Exhaust heat for a water heater?

[Metal Twister]

Ive been toying with the idea of making a hot water heater using my exhaust pipe as the heat source. Has any one here done such a thing? It seems to me that I could build a copper coil wrapping around the exhaust pipe. Then cover with sheet metal so that it would look much like a muffler. I could then use a fuel pump to circulate the water to a insulated holding tank on the vehicle anywhere. It seem that with in a few miles it should heat up the water enough for a shower or not?

What do you think, Would you get enough heat transfer to make it worth while. Solar showers are ok but Im looking for a little warmer water. LOL :wings:

 

[java]

lots of people do the same idea but with the coolant. use a copper pipe with coils in it as the exchanger. no need for a holding tank and they get hot quick. can usually use the heater controls to adjust the temp.

 

[Metal Twister]

lots of people do the same idea but with the coolant. use a copper pipe with coils in it as the exchanger. no need for a holding tank and they get hot quick. can usually use the heater controls to adjust the temp.

If I had a holding tank I could use it for washing dishes or what ever, maybe even preheated water for use with cooking that would save allot of propane... Then use the heater control to run the pump... Right?

 

[corax]

Use a heat exchanger in the engine coolant, not the exhaust - exhaust gets way too hot for water heating duty. Someone already did all the work designing the system for you, here's an install write-up on 4crawler.com of the R&M Vehicle Mounted Hot Shower System

 

[ntsqd]

There is a lot of energy to be found in the exhaust. The problem is harvesting it.

Idle exhaust temp isn't going to do much, so a water reservoir would be needed to take advantage of the higher exhaust temps generated by driving. The next hurdle is surface area and thermal transfer. The exhaust has to heat the exhaust tube, which then has to heat the copper tube, which then has to heat the water. There is an inefficiency at each material transition. Unless mods are made to the copper tube, the contact area between the copper tube and the exhaust tube will not much more than a spiral line. Flattening or 'D' shaping the the copper tube and wrapping it around the exhaust tube such that the flat area was in contact with the exhaust tube would dramatically increase the contact surface area. I would also look into thermally conductive adhesives. See if there is one available that can withstand exhaust temperatures. Using one of those, if possible, would also increase the contact area. Even though there is a large delta in temperatures, my impression is that the conductive transfer area between the copper tube and the exhaust tube will be the largest point of inefficiency and needs to be optimized if the project is to work as expected.

 

[Metal Twister]

There is a lot of energy to be found in the exhaust. The problem is harvesting it.

Idle exhaust temp isn't going to do much, so a water reservoir would be needed to take advantage of the higher exhaust temps generated by driving. The next hurdle is surface area and thermal transfer. The exhaust has to heat the exhaust tube, which then has to heat the copper tube, which then has to heat the water. There is an inefficiency at each material transition. Unless mods are made to the copper tube, the contact area between the copper tube and the exhaust tube will not much more than a spiral line. Flattening or 'D' shaping the the copper tube and wrapping it around the exhaust tube such that the flat area was in contact with the exhaust tube would dramatically increase the contact surface area. I would also look into thermally conductive adhesives. See if there is one available that can withstand exhaust temperatures. Using one of those, if possible, would also increase the contact area. Even though there is a large delta in temperatures, my impression is that the conductive transfer area between the copper tube and the exhaust tube will be the largest point of inefficiency and needs to be optimized if the project is to work as expected.

Ok, so what if I opened up the exhaust pipe enclosed the coils in a muffler shaped shell and allowed the exhaust gas to flow over the coils? Seems the contact surface area issue would be fixed? I could flange it on both ends so it could be removed once in a while to clean, and remove any soot build up. A flow diversion not allowing straight through flow of the exhaust gasses might help? Seem that way I could even douple or triple the coil volume and increase the contact area by quite abit?

 

[dzzz]

Marine water heaters use engine coolant. They also have electric as an inexpensive option. 6 gal is the minimum size. They hold heat for about a day.
A nice benefit for diesels is that they warm coolant in the morning when starting the engine.

 

[ntsqd]

Ok, so what if I opened up the exhaust pipe enclosed the coils in a muffler shaped shell and allowed the exhaust gas to flow over the coils? Seems the contact surface area issue would be fixed? I could flange it on both ends so it could be removed once in a while to clean, and remove any soot build up. A flow diversion not allowing straight through flow of the exhaust gasses might help? Seem that way I could even douple or triple the coil volume and increase the contact area by quite abit?

That would work better, but you would need to build in support for the copper coils. Copper tube will fatigue from vibration and I do not think that it would reach the annealing temperature, so brittle failure of the tube is very possible. How long would be the question, not if.
Alternately you could switch to a SS tube for the coils, but then the thermal transfer will be remarkably lower.
Ideal location would be directly after a catalytic converter, if present. Locating it ahead of the converter has the potential to delay converter 'light-off' which increases emissions.

A liquid to liquid Heat Exchanger is a known path, but your idea has potential. I just don't know if it will or won't be worth the effort and suspect that the only way to truly know is to try it.

 

[Metal Twister]

Im on it... thinking that I could make it quiet enough to replace a Flowmaster style muffler and maybe kill two plastic birds with one stone. (PC correct version):victory: Thanks for the input.

M.T.

 

[michaelgroves]

Is there any advantage to using the exhaust compared to using the coolant? It sounds complicated to make use of the heat compared to a water/water HE.

The advantage of the coolant is that it is at a pretty well regulated temperature - the exhaust gases change temp a lot, and will lose heat very fast when you are at idle. Once the engine has been running and is up to operating temp, you can use the hot water system at idle indefinitely.

The exhaust system also cools down rapidly as you move away from the engine - the manifold is the best place to draw off heat, but the pipes are relatively cool.

 

[ntsqd]

I've never heard of anyone actually doing it or the results though I have seen it discussed a couple of times. I figured that M.T. has a reason for wanting to try it and if he has the time and resources to pursue it why not?

Right at the manifold would be the hottest point. If no Cat I'd certainly get as close as reasonably possible.

 

[Metal Twister]

I just spent some time with our local exhaust Guru... He seems to think it could work rather well. In fact he has given me a few cut away mufflers built by Flowmaster and a few others. I think I will make this a complete muffler as well as heat exchanger. Ed claims it would be 500 degrees within the muffler just about anywhere in the system. Obviously it would have to be aft of the cat if I had to run one. As it is I have a 1974 pre California smog required vehicle. I wont be getting around to it for a while but it is stewing and it will for sure be on the rig when it gets its new LS1. Hope within the next few months... Thanks for the help guys, Ill post up a good photo build thread when its complete and let you know how it works... Ive gotta try.:coffeedrink:

 

[bucketosudz]

So if you were indeed going to get an output temp of 500 degrees, or anything over lets say 120 degrees, how will you regulate it?

 

[Metal Twister]

Have the water pump only circulate the water into the holding tank when the thermostat says that the water is too cool. I can set the thermostat to turn on and off at any water temperture. Another water temp gage could easly be mounted on the dash to monitor the temp and a manual on off could be used if desired. There shouldnt be any problem just turning it off if not in use. Heck im thinking about building a false floor in the back to store my equipment. Maybe place a big holding tank right in the center under the floor. Throw the foam mattress over it and it would be like sleeping with a hot water bottle. LOL well maybe not...

 

[Metal Twister]

Is there any advantage to using the exhaust compared to using the coolant? It sounds complicated to make use of the heat compared to a water/water HE.

The advantage of the coolant is that it is at a pretty well regulated temperature - the exhaust gases change temp a lot, and will lose heat very fast when you are at idle. Once the engine has been running and is up to operating temp, you can use the hot water system at idle indefinitely.

The exhaust system also cools down rapidly as you move away from the engine - the manifold is the best place to draw off heat, but the pipes are relatively cool.

Thats the problem with us Tinkerers. There might be a perfectly good way to do it already... But anybody can do that! :wings: