Experimental Heat Recovery Furnace Exhaust! Preheat water heater 

My father led a one-inch iron pipe through one side of his wood-burning furnace and out the other, preheating cold water and passing hot water into the gas water heater. The pipe was about two feet long. While he used the furnace his hot water was free and he only paid for the pilot light and the service fee.

Gotta insulate all those pipes.

I love it! Very well done sir. I will be doing this on my multi purpose van’s diesel heater and I am so excited to be creating a towel rail as my wife & I like cold water swimming 🏊🏽‍♂️🏊‍♀️

Just what I was looking for, thank you so much!

Cool, thanks for sharing. In Alaska many years ago I built some similar heat recovery 'Stack robbers' for generators and oil/diesel furnaces using stainless steel tubing, and on generators an exhaust diverter was required so as not to boil the glycol in the robber. We generally used them to supplement hydronic heating systems with a large open loop storage/mixing tank systems with zone pumps for each loop. The worked great for many years but required a bit of maintenance.

Thanks so much for sharing.. I've been looking at this concept for quite sometime. Btw, did you measure the temperature before and after adding this heat exchange? I wonder how much heat can you really recover from this exhaust. And as of now, what are the inlet/outlet temperatures from this exchanger?

I like the concept. You could run the exhaust through something with fins on it - like an automotive oil cooler in a box. You would still have corrosion issues. Main question is why not put this in the drain water pipe instead. Building codes in some places are requiring drain water heat recovery systems. Building with staggered stud/ double stud walls also dramatically increases insulation value - potentially saving bundles in heating and cooling.

Wait, so is this preheating the water going into the water heater, or is it just warming the cold water going to the rest of the house plumbing? Also, have you tried wrapping it insulation to save the heat that makes it too hot to touch?

95% efficient boiler is a myth. It takes time for the boiler to get to a temperature at which it starts to condensate. By the time it gets there, the call for heat is mostly over, so you will not get the promissed high efficiency. You need long cycles to achieve high efficiency. The setups used by manufacturers of high efficiency boilers and furnaces are ideal conditions which most homes don't meet in real life.

That’s cool. We have a 96% furnace. We have a 20foot pvc exhaust for it and it doesn’t feel super warm near the end, so I imagine a lot of the heat radiates into the cold basement. It’s the only thing stopping me from putting an air to air metal heat exchanger on it.

I like to do recovery on 4 " exhaust on furnace , no water just air go through the copper , then a small fan pulls out the air out of the end of the copper pipe into the living room . I will wrap the outside of the copper pipe to capture more heat . I do not know if I can get soft copper pipe so i can wrap it around by myself

You will need to add a 3/4" sweat globe valve between the hot and cold take-off ports. Water will take the path of least resistance. The coil tubing has 60ft of equivalent length, which has a higher pressure drop than the small short run of pipe between the hot-cold take-off ports. You will have to throttle the globe valve to force water through the exchanger...

Did you solder the ends of the 1/2 inch spiral copper tubing you are running water through to keep the loop tight on the 3 inch pipe ????

Thanks for sharing!

I really like this, do you think you could squeeze a little more BTU's out of it if you insulated it?

I think I’m going to wrap this around my fire pit lining and bury 100’ of this in my gravel around the fire pit. I’d imagine the convection exchange would be great on a cold snowy night

What was the temperature of the exhaust before you installed the mod? What is it now? I'm pretty sure I can hold my hand an inch from the exhaust of one of these burners, indefinitely. This is why they can use plastic pipes for venting the exhaust outdoors. The reason no one was stupid enough to do it, is because the copper alone costs more than any energy ever recovered.

Love the system, I am looking at installing a heat recovery to heat air rather than water

very nice!

Can you use this on a propane tankless condensing water heater? If so what brand heat exchanger are you using?

a few degrees? How many? I'd really love to hear some concrete numbers on systems like this.

sweet man

Could you plummet directly to a heat exchanger connected to your hot water heater? So, you're not heating the whole homes water, just the hot water that you're Using.

Nice Coil, Did you make it yourself? Why is there no insulation on your pipes, there must be heat loss on them.

Do ypu have a material list links where ypu found them

What kind of paint did you use to coat the inside of the pipe? Can it be covered in insulation without causing damage to the pipe or insulation and prevent accidental burning while keeping more of the heat in the pipe?

I was thinking of something similar with a generator that you could run off natural gas. Hot water, Heat and electricity pumped back into the grid. Effectively a on-demand water heater that pays for itself.

A tee, even with some vanes in it, would not ensure that there is an equal flow going through the exchanger and the bypass. There will of course be equal pressure drop, but due to the low flow going through such a long run you have to the exchanger.

Why not use this on campers on the exhaust pipe to store heat.

How did you wrap the 1/2 inch copper line around the inner pipe?

Why don't you heat up the cold air coming into the furnace ?

It would be good to have that heat exchanger coil wrapped with a ceramic insulation blanket so Little heat is wasted

I’m converting a bus and thinking of using a concept like this to heat water using exhaust pipe heat. any thoughts or concerns about that?

Without any numbers it is subjective at best. I don't believe it has much of an impact as you only send water through your coil when there is flow, faucet of dishwasher for example. The rest of the time you are heating a couple of liters of water a few degrees that will vanish rapidly when in contact with colder pipes. What you need to do is have the inlet of your coil on the cold side of your water boiler and the warm side on the hot. A circulation will be created as warm water rises and flows to your boiler creating a draw on the bottom, maintaining the system as long as your heat source is active. As I lived in Hawaii I had made a solar collector on my roof working on that principle so I know it works. My energy bill dropped by 33%. Of course your exhaust pipe will not generate 1KW /Square meter like the tropical sun but it "could" have a positive impact. Also. Wrapping a tube around a tube gives you a very small contact surface, just a thin line if you think of it. You could better this by either soldering, insulating around the whole device or better, running your coil inside the exhaust as total immersion will give you a 100% exchange surface

Where can i find this? please help....

If the exhaust is too hot to touch....This would possibly be high-static in the air handler. When this happens the 95% efficient furnace is no longer 95% efficient. Most of the efficiency from the 80% to 95% efficient furnace comes from the secondary heat exchanger when the vapor in the flue gas condenses. When the air handler static is high the airflow is reduced thus raising the flue temperature. I would be curious, what is the Duct leakage number (total and leakage to the outside), then total external static compared to mfg data, temp rise, and watt draw on the furnace?

This is used often. It is a economizer .

How hot is the exhaust afterwards?

I really like cold water coming out of the tap in winter. I would have put this just upstream of your water heater and leave the cold alone. Nice work, though! One concern I had is about heating the water when it's not moving. In your system, it looks like any time water is coming into the house, water is moving through your coil. Presumably any time the water heater is on, water is moving through the pipes, but what about when the water heater turns off? Then the exhaust pipe is still getting warmer and the water in the coil is still heating. That doesn't seem so bad, but you talked about applying this to a furnace exhaust. When the furnace is on and producing heat at the exhaust, it is likely the water is *not* moving. This can cause a dangerous pressure build up in the copper pipes, right? How is that mitigated?

1 issue here, the exhuast of a furnace needs some amount of heat energy to draft correctly, by extracting heat energy you can greatly effect the flow of a gas stream and can f-up fhe furnace efficency so by "recovering" 1-2% of the heat you lower the furnace effeciency by 10-15%. Its a net loss. If it workled well dont you think they would be included already when you buy a furnace so a company can claim 98% efficiency?

Don't you worry about exhaust gas blowing through the water in your p-trap and letting CO into your house? Why not tilt the whole thing towards the outside of the house and let the gas + condensate run outside?

The 3” copper will last a few weeks with the acidic condense in the flue exhaust it will rot - band using copper in condensate connections

How has this system held up over the last 4 years

It seems to me there are a couple of weak points to this installation: 1- there is no insulation, so heat recovered by the copper pipe is simply dissipated into the air. 2- the bypass you installed does not ensure single direction flow, so it is possible that the pipe is taking heat from the water rather than giving it.

Anybody try this on an automotive exhaust as a heat exchange water heater? safe? I'd like to just warm some shallow well and creek water from around 75F to comfortable.

insulate the coil for higher recovery

You need to insulate your pipe in basement my friend...save energy,,unless you want waste in there..cool thing though

It would be more efficient if you had insulated it, just saying. Also highest efficiency is with the largest Delta T which means if you turn it the other way, you will have a greater success. The coldest part should hit the hottest part first.

Where can i get the heat exchanget pipe?

you havent seen anything like this because you dont know the names of the devices. on a normal boiler this is called an economizer, economizers are never used on high efficiency boilers because your exhaust temp cant be higher than 180F or the thermal fuse will blow and the unit wont run. if this were an old atmospheric boiler or just anything that has a significant stack temp this would be a flawless installation aside from the fernco. may i sugest that you build a rocket stove and use your heat exchanger there as an emergency heat source with isolation

Interesting, reason y I looked , exhausted of my furnace is crazy hot , can’t touch it, was thinking maybe put a small fan blowing on it., wasting electricity…I ll think of some coils with wings maybe …tnku

any ideas of drain heat exchanger?

They sell these in HomeDepot, but the installation should be done vertically. As the water drops, it is thrown against the inside of the walls. In this case, you are only using half of the interior walls since water is probably only reaching half the diameter of the pipe, This means you could still increase the efficiency of this setup. Check with the company to be sure. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-6VITshAnTY8.html ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-nyAoT9748lU.html For the HVAC, there is a different heat recovery device, also, at Home Depot. They claim a heat recovery of over 30%, and in addition, it reduces the HVAC cooling energy. This is measured as lower amps for the HVAC units. I read somewhere that heat recovery is a little over 2,000 BTUs per kW or per ton (don’t remember well) rating of HVAC. There is a news clip somewhere in RU-vid about these units in Florida. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Td00sJZwKQo.html

It would make more sense if that was for your preheat for the water heater tank or boiler. That way neither has to raise it as much, and you're getting that return. Warming up the cold water is couter productive as people will just, run the water longer in order to get a cold to drink. That being the case you're just running it down the drain. Heated up and put it where it's going to be needed or utilized so it means that your water heater or your boiler is on less often or not as much to bring the water up to temp. Great idea. Not utilized as well as it could be.

No one is doing this because the energy available for recovery is so low. If your boiler has a 100k BTU input, and it is 95% efficient your looking only 5k BTU available for recovery. Call your energy recovery efficiency 50% on the absolute high side. Assume it's crazy cold and your boiler is working 75% of the time. That's 45,000 BTU per day or about 38 cents saved per day on natural gas in absolutely ideal conditions. You're looking at a 6 year ROI at a minimum. In a modern climate is more like 12 years.

I quit watching after 18 seconds. There is no t at the end of furnace. After I heard that I hit to stop button. 👎👎👎👎👎

probably a good reason why no one else is doing it. unless you think you have had an einstein moment and the world will for ever wonder at your genius. heat exchanges have been around forever. heat exchanges on a plastic pipe boiler exhaust not so much lol

Noble idea but I would shut that appliance down so quick without approval from the respective authorities. They likely never would approve this due to the risk of exhaust gasses venting into the living space and the increased likelihood of freezing of the exhaust termination. Interesting but definitely not safe.

Yeah that's not doing anything dude. Water only flows when when you're using water and then its less than half id say closer to 2/10's of the water flowing to your tank that flows through that heat exchange and that's why its hot to the touch. You're much better off with a tempering tank made out of an indirect water heater with a pump to move water or a thermal fluid through that heat exchange. This setup you have is doing next to nothing and I'm sure if you take a before and after temperature reading of your main line you'll see that there is little to no difference at all. Not trying to shit on your idea I do something similar but this is just not an effective way to recycle heat.

Yes people....you can pump carbon monoxide into your drain......and you need to spend more to heat your basement...... Lol. Think this out dude. Fail.

Dumb. The low grade heat from a condensing boiler is negligible and the rubber fernco fittings are illegal on a boiler exhaust stack. Put the shower drain economizer on the shower drain and you will actually save energy. 100% of your hot shower drain water is going out to the sewer.