exhaust heat capture

[Living31]

As propane prices rise this may become a more viable option, but just to spitball for a second:

The exhaust vent of my new Suburban furnace puts out heat in the 300 degree range.

Take one 105mm aluminum CPU radiator and position it 3 inches in front of the exhaust vent. (as not to greatly impede exhaust gas escape)

Insulated 3/8 inch tubes carry glycol mixture from exhaust vent radiator to a twin radiator with CPU cooling fan, positioned under the trailer.

A micro pump on a delay timer and wired to the Suburban blower fan circulates the fluid; pump and under trailer CPU cooling fan, are started shortly after the Suburban blower fan engages and stopped shortly after the Suburban blower fan disengages.

Why? I hate to see all that heat just shoot out into the world. I have some of the above parts and can source the rest for under $50.

To the great minds of airforums, what am I failing to consider?
Do you think this would provide more efficient and/or greater heating than the tried and true 100w bare lightbulb under the skirt? (My skirt is snug and well insulated)

Thank you

[perryg114]

There is a fair amount of wasted heat as anyone who left their door open and has a scorch mark can attest to. It comes down to size versus efficiency. A Catalytic converter in the exhaust would extract any heat energy and chemical energy left in the exhaust. However, this would double the size and cost of the unit. How much propane can you buy for $500? I expect most folks don't go through that much propane in a lifetime unless they are full timing. Now lets get into how poorly insulated Airstreams and any other RV or mobile home is.

The heat exchanger on these RV furnaces is not large enough to extract all the heat. To do this you would have to double or triple the size or at least the surface area which doubles or triples the cost. Propane is still cheaper than electricity or it should be if you are not being gouged. It is not uncommon to be charged $30 for $15 of propane. Propane is less than $2 a gallon if you don’t live in a nanny state where they tax you to death. A 30lb gas bottle is around 7 gallons of propane. We should be running our cars off of propane and natural gas.

Perry

[markdoane]

I've built a few of these type devices, and I think you will be disappointed with the total amount of energy you can transfer. A 4 inch square radiator isn't very big for absorbing heat from an air source, even at 300F. And I think you may have even more problems on the glycol side - just not enough surface area available.

I would try adding more CPU radiators, if you can get them cheap.

[perryg114]

I would think you would not get more than 100W of heat from such a small device.

Perry

[idroba]

Don't confuse temperature with total energy. Just because something is hot does not mean it has much energy in it. A wooden kitchen match burns very hot, but only produces about one BTU of energy. Your furnace takes about 30,000 BTU's of propane in one hour, or 30,000 matches worth. It may burn at about 70% to 80% efficiency, or put out 21,000 to 24,000 btu's into the trailer in an hour, so your 'loss' is in the range of 6000 to 9000 btu's each hour of burner time. About half of that 'loss' is in the water vapor that is contained in the exhaust gasses. Without condensing the water vapor that energy is virtually alway lost. Unfortunately, condensing furnaces for RV's are not available and would be very expensive if they were.

Just a whole lot of practical and technical reasons this would be hard to make work.

[dwightdi]

If you want to just save the cost of the wasted gas used to generate the heat which is pumped out the exhaust of the furnace, the best answer is to invest in a smaller auxiliary catalytic heater for your Airstream and not run the wasteful furnace, except when it is necessary to protect the tanks from freezing. Early Airstreams got quite a bit of heat out of the propane powered lights. They had no heat being ejected to the outside of the trailer. Another way to save on gas is to use electric blankets and/or electric mattress heaters.

[Living31]

Thank you for all the thoughtful replies. Looks like I need to think on this some more. At first it seemed a natural, I would like some heat under the trailer, there is some unused heat shooting out my furnace, bingo. Sadly, all is not as simple as it would seem. Thanks again

[HiHoAgRV]

As pointed out, there isn't a huge pile of available energy left in the exhaust but the main reason for not pulling more out in the heat exchanger is economics. It's not the extra cost to pull out the energy but the ugly issue of what happens when you do. As soon as the exhaust cools below a certian point, the gasses start condensing out the nasty side effects of combustion. This requires all sorts of material upgrades to deal with the corrosive nature of the liquids. It's cheaper to just whistle it out of the tube, fully evaporated in the hot gas form.

Plus, in it's current form, I can warm up my hands while standing in the cold...