Section Four - Hot Water Pre-heaters
Keeping with the theme of water usage and energy, this blog section is going to explore hot water pre-heaters. Now most of us are familiar with a hot water tank. Energy, commonly gas or electricity, is used to bring the water from the temperature it enters the system to a scalding temperature that we cut with cold water to bring to a temperature suitable to our needs when we use it. There are also tank-less hot water heaters that take cold water and heat it in a coil of pipe before delivering it to the end use. These have the advantage of greater energy efficiency than a hot water tank because you aren't keeping a large mass of water hot in for later use. However, in practice many people have found that tank-less hot water heaters can have a hard time keeping up with demand.
The hot water pre-heater is a piece of equipment that can integrate with either of the above systems increasing their efficiency and mitigating their weaknesses. In its simplest form, a hot water pre-heater is a barrel hooked up to the hot water feed pipe before the heater proper in such a way that it holds water prior to it moving into the heater unit. You paint the barrel black and leave it out in the sun. The water in the barrel picks up heat from the sun and as a result doesn't require as much energy to heat up to its final use temperature.
In practice solar pre-heaters are a little more complex, but not by much. A very simple and effective version consists of two long tanks mounted one above the other in a box with a glass top. The tanks should be black and the back of the box should be covered with a mirrored substance, like aluminum foil or a space blanket. The tanks are plumbed together so that cold water flows into the bottom of the lower tank. The top of the lower tank, on the end opposite the water input, is connected to the bottom of the top tank. The top of the top tank, on the end opposite the water input, is connected to the water heater.
The pre-heater assembly should be mounted facing south with the collection glass angled for maximum sun exposure. This will generally be your latitude + 15 degrees. This is best for winter. If you want to get fancy a tilt can be added so you can compensate for the changing height of the sun in the sky over the year thus maximising the amount of solar energy the system collects. It should go without saying that the unit must have a clear exposure to the sun. As a result you may wish to angle the box off south if the direct sunlight is blocked from certain angles. Common sense and logic will dictate what should be done.
In some climates units like the one described above will only be of value from mid-spring to mid-fall and if used in winter might actually cost you money by losing more heat then they gain. To deal with this it is suggested that if your area's winter temperatures routinely drop below the temperature of your input water that you install a diverter pipe so you can bypass the pre-heater in the cold months and run your water directly to the hot water heater. If you use a system like this, it is advisable to have a drain valve for the pre-heater as well as a bleeder valve at the highest point of your pre-heater's tanks to let the air out of them in the spring when you refill the pre-heater.
The efficiency of the pre-heater can be increased by placing mirrors that will direct more solar energy onto it. A slight variant of this system adapted for purifying water for human consumption is described in my book Tinker's Plague. (Shameless plug, shameless plug, shameless plug.) Did I mention that this is a shameless plug and that Tinker's Plague is available from Draumr Publishing, Amazon, Indigo and Chapters, Barnes and Noble and other bookstore outlets?
Another type of pre-heater is again quite simple. One takes flexible copper tubing and wraps it around the drain pipe then encloses the drain pipe and wrap of tubing in insulation. You run your input water through this tubing before it goes to the hot water heater. Some of the heat in the waste water is transferred to the incoming water.
More advanced systems use heat pumps attached to gray water tanks. Systems like this can be used in some climates as the main water heater, especially if there is a solar pre-heater, because they simply concentrate the heat energy into a smaller volume of water thus increasing its temperature.
Advantages
The advantages to hot water pre-heaters are many. If used with a tank-less hot water heater, it decreases the amount of work the system has to do to supply water at the desired temperature. In short, it's easer and takes less energy to heat water ten degrees then twenty. As such this can alleviate many of the inadequate flow rate problems that sometimes plague these systems.
With a hot water tank the pre-heated water requires less energy to be brought up to the tank's operating temperature than non pre-heated water thus you use less electricity, gas or whatever. It also means there is a greater volume of pre-heated water so it will take longer to run out. Depending on your local climate even a simple hot water pre-heater can have a significant impact on the money you spend to heat your water.
Another simple, solar water-heater, which is popular for use with swimming pools, is to take loops of black pipe and put them on a south facing roof. The pump water is diverted through these pipes which pick up the sun's energy which heats the water that flows into the pool. A system like this with appropriate diverter pipes could be set up to do residential water during the warmer parts of the year.
Objections
Objector. It sounds complicated.
Answer. It's not. Any competent home handyman with basic plumbing skills could knock a solar pre-heater together in a few hours.
Objector. Won't those tanks become breading grounds for bacteria?
Answer. Fair enough, if they sit full and stagnant for long periods of time that could happen. I'd suggest draining them if you're leaving the home unattended for a week or two. Other than that, the constant flow of fresh water into the system should keep it pretty clean.
Objector. I don't have a south facing roof.
Answer. I feel your pain. Maybe in your next house. The goal is for us all to do what we can. Some can do a lot, some only a little, but if we each do what we can, the cumulative effect just might save the Homo sapiens species.
