Sealing/insulating open chimney

Discussion in 'Eco Talk' started by gingertimmins, Oct 6, 2018.

  1. gingertimmins

    gingertimmins Member

    hi all, I have an open chimney in my living room and in the new year we’re having a multi fuel stove installed.
    At the moment I’m really concerned about heat loss uk the chimney and I’m wondering how best to temporarily stop it.

    I have some knauf glass mineral wool to hand and I’m wondering if I can just stuff some up the chimney and seal off with some wood batten and a wooden panel.

    I understand that the chimney needs air flow so in the wooden panel I would leave some gaps/air vents but I’m wondering, Is glass mineral wool breathable?
    If not I have some pvc waste pipe that I could run up the side of the insulation to allow ventilation.

    I’ve also thought about putting the mineral wool into a pillow case.

    Does any of this sound mental?!

  2. Google "chimney sheep"
  3. MGW

    MGW Well-Known Member

    I think there is a special blow up bung, designed so if some one does light a fire, it will burst and cause no problem.

    For any open flue, there must be a path for the replacement air, a vent near the fire is best as without it the fire will draw air under doors etc and cause unwelcome drafts.

    There are two basic solid fuels, those with all the nasty stuff left in, and those with nasty stuff removed. So coke and charcoal have nasty stuff removed so as long as it will burn and the flue gas goes up the chimney then no problem, an open grate would do, although in real terms coke will not burn in an open grate.

    However for coal and wood, where the nasties are left in, the burner has to be rather complex to ensure there are no particular emissions, in the main this means adding air after the main burn so all the fuel is used, this causes a problem, as it means the unit has to run at a set rate, you can't run it cooler as then you get particular emissions or hotter or you are wasting fuel, in the main they use a back boiler so excess energy is stored for future use.

    However these complex units often draw air directly from outside to stop drafts, so you may need a vent for then multi fuel burner, and putting that vent in now would mean no problem temporary blocking flue.

    I was asked to make a web site saying how bad wood burning is. So I went to check facts, and I found there is a huge difference between different wood burners, some are really bad, but some are also really good, but does not matter is coal or oak or pine, they all produce around the same amount of energy per unit dry weight, however pine is lighter than oak for same unit volume, so to be able to burn both, the grate needs to be adjustable to take higher or lower bulk.

    When I asked around I got conflicting information, some would say you needed hard wood, others a mixture, and others soft wood, and I realised they were all correct, it was dependent on how their unit was set, often the sides of the fire box should be moved in for hard wood and out for soft wood so it gets the burn rate required to ensure no particular emissions.

    Also very important was the correct draft in the fire, and for these complex fires if they draw air from within the room then opening or closing a door can alter burn rate, so it is far better to draw combustion air from outside, so back to same thing, likely a hole will need to be bored to outside for the fire, so if you find out where it will be required it could be installed now for ventilation while you wait.

    Pre-war it was common to have a vent near a fire, it stopped drafts, but post war it seemed builders were designing houses rather than a trained architect and these vent were often missed, and even when fitted common to find them now blocked with expanding foam.
  4. sospan

    sospan Well-Known Member

    For some of the properties I am working on, I have temporarily filled some black refuse bags with insulation or bubble wrap and pushed them a little way into the opening. The bags reduce the airflow but still leave enough to ventilate the flue and also are easily removable once you fit the fire.
    KIAB likes this.
  5. gingertimmins

    gingertimmins Member

    Thanks for the advice. This is basically what I have done except I used pillow cases which in themselves are a bit more breathable. I’ve also tied a rope around the first pillowcase which is just visible.
    The difference it has made is astounding!
  6. sospan

    sospan Well-Known Member

    The number of times over the years, I have been to peoples houses and they complain about it being cold or drafty and they have an open fire with a screen in front of it. Even while explaining it takes a while for the penny to drop that all their heat is going up the chimney :rolleyes:
  7. Richard_

    Richard_ Active Member

    Even worse is that the intense radiant heat of the fire means the air in the house feels cool, plus you have to sit on the draught of air pulled in by the fire.
  8. MGW

    MGW Well-Known Member

    I think it is called "the chimney effect" the main problem is once started it's hard to stop, when heat treating pipes I had to be very careful to stop draft starting, with a 36" pipe if the bung came off, it would take two of us to replace it holding against the air flow. Yet it could normally be held in place with masking tape if done before the pipe was heated.

    I think the main advantage with central heating was the removal of the draft, today [​IMG] we have room sealed fires which draw their air from outside, so much better, note how air is fed both under the fire and on top, the latter reduces particular emissions. Today there is no need for a chimney with a wood burner [​IMG] this unit cools the flue gas to a point where it can be lead outside with a plastic pipe, however I am not sure what happens in the case of a power cut? The does go through burning wood, and it is not as easy as you first think. My brother-in-law had a wood burner in his old house which worked very well, it had a boiler built in so that removed most of the heat produced, so lit at 6 pm it would burn to 9 pm and heat up the water store, then as the house cooled the pump would run circulating the stored water keeping the house warm, also there were immersion heaters in the water store, and if the output from solar panels was not being used for other things, then it also heated the water store, and if all that failed there was LPG boiler that could also heat the water store.

    It was installed when the house was built so cost of installation was on the mortgage, and room was allocated for the two huge tanks which stored the water, hard to cost as fitted as house was built, but looking at around £16k to fit the system, even after 25 years, not sure you would ever get that investment back. To comply with the clean air act solid fuel fires are so expensive today, not really worth it.

    You can get cheap fires which add ambiance to the room for use mainly as an emergency when central heating breaks down, but are often not legal, and all it takes is one neighbour to complain. And to have it, means draft up the chimney so not really a good idea.
  9. sospan

    sospan Well-Known Member

    Be interesting to see a link. I haven't seen any wood burners that can run without a flue apart from ones which have been converted to run on bio-ethanol which isn't the same
  10. Richard_

    Richard_ Active Member

    A few houses on our street have had wood burners installed on houses that are only 15 years old. So they are plonking a concentrated heat source into a house with balanced CH system and well insulated walls. It looks petty but they need the living room windows open in winter!
    Last edited: Oct 17, 2018
  11. sospan

    sospan Well-Known Member

    Seen a lot of new homes having them installed recently. Because they don't have conventional chimneys they have some really expensive external chrome flues fitted. Some of them paying £9k for the installation.

    As you have said, sizing of natural fires is key, the wife wanted a particular stove when the shop worked out the dimensions of the room and the heat output of the fire - to avoid being cooked she would have had to sit in the other room across the hallway
  12. MGW

    MGW Well-Known Member

    The most well known is the Rocket wood burner, this is in incessant a home made unit so I would question how one would insure your house if you built one. [​IMG] The is as far as I can see at the moment only an idea, as I said what I question is what happens if the power fails, how does is fail safe.

    In general if you burn wood or coal (which is after all only compressed wood) you have three problems.
    1) To ensure flammable tars are not deposited in the flue.
    2) To ensure the heat escaping out of the flue is kept to a minimum.
    3) To ensure you have limited particular emissions to a minimum.
    One extra could be added, and that is to limit drafts within the room.

    So the fire in the main needs to be two stage, with extra air added to ensure a very complete and hot burn, it needs to draw air from outside, and the rate of burn needs controlling so the flue is kept at around 150 deg C hot enough to stop condensation and cool enough to mean not too much heat is wasted. And the air control needs to be automated to ensure it stays within strict limits.

    The stoves may look old world however to get 80% efficiency your looking at something like this [​IMG] and still only 80% efficient, now I think you can go down on efficiency rating, however there is a limit of something like 5% so even with that unit your old heater would have to be less than 85% efficient, now a condensing boiler is around the 90%, so once a condensing boiler is fitted, you can't return to non condensing type what ever fuel it burns. (In theory although I am sure many brake the rules.)

    So we walk through B&Q and we see loads of fires for sale, but that does not mean we can legally fit or use them, be it electric or real there is nothing to stop a stove being fitted for show. So there is nothing to stop their sale, however using them relies on no one complaining. this website was commissioned by a guy who lives near to me, he has made some very valid points, and after meeting him I realised to burn wood you are looking at spending at least £1000 on the burner, often a lot more, if you think you can save money by burning wood think again, there are however some half truths.

    For example burning charcoal does no release all the nasties, which is true, but it does depend on where the charcoal comes from, made in a proper retort where the tars and oils are collected and fractional distilled yes it is very good, but from a traditional charcoal burner, [​IMG] this is not good for the environment, all it means is the nasty stuff is in Poland not the UK. Units like this [​IMG] are good, every bit of the wood is used, but most of the charcoal we buy is not made in something like this. The bigger the better for wood burning, and to supply a power station with coppiced wood is environmental friendly, unless you consider to make enough biol-diesel to power just the trains in the UK it would need 5 times the size of UK to grow it. So it's a carbon neutral or feed the world, at least as far as wood burning goes.
    Richard_ likes this.
  13. sospan

    sospan Well-Known Member

    So there isn't a flueless wood burner for the home ?
  14. Richard_

    Richard_ Active Member

    Thanks MGW, that was a great post.
  15. MGW

    MGW Well-Known Member

    I have seen very small gas fires with no flue, and I suppose you can light a candle without requiring a flue, or a cigarette, but to burn any fuel one really has to have some type of flue, even a gas stove often requires a cooker hood to extract fumes.

    The question is more does the flue need to be in the main vertical? To use a horizontal flue in the main you need either a motor or a method to extinguish the burning if certain safety criteria is not meet. With liquid or gas fuel it is easy enough to have some sort of valve to turn off the fuel supply should safety requirements not be meet.

    Although some hopper fed systems can have a fail safe system built in, most solid fuel burners have a problem switching off quickly, I have lived with solid fuel, and I have had to drop the fire bars and rake out burning coke onto a shovel and carry it outside, to allow for this we had quarry tiled floor in the kitchen, other fires in the house did not have a water heater built in, so could be allowed to burn out.

    Since those days the whole of the home has become more sealed, and to counter this we install items like CO2 alarms, as we restrict ventilation using things like heat recovery units we also become more and more reliant on the electricity supply to keep our homes safe.

    We have seen even gas appliances banned in certain homes, high rise flats, and narrow boats for example, the narrow boat clearly can't have a vent low down, or it would sink, and one can't rely on the electric supply, so gas fridges are banned.

    When my son fitted a wood burning stove to a narrow boat, it required twin 12 volt water pumps, from two independent supplies, as if the water did not circulate so much damage could result, even then he never left the boat with the fire lit.

    We see restrictions on open flues, no tumble driers venting to outside, as that fan could cause a depression and draw flue gasses into the room. Years ago our houses were not sealed, they were draughty homes and we did not really need to worry when using an open flue, those days are gone, even the bathroom fan can cause problems.

    We still see open flues, the problem seems to be people don't understand what an open flue is, it does not mean you can stick a sweeps brush up the flue, it simply means combustion air is drawn from the room rather than from outside, and with the traditional place for a fire being the centre of a house, getting a supply of air from outside is not always easy.

    It is not a case of vents being blocked, although that often happens, the number I have found filled with expanding foam, but often builders simply never fitted them. We break the rules so often, even simple things like replacing a bathroom window with a non opening one without fitting a fan.

    I like the ambiance of a wood burner, however I don't want the dust, drafts, and dangers, so I do without one.

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