Recovering exhaust heat from natural gas forced air furnace?

[BioHazard]

I have a natural gas forced air central heating system in my house with a natural gas water heater. They're both almost 15 years old, and, rated only at 80% efficiency. The rating on my furnace shows 92,000 BTU input and 74,000 BTU output, so 18,000 BTU is going up the chimney. I'm not sure how much the water heater wastes but it's probably alot.

Anyway, both appliances are in my garage, the furnace has a 4" double wall exhaust pipe, the water heater has a 3" double wall exhaust pipe, and they both feed into a 6" pipe that goes out my roof. I'm wondering if anybody has any thoughts on recovering some of this 20-some thousand BTUs I'm pumping outside?

There are several different strategies I could use, mainly I'm considering:
1. A heat exchanger located on top of my furnace in the garage that would be used to heat the air in my garage. (small 2 car, well insulated, would love to keep it warm in the winter)
2. Capture the heat with water and send it back into the water heater.
3. Tap into the 6" exhaust pipe located upstairs in my house, and attach some sort of air heat exchanger there to heat the house.

Mainly I'm just thinking outloud here. Does anybody have any thoughts on any of those ideas? Does anyone know how hot the exhaust gasses should be leaving my furnace? I know they sell "heat recovery units" for wood stoves with a 6" pipe, I wonder if I could use one of those with my gas appiances? Maybe something as simple as coiled copper tubing inside the exhaust vents to heat water? Also, if I try to steal the exhaust heat, am I going to have to come up with some sort of induced draft fan?

Of course, CO detectors everywhere in the house before I even think about touching it...

Please, poke holes in my ideas.

[Geno]

I have one of the flue type heaters and it works well on a wood stove. I had it on my heating oil furnace for a while but it didn't get hot enough to turn on. This could be fixed with a different snap switch. You don't want to lose draft though.



Thanks, Geno

[Crofter]

If you take heat out you will change the draft so possibly flame adjustment. If you lower temperature enough you will have condensate to deal with which is quite corrosive so generally mild steel should not be part of the flame enclosure. The newer high efficiency devices are built to handle this.  Many older flue devices were subject to failure and plugging with deadly results. I dont think multiple units are allowed by code on a common flue either. Many potential problems with what you are considering.

[BioHazard]

I dont think multiple units are allowed by code on a common flue either.

I don't know if it's "correct", but that's the way the builder did it 15 years ago, and it has worked fine ever since. Seems to be common practice around here for a furnace/water heater to share a flue. (being parked next to each other in the garage like mine)

If I'm currently sending 18,000 BTU out the roof, I would be happy if I could just harvest 8,000 BTU. That would certainly warm up my garage. Would the remaining 10,000 BTU be enough to force a draft?

Also, if I cooled the exhaust enough, it might be possible for me to vent it directly out through the wall, rather than up the roof 2+ stories high. This is how my natural gas fire place vents. (which I rarely use, because the flame is for decoration, not heat)

[Crofter]

You would have to check your current local code; what was acceptable 15 years ago may not be now. In general terms with the current code you cannot put any add ons to the flue of any appliance without voiding its compliance certification. Many older gas appliances used a standing flame pilot light that could be extinguished by the woof of another appliance firing up, etc. New units with electronic ignition not so critical.

Direct low temperature venting would not pass any code with a home made device that lowered the exhaust gas temperature. If that part failed the exhaust temp would revert and the stack would not stand it. Many things will "work" but create the odds of a problem. If you made it, you responsible. Any bad consequences be on your dime or your conscience.

[Carlb]

The easiest way if you can do the work yourself is to swap out that 98k 80% furnace for a new 95+% 2 stage furnace of around 75k.  You can buy a goodman GMV series for around 1.000 dollars and if you do it this year it will qualify for a 30% federal tax credit and there may even be state credit available depending where you live.  This would also allow you to vent it straight out of the wall using PVC pipe.

[mike90045]

And don't forget the hi-eff heaters and furnace have a "forced draft motor" that kicks on 30 sec before the burner lights, so the airflow is pre-established.  So the BTU is 95%, but the electrical demand went from 0 watts, to 180W for the motor and electrics.