The number of zone valves you put on a transformer depends on the situation.

A load is the user of electricity. Loads come in many familiar forms such as light bulbs, motors and doorbells. A load changes electrical energy into another form of energy, such as light, heat, sound, motion and magnetism.

The heart of a load is almost always a coil of wire of some sort. Electricity travels rather effortlessly along a straight wire. But when the wire is coiled, electricity has to work to get through. It's a little like when we take a walk. Moving along a sidewalk doesn't take a lot of effort. But if we come to a twisty path going uphill, we're suddenly working a lot harder. In fact, like electricity, we can create a lot of heat (illustrated by sweat) when we work hard. Since a load is usually a coil, I like to think of it as a spiral staircase for electricity. Running up a staircase creates heat.

In a wiring diagram, a load is usually shown as a squiggle. There's the spiral staircase idea again. There are lots of varieties of squiggles, but the idea is to show the resistance that electricity comes up against as it goes through the load. An exception is the motor symbol, which often doesn't show the squiggle. But you can mentally draw it in.

Zone Valve

A load we see very often in hydronic control circuits is the motor of a zone valve. A zone valve has two completely separate parts. There's the body, where the water flows through. And there's the powerhead, where the electricity goes. Of course the motor is in the powerhead.

Let's look at the valve body for a moment. The opening inside the valve is the port. The amount of water than can flow through the valve is called the flow rate, or cv rating. The definition of cv is “gallons per minute (gpm) at a pressure (called pressure drop) of 1 pound per square inch (psi) at 60 degrees F.” A typical flow rate is 3.5 cv. That means that at 1 psi and 60 degrees F, 3.5 gpm can flow through the valve.

Of course there are larger capacity zone valves. The same valve with a larger port can be larger capacity. But no matter how large the port or capacity, the valve can have only as much water going through as the external piping will allow.

A valve is also sized by the diameter of its pipe connection, such as 1/2 inch or 3/4 inch. Different-sized valves may be identical on the inside, depending upon their cv rating. Different-sized pipe connected to them, though, may make a difference.

Close-off rating is a measure of a valve's ability to close against water flow. The rating is given in psi. If the flow of the water is stronger than the close-off rating, the valve can't close smoothly or completely. The close-off rating depends a lot upon the valve's type of closing mechanism. For example, the turning action of a cartridge is much stronger than the swinging-gate action of a ball or flapper plug. The most common cause of close-off problems, though, is oversized pumps.

The electrical part of the valve is called the operator or powerhead. It contains the valve motor. In zone valves up to about 1 inch, the powerhead and the body are contained in the same unit. Larger valves may have a separate motor, sometimes called an actuator. When the motor is separate, a linkage may be required to connect it to the valve body. If it is a “direct coupled” actuator, no linkage is needed.

Regardless, no water comes in contact with the electrical portion of the valve. When you look at an ordinary zone valve, notice that wires go only to the top part of the valve and water goes to only the bottom part.

A typical zone valve is a two-way, straight-through valve. The water comes in one side and leaves the other. Nothing much happens in between. The valve serves as a gate that opens and closes.

Mixing valves are three-way or four-way valves. Water of different temperatures comes into the valve, mixes together and leaves the valve at a desired temperature.

A diverting valve is also a three-way valve. The water enters the valve from one direction and leaves in two separate directions.

Zone valve noise or water hammer is usually caused by one of two situations. One is the “bigger is better” approach to pump sizing. An oversized pump is likely to cause noise when the valves try to close, especially if only one of several valves has been opened.

The other common cause of water hammer is valves installed backward. If there's an arrow on the valve body, it's pointing the way the water is supposed to go. When analyzing an existing job, don't forget the possibility that the water is not going the way you think it is.

Making The Circuit

Now let's put a zone valve in a circuit. Remember that, for a circuit, we need a power supply, a switch and a load. The zone valve is the load. The switch is probably a thermostat. Each zone valve has its own thermostat. The power supply is a transformer.

One transformer can be the power supply for more than one switch (thermostat) and load (zone valve) combination. The much-debated question is, “How many zone valves can you put on one transformer?” Some say three. Some say five. Some say seven or more. How many do you think?

The answer is, “It depends.” It depends on the VA rating of the zone valves and transformer. A transformer comes with a VA rating. Let's say ours is the very typical 40VA. “VA” means volts multiplied by amps. Think of VA as a measurement of capacity like ounces. So let's say the transformer is like a pitcher that holds 40 ounces of any liquid (your choice!). That's a given - it's just how much it holds. Now the question is how many zone valves can we “fill” with that 40VA transformer. Let's go back to the pitcher. How many glasses could we fill with the pitcher? It's easy to see that “it depends” upon the size of the glasses. Let's say that the glasses will each hold 8 ounces. The math then is simple. We'll just divide the 40-ounce pitcher by 8 ounces. That gives up five glasses that can be filled.

Wait now. What's the VA rating of a zone valve? That number isn't given anywhere on the device or in the literature! But we can handle this. V = volts and A = amps. We know the zone valve is 24V. And we can look on the zone valve or in the literature and find the amp rating. Let's say the amp rating is 0.32A. When two letters are side-by-side like VA is, we multiply the two. So 24V x .32 A = 7.68 VA for one zone valve. Divide the 40VA transformer by the 7.68 VA zone valve, and we get 5.2 valves. Rounded down to a whole valve, the answer in this case is that five zone valves can be used on one 40VA transformer.

Some of you may have some yeah-buts about that:

  • “Yeah-but, aren't you supposed to have a margin of error?”

    Yes, it's good not to cut things really tightly. However, unless the wire runs are extremely long, and if you're sure there's nothing else wired to the transformer, it's OK.

  • “Yeah-but, what if there's already something else on that transformer, like maybe a door bell?”

    You have to subtract the VA of that something before you figure how many zone valves.

  • “Yeah-but I've been using six zone valves and they work just great. Then you haven't yet met up with the 'coldest, darkest night' factor. Why do transformers wait until the middle of the coldest, darkest night to go out?”

    Because that's when all the zones finally all call at once. Until then, only a few have called simultaneously.