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Throwing the BTU’s out with the bath water

Okay so on my wood fire hot water heater post I said  that we  could get up to a 30% energy saving on heating water and the hint was, what do we do after we’ve just spent so much time and energy getting it up to temperature? The answer as everybody knows is we let it run down the plughole in an attempt to heat the sewer or septic tank or whereever it is your waste water goes. Lets face it, rats and microbes should be entitled to a hot bath too! Seriously though its crazy, we just let it go, the waste!  I’ve long thought we could do better but the pattern in general has been to concentrate on generating the heat directly not on waste heat recovery, its just not as sexy as the burning wood bit. I’m always on guard against falling into the pattern of …well… falling into a pattern!  You know the old saw about when you have a hammer the whole world looks like a nail. Saw ? Hammer? What? What I’m trying to say is don’t always try to solve all your problems with those solutions you most like or are identified with. Case in point I always ask students what is the most efficient rocket stove? I get many and varied replies but the correct answer is “the one you don’t have to light” People like burning stuff so we tend to gravitate towards solutions that favor this but honestly if I can get the same result with less wood burnt then I’m very happy.  So I think the goal of using up to a third less wood for the same end result is a great one to pursue.

Of course not burning any wood to heat our water is even better and we will get to that in a future post.

Now first the concept of the heat exchanger,  the radiator in your car is a heat exchanger. Water heated by the engine is piped through the radiator, air is drawn through the radiator by the fan and the air picks up the heat from the water in the radiator and carries it away. The copper coil inside the hot water tank in my last post  on wood fired hot water systems  is also a heat exchanger, the heat in the water outside the coil passes through the wall of the copper pipe and into the water in the pipe. So a heat exchanger is a device that removes heat from one fluid and conducts it into another.

A few factors affect how well this can happen:

How good a conductor the material that the heat exchanger is made of is. Copper or aluminium would be good, whereas plastic or wood would be bad as they seriously slow the passage of heat.

The temperature difference between the hot and cold.   If we have water at 70 deg c on the hot side and 50 deg c on the cold side were not going to get as much heat transfer as if  we had 70 deg c water on the hot side and  10 deg c water on the cold side. Think of it like this, the greater the temperature difference the greater the ( to use a word incorrectly but descriptively) “pressure”.

The surface area, the larger the area of contact between the hot and cold the greater the heat transfer.

The time. The longer the hot and cold sides are in contact the greater the heat transfer.

So within reason we want heat exchangers that have a large temperature difference between the hot and cold sides , we want the exchanger to be made of the most conductive materials possible , we want the largest surface area we can and we want the fluids we are exchanging the heat between to be in contact for as long as possible.

Back to our shower.  Water flows into our hot water system at ambient temperature which will depend on where you live and time of year, for arguments sake let’s say 15 deg c. The water is then heated to a maximum of 80 deg c . The water passes through a tempering valve and comes to the shower head at a maximum temp of 55-60 deg c. By the time it hits our body and then the floor of the shower its at about 40 deg c. It then runs down the drain and is lost.  The trick is to use this 40 deg water to heat up the water flowing into the hot water tank at ambient which as mentioned is 15 deg. As a side note, in winter because the ambient temp of the water is even lower it will give us better heat recovery due to the higher temperature difference. Now that doesn’t mean it will use less energy to heat the water in winter, just that we will recover more waste heat due to the greater temperature difference. One important thing that hasn’t been mentioned and is very close to criminal is not insulating the pipe from your hot water system to where you are going to use it ,this is a huge waste of precious heat so if you haven’t done so, do so, pronto!

Now if we are using off the shelf equipment to build our heat exchanger then the humble copper water pipe is hard to beat. For a start its designed for potable (drinking ) water, its readily available and copper is a great conductor. Now the easiest system is simply to have the copper pipe feeding your hot water system take a detour along a section of your drain pipe. Typical drain pipe is around 40-45mm so the 13mm copper pipe will sit nicely along the bottom of the bigger pipe .From experience a 5 or 7 mtr length will give approx 50% heat recovery. Now don’t go getting all excited and think you can just double the length and get all the heat out.  The universe has specifically been designed so that you can’t.  As Robert Heinlein once wrote T.A.N.S.T.A.A.F.L (there ain’t no such thing as a free lunch).The energy recovery is exponential which means that the next 5 mtrs will get you an extra 25% the next 5 , 12.5% and so on.

If we harken back to one of our factors affecting heat transfer, the greater the temperature difference the greater the heat transfer. Now imagine our drain pipe with its 40 deg c water and the water in the heat exchanger pipe flowing in the same direction, the drain water loses a little heat, the water in the exchanger gains a little heat, they flow on a bit more one steadily losing temperature the other slowly gaining. What is happening to our temperature difference? All the time it’s steadily decreasing till at some point the temps are the same. Now imagine the identical situation except that the water flowing in the heat exchanger pipe is flowing the opposite or counter to the flow of the drain water. You now have a counter flow heat exchanger. This time as the drain water loses heat it encounters cooler water in the copper pipe the further it flows, conversely as the water in the copper pipe follows the drain it encounters progressively hotter water. This means we are maintaining as large a temperature difference as we can and so we are transferring more heat.


Now a few real world caveats, the first being that shower drains generally get lots of hair and stuff down them, so while having a spiral of copper pipe or some other high surface area arrangement will recover more heat it will also block more readily so be happy with the straight copper or if your a bit handy I’ve always thought it would be great to make a shower base out of an old copper hot water tank or sheet of copper with a large coil of copper pipe soldered to the back and then down the drain. Commercial units are available that have a spiral of pipe around a section of large diameter copper pipe through which the drain flows and this is ideal and can fairly easily constructed by those with access to welding equipment.

water flows

The second caveat is also about blocking but this is concerned with where and how your copper pipe enters and exits your drain pipe. If you drill the holes in the drain pipe in the top of said pipe then again it will almost certainly block due to the copper pipe hanging down into the drain so while its harder to seal its far better to have the pipe enter and exit from the bottom.

Now if your talents lie in other areas, as stated earlier commercially constructed units are available and are generally well engineered, but will cost you. If you’re like me though you wont be able to resist the temptation to tinker.

Happy Thermodynamics !


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