Extracting waste heat from central gas heater flue. Silly?

[wey2go]

I am in Melbourne. Am thinking of ways to reduce heating bill if I need to maintain optimum water temperature for the fish tank.

Is it a silly idea to extract the heat from flue gas for a central gas heater of a home?

I would imagine it may be dangerous as it may contain Carbon Monoxide.

Or is it not a silly idea after all if precaution of good heat extractor is designed and operated?

This has been in my mind for a long time (even before thinking of using the waste heat to warm up fish tank) as I see that the flue gas still has lots of heat energy in it.

[Murray]

I have used a similar idea on my wood fired "pot belly" stove. I made up a copper coil from 25mm copper pipe into the body of the stove and joined that into the solar HWS. It worked well for 15 years until the solar HWS died a few months ago.
The system delivered a load of hot water. On overcast days during winter we only had to run the wood stove for about an hour and a half to completely heat the 300 ltr solar HWS.
I have had the electric booster in the solar HWS turned off for years. Just not needed, so over the years we have saved a heap on the power bill.
My pot belly wood fired heater will be replaced before next winter by another wood fired heater and I intend to run a 25mm copper pipe up the inside of the flue to heat water for our domestic use, rather than have the heating coil in the body of the stove.
It has worked very well on the old stove and solar hws. I suspect the pipe running up the inside of the flue will be very, very effective. Cold water coming into the bottom and out the top delivering hot water to a hot water tank by action of convection.
So with a bit of thinking it would not be too hard to draw off some of that heat and run it through a heat exchanger (stainless steel) in the fish tank. How one would control the temp is another matter....you could end up with poached fish !!!!

[GaryD]

Hi wey2go,

As Murray has pointed out, you can extract waste heat from a flue to heat water.

You would avoid the "poached fish" problem by heating a separate container of water (to your fish tank) and then having a temperature controlled pump move water from your separate container into your fish tank until it reached the right temperature range.

The main issue with this solution is the amount of time that it would take to get a return on your investment in controllers, pumps, etc.

A more cost effective way to go might be to have a small tank inside your centrally heated environment in which you grow fingerlings at an accelerated rate in advance of the warmer weather.....at which time you'd move them outside to finish them off.

Gary

[josh]

Winter heating will be a problem here where I live in Stanthorpe. I think the idea of a large body of water within the greenhouse to act as a heat sink might help take the edge off the low morning temps.
Perhaps this large body of water will be heated from the wood stove and just retain heat for later dissipation. A bit like a radiator of sorts.
josh

[jlwebb]

Josh,

I suspect you might be on to something with tank acting as a heat sink. I don't know this to be true from personal experience (yet), but in researching a way to implement an aquaponics system here in Oklahoma (where the winters will get cold enough to kill the fish), I ran across this dome greenhouse system: http://www.geodesic-greenhouse-kits.com/. It uses a large tank of water to store solar energy during the day, then release it at night.

I'm not sure just how critical the dome shape is to to making the system work.

Gary and Murray, I would love to hear your thoughts.

Jason

[Murray]

I reckon it has good potential. The large body of water being the fish tank or fish tanks. Water holds heat very well compared to the earth (soil)

I had a look at the dome site, very interesting. The dome shape would allow sun to penetrate the dome house at very low angles so it would be more efficient in higher latitudes. Every minute of sunlight would count.

But having made that observation, any well constructed greenhouse would work I am sure.

Oklahoma.... That is where "the wind comes right behind the rain" and "where the wind comes sweepin' down the plain"

Must get pretty cold there in winter.

[GaryD]

Hi Jason,

A dome greenhouse would work well but the most efficient greenhouse model (on a small scale) is the sun-facing greenhouse.

this is a lean-to shaped greenhouse that (in your hemisphere) attaches to the southern face of a structure (in Australia it is the northern face). It takes advantage of the winter sun. You could place your fish tank and grow beds in it or just the grow beds.

The Sun-facing greenhouse works most efficiently when it is attached to a solid masonry (stone, brick or cement) wall. The wall heats up during the day and radiates the heat back out into the greenhouse at night. As it is, you'll get something of this effect because of the mass of water and gravel (or both) in your tank and grow beds.

The efficiency of these structures is leveraged even further when they are used to assist climate control in the structure to which they are attached.

Gary

[anniefish]

Hi all,
Came across this article in Mother Earth News - http://www.motherearthnews.com/Alternative-Energy/1979-09-01/A-Homemade-Solar-Water-Heater.aspx I think some may find it interesting, you can do a solar heating search and find more articles. I buy this mag at the Newsagents, has some very eye opening articles and different ways of doing things. Cheers Ann

[Murray]

Thanks for that link Anniefish.
I have read it and it is great. I have a couple of old collectors from my solar HWS that blew up last winter. I must get them down off the house roof and see if I can fix them up before this winter to heat the fish tanks.
Thanks for the reminder. Winter is not that far away really.