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General: Log burning fridge

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    • CommentAuthordickster
    • CommentTimeJan 25th 2012
     
    Been doing calcs re heat loss of new house, all good stuff, but thought this tid bit might be useful.

    We have 75mm external air inlet pipe connected to our log burning stove and were quite happy to let fire go out overnight. It was only after a couple of below freezing nights that I realised that once fire has died, the chimney was pulling below freezing air into the stove and up the chimney, creating the perfect fridge for our front room.

    We now close the vent on the stove when we go to bed.
    • CommentAuthorowlman
    • CommentTimeJan 25th 2012
     
    Is the 75mm air inlet pipe accessible?
    • CommentAuthordickster
    • CommentTimeJan 25th 2012
     
    Yes it is, but shied away from blocking it externally (dodgy practice)as the control vent at bottom of the stove does the same thing. Although there is still a cold bridge through pipe, there is no flow of air.
    • CommentAuthorowlman
    • CommentTimeJan 25th 2012
     
    What I was going to suggest was to cut the pipe and insert a "blast gate", these are commonly used in dust extraction systems, I've got them on my workshop centralised dust extraction. They are simply a sliding metal plate which blocks off airflow to a particular pipe/duct. I'll have a look if they are available in 3"-75mm. They will probably be more effective than the stove damper especially if you place it near where the air inlets the house.
    • CommentAuthordickster
    • CommentTimeJan 25th 2012
     
    owlman. That's exactly what I've got in the control under the stove. Thanks for your help anyways.
    • CommentAuthorStuartB
    • CommentTimeJan 25th 2012
     
    Best topic title of the year so far...:bigsmile:
  7.  
    Most disappointed I was expecting pictures of a gas fridge running on woodgas
    • CommentAuthortony
    • CommentTimeJan 25th 2012
     
    or a log fired heat pump!
  9.  
    I question whether you actually need an inlet pipe, I find them troublesome and a bit of a gadget. I've seen stoves in airtight houses working fine without them.
    • CommentAuthorJonti
    • CommentTimeJan 25th 2012
     
    I have wondered about that to Viking House. I saw a minenergie place in Switzerland where the owner used to light the wood burner now and again. Again anothe regulation that is based on theory but devoid of any conection with reality:confused:
    • CommentAuthorpmagowan
    • CommentTimeJan 25th 2012
     
    The stove would act like a pump pushing air out of the building. If you have MVHR and an airtight house then (theoretically) the only inlet for air would be via this. Perhaps the negative pressure would assist the inlet and impede the exhaust of the MVHR and thus the system would balance. You would hope that the MVHR would not win the battle and draw fumes down the chimney. In reality air can expand quite a bit even if a negative pressure was created so in a large house it may not be an issue. Who is prepared to give it a go?
    • CommentAuthorJonti
    • CommentTimeJan 25th 2012
     
    Lets face it. A house maybe airtight at the time of testing but 10 years down the road is it still lickely to be?

    Jonti
    • CommentAuthordickster
    • CommentTimeJan 26th 2012
     
    Have a front room/kitchen with MVHR and seperate cooker hood extract. I think regs state that over 5Kw stoves should have dedicated air supply. With stove ticking over and cooker extract on, it's impossible to put another log on without a cloud of smoke billowing forth. V pleased we have external air supply to stove, don't like to think of my stove hungrily using up our oxygen.

    Haven't used the MVHR, (have managed very well with windows cracked open a bit) but guess this wouldn't effect anything much.
  14.  
    We have a 10kw stove in the lounge and a 16kw Rayburn in the kitchen. We have no kitchen or bathroom extracts and hardwood double glazing everywhere with good seals on the opening parts. Solid floors. Doors have seals. We have no permanent openings for combustion. With the stove in the lounge running hard, it can be difficult to get the Rayburn to light from cold. The Rayburn must be under some negative pressure as it will smoke until the fire get going. I ussualy open the back door to equalise the pressure for a couple of minutes until it gets going. Once it is roaring their is no problem. We have a dog flap in the back door. This has good seals and a lead weight as it tended to be sucked open! There is a hell of a draft if you open the dog flap!
    Bottom line is even in a reasonably air tight house combustion air comes from somewhere! Sod Part J of building Regs. It is in direct conflict with Part L anyway. One says leave a big hole for combustion, other says seal it up cos its not energy efficient!
    • CommentAuthorpmagowan
    • CommentTimeJan 26th 2012
     
    I was in a house that had new windows and doors fitted with good seals. They had a large open fire and a woodburning stove. Both fires needed lit to maintain the heat as there was no insulation and no other heating source. In the middle of the night we woke up and the house was full of smoke. The large fire had won the battle with the stove and was drawing all the fumes down the flue. Once we opened a window everything equalised but of course lots of cold air came in. I suppose there was some risk associated with this event. The problem is also greater with the gasses you can't see. For the sake of a simple air connection I don't think I will take the risk (and I am not risk averse).
  16.  
    From the above it is clear that each stove installation is different and the owner has to know the characteristics of their installation and what works best for them.
    I would favour a direct air feed pipe but finding a reasonably priced small stove (below 8kw) with direct air feed is difficult - and I've not seen any direct air retrofit options
  17.  
    Bottom line is even in a reasonably air tight house combustion air comes from somewhere! Sod Part J of building Regs. It is in direct conflict with Part L anyway. One says leave a big hole for combustion, other says seal it up cos its not energy efficient!


    I will run this past my building inspector tomorrow. I am sure he will agree!!! I agree, but regs is regs most of the time,
    Gusty.
  18.  
    Interesting comments! The stoves I saw without external air supply were less than 5kW, so there may be a case for bigger stoves. Bigger stoves are usually in houses with bigger demand that are probably leakier but It was interesting to read pmcgowans experience!
    • CommentAuthorpmagowan
    • CommentTimeJan 26th 2012
     
    This was in Italy and the standard open fire there was a massive square chimney with large opening to the room. You burnt logs about 1m in length. I think what happened is the small stove burnt down and the draw reduced whereas the fire with large logs kept going and required lots of air. It is a bit of an unusual circumstance for most buildings but demonstrates the theory. We also noticed that there was a draught hitting the tops of our heads where ventilation was being sucked in through the roof-tiles and then the ceiling so obviously not a completely airtight house.
    • CommentAuthorSeret
    • CommentTimeJan 26th 2012
     
    Just thinking of the stove itself, burning the cold dense air from outside will be more efficient than using warm air from indoors.
    • CommentAuthorpmagowan
    • CommentTimeJan 26th 2012
     
    Can the inlet pipe be insulated with rockwool or similar and then shut off when not in use. I looked at a device for open fires which had a cowl cover to seal the chimney when not in use. When you lit the fire you had to pull a chain that went up the chimney to unseal the cowl.
    • CommentAuthorJonti
    • CommentTimeJan 27th 2012
     
    Is there a reason why the inlet pipe for air couldn't be run down the chimmney?

    My fire place is in the middle of the house with no external walls and a solid floor so I do not know how I would get an external airflow if I did need one.

    Jonti
    • CommentAuthorCliff Pope
    • CommentTimeJan 27th 2012
     
    If cold air burns more efficiently, why are some fires designed to pre-heat the air?
    • CommentAuthorSeret
    • CommentTimeJan 27th 2012
     
    Posted By: Cliff PopeIf cold air burns more efficiently, why are some fires designed to pre-heat the air?


    Because they direct the jets of hot air into specific parts of the stove to ignite unburnt volatiles, causing a secondary burn.
    • CommentAuthorRobinB
    • CommentTimeJan 27th 2012
     
    Posted By: dicksterHaven't used the MVHR, (have managed very well with windows cracked open a bit) but guess this wouldn't effect anything much.


    Just out of interest why would you have MVHRbut leave it off, and open windows "a bit" instead? Assuming it's winter where you are too.

    RobinB
    • CommentAuthorowlman
    • CommentTimeJan 27th 2012
     
    Posted By: Jonti............Is there a reason why the inlet pipe for air couldn't be run down the chimmney?

    I don't know about solid fuel fires but certainly I've used this form of "balanced flue" for a gas fire. For solid fuel I guess subsequent sweeping of the flue would present problems. A rigid pipe may be a possibility, but I suppose it would need to be fan assisted as the heat from the flue would cause the intake air to rise.
  27.  
    Like Jonti I thought about a 'balanced flue' but I think it wouldnt work as the stove needs the 'chimney effect' of discharge elevated above suction, to make sufficient draft and draw air through the fire. A balanced-flue gas boiler has a fan to positively draw the air through, maybe that could be done.
    I wouldnt diy it, as am scared of filling house with carbon monoxide if I'd not understood it.
    • CommentAuthordickster
    • CommentTimeJan 30th 2012
     
    Robin B, it is winter here (Hants)

    Had MVHR installed for three reasons,

    1. To gain points on our SAP rating (super insulated by itself not quite good enough).

    2. Can't put it in after house has been built.

    3. We might need it.

    Aren't using it at the moment because.

    1. Just moved in, can we live without it?

    2. 24 hours x 60w x a year = 525Kw of electricity, just a bit bonkers (using 3-4 Kw per day in total, so using MVHR increases usage by around 40%) Maybe prefer to lose Kw in terms of logs through natural ventilation rather than electricity.

    3. You can hear it.

    4. Something inside me says "try and get away without using it, rather than do as the MAN says)

    Ditto by the way re underfloor heating. It's there, but not used (until logs no longer available/can't manage the splitting maul any longer).

    Small house, 2 peeps, just a log burning stove, have no stored hot water and a combi boiler. Simples!
    • CommentAuthorEd Davies
    • CommentTimeJan 30th 2012
     
    Posted By: dickster2. 24 hours x 60w x a year = 525Kw ... using 3-4 Kw per day


    kWh :devil:
  30.  
    For the house I'm planning to build the MVHR is forecast to consume 525kWh per annum & recover 9590kWh per annum.

    You can turn it off in summer & open the windows, but I don't know why you wouldn't use it in winter when you've already paid to have it installed.

    David

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