Boiler Flue Heat Recovery

[baggy99]

Hi All
I have been thinking about how to rig up a heat recovery system to recover some heat from my boiler flue.  At present it seems to be venting vast amounts of heat into the atmosphere     I HAVE, I believe, thought about the potential downsides and dangers of such a system, which as I see it are as follows:
1 - Don't interfere with the flue gas itself - any installation would be on the OUTSIDE of the flue
2 - Don't pull too much heat out of the exhaust or the flue will fail to draw and/or condense sulphuric acid back into the boiler
OK, so having got that out of the way, here's the scenario:
Oil fired non-condensing  boiler in the garage (part of the house)  Thermal store immediately above the boiler.  Twin coil solar tank about 10 foot away.  Boiler is on a twin flue (ie separate uninsulated stainless steel pipes).  Pipes run (almost) vertically for approx 3 foot inside the garage before exiting horizontally through the wall.
My thought is to coil some microbore tube (or something more thermally efficient) around the flue and wrap the whole thing in insulation, so that the flow through the microbore picks up the heat from the flue and transfers it to the DHW (haven't worked out how best to achieve that yet!)
BUT before I consider the matter any further:
1) Has anyone tried this or something similar?
2) Are there any other downsides I have missed?
3) If its a goer, how much heat can I afford to extract from the flue before I get problems with draw, condensing acid, etc?  I could presumably just stick a thermometer in the flue outlet to check the flue gas temperature was high enough?
Any help would be appreciated.
Thanks
Mike

[desperate]

I would be vary wary of modifiying the flue, almost certainly the design relys on the amount of air being drawn through the casing for varoius functions, cooling, combustion, clearance of flue gasses etc. If you really are loosing sleep about the wasted heat why not get a condenser? It,ll probably be more cost effective in the long run, and safer.

Desperate

[rogeriko]

Oil burners are pretty efficient they transfer almost all the heat to the water I know this because I have a woodburner that really used to roar when burning pallets etc and the bottom meter of chimney would glow red continuously. So i thought like you lets capture the exhaust heat which I did by mounting an old oil burner so that the exhaust from the woodburner went through the core of the boiler. The result was a cold chimney and lots of sooting up. I had to cut extra holes in the metalwork inside the boiler between the water channels to let more heat through and keep the chimney clean. I dont think your going to get a lot of heat from the chimney because the designers of the stove have already taken all they can and leave just enough to make the chimney work.

[Alan]

Perfectly do able.

As you have all ready said.


Dont add additional resistance to the flue.

You could make a heat exchanger with internal pipe work.


The exit from the flue top must not drop below 116 Deg C.

 Worcester-Bosch
Condensing oil boilers, Just over 1 year old with the boiler back and start of the flue system totally eaten away.

Save your money and extract a bit of heat from what you have.

Fitted motorized damper to shut off the flue when boiler is off on temperature and heat exchanger on boiler in the last house. Was still working after fifteen years when the house was sold.

Regards

Alan

[Ivan]

If you are going to 'condense' you'll need to have a drain (with water trap) at the bottom of the flue. and a bend to avoid the condensate falling back into the boiler.

There is already a product designed to do EXACTLY as you require, and it fits between the boiler and the flue. The only problem is that I cannot remember what it's called. It is designed to both cope with the condensate, and it uses this in an ingenious way to provide additional heat to the coil of convoluted stainless steel pipe which is used for heat transfer.  (ie the same as the stainless steel pipe used for some solar installations). Anyone know the name of the product?

[Flamethrower_]

Ivan,

I think this is what you are referring to:

http://www.zenexenergy.co.uk/Zenex1/index.php?option=com_content&view=article&id=9

Rob

[baggy99]

Thanks all - yes I'd already looked at the GasSaver a while back.  The last time I looked at their website it was only suitable for certain "new" boilers, but it looks like they are now happy for it to be fitted to any boiler.  Great solution - only problem is it's £600.......

Ivan - why would I be 'condensing'?  I was just going to coil the microbore around the flue and use it for preheating.

Alan - thanks for the outlet temp info.  Can you explain a little more about the system you installed at your last house - sounds intriguing!

Thanks
Mike

[Flamethrower_]

Baggy99

Mike,

You might not be condensing but if your heat exchanger pipe work was very efficient your flue gas temperature  may drop below its latent point of condensation which could cause you all sorts of problems (corrosion etc.)

Rob

[Ivan]

Ultimately, if you are after efficiency, you need to condense your exhaust. Water gives up a *LOT* of energy when it condenses from steam - so you want to aim to do this if you want to get good efficiency.

Rob, yes that's what I was thinking of - thanks. Yes, they're expensive. You could probably fabricate one youself if you really had the inclination. They're nowhere near as fancy inside as the diagram suggests - flexible stainless steel solar piping is the same stuff as their heatexchanger. I'm not sure I believe their claims for efficiency improvements for condensing boilers (or the 50% improvement they claim for 'conventional combination boilers' - I think they're quoting very special situations for these), but I do believe it can make a good reduction in losses for non-condensing boilers.

[Rupert]

There are lots of "heat exchangers" which have been designed for flue heat extraction and they have been around for donkey's years (look for manufacturers)........but when you take heat from the flue then you cool the exhaust gasses so you have to make sure that the flue gasses vacate the flue so you have to imput a forced extraction unit (fan) which will greatly affect the burners/stove/whatevers  performance with all associated problems etc etc etc etc.........SO in otherwords you may as well or have to completely redesign the whole heating appliance/system from scratch which is what always happens otherwise you are wasting your time.

I have said this before.....from an efficiency design point of view your (most peoples) whole house never mind the heating system is a complete disaster

[baggy99]

Ivan - I'm probably being thick, but HOW do I get to condense the exhaust?  I'm obviously missing something.  I like the flexi s/s piping idea.  Have you seen inside one?  Any idea how long or how many turns they have in the coil?

Rob/Rupert - I quite agree that messing with the flue dynamics to the point where the exhaust condenses / it needs a fan / etc is a non starter.  I only want to take a "little" heat out of the exhaust.  Zenex can obviously make it work, even if their figures aren't 100% reliable.  And if it's as simple as Ivan suggests it has to be worth a punt?

[Rupert]

If you only want a small amount of heat recovered then the simple and cheap but ugly way is to tightly coil copper pipe around the hottest part of the flue and cover it with fire resistant insulation......this would be the first meter or so from the stove.

[KLD]

Ivan - I'm probably being thick, but HOW do I get to condense the exhaust? 

Mike, the exhaust gases contain a lot of water vapour. As long as the temperatures of the flue stay high, the water will remain in the vapour phase. If you cool the gases down, the water will condense (form a cloud  ). The good thing is that while condensing a lot of heat is released! That's the same heat that you'd need to boil a pan full of (the same amount of) water dry.

So, if your flue gas heat exchanger is large enough and the water flowing through it is cool enough, then you get condensation. No magic there.

Klaus

[wexford]

I know this is probably stupid but if the flue ended in a snorkel where the flue went down wouldn't cooling it like mad on the downward section be beneficial.

Imagine an upside down capital J or umbrella handle. Flue rises for 3mtrs forms an upside down U and goes down for 0.5mtr. Bang as much heat extraction on the last 0.5mtr as you can.


Wexford

[baggy99]

Mmmmm.....  Much food for thought.

Klaus - to condense the water vapour back to liquid I presumably need to get inside the flue.  According to Alan's post above, the exhaust can't fall below 116 Deg C at the exit or the sulphuric acid will condense.  Wouldn't I need to extract MORE heat than this to get the water to condense?  ie I would end up with condensed sulphuric acid as well as water.  And I'm still confused as to how I would capture the massive amount of hear released at condensation.  Hypothetically (ignoring the issues of CO poisoning, acid rot, no draw through the flue, etc) are you thinking of a coil of cold water flowing inside the flue?  If so, are you saying the heat gets passed directly to that same cold water when the flue gasses condense on the outside of the coil?  Before anyone starts shouting about the risks involved in this, I AM NOT ABOUT TO DO IT, I'm just interested in the physics!!

Wexford - Not sure I see the point of your design, unless you are trying to keep the condensed liquids away from the boiler, so they run DOWN to waste through the upturned umbrella handle?