Recently, I had a call from a contractor, saying he was having issues with a unit making a lot of noise and the blower “bouncing off and on”.
The furnace had been in for a few years and, all of a sudden, developed this operating condition. We discussed the usual things to check, polarity, voltage, grounds, etc. I asked if anything new had been added or changed on the system? The only thing that had changed, as far as the contractor knew, was that the homeowner installed one of the new NEST Thermostats. These are those fancy thermostats that give the customer access to it from their smart phones or the internet so they can “play” with the setting even when they are not home.
In the instructions that come with the new thermostat, it boasts No Problem Wiring To A 4 Wire System. Well….. that is yes and no!
Depending on the control system in the equipment, it may work perfectly fine on the 4-wire hook-up. But, in some system it may need the C wire due to power stealing battery charging causing disruptions to the control programming. What happens is, when the furnace is At Idle, it Steals MillIamps from the control board to keep it operating and charged.
Again, on some system, this may or may not be a problem. Well, on the system our contractor was working on — it was a problem. When there was a call for Heat, and the thermostat was in a “charging mode”, it rattled the relays on the board, and the blower motor bounced off and on and actually sounded like thunder through the duct work.
All I want you to be aware of is that the newest technology is a great thing and can make our lives easier but it can also create problems. The solution here was simple, remove the “power stealing” aspect of the stat and “hard wire” it so it is powered and it worked just fine and so did the furnace and air conditioner it was intended to control.
(Thanks to John of Effective Air for providing the information for this post)