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  • wood burner / heat exchanger idea

    Discussion in 'The main mechanical design forum' started by oemodm, Dec 22, 2014.

    1. oemodm

      oemodm Active Member

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      Hi everyone,

      Looking for help on wood burner heat exchanger. Wood burner is on level 2 (no space for it on level 1). Would like to channel heat downwards to level 1. Idea is to make heat exchanger from tubes, push air from level 1 through it, heat up the air, then returns back into level 1 hot.

      [​IMG]

      >>>

      Cannot push air directly downwards to level 1 as:
      - risk of negative pressure on wood burner (i.e. suck air from inside burner itself)
      - burner needs its own air supply from within level 2
      - I also do not want to implement a hot water system as its quite complex. Having said that, I could mount a bath rail behind the wood burner, plumbed into a radiator in room 1, pumping water between the 2. This might be ok?

      >>>

      - Fan will be 50m3 per hour,
      - Level 1 room is 75m3
      - wood burner surface can get to around 600deg C.
      - exchanger 50mm tubes, steel, 2mm wall thickness.

      >>>

      1. air enters exchanger probably around 10degc (level 1 is cold in winter)
      2. exchanger is right next to wood burner, guess exchanger will be 100 - 200 degC?
      3. Air exiting exchanger could be around 40degc?
      4. If room 1 can warm up in 1 or 2 hours, that's fine.

      2 & 3 are a complete guess.

      >>>

      Is it worth it, how effective what it be & any ideas?



      thanks / simon
       
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    3. brads

      brads Active Member

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      You say that you don’t want go for a wet system as it would be too complicated but I would guess that would be simpler than a forced air system. I am aware that log burners that have ‘wet backs’ are already on the market and can be plumbed into an existing central heating system or used to create a new one.

      Not sure I can offer much help with implementing a hot air system, but I will say that you need to be carefully with building regulations. They are very strict on the flues from combustion products and will not allow any kind of obstruction inside a flue so will make you pass the air around the outside of the flue, so not sure how much heat you would extract using this system.
       
    4. oemodm

      oemodm Active Member

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      The burner is already in place. It does not have a wet back. I am not extracting any heat from the flue.

      Regarding wet system. I had idea to place a bathroom towel heater behind the burner. Like an after-market wet back. Plumbed to a radiator below. A pump circulates water between the 2. Burner heats up the water in the towel radiator, which is then pumped downwards.

      [​IMG]


      How much heat gets sent downwards I have no idea......
       
    5. brads

      brads Active Member

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      Your second diagram looks similar to what I pictured. With a wet system you also need to consider the pressure within the system due to the expansion of water while being heated. So if you are going for a sealed system then you will probably need an expansion vessel, pressure relief valve, pressure gauge and a filling loop. If you go for a vented system then you will need to find somewhere to put your expansion tank which will require a water supply. All of which I can see why you think an air system would be simpler.

      If your log burner is already installed, what space do you have behind to put your heat exchanger? I always picture air systems being massive ducts which would not typically fit behind a log burner.

      If I was doing the project at my house I would try and find out how much heat would you actually gain from a heat exchanger (air to air, or air to water) sited near a log burner? Can you do some tests? Such as maybe if the log burner is installed and working can you put a bucket of water in front and see how long it takes to heat up?

      In terms of a heat exchanger to go behind a log burner, your best bet might be to contact a heat exchanger company. Really to maximise heat transfer you need to use as small as possible tubes to maximise surface area (however this increases the pressure drop of your heating fluid [air or water]), also to maximise surface area it would be good to introduce fins to the tubes but this is not a DIY job.

      All of which is probably no help to you, but good luck with your project, sounds interesting. It would be good to hear how you get on.
       
    6. oemodm

      oemodm Active Member

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      Agree on expansion vessel. Can buy a standard one used for electric water heaters. But yeah.....air seems simpler.

      Burner is about 10cm from the wall behind. There's enough space to make a vent in the floor, to pull air downwards from behind the burner, to the room below.

      With regard to testing. I like your train of thought. I thought about making a quick cardboard duct, 2metres long, place one end behind the, pull air from behind the wood burner sideways, testing temperature gain at the output end. Lets say the temperature gain is 10degC on the output. How would that effect the downstairs room of 75m3 if its 10degc? If the fan runs at 75m3/hour, I could assume the whole room downstairs to increase in temp by 5degC, 10degc?

      I thought someone on this forum might be able to help with some calculations. Wood burner surfaces can reach 800degC. So if I said the surface was 400 degC, there must be a way to average out a rough estimate on the radiant heat, directly behind it........... or maybe I'm being simplistic. :):):):)
       
    7. zaccutt

      zaccutt Member

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      I didn't have time to read all the replies so I apologize in advance.

      I just installed a wood burning furnace in my shop. It's got a separate fire box with an upper plenum that extracts the heat and a standard furnace blower. I think it's designed to heat 2000 Sq ft. It heats just like a gas furnace. So this exists no? Did I miss some important info? You can get wood furnaces very cheap used because people are too lazy to run them these days. We got ours for free.
       
    8. brads

      brads Active Member

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      Did you get this working?
       
    9. Strambo

      Strambo Member

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      Combustion air supply

      Hi Simon,

      Sounds like a good idea; seems like it should heat the downstairs room nicely.

      The one tip I have for you, based on experience, is to add a tube for taking your firebox feed air directly from OUTSIDE of the building. Tubing can be half of the diameter of the chimney pipe. This will increase overall heating effectiveness greatly.

      If Room 1 were the source of combustion air, the amount of air you suck out of the room is replaced by cold air sucked in through cracks and door edges all around the outside wall. Pulling combustion air from outside = no negative pressure from the firebox = no chilly makeup air entering the living space.

      Good Luck!
       

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