There is no such thing as an air tight stove, for a fire in a stove we need Fuel and Air.
There are a number of different stoves manufactured to work with an external air intake for combustion.
It is very important to bring your Mechanical Heat Recovery / Ventilation Supplier on board by getting an assurance from them that the air pressure in the room where the stove is installed is not compromised.
One of the problems we have found in rooms without a permanent external air vent is the stoves can smoke when being lit or re-fuelled.
Some suppliers / installers of mechanical ventilation understand how their system should be set up to ensure the air pressure is at least equal, others dismiss the potential problem as a non issue.
Some consumers resort to opening windows when lighting or refuelling the stove to prevent smoke pouring into the room, it is not necessarily a bad stove or chimney / flue just a difference in air pressure between the sealed room and open flue.
Your correct, the right term would be room sealed instead of air tight.
I think this issue is a massive one, your dealing with carbon monoxide and poorly ventilated spaces by design. I`m amazed at how blase some vendors are about this, as it would be very easy for somebody to get killed.
I have come across 3 types of stove, room sealed, non room sealed with external air and room intake stoves. I personally think the only real choice is the first one.
There are lots different brands designed with external air intakes, it is not new technology, houses built in the 1940's had external air delivered under the hearth of standard fireplaces.
The room still needed proper ventilation which served the obvious purpose of air changes and balancing the air pressure for the flue.
Many people still remember the warnings about not closing or allowing doors to slam because the fire was being lit?
For a new build am looking to leave a duct(s) in the slab for the future install of an air supply to & exhaust from a room sealed stove
(1) approx what size of ducting to I leave
(2) can the supply also double up as the fire exhaust in the slab?
I left a 4" duct in the slab and it's worked out fine. Although if you have a particular stove in mind then ask the vendor re the duct size.
I've no idea regarding the exhaust, I just used a flue to the roof.
Different manufacturers use different sizes and it depends on the stoves output, to be safe I would recommend 150mm (6").
If you intend using an inset stove with ducts I suggest you plan the installation before you build the chimney.
They work very well if you follow the manufacturers guides but can be a long term nightmare if you have to adapt chimney breasts and flue liners later.
Is the 150mm enough for the double flue & insulation? or would you suggest splitting the air inlet line & fire exhaust pipe.
I would prefer to run it all though the slab rather than have to puncture the air tight membrane etc in the future
Definitely split the two, the air supply is usually ducted under the floor, the size of the duct depends on the stove output and the distance from the stove to the outside air intake.
The exhaust / flue pipe will usually be taken off the top of the stove because the max length of pipe from the back of the stove is 150mm and that will not take you through the wall unless the flue is fan assisted which it is not on standard stoves.
If beside the outside wall (air source) 100mm duct should suffice but check with the manufacturer or retailer !
The flue diameter could be as much as 180 > 200mm for twin wall to suit a stove with a 125 > 150mm spigot, take care to allow extra if you are going through the wall to allow an extra 30mm for the pipe sockets.
..I've seen 3 ways to provide air to appliances in airtight houses lately:
1. Duct under raft, directly up under willow-burning stove (not against a wall, hence the need...)
2. Duct in plinth, over footings, under ffl, up under the grate (open fire)
3. Duct built inside falsework chimney breast, up to ceiling level. In the attic the duct went to gable vent. Simple, effective, and no issues of dampness, or little furry guys !!
There is a good reason for
The provision of ground level external air supplies to stoves needs to be considered in the context of the risk of wind induced negative pressure on the air intake, with consequential flue problem.
This negative pressure can occur if the air supply in on the leeward side of the house when a strong breeze is blowing across the house, particularly 2 storey.
The recommended solution for ground level external air supplies is to provide an air intake from both sides of the house meeting at a tee or similar.
The system mentioned above addresses this issue by have the air intake and flue exit at the same pressure, regardless of wind.
PS I see 50mm dia mentioned as an air supply, if the run is anyway long the pressure loss will make it very ineffective.
90 mm is the normal minimum
Can an air intake pipe be put up the chimney parrallel to the flu? Would this work?
Carlow52, Im assuming the same problem will occur if the external vent duct is taken straight out through the wall behind the stove. Duct would be the thickness of the wall plus about 400mm. I was proposing to have a 100mm duct. I didn't provide for an underfloor duct nor could I now retofit the option you are talking about, i.e. duct from both sides of house. My proposed duct will be on the west facing wall of the house.
There is a lot of miss infomaiton about this topic
REMEMBER CO (Carbon Monoxide) is tastless, oderless, and can kill - correct ventalation is critical
Also having a practice of opening a window or stopping the MHRV while lighting the fire is really unacceptable - because - your cannot expect everyone to follow the procedure
Finally certain external air kits do not meet the requirments of a MHRV, airtight house.
Now read rhttp://www.boards.ie/vbulletin/showthread.php?p=71794885 - although h.... cannot reply currently - he has a lot of good advice here on the saftey of MRHV and stoves
I also made an enquiry of SEAI on this topic - The relevant guidance is given below.
Where MVHRs are being used room sealed stoves should be used(BR 398 below recommends that open flue applianaces are avoided). This means the combustion air is taken from outside the building.BS 5410 advises that the flue gas exit terminal is positioned on the outside of the building concentric with or adjacent to the combustion air inlet. This arrangement is designed to minimize the effect of wind on appliance combustion performance.
The stove manufacturers instructions should also be followed.
Ref. BS 5410 code of practice for oil firing —Part 1: Installations up to 45 kW output capacity for space heating and hot water supply purposes.
"4.3.2 Room sealed appliances It may be desirable, or indeed preferable in some circumstances, to avoid taking air for combustion or for draught control from within the habitable space. To meet this need, room sealed appliances are available that take their air directly from outside the building."
A room sealed balanced flue appliance is an appliance with the flue gas exit terminal positioned on the outside of the building concentric with or adjacent to the combustion air inlet. This arrangement is designed to minimize the effect of wind on appliance combustion performance. Room sealed balanced flue
appliances may discharge flue gases at high or low level. An appliance designed to burn class D fuel should discharge its flue gases at a height of 2 m or greater from the outside ground level. No such limitation is required for an appliance designed to burn class C2 fuel. NOTE Further information on room sealed oil fired appliances is given in BS EN 303-1, BS EN 303-2, OFS A100  and OFS A101 ."
Following a prompt from fclauson and a discussion between the moderators it was decided to start a new thread here using the off-topic posts from a thread in the prices/costs forum called "Airtight Stove for House with HRV" which was actually looking for prices.
Anyway, please read through the posts above before contributing and stick to the topic which is in the title and the posts above.
You need to be careful with this type of air supply, it would need to be very well insulated and in a seperate duct to the flue because warm air rises and the air pressure in the room could "push" the air out instead of allowing down the pipe.
It will work on fan assisted stoves such as wood pellet because they have a fan, there are also wood burners designed with fans for such situations.