@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.