Digital Thermostats 101

tsatCustomer: “My ski jacket zipper thermometer says 70 degrees, the new thermostat you installed says 71 degrees…something’s wrong with it”

Contractor: Why me?

While the infamous $10 ski jacket thermometer is about as accurate as divining rod, you may be surprised to know a digital thermostat’s display may be slightly… ‘optimistic’

There’s more to digital thermostats than you may realize.

TRIACs, power stealing, and temperature display rounding. We are going to cover what you need to know to apply, install, and troubleshoot them correctly.
TRIACs, almost every electronic thermostat has them, and they can cause a lot of confusion when troubleshooting. Let’s start with electronic thermostat history. First generation electronic thermostats used small mechanical relays to energize the Y, W, and G outputs. if you listened carefully you could hear a faint click as they were energized. If you wanted to check if the thermostat was calling for cooling you would place your meter leads across the “C” and “Y” terminals. If the meter read 24 volts, the cooling output was energized. Checking the other outputs was the same, 24 volts meant the output was energized, zero volts meant it wasn’t.

The relays worked great, but they were big. (you may remember the old thermostats were huge). Along came the TRIAC or “Triode for Alternating Current” Triacs were small, reliable, quiet, and cheap and soon replaced the mechanical relays in thermostats. They’re small, reliable and quiet because there are no moving parts.

triacSo what is a TRIAC? It’s basically an electronic version of a relay, kind of. A TRIAC is a resistor that changes resistance when power is applied to its gate. When no power is applied to the gate, the resistance is very high and current can barely pass through it. But, apply power, and current can pass through easily.

The confusion comes into play when you’re checking the outputs with your meter, if there is no load on a TRIAC it will “bleed” voltage out even though it’s not energized. There won’t be enough current to energize a load, but there will be 24 volts, some refer to this as “Ghost voltage”. Many technicians have replaced thermostats because they thought the outputs wouldn’t turn off, in reality they were seeing ghost voltage. Just remember, when you check a thermostats output always make sure there is a load on the output by leaving the equipment wires connected to it.
Next on our list is the power stealing thermostat. You can identify these easily, they have an “R” terminal for the 24vac power supply but no “C” terminal for common. How do they do that?

The thermostats “steals” the common through the unused circuitry of the opposite mode. In other words, if the thermostat is calling for cooling, it steals the common through the heating circuit, if it’s calling for heat, it steals common through the cooling circuit. Remember, it’s only using common to provide power for its programming and to activate TRIACS.

How can you get into trouble with power stealing stats? The most common way is to try and use the thermostat in a heating only application where there is no cooling circuit to steal from. The second most frequent mistake is using the “RC” terminal for cooling, and the “RH” for heating, with two separate power sources. An example of this would be a hydronic heating system’s oil burner “T-T” terminals connected to “RH and “W”, and the cooling air handler connected to “RC” and “Y”. Each piece of equipment has its own control transformer so there is no shared common. In this situation you would either need to run a common wire to the cooling unit’s transformer, or use a 24vac isolation relay to control the heat. The coil of the relay would be connected to the thermostats “C” and “W” terminals, and the normally open contacts would be connected to the boilers “T-T” terminals. In this application the relay coil would provide the path to steal the common.
The last thing you should know is how an electronic thermostat displays room temperature. If the room is 70, then the display should read 70, right? Well, not necessarily.

You see, when you or I think about temperature we think in whole degrees like 68, 72, or 75, but a thermostat thinks is fractions of a degree. What you see as 70 degrees, a thermostat sees as 70.8 degrees, or 70.1 degrees, but it only has a two digit display, so it rounds to a whole number. Here is where it gets slightly confusing. You would assume it would round toward the nearest tenth of a degree, so would I, but most dont. they round toward the temperature set-point. If the room is 70.1 and the thermostat is set at 71, the temperature is rounded to 71. It does this so the home owner doesn’t see a huge difference between their chosen setting and the room temperature and think something is wrong with the system. This can cause problems when a home owner has a thermometer in their home that reads to the tenth of a degree. Always remember that although the thermostat reads and rounds in whole degrees, it controls to a much closer tolerance.

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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