Homemade flue heat recovery?

Roadie

Hi all! I had an idea but wasn't sure where to ask if it was at all feasible.
My hydronic oil boiler is on the old side and horribly inefficeint, but I can't afford to replace it with a newer condensing one. I do however have easy access to some raw materials, so what I was thinking of making a heat exchanger coil from copper pipe and putting it inside a metal box which flue gas would go through before venting to the atmosphere. Was thinking have the cooler return water first go through that coil to harvest some of the waste heat, then in to the boiler.
I have seen some posts on here about heat recovery, with some problems pointed out. One was a risk of CO being formed, I'm not sure why that would happen since it wouldn't change the boiler itself, but the boiler is located outside in a separate boiler house, so if any was formed, it wouldn't be a risk to people in the house. The other was that heat wouldn't be able to rise out of a flue, but it currently comes out of the top of the boiler, immediately turns 90 degrees and exits horizontally out of the wall, so that isn't a problem either.
My main concern, will the flue gas be able to corrode its way through copper pipe? I was thinking of having the front of the box able to be opened so it can easily be checked/serviced, but I'd like to have some idea of how long it might last before starting trying to build anything.
Sorry this was a long post, but hopefully it at least sparks a bit of interest. I haven't seen anyone else doing this but can't find a reason. If it's a stupid idea, I'll accept that but will be interested to know why.
Thanks,
Roadie

JohnNY

Roadie said:

My main concern, will the flue gas be able to corrode its way through copper pipe?

Yes. Quickly. I've seen it many times. The collecting condensate makes quick work of destroying the copper.

Gordy

Not only that, but the flue gas will be cooled faster, and then condense faster inside the chimney. That condensate will eat away at the chimney liner if it's fire clay tile. Which I'm not sure of your flue set up.

Can effect draft, and combustion of the boiler. Not a good idea.

Roadie

Thanks for the quick responses, despite it being bad news! Do you know of any material which could be manually formed which I could use for it? Or any "universal" ready made solution? Doing a bit of searching on ebay, there are some designed for specific gas burners, I was thinking if it still worked to an extent, it would be better than nothing. There's a ridiculous amount of heat wasted out of it, I don't like that from either an economic or environmental standpoint.
Not worried about it eating any liner or anything, there's about 7 inches of pipe (basically just going through a single layer brick wall) to the outside straight from the boiler. Very little to go wrong there.

Ironman

Actually, there's a lot to go wrong. I'm not sure from your description if your saying that your boiler vents directly out the side wall or vents into a chimney outside the wall. In either case, altering the boiler into a condensing appliance is something that's fraught with consequences that will cause you to soon be purchasing that new boiler which you say you can't afford.

Has this boiler every been properly set up with a combustion analyzer? Do you have a high quality oil filter or a Tiger Loop on it? Is it completely cleaned of any soot between the sections? 1/16" of soot can cause an 18% reduction in efficiency. Have you ever done a heat loss calculation and radiation survey to see if the boiler is properly sized? If it's like most, it's probably 1.5 - 2 times the size it should be. Does it have an outdoor rest control? That alone could save up to 30% in fuel.

What I'm trying to get you to see is that there are other reasonable and safe methods of saving energy that are available. There are numerous technical reasons why the boiler has to have a somewhat elevated stack temperature. Don't have tunnel vision on that and miss the other things that can be done safely. Your not by any means the first to think of recovering heat from the flue, but the only ones who are successful in doing it are the appliance manufacturers and many of them have had their woes in attempting it.

Roadie

It's old and was there when I bought the house so I have no idea how it was initially set up, my guess is not very well since there's no cap or anything on the end of the flue, the pipe just goes out through the wall and ends, also there is no filter on the oil feed. Not sure what a tiger loop is but I have the feeling I won't have one.
When I had it serviced, the guy used a gas analyser, it was sitting around 90% combustion efficiency. It's clean inside, I managed to get pretty much all the soot out of it as my first effort to get it wasting less heat, no idea how much better I made things since it being an old design, it's never going to be great (rated at .
I also suspect you're right about it being oversized, it no longer heats a hot water tank and I removed one radiator from the system.
Did a quick search for a rest control, I don't have one of those, I just have a basic sitting on top of the boiler controlling water temperature. It's set at its minimum which I think is 65C, thinking the largest temperature difference between the flame and the water should transfer the most heat.
I wasn't dead set on my initial plan, I had just exhausted every other option I could think of. That's why I'm on here, for everyone else's collective knowledge.

JUGHNE

90% efficiency on an old oil boiler seems high.
If true you would be almost condensing flue gasses?

Do you know where did he drew the flue gases from?
How about some pictures of your boiler.

Did he leave you a print out of the testing?

Roadie

That was listed as combustion efficiency (so I guess how much of the fuel was being fully burnt?) because I had initial high hopes when I saw the printout saying that as well, before noticing the boiler's rated efficiency when new is something like 80%, then looked a bit closer and saw that it was combustion not system efficiency.
I can get some photos of the boiler itself when I go home if it'd help, I think the printout might still be up at my mum's house, I'll look for it as well but doubt I still have it.

Jamie Hall

Something is wrong with that efficiency reading. You can't get an overall efficiency in a combustion appliance much above 85% without condensing some of the water vapour in the stack gas. Only boilers which are specifically designed to do that should do that, and should only be used with flues which are also specifically designed and installed to handle the cool stack temperatures.

As @Ironman said (and he is one of the best in the business), there is a lot that could go wrong with trying to condense that water vapour, and it is that water vapour that contains the heat which you are worrying about. I can't recommend that you try to do it.

Roadie

I might be using the wrong words for things, I've never looked at a boiler before this. I've been assuming combustion efficiency is how much of the injected fuel was burned and system or overall efficiency was how much fuel was turned to heat which is actually kept in the system and transferred in to the water. The one I was given after the service I'm sure was just combustion efficiency, it was just a probe in the exhaust gas stream so it had no idea how much heat was actually being passed in to the system. The rating of 80% overall efficiency was taken from a website of that model when it was new - Firebird popular 82. I've tried to attach a photo I took of it when buying the house, it's dark outside now so probably wouldn't be able to get a worthwhile photo. It shows how the flue is, that wall is just single layer brick, the other side of it there's just a circle cut the size of the pipe. All bare water pipes have now been insulated but I'm thinking the biggest issue is with the boiler itself since if I hold my hand even a couple of feet away from the exit of the flue, it would burn. My dad's boiler is of a similar age and the exhaust gas coming out of it is far cooler.

Gordy

Is that day light I see around the flue pipe?

Roadie

Yep. Told you it wasn't an especially good installation. I'd have done something about it by now if it wasn't an external thing (or if it ever got used)

Jamie Hall

There's a misunderstanding there. Unless the boiler is very very seriously out of whack, all the fuel which is injected is burned. The result is a mix of carbon monoxide (CO), carbon dioxide (CO2) and water. There are also some nitrogen oxides and some minor compounds, which don't count. A proper analysis of the combustion will measure those three ingredients plus excess oxygen and the flue gas temperature. From those measurements, the efficiency can be calculated. A proper adjustment of the burner will result in relatively low amounts of carbon monoxide, but not zero, with some excess oxygen.

The construction of the boiler is the single biggest factor in determining how much of the heat of combustion winds up in the circulating heating medium -- water in your case -- but the temperature of the returning medium also has an effect. More modern boilers will have lower stack gas temperatures, which means more heat going into the circulating medium.

What they don't do, unless specifically designed to do it, is reduce the stack gas temperature to the point where the water from combustion starts to condense -- typically around 350 to 400 F. The reason is that the resulting condensed water is extremely acidic and corrosive. Even with a short (and, in my humble opinion, somewhat dubious) flue such as you have, it would cause the flue -- or the boiler, should it get into it -- to corrode and fail remarkably quickly.

Economisers -- which is essentially what you were thinking of -- condense that water, and can be used and are used. They require corrosion resistant materials, such as stainless steel, and also require either forced draught or induced draught to ensure that the flue continues to exhaust properly. The condensate has to be neutralized and disposed of properly.

Purely incidentally, but somewhat along the same lines, the various devices used by some on wood stoves have somewhat similar problems, with the added problem of allowing the creosote from relatively incomplete combustion of wood (particularly in airtight stoves) to condense in the chimney. That stuff is flammable, and the result usually is a chimney fire...

Zman

The combustion efficiency is just calculated by the device. The number is totally meaningless if the wrong parameters are programmed. Even with everything correct the number is suspect at best.
What you are attempting to do is not a great DIY project for the reasons previously listed.
Adding a condensing portion to a conventional boiler is frequently done on large commercial boilers. There are some pretty unique materials and engineering know how that goes into the design.

Solid_Fuel_Man

Are you located in Europe?

Roadie

Yeah, Scotland specifically. I can't find any pre made economiser for that boiler so I guess I'm out of luck. Was worth asking about anyway so thanks all.

Mark Eatherton

Your boiler is missing a critical device called a draft regulator. If it is connected to a tall chimney, this is a must.

ME

Ironman

Mark Eatherton said:

Your boiler is missing a critical device called a draft regulator. If it is connected to a tall chimney, this is a must.

ME

If I understand his description correctly, he has no chimney; the pipe just goes out through the side wall.

HVACNUT

Sorry to be blunt Roadie but take your raw materials and your labor and put it into a new boiler with an indirect water heater and outdoor reset. From the pic I saw, you would not benefit from a mod con boiler. Looks like you've got finished tube baseboard or a hydro coil. Both high temp applications and return water temperature will not drop below the dew point of the flue gasses.
In years past I've seen some crazy setups using ribbon metal around the flue pipe with a fan box to distribute warm air to the space. But as the others have said, unless you want your chimney to rain, knock off the MacGyver routine and invest in a new system.

Roadie

Ironman: correct. From the picture, after it turns 90 degrees (away from the camera), it just goes through the wall and ends. I'll try and remember to take a photo tonight, it seems very wrong to me, if nothing else having no weather protection on it.
Finished tube baseboard from what my quick search turned up, looks like heating pipes running through what we would call the skirting boards - I don't, each room has a big panel radiator. Not high temperature, currently it's set at 65 degrees.
What does a hydro coil do, is that not for hot water through faucets? If so, the boiler is nothing to do with that supply of hot water. That's one reason I think the boiler is probably oversized for my needs.