Building an air pipe heat exchanger over an open fire

Discussion in 'Eco Talk' started by DavidDavidson, Nov 15, 2018.

  1. DavidDavidson

    DavidDavidson Member

    So, I've had the idea in my head for a while (it was more of an important concern before the landlord switched the house to pay as you go gas) though I'm still intrigued by the idea of doing this as a DIY project.
    The essential idea is to have a few pipes run under your fire (though not under flooring, so embers can land on them) then have the pipes run along up the rear of the fire and bend over the flue and then end just before or just after the top of your fireplace. These pipes are then all coupled to one pipe at the bottom part of the fire, this pipe then runs to a box where there are a few fans designed to blow room temperature air into the pipes which then run under up and over the fire blowing heated air out the top pipes. This means you save at least some of all that heat that escapes up the flue and have it blown back into the room (minus the smo
    ke and poisonous gases, of course). Pretty much every design I've seen uses steel piping that has been welded together and because I don't have a welder, nor have I learned to weld. I'll link a video down below so you get an idea of what I'm talking about.
    I was thinking of maybe going with copper due to the fact that you can easily bend it or use 90 degree fitting to change the angle though my fire can get hot (that said I tried to melt a copper penny when I was about 14 and it just got red hot) so my concern would be that the copper may melt, however if it's constantly having cool air blown through it so that may stop the copper reachkng a melting point?
    Or would I have to go with a completely different metal?

    Let me know what you think; is it an interesting project or a no go since I don't have welding equipment or experience.

    Here's what I'm talking about:

    All the best
  2. DIY womble

    DIY womble Well-Known Member

    Great fun , I've always thought to add a cold air feed from outside to the base of fire would reduce cold suction throughout the building .
    Dont forget Carbon monoxide detector
    DavidDavidson likes this.
  3. DavidDavidson

    DavidDavidson Member

    Yeah I have one that was put up when the gas boiler was installed, a cheeky little thing too! it literally tells tales on you; I took the back mounting plate off to see if it took batteries as a "disposable" CO detector seems really wasteful, apparently it is disposable, it's the cheapest of the series but it still syncs up to an app on a smartphone by making 56k modem/R2D2 noises which are picked up by my phone.
    I checked it when it was installed by the contractors the landlord brought in and it gives a whole heap of stats like uptime, battery life left and if there were any alerts and if so the CO amount in ppm. After removing the backpback to see if it did have a replaceable battery I synced it with my phone again and immediately it flagged the app with "Tamper alert! Mounting plate removed!"
    I wouldn't want to move that from the room my gas boiler is in though (bathroom, in the cupboard which previously housed bath towels and the like, I have just enough room left now for one shelf only 7.5 feet off the floor.

    I have another, older one which I got off my grandmother (she had gotten new ones) and it takes replaceable batteries but it seems to be a little overly sensitive in that smoking a hookah (with really good hookah coals) would set the thing off.

    If I do decide to set one of these up for the winter (which is tempting as gas prices just went up by 10% for me) I'll definitely be getting a new brand CO alarm specifically for the project as I have a tendency to doze off on the sofa.

    That said I should probably fit smoke alarms to the house first. I had to remove them as the americium (a radioactive alpha particle emitter) in them had gone through about 1.5 half lives meaning they would start going off at random (often waking me up thinking there was a fire) and always when trying food.
    My landlord sent someone out to inspect the house, he didn't seem at all bothered by the lack of smoke alarms, more that the place needed repainted (which I said I would do; I've lived here all my life and repainting the place does seem like it should be my job; it has been for nearly 2 decades anyway) and recarpeted; which will be an awful job for the guys who have to do it as the previous "removators" who did the carpeting simply glued one very thin layer of carpet to the floorboards.
    Last edited: Nov 16, 2018
  4. MGW

    MGW Well-Known Member

    I had a friend ask me to do a web page on wood burning, so step one was a lot of reading up. It seems there is one commercial design and one DIY design which has a condensing wood burner, but neither is really open to you, however the big problem with wood and coal is particular emissions, there are different ways around the problem, but the main way is a set burn rate, and an after burner, so no alternative fire must have doors, open fires are out. The set burn rate is a problem, as we want to control the output, so the main way around the problem is circulating water to absorb the extra heat and store it for future use. This water store also means other heat sources can also be added. Be it solar, a gas boiler, or electric (normally electric produced by solar panels) and the central heating draws on the heat store. There are many advantages including ability to control each room independently. The fire is controlled so the flue temperature is around 150°C and fresh air is drawn from outside, which should always be the case. It seemed ideal, that is until I saw the installation cost, it would take around 25 years to pay for its self.

    So step one is look at the cost of the water store, once a boiler, well we hope it will not boil, is added to a fire, that boiler needs cooling so there has to be some where to get rid of the heat, my son has underfloor heating upstairs it is there to simply get rid of excess heat from Aga.

    Racking out burning coals when the water circulation has stopped is not fun, I have done it, lucky the Aga was on a quarry tiles floor so any burning coals spilt would not cause a fire, but once you add a boiler, then water must flow. Before my sons Aga was in the house, it was in a narrow boat, he had two 12 volt pumps if one failed the other auto took over, unlike the house thermo syphon could not be used to ensure circulation.

    On a larger scale with Sizewell 'B' power station there are 12 x 1.5MW diesel generators to keep cooling water flowing in case of an emergency. And of course multi pumps.

    So 300 litre water store will cost around £1000, I suppose you could make one, but it needs space to put it in and very well insulated. If you consider 1 kg of fuel then it will produce around 4 kcal of heat so if water is at 30°C and is heated to 80°C so 50°C temperature raise you will need a set amount of water storage, I am not working it out, but sure you can see where I am going, now my son says he wish the Aga did not have a water boiler as it is such a problem keeping it cool.

    It seems he should use smokeless fuel, as to control the oven temperature you can't run Aga at the set rate required to stop particular emissions, so it should be run on charcoal or coke not wood, OK he may use wood, but all it takes is for one person to complain.

    Since not your house, I would say project is really good, would love to see it working, but in practice not really an option. Look at the Rocket wood burner DIY heater, been done and clearly works, but do you think you would get insurance to fit one in a UK house? [​IMG]
  5. Bob Rathbone

    Bob Rathbone Well-Known Member

    MGW, I would suggest that the outlet for the gasses was higher than the air intake, this would utilize the remaining heat in the flue gasses to induce convection in the flue and ensure that the flue draught would remove all products of combustion from the room.
    DIY womble likes this.
  6. kiaora

    kiaora Well-Known Member

    I’d just like to point out a couple of things.....
    The old gas boilers would heat the water, and discharge the products of combustion to the atmosphere.

    The boiler design was approximately 70% efficiency

    The reason for designing the boiler to run at 70% efficiency, was to ensure the flue gasses where around 130/140 c

    As you know, the products of combustion are co2 and h2o.
    If you make the appliance more efficient, ie lower flue gas temperature, the h2o in the gas, will condense and produce water, and rust the appliance, !

    It’s the same with solid fuel !

    So be aware with how it actually works, if your appliance is natural draught design, you are putting your family in danger,

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