You are the master controller because you set the temperatures and determine the pump speed (start #3, Figure 86). But when you’re not around to oversee things — which is most of the time — a small electrical box with a differential thermostat takes charge. Out of this box run 4 electrical lines. Line A plugs into a regular wall socket. line leads to a sensor in the collector’s absorber plate. Line C goes to a sensor against the side of the solar hot water tank, and Line D leads to the pump.
Electricity runs through the differential thermostat to the wire leading to the thermistor sensor on the absorber plate. This sensor should be located on the absorber as close to the outlet of the collector as possible, because its job is to know how hot the fluid is as it comes out of the panel.
Meanwhile, there’s a second sensor at the end of a second wire circuit. It has been installed in the sensor port, located part way up the side of the storage tank. This second sensor’s job is to know the temperature of the water in storage. (Both end sensors should be mounted with silicone fiberglass tape, or better still, liquid aluminum epoxy, so there’s good thermal contact.)
Each of these sensors — the one in the collector and the one in the tank — is about 4 inches
Figure 93. Thermistor sensors look like this. They’re about 4 Inches long and have a tiny electrical resistor in one end. This Inexpensive part is small, but it’s critical to your solar water heater’s control system.
long and has a tiny electrical resistor in it (Figure 93). When there’s more heat in the resistor, there’s more resistance to electricity passing through it. Likewise, when there’s less heat, more electricity flows back through the return wire to the thermostat. This is how the thermostat "knows" the water temperature in both places.
When the sensor in the collector tells the thermostat that fluid there is, say, 10 degrees hotter than the fluid below in storage, the thermostat sends an impulse to the pump. As soon as the pump receives this electrical message, it comes on — circulating fluid through the collector loop. When the sensor mounted against the side of the storage tank says the temperature there is getting close to the temperature in the collector, the pump is instructed to turn off (Graph 4).
In other words, the antifreeze-and-water fluid circulates whenever the temperature in the collectors is hotter than the water storage temperature by whatever preset amount you choose. (A 10-degree temperature differential to start the pump, and a 3-degree differential to stop it, are typical settings.) That’s how the control system works. It’s pretty foolproof.
A slightly more expensive variation on a simple differential thermostat has a feature that automatically turns on the pump whenever the collector temperature gets lower than 36 degrees Fahrenheit. This causes warm fluid to circulate through the panels, to prevent snow build-up, until the collector temperature reaches 37.5 degrees. Then the pump shuts off again, because the collector is warm enough to melt any snow
Graph 4. The heavy lines in this graph chart two different temperatures in a solar hot water heater during a 24-hour period. One line represents the collector temperature, the other the temperature of the water in the storage tank. Here’s how the day went:
1. At about 8;30 in the morning the absorber plate in the collector got enough warmer than the tank water to allow the pump to turn on.
2. Notice how the collector temperature took a quick dip as soon as circulating fluid started to cool it off. You can see that it recovered quickly.
3. At about 5:00 that afternoon, the collector temperature dropped to within a few degrees of the water in the tank, as the sun got lower in the sky. At this point the differential thermostat instructed the pump to shut off.
4. Because there was no longer any fluid circulating
that lands on it. (A differential thermostat like this costs about $40, as compared to $35 for a regular one.)
Another more sophisticated thermostat has an "automatic off which doesn’t allow the pump to turn on if the temperature in the collectors falls below 80 degrees F. (This will prevent the system from fruitlessly trying to collect heat on a warm night after you’ve used up all your hot water in storage.) The same device can also be set to refuse electricity to the pump whenever the storage temperature gets above, say, 160 degrees.
Any of these differential thermostats should, of course, have a manual override. The cheapest and best ones come from companies like Rho Sigma and Heliotrope General, both of California (see appendix).
The slower the fluid goes through the collector, you’ll remember, the hotter it gets. But you should also recall an earlier lesson, that a collector is more efficient when more fluid is running through it at a faster rate. You can make your whole system more productive simply by adjusting the speed of the pump. On a warm sunny day, you may set it to run faster; when there’s haze, it should run slower. Your in-line thermometers will help indicate what to do — if you
decide that varying the flow rate is necessary at all.
There are some special variable-speed pump and differential thermostat arrangements that coordinate with a "rheostat" unit. A rheostat works like a dimmer-switch for lights. When the collector sensor indicates lots of heat in the panels, the rheostat feeds more "juice" to the pump and circulation speeds up. When there’s less heat in the collectors, the pump gets less electricity and slows down again. These are rather sophisticated and hard to hook up. Besides, there are very few thoroughly tested systems of this type available at this time. One we do know about is made by Rho Sigma. Contact them for more details, if that seems like too much trouble, stick with manual speed control of your circulation pump — or none at all.
That’s all there is to it. It wasn’t as complicated as you thought. We still bet there’s no reason you can’t put this whole thing together yourself, once you have the right parts. To be on the safe side, though, sketch your intended system on paper and show it to an experienced plumber. He’ll be able to point out any obvious goofs and weaknesses immediately.