Recently I wrote in this column that every component in a controls circuit is a power supply, a switch or a load — no other possibilities. And that means that every control, hydronic or otherwise, is one of these three. For example, a transformer is a power supply. Thermostats and aquastats are switches. Motors are loads, and that means that zone valves and pumps, which contain motors, are loads.

There is a rule that there can be just one power supply and just one load in a simple circuit. But several simple circuits can share a common power supply. When we do this, the combination of the simple circuits together is called a parallel circuit. We frequently wire several zone valves or zone pumps to one transformer in a parallel circuit. Again, the transformer is the power supply. The zone valves or zone pumps are loads. Each zone valve has its own switch, which is a thermostat.

Get a room full of plumbers together and ask, “How many zones valves can we put on a transformer?” Answers fly out with absolute certainty: “Three, five and finally, it depends.”

It certainly does depend. It depends upon the size of the transformer and the zone valves. Then what?

Let’s use an analogy. Say you have a pitcher of water and you want to know how many glasses you can fill. Right off you know to find out how big the pitcher and glasses are. Let’s say the pitcher holds 48 ounces, and the glasses hold 8 ounces each. You instantly know then that the pitcher can fill six glasses. How did you get that answer? You divided 48 ounces by 8 ounces and got six.

It’s the same set up for a transformer and zone valves. It depends upon their sizes. The transformer has a limited amount of electricity. How much does each zone valve need?

Transformers are measured in VA. The most common control transformers are 40VA and 20 VA. The number is right on the transformer, on its box, and in the packing literature. VA stands for volts multiplied by amps. VA for transformers is a size measurement like ounces is for a pitcher of water.

That was easy. So now we just need to find the VA of a zone valve. Look all over the valve, look on the box, study the packing literature, and you will not find the VA of a zone valve. It is not there. So what to do? Well, VA stands for volts x amps. Volts and amps are printed on the valve, on the box and in the literature. A very common zone valve is 24V and .3A. (Be sure to notice the little decimal point dot in front of the 3.) Since volts x amps = VA, multiply 24V by .3A, and get 7.2VA.

We’re getting close to how many of these zone valves we can put on a 40 VA transformer. Returning to the pitcher and glasses analogy, to know how many glasses could be filled, we divided the 48 ounces of the pitcher by the 8 ounces of each glass and got six. Let’s do the same arithmetic for the transformer and zone valves. Divide the 40VA of the transformer by the 7.2VA of a zone valve and you get 5.555. We can only do whole zone valves, so the answer is that five of these zone valves can be wired to a 40 VA transformer.

But not all transformers and zone valves are the same. How many of these zone valves could we put on a 20VA transformer? Divide 20VA by 7.2VA and we get 2.77. Rounding to the whole zone valve gives us two zone valves.

There are common zone valves that are .8A. How many of those could we put on a 40VA transformer? First multiply .8A by 24 V to get 19.2VA. Divide 40VA by 19.2VA and we get two zone valves. How many of these could we put on a 20VA transformer? One.

What if you have six 24V .3A zone valves and two 40VA transformers? We already found that only five of these valves go on one 40VA transformer. Some people would try to wire the two transformers together to accommodate all six valves. But then you would have to know how to wire the two transformers together, which most people don’t know. The simple solution is to wire three zone valves to each transformer. That gives the insurance that if one of the transformers happens to burn out, at least three of the zones would still heat.

Speaking of transformers burning out, why is it that transformers always seem to burn out in the middle of the coldest darkest night? Before answering that, I’ll remind you that plenty of people have successfully put more than five 24V .3A zone valves on a 40 VA transformer.

How do they get away with it? The valves have not yet all called for heat at once. Perhaps they won’t, until the middle of the coldest darkest night. When they all call for heat simultaneously, they try to pull more electricity out of the transformer than the transformer has, and the transformer burns. That’s plenty of reason to use two transformers when there is a doubt. Better yet, do the simple math and be sure!

Carol Fey is a technical trainer who has been in the HVAC industry for over 25 years. You can find her books and DVD at www.carolfey.com. To see her adventures while a heating mechanic in Antarctica, go to www.carolfey.blogspot.com.