HPAC Magazine

Ones and zeros

How to incorporate relays into your design strategy.

February 1, 2015   Adam

Since this is the introductory Ones and Zeros article, it is best to go back to when the controls world started to get a little more interesting with the introduction of the electromechanical switch, or the relay as it is better known. It is a device that really did change the world. While simple, it can still make or break your control strategy.

Generally, if you know the details of how something works you are better able to visualize when you design with it. The same is true of relays.

Love it or hate it, the relay is here to stay. Binary code, which is just a series of ones and zeros, is the base language of all computers and can be done with relays. In fact, the first computers were built with only relays. Scientists would use thousands of relays turning on and off to do simple calculations such as adding and subtracting. Computers at their very essence are just billions of tiny relays making ones and zeros into words, moving them back and forth, turning those words into decisions and running Google. None of this would have been possible without relays.


The relay is an electromechanical switch with two sections. In its simplest form, a coil and a switch. When the coil is energized the switch moves, creating a contact closure moving the electrical signal from one point to another. The coil is the portion of the relay that is the electrical and the switch is the mechanical part. When voltage is applied to the coil it creates a magnetic field, which pushes or pulls the switches (see Figure 1).

Think of the coil as that experiment that we all did in grade three science; the one in which we wrapped a nail with wire and connected each end of the wire to a battery. By doing this we created an electromagnet that could be used to pick up pieces of metal. The metal pieces would fall off when the battery was disconnected.

The coil of the relay works with these same principles. When we apply power the switch moves one way and when we take the power away the switch moves back to where it started.


Relay coils can be made to energize from DC voltage or AC voltage. DC voltage relays are generally used on circuit boards. AC coil relays are the main choice in our industry. The primary reason is that those voltages are readily available in the boiler room. These voltages are 24VAC and 120VAC.

The next section of the relay is the poles and throws. I am sure you have been in a boiler room and heard the expression double pole double throw. That is a very common relay type and refers to the relay having two switches, each having a normally open (N/O) and normally closed (N/C) connection on each switch (see Figure 2,2a).

Pole basically means the number of common terminals there is on the relay or the number of switches it has inside. Throws mean the number of directions the switch can move. If it is a single throw, the relay switch can only move in one direction and generally only has an N/O terminal. If it is a double throw the switch can move in both directions and the relay has an N/O terminal and an N/C terminal. There are different reasons that we would use the N/O terminal and not the N/C terminal. We will elaborate on that but this gives us a good understanding of the inner working of the relay.


If you take away only two tidbits from this article let it be these. Firstly, coil voltages can be different than pole voltages and that can create some confusion. Secondly, remember that a relay can have different voltages on different poles.

To clarify, start with a 24VAC double pole double throw relay. Remember that the 24VAC is the coil voltage rating. When we apply 24VAC the relay energizes, closing the N/O terminal and completing the circuit from the common terminal to the N/O terminal. At the same time the N/C connection opens stopping the connection from the common terminal to the N/C terminal. The pole can only connect to one terminal at a time. Either the common connects to the N/O terminal or the N/C. A pump could be running on the first pole, which would use 120VAC and a valve could be running on the second pole, which would use 24VAC. This is also a very good reason why more than one pole relays are used so the same coil can be used to push and pull multiple switches inside the relay. Otherwise we end up with a real mess of relays in the boiler room.

So, where do we use relays? Relays can be used to take a 24VAC coil voltage to make a pump turn on. They can be used when the pump output on the control is not big enough to run either that pump or you are trying to run two pumps at the same time. As mentioned previously, coil voltages do not have to be the same as pole voltages. So, in this case we can apply 24VAC to the coil and run 120VAC through one of the poles to make a pump turn on when we energize the coil. There may be many devices to energize the coil, like an end-switch on a valve, because in that case the end-switch would not be able to handle the current needed to run the pump so a relay is used (see Figure 3).

Relays are also used in cases where we may need to turn on a valve with 24VAC and a pump on with 120VAC at the same time. We could take the coil signal from a control, but the control only has one output. In this case, we would use the control output to energize the coil to turn both the valve and the pump on at the same time. This is where most of the “slip ups” in relay logic occur (see Figure 4).

The most difficult part of relays would be using them for interlocking or, in other words, turning one device on when the coil energizes but at the same time turning another device off. An example of this might be in DHW. Keeping in mind that in different regions system designs differ, but in situations where both the system pump and the DHW pump are able to pump through the boiler some controls can do this automatically, while most non-condensing boilers cannot. This control scheme must be done externally. In this case the system pump would be normally on if the control needed it on but if the DHW calls it will turn off the system pump and run only the DHW pump, essentially prioritizing it (see Figure 5).

This is a little more complicated than it looks. The line voltage for the system pump actually comes from the pump output on the control. Two things have to happen for the system pump to be able to come on. Firstly, the system pump output from the control must turn on and secondly the aquastat cannot be calling. You will notice that we are using the N/C terminal for the system pump. This is because we want the system pump to “be able” to come on when the aquatsat is not calling. If the aquatsat calls, the N/C terminal will open, turning off the system pump and the N/O terminal of the second pole will close, turning on the DHW pump

In the heat pump world there are complicated strategies that always require relays. An example of this is where an installation has only one heat pump and one boiler. In winter there is generally no need for cooling, so using the heat pump and the boiler together is easy. In the summer when the heat pump is used for cooling the heat pump cannot do both heating and cooling at the same time.

While the heat pump is cooling there is no way to make hot water. In this case a relay would be used from the reversing valve or cooling contact on the thermostat, to direct the heating call to the boiler. In the summer or when the heat pump is cooling and we get a heat call, the relay will turn the boiler on instead of using the heat pump in heating.

To add another twist, the boiler needs dry contact so the 24VAC signals have to be isolated from the boiler. A relay will be necessary to accomplish this as well. A dry contact is essentially just using the switch of the relay without adding 24VAC or 120VAC to the common terminal.

We would use the common and the N/O terminal so that the device we are connecting to sends its own voltage or signal through the relay. In this case we would most likely either blow the transformer on the boiler or the transformer feeding the thermostat if we did not isolate and make a dry contact (see Figure 6).

At the end of the day control options are getting better at offering solutions and outputs for all applications but there are always those contractors who like to push the envelope and do something slightly different. For that, luckily we have relays. <>

A graduate of Southern Alberta Institute of Technology (SAIT), Curtis Bennett, C.E.T., is operations manager and a product developer at HBX Control Systems in Calgary, AB. He can be reached at curtis@hbxcontrols.com. Look for more electronics and controls articles by Curtis in future issues.