Making use of steam boiler flue gas energy - residential

jfleming

I was wondering if it is at all possible to make use of the energy in the steam boiler flue gases. So the boiler wouldn't be any more efficient, we'd just capture the waste heat. This would be for a residential, 1-pipe system.

The goal is energy efficiency -- assume the system is otherwise well-insulated and functioning efficiently.

The only thing I could think of is running the gases through a heat exchanger in a tank of water, then making use of that water to heat something else.

The Steam Whisperer

Another option is to preheat the incoming combustion air. If done like the old Dunkirk Quantum leap hot water boiler, which had both a condensing heat exchanger and a exhaust heat recuperative heat exchanger, efficiencies even at higher water temps were very high.

pedmec

Please do not do. This will create more dangerous problems than it solves. atmospheric boilers rely on the hot flue gas to create the draft necessary to draft the flue gases. by removing the heat from the flue gas you decrease the the temperature which will reduce the draft. That is just one issue and a big one at that. this can lead to no draft and flue gas spillage out of draft hoods. And also remember this is assuming you have proper air for combustion and enough left over for dilution air. this is a big safety issue.
Secondly, you can cause condensation in your chimney. you can and most like will reduce the temperature of the flue below its dew point. this will cause condensation in the chimney which is highly acidic. this will eat the mortar and brick and if enough is absorbed into the brick and mortar it will freeze and crack the chimney. and now depending on where it cracks and how well your boiler is tuned up you could be leaking carbon monoxide into the living space.
i speak from experience. i have i been in the trade for 30 years mostly doing troubleshooting and installing heating systems. i have first hand knowledge of the destruction that can happen as i had one of the systems your looking for in the first house i purchased. it was called 'magic heat' if i remember the name right. it was in-line with the flue and basically was just a little fan box with a heat exchanger built-in. it removed heat from the flue gas. just plugged into an outlet. the chimney was a mess. mortar was dust and bricks would crumble to pieces with a slight tap. had to abandon chimney and go with high efficiency equipment. which i had planned on doing anyway.
just upgrade to high efficiency equipment. its engineered to be efficient and can save you money safely

Steamhead

Those Magic-Heat and similar devices would probably not have caused a problem if installed on an old snowman or other dinosaur with insanely high stack temperatures. This is, of course, not the case with newer equipment.

ISTR Hoval and GasMaster marketed commercial steamers for a while that redirected some heat from the flue into the incoming combustion air, returning water, or both. They were never extensively marketed in this country though.

JohnNY

I've seen people over the years get all kinds of creative trying to recapture heat from the flue. None of it ever seemed to pay for itself, and as mentioned above, some of it has created less-than-safe conditions. Running the flue through some type of heat exchanger is easy. Dealing with the condensation and reduced chimney draw makes it less fun.

Jamie Hall

Looking at it from a slightly different point of view... with any decently engineered and adjustted modern steamer, the boiler is already extracting virtually all of the sensible heat of combustion which is available. Any increase in efficiency beyond that will require capturing some or as much as is reasonable of the latent heat of combustion, which is done by condensing as much of the water vapour in the flue gas as possible. This feasible, of course -- and is how condensing hot water boilers or hot air furnaces achieve their high efficiency.

But.

It comes at a price. At least two, in fact. First, the combustion chamber and flue must be use materials and be designed to handle the condensate (in the case of an outboard unit, such as a feadwater preheater or combustion air preheater -- or even a small space heater such as has been mentioned -- this would apply to that unit and the flue, of course, rather than the boiler). Depending on the specific fuel being burned, the condensate will always include a fair amount of nitric acid, and with some fuels sulphuric acid, as well as carbonic acid. There are a few grades of stainless steel which can handle this stuff, as well as most ceramics, but most other materials can't. The stuff is ferociously corrosive. Second, as has been mentioned, you inevitably reduce the temperature of the flue gas. In simplest terms, this means that you are going to have either have sealed combustion with a forced draught or you have to have an induced draught fan.

The Steam Whisperer

Jamie hits most of the details. This is definitely induced draft/ power draft territory. Heat Sponge make condensing stack economizers... but for big boilers. The Art Institute of Chicago has one of these on thier primary boiler. I am not sure where they are dumping the heat, but ( reportedly) they are getting around 92% out of the boiler.