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I just bought a Nest thermostat and love it so far. I know it has great reviews and customer ratings anywhere I look online. However there are multiple discussion threads I have seen online (especially from a few years ago) where people are suspicious that Nest thermostats are harmful to HVAC systems- prematurely burning out circuitry or fan motors due to how it functions by supposedly switching the equipment on and off often.
So my question is, to anyone out there who is a bit of an HVAC expert and can speak with true knowledge on how these work, is there any truth to this concern? Whether using a C wire or not (using the power stealing method, I believe is what they call it)?
My house is only 4 years old, so obviously my HVAC equipment is quite new and should have many years of life in it yet, so I am a bit paranoid about doing anything such as using the Nest if it's going to cause any trouble at all.
And secondly, if there is a minor threat to the equipment when not using a C wire, would any chance of trouble be resolved by using a C wire? I ask because as I look at the wiring there is one unused blue cable that appears it would be the C wire- so I could probably hook it up (assuming it's hooked up on the other end, in the attic at the furnace itself).
Anyway sorry for being so wordy, and thanks in advance for any information anyone can provide!
And secondly, if there is a minor threat to the equipment when not using a C wire, would any chance of trouble be resolved by using a C wire? I ask because as I look at the wiring there is one unused blue cable that appears it would be the C wire- so I could probably hook it up (assuming it's hooked up on the other end, in the attic at the furnace itself).
The C wire provides power to the nest, how many wires do you have hooked to it and what are they for?
The nest has a rechargeable battery in it and without the C wire it will siphon off power to charge that battery but this only occurs when there is a call for heat/AC. Two problems with this, if it's taking too much power it may be hindering the operation of what that power was intended to be used for and secondly if there isn't enough cycles it may go dead.
They are walking a fine line with this, I know a few years back they had firmware upgrade because it was taking to much power. That resulted in lot of non functioning thermostats. If you have a C wire it should be connected.
The C wire provides power to the nest, how many wires do you have hooked to it and what are they for?
The nest has a rechargeable battery in it and without the C wire it will siphon off power to charge that battery but this only occurs when there is a call for heat/AC. Two problems with this, if it's taking too much power it may be hindering the operation of what that power was intended to be used for and secondly if there isn't enough cycles it may go dead.
They are walking a fine line with this, I know a few years back they had firmware upgrade because it was taking to much power. That resulted in lot of non functioning thermostats. If you have a C wire it should be connected.
I currently have 4 wires that were used by my old thermostat that I am currently using on my Nest. There are two others- a blue one and a black one- that were unused, tucked in the back that as I read about it now, seems one of them must be the C wire. I didn't know that at the time I installed the Nest.
I had read that without using the C wire, it can occasionally cycle on the HVAC equipment just to get power, and that frequent cycling like this can damage the equipment- I assume with the C wire connected there would be no concern over it doing anything like that?
I'm not in HVAC but I don't think you have anything to worry about. We have two Nests, one upstairs one down, and if anything I feel it protects the HVAC.
For example, it won't let you short cycle the system. If you turn the a/c on, then off, then back on again within 5 minutes it will delay the system to prevent damage.
They pretty much thought of everything. I can't imagine my life without Nest, and I wasn't sold on it until I got home from work and my DH had them installed. The rest is history.
The Nest thermostat actually charges its battery when the HVAC is NOT running. If an HVAC unit runs all the time the battery will not charge. The NEST Thermostat grabs a few miliamps of power to charge its battery when the switch in the thermostat is open; that is, not calling for the HVAC to turn on and run. When the switch is open there is a difference of potential (24VAC) across the switch. That's where the NEST gets its power from to charge the battery. When the switch closes and calls for the HVAC to run, the voltage across the switch drops to zero so there is no power available the charge the battery. This is one way it could be done; depends in how smart the charging system is. If there's no active current control and learning circuit it could be made much better.
Ultimately a thermostat is just an ON--OFF switch. HVAC units have built in protection to prevent short-cycling. So, the thermostat can switch on but the HVAC unit may delay for a bit until it's ready to run again.
The Nest thermostat actually charges its battery when the HVAC is NOT running. If an HVAC unit runs all the time the battery will not charge. The NEST Thermostat grabs a few miliamps of power to charge its battery when the switch in the thermostat is open; that is, not calling for the HVAC to turn on and run. When the switch is open there is a difference of potential (24VAC) across the switch. That's where the NEST gets its power from to charge the battery. When the switch closes and calls for the HVAC to run, the voltage across the switch drops to zero so there is no power available the charge the battery. This is one way it could be done; depends in how smart the charging system is. If there's no active current control and learning circuit it could be made much better.
Ultimately a thermostat is just an ON--OFF switch. HVAC units have built in protection to prevent short-cycling. So, the thermostat can switch on but the HVAC unit may delay for a bit until it's ready to run again.
Thank you for this explanation! I wondered if a lot of the speculation I have seen elsewhere is baseless, where people think that the Nest is switching the HVAC circuitry on and off frequently to get power and blowing out boards or somehow even blowing out fan motors. I wasn't sure how that would even be possible, when originally buying this I, too, thought it was nothing more than a smart on/off switch.
With that being said I do hope to get a C wire hooked up. As noted, I saw a black and a blue wire unused behind the thermostat, I will just need to figure out which one of those is in fact the one I need and hook it up.
With that being said I do hope to get a C wire hooked up. As noted, I saw a black and a blue wire unused behind the thermostat, I will just need to figure out which one of those is in fact the one I need and hook it up.
Most likely they aren't hooked up at either end, so just pick one and connect it at both ends.
NESTs were blamed for compressor failures when they first came out but I haven't heard much about that lately. If you're not going to use a "C" wire you're better off taking the nest back to where you bought it....it needs a complete circuit all the time to be (less) trouble free. I condemn more nests that any other thermostat thats 5 years old or newer. Needless to say, I won't sell them.
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