24 volt transformers

Here is one for you, Carol. I had to trouble shoot a system today. The system has 13 zone valves. Each valve is in the room it serves. So, what I have in the boiler room is a pair of wires hooked to a xformer. The wires simple run through out the building, connecting to all the zone valves in a parallel circuit. Each zone valve is switched with a t-stat in the room. As these were Honeywell zone valves, I decided that I needed at least a 100VA xformer to drive them all when they are all open at the same time.

Well, the xformer was not 100VA. It has slowly melted down over the years. They have always had trouble with some zones not working sometimes, and then they seem to just start working again. I am sure that as more and more of the stats called for heat, less and less power was available for each valve. Last night I have the people turn off all the stats except only the ones they needed, and the heat did work in those zones.

So, here is the questions. As the xformer becomes over loaded, what happens to the valves? Do some of them remain open and others not? Or do they all begin to loose the ability to stay open? Will each valve see a drop in voltage as more call, or is it a drop in amps? Can this burn out the motors as well as the xformer?

Now, I had some difficultly finding a 100VA xformer. Is it possible to connect 2 of them in parallel in order to get the required VA?

Dave in Denver

The cool thing about this situation is that the zone valves don't all have to go on the same transformer. You can use regular 40VA transformers. And there's no need to connect the transformers to each other. You can put five .32A zone valves on one 40 VA transformer and five on another. One good reason for not connecting the transformers is that if one burns out, the other isn't affected--the users still have heat. And of course 40VA trasnformers are easier to find and cheaper than the big ones. But if your situation gives you just one set of wires from all of the transformers, yes you can connect a couple transformers together.

When a transformer is overloaded, the damage happens to the secondary side of the transformer. The usual result is that the winding burns, and there just isn't any power available to operate any valves anymore. The valves should be perfectly OK except that they can't get the power they need. The situation you're describing where some work and some don't is a new one for me.

The valves are pulling "VA" from the transformer. That's volts multiplied by amps. It also happens to be exactly the same as watts. For some mysterious reason, when the load is inductive, the measurement is called VA, and when the load is resistive, the measurement is called watts.

hooking transformers together

There's one little trick to that -- be darn sure that the secondaries are hooked up so that they are in phase. Otherwise you will fry one or both of the your nice new transformers...

It seems that 3 valves is the common belief, but it all depends upon the VA of the transformer and the valves.

Here's the simple math to use with any zone valves and transformers.
1) multiply the amp draw of the valve by its voltage to get the VA of the valve (e.g. .32A x 24V = 7.68VA)

2) divide the VA of the valve into the VA of the transformer ( e.g. divide 7.68VA for the valve into 40VA for the transformer = 5.2)

3) the result of the division is how many zone valves you can put on that transformer provided there's nothing else wired to that transformer. In this case it's 5. But try the math for a .8A valve. 'Way different answer!

xfmrs

Hi Dave. Yes, drawing too much current from the transformer will cause the voltage to drop. Measure the no-load voltage. I'm guessing around 28 volts. Keep adding zone valves in parallel until the reading drops to near 22 volts- that's how many I would connect. They should work down to near 18 volts but, why tempt fate. This should also protect you from in-rush current if they all happen to call for heat at the same time. If you have too many on-line there will not be enough voltage left to open the extras. Some of the others may also drop out due to low voltage.

To check the phase: First check that both transformers have 24 volts at the secondary. Next, connect the secondary commons together with a wire and place your volt meter across the other two terminals. If you read 0 volts, you are in phase and all is OK. If you read 48 volts you will need to reverse the polarity of one of the transformers.

Good luck.

\"In Phase\"?????????

What exactly do you mean Jamie? Don't wanna fry anyomr ethings than I have to. We were always taught that you don't power more than 3 zone valves off of a 40 VA. I guess that was a very safe fudge factor. Mad Dog