We are struggling to find exactly what we want as far as wood burning stoves are concerned, we have put in the hours trawling the web and leg-work at various eco shows but the option for external air ducted fires seems at least for the present, limited.

So maybe someone can help. If we kept below 5kw would we even require an air duct? Bearing in mind we are making substantial improvements throughout, airtightness and super insulated being probably the highest two on the list. Hopefully getting an airtightness of less than 1, but probably something between 1-2 air changes per hour. Will the fire work at these levels of air tightness without an external air duct?

Er, Tony, that's exactly the point.

I would imagine 1 or 2 houses worth of air would be plenty to keep a fire burning. Are you using any kind of mechanical ventilation? Even if you're not required to have a permanent hole in the wall for your fire you could install one that shuts. It'd be a cold bridge but if it's supplying the fire you'd still end up ahead energy wise.

Posted By: SeretI would imagine 1 or 2 houses worth of air would be plenty to keep a fire burning.

Are you suggesting a sub 5kw fire in a sealed house would work? house is around 250m2

Posted By: SeretAre you using any kind of mechanical ventilation?

Sorry forgot to mention, a full MVHR system is planned.

Posted By: SeretEven if you're not required to have a permanent hole in the wall for your fire you could install one that shuts. It'd be a cold bridge but if it's supplying the fire you'd still end up ahead energy wise.

Not sure i could spend all the time and effort to seal tiny pin holes and cracks only to create a large vent, even if it could be closed off.

If it's any help, a 5kW stove would need in the region of 10 to 15m3/hr air when running at rated output, ie quite a small proportion of the airchange rate.

My Contura 5-8kW has an exernal kit fitted. Most of the Contura range are available with external air kits. I am not linked to them.
Gusty.

Posted By: an02ewNo CPB youve missed the point,

Au contraire. Just because the building regs say that you can install a sub 5kW stove without an external air supply doesn't make it safe.

The UK building regs need to be urgently updated in this regard, they were written at a time before houses were hermetically sealed.

If you are building an air tight house for safety's sake you need to have an external air supply, end of story.

Firstly thanks to Keith it now seems possible to answer posters questions in the spirit of the forum! Long may it continue..
Some good advice above already. That 4 Kw woodwarm looks very good.
Firstly, is your installation to be signed off? If so, by whom..If BCO then ask them what they want to see, if a HETAS installer then ask them. You will surely get a different answer as they are interpreting Part J in their own way. I would expect a HETAS installer to object to a MHRV unit and a stove vented from the room. As Tony says, how about direct air feed? This will certainly help compliance, but you may find spillage from the stove when you open the door to refuel. A word of caution about direct air feed though, do consider using non-combustible ducting as there is evidence that under extreme air pressure scenarios combustion gases can be drawn back through the ducting

Finny. i agree, my only option now is to choose one of the few suitable, will also take on board your point about duct pipe, do you think the problem with spillage due to MVHR when i open fire door could be dangerous? could the MVHR be balanced to avoid this?

Posted By: Chris P BaconThe UK building regs need to be urgently updated in this regard, they were written at a time before houses were hermetically sealed.

Chris,

sorry to ruin your illusions but the vast majority of new houses built in the UK are no where near being hermetically sealed and even those that are most will not stay so long term. The building trade should be investing far more in getting their people to realise the whys and hows of modern building

Jonti

When I last read the building regs, the exemption allowing to exclude a air vent for a sub 5Kw did not apply to a new build house, only retro fitting.

Wrong to think that 'a houseful' of air is available to a stove when there's no lo-resistance inlet. That air may be there but it won't be flowing into the fire and up the flue. If you get it to burn at all, the combustion gases will find it just as easy (or difficult) to flow back into the room, as up the flue. The suction created by buoyancy of hot gas in a flue is a weak force, so the whole flow path, from outside air into the room into the stove and up, must be of comparable or lower resistance - i.e. wide open. Even worse, the weak draw of the flue can easily be exceeded (creating back-flow) by an extract fan anywhere in the house (or switching a MHVR to 'boost'?) - in which case the combustion air inlet must provide a very-much-easier route (than the flue) to supply the extract airflow in addition to supplying combustion air.

All of which disappears as a problem, with a balanced flue stove. Good luck! The difficulty is that you want the stove to be double-fronted? A single-fronted stove will still kick out heat from its back - sure you have to actually see flame from both rooms?

I don't know how 5 kw compares with an ordinary open fire, but that can make our uninsulated draughty sitting room feel very stuffy and headachy if we don't open the sliding vents in the floorboards next to the fire.

<blockquote><cite>Posted By: Cliff Pope</cite>I don't know how 5 kw compares with an ordinary open fire, but that can make our uninsulated draughty sitting room feel very stuffy and headachy if we don't open the sliding vents in the floorboards next to the fire.</blockquote>
I would condemn this appliance immediately. You should not use it until it has been thoroughly tested by a qualified person. Please tell me you at least have a working Carbon Monoxide alarm in the room..


<blockquote><cite>Posted By: mike7</cite>Hmmm ... I can see that Tom's explanation could be expressed in other ways, but it doesn't seem wrong in principle. Care to expand, finny?</blockquote>
It is not the buoyancy of hot air that creates draw.. if it was you would be unable to light it..


<blockquote><cite>Posted By: mike7</cite>How does an unlit stove create any draw without a temperature difference between external air and air in the flue?</blockquote>
The density of air is different at the opening of the flue and at the termination of the chimney. This creates draw..commonly called "the stack effect" in ventilation terminology. This is the reason for a recommended minimum flue height of 4.5 metres.
Hope that helps

Sounds great Mike but this effect is borne out by experience. Having installed many flues of all shapes and sizes over the years I can categorically say that a flue will draw without any difference in temperature between the air at the inlet and the outlet. The reverse stack effect you talk of can occur, generally in poorly constructed flues. This is the reason why a properly sized and installed flue, ideally insulated elicits such wonder in customers used to the traditional "poke a bit of flue thro the plate only" type installation.
PS just tested my rayburn flue.. 12 metres, 6 inch liner, insulated over entire length, one 45 degree elbow. indoors 17 degrees, outdoors 22 degrees. 15 pascals..

<blockquote><cite>Posted By: finny</cite>PS just tested my rayburn flue.. 12 metres, 6 inch liner, insulated over entire length, one 45 degree elbow. indoors 17 degrees, outdoors 22 degrees. 15 pascals..</blockquote>
What difference doea putting a house on it make? Do the laws of physics change? Am I wrong?