An Olympic caliber relay for heating

 

By PAUL ROHRS,

contributing writer

 

Have you ever run into a situation where voltages were “butting heads” against each other, causing circuit breakers to trip or “letting the smoke out” of a transformer? You are saying to yourself, “Self, I am a mechanical contractor, not an electrician.”

 

I agree with that statement, but there may come a time in your hydronic career when you will need to troubleshoot or incorporate relays to isolate power or to send existing power where you want it, when you want it. A relay might be the answer to the questions you have about switching power.

 

A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. These contacts, whether normally open (NO) or normally closed (NC), can switch power or just close a circuit, providing continuity.

 

This article will talk about relays, explain why they are useful and give examples of how you can incorporate relays into your jobs to achieve specific results. For our discussion, we are going to talk about double-pole double-throw (DPDT) relays, mostly because I have the Visio stencils from Tekmar that make it easy for me to illustrate relay operation.

 

Figure 1 illustrates what a DPDT may look like. The type of relay you need may vary. There are several types, such as solid-state and ice-cube relays. Relays come in a wide variety of “poles and throws” as with a single-pole single throw (SPST), triple-pole triple-throw (TPTT) or the DPDT, which I already mentioned. To order the correct relay, you must specify the coil voltage. When the proper voltage is present at the coil, it becomes an electromagnet and pulls in the contactor arms. These arms should be viewed as connecting bridges that allow electrical current to the desired flow path.

 

Figure 2 is a mechanical representation of a DPDT relay. As this is a 24V coil relay, the coil needs to be energized with 24V (ac) on terminals 7 and 2. Terminals 1 and 8 allow us to switch power through the relay when normally closed (NC). Terminal 1 illustrates a 120V (ac) source, allowing power through the relay and exiting on terminal 4. Additionally, with the coil still closed, we can switch 24V (ac) power from terminal 8 on through to terminal 5. Again, this is while the coil is not energized. Upon energizing the coil, the normally open (NO) contacts now close and the 24V (ac) power that was exiting the NC terminal 5 opens; terminal 6 closes and allows the 24V (ac) on through. Conversely, the 120V (ac) power that was exiting terminal 4 now opens and can only exit via terminal 3, which is now closed. So, with the closing of a coil, we can switch voltages through a relay and make normally open contacts close.

 

Maybe you want to turn on a light, start a timer, actuate a solenoid or switch heat sources when your thermostat calls for heat. You can accomplish this with a simple and inexpensive relay. For instance, we just finished updating a system for a gentleman with a multi-zone system of radiant floor heat. This system was being heated by a residential water heater and hadn’t worked properly from its inception.

 

A standard residential water heater has two 4.5kw elements that fire non-simultaneously, as an upper and lower thermostat controls power to each element. This homeowner was not getting enough heat when the temperature was below 32 F, as the 15,358.50 Btu fell short of the load. 1kw = 3,413 Btu, so one 4.5kw element 5 3,413 Btu =15,358.50 Btu. The homeowner called an electrician, who wired the elements for simultaneous firing, but there was still not enough heat at 30,717 Btu.

 

Now it was our turn. After a simple heat loss and budget meeting, the homeowner contracted us to install an electric boiler. The water heater did not have contacts to receive a call for heat, so I needed to provide them to the boiler. What to do? The answer is a simple DPDT (ice-cube) relay to send a call for heat to the boiler TT. Thermostats in the residence were feeding a signal to contactors that would then energize 120V (ac) to a space-heating pump. Why not use that to our advantage?

 

I bought a simple relay with a 120V (ac) coil. Anytime the thermostats called for heat and sent power to the space heating circulator, that same power would close the 120V coil on my relay and make the normally open contacts close, providing the necessary switch I needed to signal my new electric boiler. See Figure 3.

 

The boiler's internal controls would tell the boiler whether it needed to send power to its elements and could run continually, until the water temperature reached its setpoint or satisfied the call for heat. As long as a thermostat was calling for heat and as long as the circulator was energized, the boiler had its call for heat. You will note that I am not switching power in this scenario. I am only using terminals 1 and 3 to complete a boiler TT circuit. When the coil closes and the normally open terminal 3 closes, it allows me to have continuity and to complete the call for heat to my boiler.

Be sure to observe the coil voltages and contact ratings. Whether you are using 24V, 120V or even 240V to energize the coil, check and see the maximum amp rating for the power you want to switch. Improper or insufficient power can damage the coil. If the voltage to the coil fluctuates, it can make the relay “chatter” or arc, making it susceptible to damage.

Handing off a baton in any competitive relay requires teamwork, and the general-purpose relays discussed today can pull a team of hydronic components together so that proper operation and switching can occur. In any successful relay, however, cooperation is essential and every member or, in this case, every component, must work together in unison to achieve their goal.

 

I hope these illustrations have been helpful in navigating the current as the coils open and close. Next month, I plan to pass off the baton to heating professionals across the country and  illustrate how to navigate hydronic systems with the help of relays.      

 

Paul Rohrs welcomes your comments. Contact Paul at paul@biggerstaffradiantsolutions.com.