Log burner underfloor air feed?

[Hastings]

Our build is not to be certified or tested for airtightness, just built along airtight principles, so I am thinking of not worrying about it.

 

Just would be an annoying and avoidable 'hole' in the fabric after so much care and extra expense with the airtight detailing.

Edited July 13, 2022 by Hastings
typo

[jamieled]

31 minutes ago, Hastings said:

 

Did you not go with a room-sealed stove because the house is not particularly airtight?

 

Only ask because I have almost completed an MVHR/airtight build only to discover  too late that the direct-air stove I bought (Morso 3116) is not suitable for an airtight building because the air supply kit does not fully seal around the stove, effectively introducing a permanently open vent to the outside.

 

I might have got the wrong end of the stick here...

 

The house is fairly airtight by modern standards but we're not at passive standard for airtightness and were never intending to, hence dibt type stoves seemed unnecessary at the outset. The direct air supply is a good fit to the stove and does primary and secondary air and tertiary air. No noticeable leakage around the direct air connection. I'm a bit surprised at the Morso not being able to seal well as they are a good make and the direct air connection should be one of the easier aspects to seal.

 

A big metal flue exiting through the roof is a big cold bridge so I'm not sure I'd worry about a small bit of air leakage.

 

 

[Hastings]

@jamieled Thanks for the extra info. That makes sense.

We have a lot of cold bridging throughout and only 150 of wall insulation as it is a conversion of a derelict old stone house so not worried about that.

 

Very windy location is why I liked the airtight approach and I can imagine a partly open flue fighting the MVHR and making quite a noise at the same time!

[JohnMo]

13 minutes ago, Hastings said:

Our build is not to be certified or tested for airtightness, just built along airtight principles, so I am thinking of not worrying about it.

 

Just would be an annoying and avoidable 'hole' in the fabric after so much care and extra expense with the airtight detailing.

The hole you make is airtight it allows a feed of air from the outside to fire and make the fire easier to start and your flue will work effectively.  Instead of using outside air for combustion, you will using your internal heated air.  You will depressurise the house and when you open the door to restock the wood, the smoke will come into the room; as its at a lower pressure.

 

Cut and paste from another forum

 

The biggest concern with woodstoves in airtight homes is if they don’t have a direct air feed from the outdoors to the back of the woodstove. A lot of people omit that one crucial feature, and in doing so you will depressurize your home every time you start a fire. It will also be hard to light, and in combination with a bathroom fan or stove hood operating, it may fill your home with smoke. At the best of times, you will be feeding the fire with heated indoor air which will then activate every tiny leak in your building envelope drawing in cold air. 

 

 

[Hastings]

5 minutes ago, JohnMo said:

The hole you make is airtight it allows a feed of air from the outside to fire and make the fire easier to start and your flue will work effectively.  Instead of using outside air for combustion, you will using your internal heated air.  You will depressurise the house and when you open the door to restock the wood, the smoke will come into the room; as its at a lower pressure.

 

Cut and paste from another forum

 

The biggest concern with woodstoves in airtight homes is if they don’t have a direct air feed from the outdoors to the back of the woodstove. A lot of people omit that one crucial feature, and in doing so you will depressurize your home every time you start a fire. It will also be hard to light, and in combination with a bathroom fan or stove hood operating, it may fill your home with smoke. At the best of times, you will be feeding the fire with heated indoor air which will then activate every tiny leak in your building envelope drawing in cold air. 

 

 

Thanks but I think you have misunderstood the issue. My stove air supply does not fit the back of the stove properly and allows air into and out of the room. The kitchen and bathroom vents do not extract directly to the outside.

[JohnMo]

Sorry missed your previous posts.

 

Just looked at the Morso instructions and worryingly it states

 

Fresh air supply A wood-burning stove requires air for combustion. As a result, firing a wood-burning stove will help create a healthy climate in your house. If the house is very tightly sealed, and particularly if there is a cooker hood or ventilation system in the house, it may be necessary to install an air grate in the room in which the wood-burning stove is installed. Otherwise there may not be enough draught in the chimney, which may result in the stove not being able to burn properly, and smoke may come out of the stove when the fire door is opened. If it is necessary to install air grates, they must be positioned so that they cannot be obstructed.

 

 

[Bob77]

On 31/05/2022 at 12:25, Bob77 said:

I am on fairly level ground, there is a small slope but if the air brick/vent is meant to be 600mm above ground then no chance! With a vent or air brick low down to the ground I would be worried about it getting blocked with dirt or leaves etc being blown up against the wall. (Or snow, if we ever get that stuff again!)

 

I will ask the fireplace supplier what they suggest. Want to get it straight in my head and on the plans before the floor goes down!

 

Just updating this, after speaking to the stove supplier and builder the solution was to run a length of 5-inch (I think) soil pipe under the new floor which then fits to a telescopic air vent connected to an air brick well above the outside ground level. (The new floor is block and beam and has several airbricks connected to these telescopic things.)