The article series listed here gives four solutions to providing a needed C-wire for a smart thermostat and includes examples of installations that worked easily, and one that did not work at all. While the Nest thermosatat will work perfectly-well without a C wire in some installations, in others it may have trouble maintaining a sufficient charge (of its internal battery) and will work more-reliably if you find or add a C wire to bring sufficient charging current to the thermostat. The C wire or "common wire" is needed to bring 24VAC power to some new wall thermostats like the Nest series. That's not always a problem but it can be. Without a C-wire the Nest can cause a furnace or boiler to short-cycle on and off mysteriously. The problem is that the heating or cooling wires connected from the furnace or air conditioner to your Nest thermostat provide power to the thermostat only when the furnace or A/C system is actually running.īut the Nest needs power all the time.
There was no "C" wire in these installations because the thermostat simply functioned as a very low-voltage on-off switch that had so little going on that it didn't need its own power source.īut when going to a new smart thermostat like the Nest, that device needs more power for its display and other features. Our photo shows a low voltage24VAC transformer mounted next to the line voltage (120VAC) electrical power switch for a heating boiler.Ī "missing C-Wire" or "no Common Wire" at the thermostat is a common problem I've run into on some older homes that had just two wires (usually red and white) running from a heating furnace or boiler to the thermostat. Connecting the C-Wire and How to Add one If It's Missing We also provide an ARTICLE INDEX for this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Sometimes the transformer is inside the HVAC equipment itself. The C-wire or "COMMON" wire in heating and cooling or heat pump applications is a source of 24-volt AC power needed by certain controls and also needed by some, but not all, room thermostats.Ģ4VAC is provided to the C-wire right from a 24-volt AC transformer usually found mounted in or near the heating furnace, boiler, or air conditioner's air-handler. This article describes the function of the C wire in heating and cooling equipment and gives four methods for adding a C wire if your thermostat needs one. We have no relationship with advertisers, products, or services discussed at this website. InspectAPedia tolerates no conflicts of interest.
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