Got 24 Volts?

Got 24 Volts?
control voltageJust about every HVAC system out there has a low voltage control circuit, and just about every one of them is 24 volts A/C.
You name it, it probably uses it: gas furnace, oil furnace, condenser, heat pump, rooftop unit, chiller, water source heat pump and zone valves use good old 24vac. Control voltage is also one of the first things you check when you’re troubleshooting.

How many times have you muttered to yourself; “let’s see, I have 24 volts between R and C….”
Here’s a question for you; is 23 volts good? How about 22 volts? Is 18 volts enough? Is 27 volts too much?

Answer; anything less than 24 volts is generally not enough, and it’s unlikely you’ll ever have too much voltage.
Back in the old days, before electronic controls, HVAC systems used electro-mechanical control systems with relays, contactors, and pressure switches. If the control voltage was too low the loads (contactors and relays) would chatter, make a loud buzzing sound, or simply not pull in.
Today, things are different. Since the 1980’s many HVAC units began using electronic controls. Electronics need the proper voltage to function properly.
Feed 18vac to a circuit board that requires 24vac and things go downhill quick. If the control operates at all, it’s going to act like it downed one too many margaritas (minus the projectile vomiting)
The control may throw random, irrelevant fault codes. It may cycle on and off repeatedly. It could do any number of things…everything except work properly.
This means it’s more important than ever to select the right primary tap on the control transformer. If the system is rated at 208 line voltage you must use the 208 tap, not the 230 tap.
While we’re talking about transformer taps, let’s take a quick look at the primary voltage. Always check the supply voltage with the heaviest loads energized like the compressor or supplemental electric heat. Why? Because a drop in the supply voltage results in an equivalent percentage drop in the secondary voltage. Think; low in = low out.
Something that’s often overlooked when troubleshooting is the operating voltage, both primary and secondary. I always take voltage readings with the equipment idle (not operating) and an operating voltage (equipment running in various modes). Remember, just because the control voltage is ok when the system isn’t running doesn’t mean its ok when it is.
This is especially important when troubleshooting intermittent problems; you know, the ones that never raise their ugly heads while you’re on site, but cause countless call-backs.
Measuring the operating voltage in every conceivable mode combination is important if you’re chasing an intermittent problem. Check the voltage with the heat running along with the humidifier. If the system is a heat pump, check the voltage with the compressor on, with the supplemental heat energized, and as the system stages in and out of a defrost cycle.
Lets take a look at a few causes of low secondary voltages that I’ve run into over the years-
• A failing water solenoid valve on a bypass humidifier.
• Shorted heat pump defrost thermostat.
• Binding zone damper actuator.
• Shorted 2nd stage heat coil on a two stage gas valve.
• Failing spark pack on a rooftop unit.
• Excessive thermostat wire length.

Take the failing humidifier solenoid as an example.
The control board had 26 volts feeding it. The control energized the solenoid valve, but the high current draw caused by the shorted solenoid coil caused the voltage to drop. Once the voltage dropped to 18 volts, the board de energized the humidity output. Once the solenoid was de-enrgized, the voltage returned to normal and the sequence started again. This all happened before the fuse could blow.

What were the symptoms of this service call? The humidity in the home was very low, and the humidifier solenoid was cycling every minute or so. What did the first technician who went to the site do to fix the problem? He replaced the control board.
Now the service company was in the position of explaining to the customer why he was billed over $500 for a new control board when the problem turned out to be a $40 solenoid valve. What was the financial resolution? The service company ate the $500 charge for the control board.

Another call involved a rooftop unit that had been in place and operating for five years without an issue. When the unit’s control board started indicating random fault codes a technician replaced it, only to have the problem continue. What was the real cause? Someone had added over forty feet of thermostat wire when relocating the thermostat.

I ran across a heat pump that would not heat the home. Every time a technician visited the home, the unit would be locked out. He would reset power to the unit, and it would work fine. Finally the technician reset the unit and waited until the heat pump entered a defrost cycle. Once the defrost thermostat made, the control voltage would drop, causing the control to lock up. What was the culprit? The defrost thermostat had moisture in it causing it to ground out the control circuit when it closed.

What did all of these have in common? None of them resulted in a blown control fuse. Why? Because the sudden drop in control voltage caused the electronic control board to de-energize its outputs before the fuse could blow. This is why these can be such a pain to troubleshoot.

Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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