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Thread: Power Stealing Thermostats

  1. #1
    HVAC Thermostat Specialist

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    Power Stealing Thermostats

    http://www.ecobee.com/blog/the-probl...ower-stealing/

    Check out this blog by Ecobee. It really explains the problems with power stealing thermostats like NEST.

    Update amazon now offers a free smart home consultation

    Q: Recently, I’ve been hearing a lot more about ‘Power Stealing’ as a power source for smart thermostats – is this something ecobee does?

    A: Good question. We’ve seen a lot of discussion about this recently because of the extreme cold weather conditions we’ve been experiencing. To quickly answer your question, no ecobee does not using power stealing as a source of power. Here’s why:

    Power stealing is a way of powering a thermostat using the circuits that turn on and off your HVAC system, rather than using a dedicated power wire or a Common wire (referred to as a C-wire). As the term implies, it involves using these circuits in a way that they were not designed to operate. There’s a lot of history and literature on power stealing dating back at least 20 years and for a while it was used for simple thermostats. We looked at this technology very carefully when we designed ecobee. However as we discovered through our research, it is very difficult to do reliably especially given all the things a smart thermostat needs to do.

    Power stealing is designed to charge a thermostat battery which then powers the thermostat. The challenge with power stealing is you can only steal a very small amount of power. Steal too much and you will turn the equipment on when you don’t want to, steal too little and the battery and your thermostat runs down and goes dead. Obviously this is not a reliable way to power a thermostat. The other problem is that batteries lose about 20% of their capacity every year, so as every year passes, the buffer provided by the battery goes down and your chances of failure goes up.

    The amount of power you can steal is different when the equipment is running versus when it is off. The amount of power you can steal is less when the equipment is running. With the recent cold weather, heating systems are running 12-18 hours per day, which is enough time for the thermostat battery to deplete its charge. When this happens, a thermostat may do several things internally to conserve power such as throttle down the CPU cycles, turn off the Wi-Fi radio and, worst case, shut down your heating/cooling so it can recharge.

    In an analysis across our customer base, we predicted power stealing will fail in 2-8% of customer installs. And a bigger problem is that when power stealing fails, it doesn’t fail deterministically. In other words, reliability or failure can’t be predicted at the time of install. So while it may happen during extreme weather conditions, it could also fail at any time.

    Heating and cooling equipment are simply not designed for power stealing. Newer HVAC equipment tends to be even more sensitive to power stealing. A thermostat that power steals might work fine for a few weeks, a year or two years, then fail. A lot depends on the equipment the thermostat is connected to, the equipment duty cycling and the customer usage patterns. This may be part of the reason some people experienced high failures during the Polar Vortex, when duty cycle and customer usage is high. For these reasons, ecobee does not use power stealing.

    In our case, ecobee thermostats require a 24VAC power supply using a ‘C wire’ to ensures constant (24 VAC) power. This provides a continuous and reliable power source, so our customers can be sure their ecobee thermostats will always stay powered on.




  2. #2
    HVAC Thermostat Specialist

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    IMHO the reason Nest has a power stealing setup is so homeowners could install thier own thermostats without opening up the furnace/air handler to deal with the common wire hookup. Most homewoners have no business opening up their furnaces and messing with wires, but they want a shiny new $250 thermostat. Look how Nest is marketed, straight to homewoners through big box stores. How many supply houses have you seen the Nest in? If you do find one in a suppy house they charge contractors retail price for it..

    Ecobee on the other hand is marketed to contractors, notice the big box stores don't sell Ecobee...

  3. #3
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    Power stealing? Personally, I'd rather borrow a small amount of current rather than to spend the money on a battery that probably took 3 times the energy to make than what it puts out. When it all comes out in the wash, you've SAVED energy on the grand scale. I'd hardly call it stealing.

  4. #4
    HVAC Thermostat Specialist

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    Quote Originally Posted by DrDave View Post
    Power stealing? Personally, I'd rather borrow a small amount of current rather than to spend the money on a battery that probably took 3 times the energy to make than what it puts out. When it all comes out in the wash, you've SAVED energy on the grand scale. I'd hardly call it stealing.
    The Nest is power stealing AND uses a battery!! Common wire thermostats require no battery.

  5. #5
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    Quote Originally Posted by Servicetech View Post
    The Nest is power stealing AND uses a battery!! Common wire thermostats require no battery.
    The nest USES a battery simply for the lack of "C" wire power in an outage. The only time it would "steal" current is if it needed a charge. My guess is the only reason for the battery would be data retention in case of an outtage.
    Last edited by DrDave; 03-20-2014 at 08:02 AM.

  6. #6
    HVAC Thermostat Specialist

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    Quote Originally Posted by DrDave View Post
    The nest USES a battery simply for the lack of "C" wire power in an outage. The only time it would "steal" current is if it needed a charge. My guess is the only reason for the battery would be data retention in case of an outtage.
    Nest claims the thermostat works without a connection to C. They are the only wifi thermostat to make this claim, and use the rechargeable battery to make it possible.

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