# Reducing Voltage to Thermostat from 28v to 24v



## km782 (Nov 2, 2015)

I am looking to install a smart thermostat (Ecobee3). It is designed to run at 24v. I've read online that it will operate at higher voltages but the temperature readings will be incorrect due to overheating inside the thermostat. I used a multimeter to measure the voltage at my thermostat and I'm getting 28.3v. From what I understand this is normal but it seems that the engineers that designed the thermostat haven't worked in the field and/or didn't do their research so it was designed to work correctly only at 24v.

Is there an easy way to reduce the voltage? I also want to be careful to make sure that reducing it won't damage my furnace and a/c units. Thank you.


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## user_12345a (Nov 23, 2014)

You would have to change the transformer in the furnace/air handler and hope to get lower voltage from it.

The control transformers aren't necessarily engineered with the tightest of tolerances.

Could have to do with the design input voltage, like if it's rated for 110 or 115v input and you have 120 to 130v the output will exceed the rating.

Best thing to do though is forget about the nest or something similar. You can


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## yuri (Nov 29, 2008)

I highly doubt it will read wrong. Sounds like a urban myth. They use a thermister to measure temperature which is a resistor which changes it resistance with temp. There is no resistance in a thermostat as it is a set of switches. There used to be a cooling anticipator which was a heater resistor in old mercury type bi-metal spring thermostats B4 we had electronic ones.

In my 35+ yrs in this biz I have never had a transformer put out exactly 24 volts as your house voltage can vary from 110 to 120+ volts and the transformer is proportionate to the incoming voltage.

I have never had any problems with the Honeywells so buy it rather than Ecobee if you are afraid. Sounds like a urban/internet myth to me.


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## km782 (Nov 2, 2015)

Thank you for the responses. I guess the best thing to do is either just live with it or get a different thermostat.


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## yuri (Nov 29, 2008)

Ecobee is a big company and it would be a total fiasco if they did not work properly due to voltage being higher. Highly doubt that would be a issue they would not correct.

There is lots of nonsense on the net from DIYers or wannabee pretender techs etc so don't believe everything you read.

Personally I like the Honeywells only. Nest has had problems and White Rodgers too. Not sure about ecobee.

Honeywell makes all the thermostats for Lennox and stamps Lennox on them and has had no problems lately. No power stealing thermostats or other nonsense/issues.


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## user_12345a (Nov 23, 2014)

km782 said:


> Thank you for the responses. I guess the best thing to do is either just live with it or get a different thermostat.


likely that these fancy t-stats have their own accurate voltage regulation components built in.

Can't see the electronics running on 24v, it likely gets stepped down and converted to DC.

All chips are DC, normally 5v or less.


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## spaceman spif (Jul 28, 2009)

user_12345a said:


> likely that these fancy t-stats have their own accurate voltage regulation components built in.
> 
> Can't see the electronics running on 24v, it likely gets stepped down and converted to DC.
> 
> All chips are DC, normally 5v or less.


Agreed, you should be okay with 28 volts as most electronics are designed to handle a range of supply voltages.


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## Bitters (May 28, 2014)

Never seen a 24V transformer putting out 24V.


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## beenthere (Oct 11, 2008)

You'll measure 28 or more volts when the thermostat is not mounted to the back plate, and when there is no load on the transformer. Once there is a load, the voltage will drop closer to 24 volts.


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## supers05 (May 23, 2015)

I use the ecobee, and I'm at about 27v or so. It reads spot on. (tested with a commercial grade temp probe) 

This really does sound like myth. They would use a switching power supply as they are cheap and readily available ICs. They at far more adaptable to source fluctuations and way more efficient then older school power supplies. All electronic stats will internally operate at 5v or less. (3.3v is another common voltage) 

Cheers!


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