Forced-Circulation Systems

Подпись: Now let's talk about forced-circulation systems, specifically, in detail. Actually, we've been discussing them all along, since much of what's been said so far leads up to the design in Figure 86. But remember that Figure 86 represents only a suggested system. It may be more elaborate (and costly) than you want, and it may not suit your house — either old or new — exactly. It's safe to say that you probably will have to make some minor modifications. For instance, you may decide, after reading the last chapter, that you don't need a circulation pump at all, in which case your system will not be "forced." Even if you decide that a thermosyphon system will work for you, you may choose to include some of the controls and other gadgets mentioned here. In many ways Figure 86 represents all of the best elements of sound solar water heating technology at the present time. At first glance it looks complicated and confusing, but it's not — once you break it down and understand what's going on. Each of the component parts — with the ex-ception of the absorber plate and the special solar storage tank — is a standard, time-tested plumbing element that should be sitting on a shelf in your local plumbing supply outlet. There's no more need to lose sleep over the possibility of your system's failing than there's need to worry about a breakdown in your regular household plumbing. That's because basically a forced-circulation solar hot water heater is regular household plumbing.
Подпись: What's in the Collector Loop Besides a Pump? The whole collector loop (start #1, Figure 86) should consist of 3/4-inch Type L soft copper or Type M rigid copper. You'll also need standard 3/4-inch sweat-type fittings, valves, threaded nipples, and other fixtures throughout. These should be joined together with high-grade 95-5 solder which withstands heat better than regular 50-50 solder. Any threads should be coated with Teflon pipe dope normally used in refrigerator tubing. Standard plumbing compound will dis-solve in propylene gycol, and threaded joints will quickly leak. The lowest point in your system no doubt will be where the cold water tap leaves the solar storage tank. Here there should be a short section of tubing and, of course, a boiler drain which can be used to empty the collector loop quickly. This is also where you will most likely fill the system with fluid for the first time, so use a drain cock with hose-connecting threads. Somewhere in the lower part of the loop — either before or after the pump — there should be a poppet-type pressure relief valve (like the Watts model #174A), set to "pop" at 30 psi (Figure 87). If this should blow for some reason, it will dump antifreeze fluid on the cellar floor, so it might not be a bad idea to keep a bucket under that valve so you can save any fluid that's released.

To keep things simple, we’ll divide the heater into its three major parts: (1) the collector loop, (2) the hot water loop, and (3) the thermostatic controls. Then, we’ll trace the flow of antifreeze fluid from the storage tank (beginning at the very base of the collector loop), to the collectors and back through the heat-exchanger coil. Next, we’ll follow the path of the water, beginning at the cold water supply, as it goes through the solar storage tank and existing hot water tank, to the house it­self. Finally, we’ll explain how the control system operates.

This pressure relief valve shouldn’t cost more than $8. But keep in mind that the best ones are not adjustable, meaning that you’ll have to buy one that’s pre-set to release when there’s 30 pounds per square inch of pressure or more in the lines. This relief valve, by the way, might be only the first of several in your collector loop. Plan others for any high, points in the system up above.

Also somewhere low in the loop — where you can get at it — there should be an expansion tank and an air purger (like Amtrol’s #443) with an automatic air vent (like Amtrol’s #701). Excellent expansion tanks are sold by Amtrol (model #15), Heliotrope General and others (see appendix). Don’t spend more on one than about $22.

slowly. The collector loop (#1), the hot water loop (#2) and the control system (#3) are all explained in the text.

Inside the expansion tank, where you can’t see, there’s an airproof rubber diaphragm with its edges fastened all around the sides of the tank. Pressurized air, pre-charged at 15 psi, is below the diaphragm, while fluid from the lines runs through the air purger above the tank. As the fluid heats and expands, the increased pressure from the expansion is taken up at the flexible dia­phragm (Figure 88).

This is how the system is able to maintain con­stant pressure at 15 psi. To check on this you may want to put in an inexpensive pressure gauge ($5) somewhere near the expansion tank. An additional option might be to install in-line thermometers at various points in the loop so you can monitor what’s going on with the tem-

Подпись:Подпись: Figure 88. When a fluid containing water is heated, it accumulates dissolved air, which can collect and form bubbles that block the liquid's flow inside the tubing. Above is a diaphragm-type expansion tank in combination with an air purger. The tank maintains constant pressure in the lines by taking up the fluid expansion. At the same time, more dense fluid flows through a lower passage in the air purger. Lighter fluid, containing air, moves through an upper channel. Here air is freed from the liquid, and released through an air vent. See the illustration of an air vent in Figure 90. perature. A tridicator, a gauge which measures temperature, pressure and altitude, is a more ex­pensive possibility. If you buy one of these you’ll be saving a tee fitting or two.

Now the pump: The flow rate through a solar collector loop is normally very low — about 5 gal­lons per minute — so you don’t need a very strong pump. A 1/20- to 1/12-horse-power (HP) circulation pump with a cast iron housing should be more than powerful enough if everything else is set up properly (Figure 89).

Whether you choose a 1/20 HP or a 1/12 HP model will depend on two factors: (1) how high you must pump fluid to the collectors, and (2) the amount of head loss in the loop. (Head loss is measured exactly with a "manometer," which you will have to rent or borrow from a plumber.) This rule may help your decision: Generally, if you’re going to pump fluid to an elevation of two stories or less — and have fewer than 6 or 7 collectors —the smaller pump, costing about $61, is adequate. Pumping to many collectors at 3 stories or higher, you’ll need the 1/12-HP pump, priced at about $69.

In either case, be sure you buy a pump that won’t be corroded by the antifreeze fluid. If some


Figure 89. A circulating pump like this can be in­stalled either horizontally or vertically, but it must never be put in upside down, or it will run backwards. Good pumps like this one have a variable speed ad­justment, and two different motor speeds. Put isola­tion valves on either side so the pump can be serviced without having to drain the whole collector loop. These valves come with the pump.

of the pumping parts are aluminum, for example, the pump will die an early death. As far as we know, the Grundfos Corporation of Clovis, California makes the best and most reasonably priced circulation pumps for our purposes here — their models UPS/UMS 20 and GP 26 (see appendix). They run on ordinary house current (115V AC). They also have a variable adjustment for flow control — which is controlled by a handy external adjustment — and 2 different motor speeds. (It’s also a good idea to use the isolation valves that are provided. Put them on either side of the pump, so they can be closed off if you ever have to do some pump maintenance.)

To keep warm fluid from rising out of the tank at night when the collectors are cold (this creates a reverse flow, remember), there should be a check valve somewhere in the lines. The little "flapper" in a check valve — which allows water to flow in just one direction — only works right when the valve is installed in a horizontal or near­level line. The most convenient place to put a check might be on the same level with the pump. If you don’t have a handy level stretch of line, put in a flow-control value, which can be installed vertically.

So fluid comes out of the tank, past the drain valve, through the pressure relief valve, the ex­pansion tank and the gauges, and is pumped through the check valve into the line of tubing leading to the collector. There it flows upward through the channels in the absorber plate and gets heated by solar radiation.

The line coming out of the collectors should run level or even slightly uphill to a float-type vapor relief valve like the Amtrol #701 ($8 to $12), which releases trapped air (Figure 90). Any air bubbles in the lines that have not been "purged" at the expansion tank should rise to this air vent, displace the fluid next to the float, and escape out the top. This type of valve not only "bleeds" air out of the system, it also provides vacuum relief, letting air into the lines when you drain the system. But caution: Float-type air vents are delicate. If the highest point in your sys­tem is outdoors and unprotected, the valve can be snapped off by snow and ice build-up on the roof. So protect it as best your can.

From there it should be all downhill — literally. (Don’t leave any hollows or dips in the return line


Figure 90. A float-type air vent like this should be in­stalled at all high points in the system. It works like this: when an air bubble from the line comes into the chamber, the water level is lowered. As a result, the float drops and opens the air valve. As the air escapes through the valve opening, the water level rises again, and the valve closes. The valve cap should be opened about 1/2 turn to permit air to escape.

where air might be trapped.) The hot antifreeze and water mixture passes back down the line to the heat exchanger coil in the storage tank, runs through the coil (giving up most of its heat), and comes out cool again at the base of the tank.

So we’re right back where we started — ready to have a look at the separate heater loop.

Cool, street-temperature or ground-temperature water (start #2, Figure 86) comes from outside the house through the cold-water supply line. As soon as it enters your system there should be a device known as a backflow preventer. This is nothing more than a double check valve with an atmospheric vent between the two checks. It doesn’t allow water to run back out of your sys­tem (Figure 91).

If for some reason your tank or heat exchanger were to fail and there were a contaminating leak


Figure 91. Here is what a backflow preventer looks like. It has two flapper discs — which don’t allow water to pass back out of your system — and a vent. These devices are required by law in many places.

between the two loops of the solar water heater, no poisonous antifreeze could escape into your well or the public water supply — although your house supply would be contaminated. Backflow preventers (like the Watts #9D) are required in many municipal systems, anyway. Even if it’s not mandatory where you live, it’s a good safety pre­caution to have one.

Once it has passed through the backflow pre­venter the water should reach a tee. Here some will be diverted to supply the house with cold water, and some will move on toward a second tee. At this second intersection (Tee #2 in Figure 86), part of the remaining water will be directed around the solar storage tank and the existing heating element to later be mixed with hot water coming out of the tanks. This by-pass meets the main hot water line at a tempering valve, which we’ll approach in our discussion from the other direction.

Water that’s to be heated passes through an open gate valve and past the tank’s boiler drain into the storage tank. (Cheaper stop-and-waste valves can be used instead of gate valves in the heater loop. These have the advantage of a small thumb-screw drain, but water has to follow an in­direct path as it passes through a stop-and-waste valve, meaning there’s lots of friction there. This much head loss is all right if there’s plenty of normal pressure in the lines — 60 psi, for exam­ple. But in the collector loop, which has only 15 pounds of pressure, this much resistance in a valve will only cause the pump to work harder than it should have to. Fluid will flow straight through a gate valve, on the other hand, so use only gate valves in the collector loop.)

once it’s in the tank, the water circulates gently, picks up heat from the heat exchanger, gradually rises to the top of the tank and out the tube leading to the existing hot water heater where its temperature will be boosted if neces­sary. This connection between the solar storage tank and the domestic hot water heater should also have a gate or stop-and-waste valve in case you need to shut down part of the system.

If it’s a sunny day, the water should have picked up enough heat so there’s no need for the oil – fired, gas or electric heating element to switch on. If not, the water will be heated some more to satisfy the heater’s thermostat, be stored, and be drawn off as needed. When someone turns on hot water in a bathroom, very hot solar-heated water will move out of the heater tank toward the tempering valve, where it joins the cold water from the by-pass line.

A tempering valve (like the Watts #70A) is nothing more than a mixer (Figure 92). It has a small spring-loaded adjusting knob on its top which permits you to regulate the temperature of the water going into the house hot water lines. By mixing hot and cold together, this little device — costing about $12 — will adjust water temper­ature between 120 degrees and 160 degrees Fahrenheit, depending on where you set it. (Right


Figure 92. A tempering valve like this one mixes hot and cold water before it goes into the house lines. The knob on the top lets you dial a desired temperature. To raise or lower the water temperature, anywhere be­tween. 120 degrees and 160 degrees, just adjust the knob.

after the tempering valve is a good place for one of those in-line thermometers, so you can check on the water temperature at the valve. Some tempering valves have no precise temperature settings, just a little dial numbered from 1 to 10.)

A tempering valve is a nice option — although it’s not vital. It prevents too-hot water from even­tually coming out of a faucet to burn kids’ hands. It also keeps more heat in storage because hot water is being mixed close to the tank rather than at the faucet.

After it’s past the tempering valve, the solar – heated water passes through normal channels until it reaches the shower head and then runs down your back, where it’s yours to relish.

Updated: August 21, 2015 — 2:29 am