installed later this year but in the meantime preparing wood burning stove. 

What are your experiences or opinions about benefit of direct air input for the stove? Was thinking to install air input from subfloor void near external vent.

House will be well insulated but far from air-tight. But considering direct air to avoid cold air being drawn into heat-pump warmed house when stove is on. 

Worth it?

Thanks all!

T

This depends on the size of your wood burner and size/type of house. If your house is a tad leaky, stoves labelled 5kw and below don't need direct air to satisfy HETAS regs.

5kw is a bit of a moveable feast I've seen whoppas sold with that label and smaller ones. 

I have a heat pump & 5 Kw wood burner with no problems (except it can get a bit hot because I have a bunch of old oak to finish up) no cold air gets drawn in at all.  

I would advise direct air for all woodburner installations.  Make sure the fitting is as airtight as you can get it by using high temp silicone where needed.

if you have a suspended floor and an older property I would advise taking the air from under the floor, make sure your air bricks are open and free to flow air.  Doing this drives c.25m3 around your underfloor which keeps it dry by removing moisture.

i do this and the result is great.  Opening the door to refuel will pull air out the room but it will be a TINY amount as the conductivity of a flue is not fantastic and diminishes with volume so no need to even consider that for a 5 second refuel cycle.

For what it's worth I installed a wood burner in this old house about 8 yrs ago.

I have oil fired CH but an abundant supply of firewood.

I bought a stove with vent and was adamant I wanted air fed from outside, the installer was adamant I didn't need it he wasn't going to the bother of installing it.

After some words I told him if he wouldn't install it I'd find someone who would, so he relented.

It's just a 4 kw stove and approximately 150 sq ft living room and after all this time I think he was probably right.

There's so much heat from the stove there's no noticeable difference either way.

Perhaps in a larger space where the stove was struggling it might be a consideration.

@blizzardgreen 

So far as I recall, a vent is only, per regs, required for an above 5kw burner.

Posted by: @morgan

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@blizzardgreen 

So far as I recall, a vent is only, per regs, required for an above 5kw burner.

Yes I get that but regs were not my main concern.

I grew up in an old house with thick stone walls and one source of heat, in one room.

A black stove, we called it "the range" it was similar to the attached pic and had a water boiler on one end. You could lift a lid and take some out with a cup or saucepan.

It generated plenty of heat but most of it went up the chimney.

A draught would enter under the door but if you blocked it the fire wouldn't draw well enough.

As a kid sitting on the floor it was noticeable.

Once when visiting a neighbour I noticed they had an opening at the opposite side of the chimney for air entry and ash removal.

It always struck me as a great idea.

BTW. I'm not THAT old really. 🙂

@lucia 

FWIW  installed a woodburner some years ago.  The combustion air is drawn from the underfloorboard space through a vent near the burner.  I cant feel a draught (because the burner itself is pretty powerful), but I can smell the damp air from the floor void being drawn in when it first starts up (thats probably good for the floorboards that air is being drawn through).  Although I cant actually pin down a reason I regret not fitting a direct air flue, there must be some leakage from the floor space when the woodburner is not running and of course Im paying to heat that.  Done at the time of installation it would have been a negligible additional cost, done now its comparatively expensive