Taking heat off of flue pipe

ethicalpaul

Hi folks-
Spending some time near my atmospheric gas boiler I was thinking about the heat of the flue pipe.

I'm pretty ignorant of the issue of condensing in flue gases but I'm aware it's not desirable in an atmospheric burner into a chimney situation. I think this is because the condensation is acidic and can be harmful to a chimney (and harmful to the boiler if condensation happens in there).

But I have a stainless steel liner above my draft hood. Are those concerns lessened?

What if I attached some fins to my flue pipe and hooked a small fan up to a relay to power up during a call for heat?

Wouldn't that let me pull some of the 18% of wasted heat (for my 82% efficient boiler) into my house?

Wouldn't there be enough heat left in the combustion gas to draft?

Tell me how dumb this is, thanks!

motoguy128

The risk is that the flue gases will condense more Of the time near the top of the chimney/liner.

From a cold start take a thermometer and measure the exhaust temp at the top of the chimney if accessible. Once it reaches A 140F it’s finally not condensing.

Biggeest concern is that with cooler flue gases, you lose draft. So the total height And length of horizontal run would be come more critical and you’d want a combustion analyzer to check if you still have adequate draft.

Even if you could maintain draft, now though need to modify the liner system to have a tee with a drain at the bottom and I’m still not sure those liners can survive a constant flow of condensate.

motoguy128

On a side note, I’d look at it as about 6-10% wasted heat. Realistically after you factor in distribution, pumping energy or blower power, scorched air and low temp hot water are only about 90-92% efficient in the real world.

I’ll bet half the mod cons install on radiator system rarely are over 90% efficient since return water temps are over 150F inmost installs.

Zman

Your reasoning and awareness of the challenges is spot on.
In the commercial market there are engineered retrofits available to do just what you describe. I do not know of a retrofit for residential boilers.
Many commercial condensing boilers and a handful of residential models are simply a conventional boiler with an extra heat exchanger installed.
All setups like this that I have seen have Cat 4 venting and power burners to deal with the draft issues.

neilc

Paul,
your ss liner above the draft hood,
that goes all the way to the top of your chimney ?
or just to the cellar thimble ?

ethicalpaul

neilc said:

Paul, your ss liner above the draft hood, that goes all the way to the top of your chimney ? or just to the cellar thimble ?

I haven’t seen it with my own eyes but when I bought the house my chimney inspector found a cracked tile lining and the seller then had the chimney lined with 6” SS

ethicalpaul

motoguy128 said:

I’ll bet half the mod cons install on radiator system rarely are over 90% efficient since return water temps are over 150F inmost installs.

Thank you very much for your replies!

can you expand on this? Do you mean that because the return water temps are over 150F that it follows that condensing temps are not being achieved because the hot return water can’t take enough heat from the combustion in order to get the combustion gas temps down low enough?

so even with the high tech boiler, they still often have lots of waste heat because of things like poorly-sized radiation loops, etc?

unclejohn

I have been in thousands of homes and have seen all types of Rube Goldberg type devices to ekk a BTU out of the flue and into the home. My advice is to take the energy that you would expel thinking up and installing something, or that someone else though up and walk to the fridge, get a beer and count your blessing. The peace of mind you save is worth the 2 btu's you lost and the equipment is operating as designed. Now isn't that beer delicious.

JUGHNE

There were/are devices like your fin/fan heat extractor with wood stoves in mind.
They worked but there was additional creosote build up in the stove pipe and even stove.
Anyone using wood burning correctly and wisely knows to clean these surfaces regulary...part of the joy of nice heat.

If there was a simple device that reliably could raise the efficiency safely even 3-4% it seems that manuafactors would include it on their units.
It would be a great selling point for steam boilers to go from 82% up to 88%.

Zman

motoguy128 said:

On a side note, I’d look at it as about 6-10% wasted heat. Realistically after you factor in distribution, pumping energy or blower power, scorched air and low temp hot water are only about 90-92% efficient in the real world.

I’ll bet half the mod cons install on radiator system rarely are over 90% efficient since return water temps are over 150F inmost installs.

The number is actually closer to 86%...

Jamie Hall

ethicalpaul said:

motoguy128 said:

I’ll bet half the mod cons install on radiator system rarely are over 90% efficient since return water temps are over 150F inmost installs.

Thank you very much for your replies!

can you expand on this? Do you mean that because the return water temps are over 150F that it follows that condensing temps are not being achieved because the hot return water can’t take enough heat from the combustion in order to get the combustion gas temps down low enough?

so even with the high tech boiler, they still often have lots of waste heat because of things like poorly-sized radiation loops, etc?

@ethicalpaul -- looks like @motoguy128 has a pretty good handle on that. To look at it another way, in order to recover the latent heat of combustion (the heat represented by the water vapour resulting from combustion, which is greater with gas than with oil), you must cool the exhaust gas below its dewpoint. The farther below the dewpoint you can cool it, the more of that heat you can recover. You start recovering some of that heat at about 140 for typical units, and the overall efficiency starts to rise, correspondingly, from somewhere between 82 and 85 percent for modern boilers to a reasonable maximum of around 96 percent -- but that latter figure assumes you have cooled the gas to around 90 to 100 F.

Trouble is, you can't get much useful heat from conventional radiators at that return water temperature. You can from a properly designed and controlled radiant floor or ceiling installation -- but only if the boiler firing rate matches the heat load. Otherwise, as we often see, recirculation is used to control the temperature of the radiant circulation, leaving warmer water to return to the boiler. Reading various threads, you will see that it's not at all uncommon for the mixing or tempering valves to be set to control the floor temperature -- and boiler return temperatures significantly higher. Like... often over 140. No condensing, no efficiency gain.

Now you can increase the overall efficiency of any boiler -- condensing or not -- by extracting heat from the flue gas and using it for something else, as you suggest. The problem there, as has been mentioned above, is two fold. First, the condensate is acidic. In fact, very acidic, and a combination of acids which is remarkably corrosive (primarily nitric, some nitrous, and carbonic; for fuel oils, unless the fuel oil is ultralow sulphur, significant sulphuric as well). Some, but not all, stainless steel alloys can withstand that -- for a time. None can withstand it indefinitely, so if at all possible you want to do the condensing within a flue pipe or heat exchanger which is accessible for inspection and maintenance -- not halfway up a chimney! Then you also have to deal with that condensate -- it must be neutralized before being discharged anywhere (this is just prudent, although if you are on a municipal sewer it may also be required). Second, since the flue gas is now cool, you will have serious draughting issues unless you use an inducer fan (or the boiler is set up for sealed forced air, with sealed exhaust). The point here is that a draught hood is designed and intended to introduce air from the surrounding space -- not to cool the stack gas, although it does, but to reduce the draught on the burner and firebox (barometric dampers do the same sort of thing). A draught hood is always open; it the stack gas is cool enough, there is nothing to make it go up a chimney, so there is a very real risk of it spilling into the occupied space instead. This is, to put it mildly, undesirable.

At least with gas fired appliances you don't have a problem creosote; the various heat extraction contraptions made for wood stoves practically guarantee creosote acculumlation in the flue or the chimney, with pretty inevitable results in terms of chimney fires. Unless the homeowner cleans the chimney religiously.

luketheplumber

https://youtube.com/watch?v=8PF2rbzCdU4&list=LL&index=664
Here is a video of one.