A mysterious caller asks a question and Carol answers.

The phone rings.

“Hey, I want you to do something for me,” said a louder-than-usual gruff voice. “I saw your column in PM magazine. Send me some of those free books and tapes.”

I chuckled, certain that this was one of those guys who like to kid around. “Well, the books and tapes aren't exactly free.”

“No, I mean it. How you gonna sell anything if you don't give anything away?”

“I see your point. But the stuff's not free.”

“Well then, I didn't want it anyway. But you gotta help me get something straightened out.”

“What?”

“You're gonna tell me how to use a three-wire thermostat with a two-wire valve.”

“A three-wire thermostat with a two-wire zone valve?”

“You're not listenin' to me. I got this situation...”

“OK, I'm all ears. Tell me about it.” I glanced at my watch. Lots of things to do today. This was looking like it could take awhile.

“I need to make this three-wire thermostat work on this two-wire zone valve.”

“Why?”

“Listen to me. You obviously don't understand consumers these days. They don't want to pay for what they don't need. He doesn't need another thermostat. I just sold him a new zone valve, and he doesn't want to pay for anything else. He's not made of money, you know.”

I was wondering how he got this mismatched valve and thermostat.

“OK. Tell me this. Did you replace a three-wire zone valve with a two-wire zone valve?”

“I already said yes.”

“Could you tell me why?”

“I couldn't find any three-wire zone valves, so I sold him a two-wire.”

I paused, trying to absorb this.

“You with me?” he barked.

“Yes, I'm here. And so you're trying to make it work with the existing three-wire thermostat?”

“You got it.”

“Did you offer him a new stat?”

“Nope.”

“Maybe your customer would like a new modern thermostat, and you could put in a two-wire.”

His voice had steadily gotten louder and louder. By now he was shouting at me.

“I already said he doesn't want to waste his money!”

I took a deep breath. “Hey, just a minute. I can help you with this. But before we go any further, would you please lower your voice?”

“I'm always loud. That's how I am.”

“Well, for this phone call, could you lower your voice a bit?”

Silence. Silence. Click.

I was stunned. He hung up because I asked him, very nicely in fact, to lower his voice? Well, that was a first. I felt relief that the phone call was over, and I could go back to what I was doing, not that I remembered what it was at the moment. At the same time a felt disappointment - I wouldn't get to give the answer and feel all good inside because I helped someone out. And yes, I'll admit, I felt a little residual fear. The rep in me was saying, “What if he doesn't buy my product?” Wait a minute, I reminded myself; he only wanted stuff for free, so that part's irrelevant.

But I still want to give the answer. So here it is.

Yes, you can use a three-wire thermostat with a two-wire zone valve. Let's look at how each operates.

Down To Business

A two-wire zone valve is “power open/spring close.” That means that when you give it 24V through its two motor wires, the valve opens. When you remove the 24V, a spring closes the valve. If there are four wires, use the two that you can follow to the motor. Ignore the other two, which are for the end switch.

The purpose of the two-wire thermostat is to let 24V flow to the zone valve if there's a call for heat. It's a two-wire because only two wires go to it. One wire connects the thermostat R terminal to the transformer. The other wire connects the thermostat W terminal to the zone valve. Another wire completes the circuit between the zone valve and the transformer. It's a perfect set-up. A call for heat causes the thermostat's internal switch between R and W to close. This lets electricity flow from the transformer to the valve motor, which causes the valve to open. Then hot water can flow to the area that needs heat.

When there's enough heat, the thermostat switch opens. Remember, a switch is like a drawbridge: open means it turns off the flow of electricity, like an open drawbridge turns off the flow of traffic over the bridge. No electricity to the zone valve means the motor doesn't hold the valve open. A mechanical spring closes the valve.

Now if you have a three-wire thermostat, you have three terminals and a wire for each: W, R and Y. Picture R in the middle. There's an internal switch between R and W. There's another between R and Y. When one switch is open, the other is closed, and vice versa. This means that when you turn one thing on, you turn the other thing off. This, by the way, is called single pole, double throw (SPDT) switching.

This three-wire switching is used in a three-wire zone valve. Instead of power open/spring close, the operation is power open/ power closed. It's like the door inside your house: you have to push (power) it open, and also push it closed. It's one or the other, and neither happens without power.

So if you put a three-wire thermostat on a three-wire zone valve, the thermostat switch between R and W closes on a call for heat. This allows electricity to flow to the valve motor, which opens the valve. When there's enough heat, the switch between R and Y closes. This allows electricity to flow to a different part of the valve motor, and the motor closes the valve.

Here's why you can use a three-wire thermostat on a two-wire valve. The three-wire thermostat has two switches in it, and you can use only one. That's the one between the R and W terminals. Don't wire anything to the Y terminal. Even though there's an internal switch between R and Y, you don't have to use it.

And here's why you can't use a two-wire thermostat on a three-way valve. A three-wire zone valve must be controlled by two switches, one to open it and one to close it. A two-wire thermostat has only one switch, and that's not enough.

Most thermostats are two-wire, and a few are three-wire. And there are some that can be either. The classic electro-mechanical mercury bulb is the T87 round. When the thermostat alone comes out of the box, it's a two-wire. There's R and W and that's it. There's a heating back plate available. Put that on the back of the thermostat, and then there are three terminals. This isn't the same as the thicker heat-cool subbase that allows you to do both heating and air conditioning. The heat-cool subbase has four terminals: R, W, Y and G. There are three switches: R-W for heating, R-Y for the air conditioning compressor, and R-G for the fan.

And there are some brand new electronic thermostats that can be any of the above. Nothing needs to be added. You just go into a simple installer setup and tell the thermostat what you want it to be. Change your mind? Just reset the stat.

So, yeah, in answer to my phone caller's question, that's how you use a three-wire stat to control a two-wire zone valve. But I want to talk about something else. Why assume that homeowner doesn't want an updated thermostat? Why not show him one designed for this century instead of last. Sure, customers don't want to waste their money. But getting something new, something that looks good and does fine things, now that may be the best offer they got this week!