Boiler Flue Heat Recovery

[Rupert]

I just gave you an example of how to do it but if you want to complecate it read on.

When you light your fire from cold you will severely affect its performance through cooling exhaust gasses delaying optimum working tempreture....thus wasting fuel/energy/money which you are trying to recover in hot water.

When working at optimum temp with solid fuel the combustion temp is up and down like a yo yo and you will need to be opening the primary and secondary air intakes to keep your exhaust temp gasses not to low and not to high thus needing an automated control system with sensors.....it would be a full time job for someone to do this and add fuel etc etc.

[wexford]

Yep it keeps the condensates away from the main run and in theory you don't have any issues cooling the flue gasses down so much they plug the top of the chimney.

Taken to a logical extreme if you cooled them down so much they were cooler than ambient air the flue gasses would fall out of the last 0.5 mtr of chimney 

[KLD]

If we do away with the precautions for a moment   then the physics don't mind whether you put a coil inside the flue, in direct contact with the flue gases, or wrap it around the outside. The latter, though, would be less efficient, so you'd need a much longer flue section to wrap the external coil around, compared to the internal one. The physics only call for that the temperature drops enough to get below the dew point.

I'm not a plumber etc, and I haven't even seen a condensing oil boiler  . There must be reasons why they aren't as widespread in use, and why they came later to the market then the gas boilers. And as Alan said, they don't necessarily stand up to their task either.

Klaus

[Rupert]

But if you added a coil inside your flue with solid fuel you may have a soot build up problem.

Think of a car radiator wrapped around your flue and there is your heat exchanger.

[baggy99]

Rupert - it's oil, not solid fuel.  Does that affect any of your observations?

[Rupert]

Sorry (should have read properly)  Yes you are in a better position than solid fuel  but in short i think the best place for a heat exchanger if in or on the flue would be the hottest part obviously.

[Ivan]

Wexford, Your suggestion would be ok only if you had a fanned system - if you did this on a non-fanned system it could be a really bad idea, as you'd lose the 'chimney effect' which is required to draw exhaust out of non-fanned boilers.

Rupert, I suspect that fanned air supply would be just as good as a fanned exhaust, and a lot easier to implement (or standard on most modern boilers).

[Solal]

The flue  from my oil  boiler  exits the top in white vitreous enamel  and  extends  vertical for about 1 meter  to an elbow  and enters the chimney.
 It throws off  a tremendous  heat into the kitchen.
It wasn't until  a boiler engineer  removed it,  that I realised  just how  much heat it released  when I was left   in the cold after his handi work.
The following day  I  refitted the  vitreous enamel  flue  to its original position  and everything was toasty again. 

[guydewdney]

I'm not a plumber etc, and I haven't even seen a condensing oil boiler  . There must be reasons why they aren't as widespread in use, and why they came later to the market then the gas boilers. And as Alan said, they don't necessarily stand up to their task either.

Theres one in my mill room - you will have seen pics of a boring white box

[sleepybubble]

having scan read this thread, his is a conventional balanced flue oil boiler in question.  SO to extract heat from the flue you would have to introduce your extraction 'whetever' into the air intake of the boiler. There is a good chance that you will reducing the temp of the intake air too, which may impact on the efficiency of the boiler, and cost you more fuel.

[knighty]

it's a bit of a rough M$ Paint picture....

but couldn't something like this be used ?



you could even have it outside as long as it's well insulated
if it's below your heat store/tank/whatever it would gravity feed / work by convection pretty well

could even be plumbed in via heat exchanger if you're worried about acid etc.. burning though the pipes...

takes care of everything ?
coil extracts the heat from the flue
drain is there incase of condensation


I know there's the worry of the acidic condensation / soot build up... but there's been enough posts by people with copper coils jammed into there flues to make it worth a go...  and as long as the add on section of flue with the heat exchanger in it is sized about right then there's shouldn't be too much soot build up ?   (could be a good idea to test it for a few weeks with no water, and check it for soot build up, then it can always be adapted without too much hassle ?

[knighty]

EDIT:  as a side note, the condensate... acidic water stuff.... it can;t be 'that' bad.... because it's being formed in condensing boilers all over the world whiteout too much trouble..... ok you might need a bit of stainless steel flue and coil to go in it... but thats not so bad ?

[odbob]

I think that the previous threads have dealt with (in detail) the dangers and condensate probs and so I would just like to add that so long as proper thought has been given, competant installation and so on then the end result can be an extraction rate of up to 85% of the boiler losses and this can be something in the order of 1.5 to 2 KW/hr even off a high efficiency boiler.
Mine has now performed successfully over the last 3 years providing very hot water for use around the home between October and April, this being virtuly free, the returns can be quite signicant, good luck to those contemplating this idea