Hot water heat exchangers

Using a heat exchanger to save on hot water heat could save a lot of energy, and also open up the possibility of a tankless or nearly tankless, electric hot water system for a tinyhouse, which you can’t otherwise have, because they require too much power. That reduces standby losses associated with a tank, and it also save quite a bit of energy overall.

2 Heat exchangers made to do something like this are:

http://www.ecodrain.ca/en/how-does-it-work

http://shower-save.com/home.html

Those were found by googling “heat exchanger shower.”  There might be more.  Anyway, they are, I assume, counterflow heat exchangers.  The thing is, the diagram on the above mentioned ecodrain page is not the optimal way to use such a heat exchanger. Not at all.   See figure 1 for the optimal config.  sorry it’s a bit messed up.  Click to see the original.

Figure 1, a more efficient hot water system configuration.

Anyway, what you want to do is put the heat exchanger on the drain line, but before, not after, the hot water tank.  Heat exchangers like the “powerpipe” use this configuration, they are connected to the sewage outlet.  The problem is, they have to be designed to allow sewage to pass, which makes the heat exchanger very inefficient.  But, if you are building a tinyhouse from scratch, it’s not hard to separate the sewage drain line, so now the heat exchanger only needs to handle greywater, and you could use one of the ecodrain devices.

There is no cold water line per se.  Only a colder line. All incoming water must flow through the heat exchanger to achieve maximum efficiency.  Another way to do it would be to control the power output of the water heater directly using an electronic control knob as your temp control knob, instead of using water mixing.  More about this later.

Now, how would this change things?  Well, let’s just suppose the heat exchanger ends up being 85% efficient.  It might easily be more than that.  If you have a 1.5 GPM (~6 liters per minute) shower head (the faucet can make use of the system, but the shower has the worst case scenario in terms of water and heat demand so let’s focus on that) and the incoming water is a typical 6 deg c and you want the water to be a comfortable 45 deg c. you would need about 15.6 kw of heating power if you didn’t use the heat exchanger.  So you now have 15.6 kw of heat going down the drain.  The heat exchanger gets back 4*0.85, so you only need to supply 4*0.15, or a mere 2.3 Kw.

That’s doable with a 120v heater on a 30 amp circuit, but it might be better to improve the heat exchanger efficiency.  I’m betting they are probably quite a bit better than that anyway.  It also makes using a solar hot water heater a lot easier because your collector is a lot smaller.

Now, but how would the temperature change with time when you turned on the tap, and when you tried to adjust the temperature? You can see it would not act quite the same as a traditional system.  What you are doing is controlling the amount of energy being added to the system, rather than the temp. of the output at the shower head.  As the temperature goes up, the amount of heat lost through the heat exchanger goes up, and eventually the amount being added and the amount being lost approach an equilibrium. With a totally tankless system, there is also the problem of heating the system up on startup.  The water standing in the supply and drain lines needs to be heated up.

Suppose you had a 2.3 kw heater. Just to estimate how long it would take,  As the water is running, 15% of the heat goes down the drain, so it’s effectively heating the water in the supply and drain lines with 1.95 kw.   Suppose there is maybe 4 meters of supply line with an inner diameter of 1.5 cm, that’s about 700 ml of water in the supply line.  Let’s suppose it’s the same for the drain line.  The water lines are indoors, so let’s suppose it’s at 21 deg c. 1.4 liters total, needs to be heated to 45 deg c., that would take 67 seconds with 1.95 kw.  Hm.  However the plumbing lines could be a lot shorter than that in a tinyhouse.  You could use more narrow lines, too. Too long, really, though, unless you clustered stuff and put the heat exchanger right nearby on purpose. To reduce water wastage and shorten the time a bit, you could put the heater directly after the output of the heat exchanger on the supply side, and turn the water on just the right amount, so that water coming from the heater was the right temp. Because the water is heated before it passes through the heat exchanger even once, you get the full 2.3 kw.

The use of a very small tank to provide that initial input energy might be a good idea.  You could even use a thermos as the tank.  That would be kind of cool.  Just use a regular hot water heating element and thermostat as the heater.  That might be pretty cheap, too.  You have to figure out a way to couple the thermostat to the temp inside the thermos, usually the thermostat measures the temp through the wall of the water heater’s inner wall.  You could drill a hole in the thermos top and put a stainless steel rod in to conduct heat to the thermostat, then put a piece of foam or fiberglass insulation over everything. The thing is, the water in the tank counts as standing water in the system, and you have to figure out how to extract energy from the tank without letting excessively hot water escape from the faucet. Switching the tank per se out of the water flow path might be a good way to do it, see below. But then you can’t use the same heating element to heat the water during the main phase of operation.

Also, you might be wondering how to reduce the temperature, what if you have the water on hot and want to switch it to cold all of a sudden? Well, you need to remove energy from the water flowing through the system.  You could turn off the heater and let it escape through the heat exchanger.  Especially if the heat exchanger is high efficiency, that could actually take a long time.  The other way is to have a third water line leading to the faucet, which skips the heat exchanger entirely on the way in.  If that water us used as a supply line, then drains out through the heat exchanger, it flushes out the hot water that was in the system, losing all that energy.  The heat exchanger has no effect, because there is no water coming in through the exchanger for the outgoing water to heat up.  You then have to put that much energy back in again when you want hot water again, too.   But the whole system would still consume a lot less energy than a normal system.

The law of diminishing returns applies here, so this might not be worth the complexity and cost, but another way is to try to store the energy.   You can use a regenerator that is switched in and out as desired. Or you could just use a small water tank.  This could be done without the use of the third water line I mentioned.  Imagine it located between the shower drain and the heat exchanger.  Water can flow through it in a sort of laminar way, with first in first out i.e. no mixing of the hot and cold water.  The “tank” could be a tube coiled up, for instance.  It only has to store about the 1.4 liters mentioned above.   Water does not normally flow through it.  It is normally filled with water at room temperature, or it could be located outside so the water is kept at the outside temp, or it could be inside a water heater tank, allowing it to be used for system startup, too.  You switch it into the system briefly, just long enough to replace the cold water with the hot water that was in the system, and it simply stores the hot water until you want it back again.  At that point the hot water is displaced with cold water and you have most of your heat back.   If the whole system was computer controlled to take care of everything, so you just tell it the temp you want… The computer could be used to control other tinyhouse systems too, so it wouldn’t be too expensive.

Just one more thing: I haven’t checked the prices of these ecodrain type things, but you could make your own heat exchanger too, just by putting a stainless steel tube inside of a plastic tube.  Stuff might accumulate on the surface of the heat exchanger, though, since you don’t have the anti stick coating (unless maybe you could get some anti stick coated tubing.)  The other problem is the resistance it poses to water flow in the drain line.  These ecodrains are designed to have a much lower resistance to flow on the drain line than a tube in tube heat exchanger would have.  However, if you used a drain pump that would be overcome, and you might need a drain pump anyway because there is very little gravity drop in a tinyhouse.   Google products shows water pumps for <$100, so maybe the price wouldn’t be prohibitive.

This post is a post I selected from the archives to repost. Since few if any of the current readers have read it, I thought it would be just as good as a newly written post.