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Milliamp draw of two smart thermostats test.

Teemok Member, Email Confirmation Posts: 358
Waiting for parts, so I rigged this up. This test is in no way approved by Central Services and none of test equipment has been calibrated. The tester is not officially vetted and is a part of no guilds or fraternities.
This is a test of the older 2nd -3rd gen Nest and a current Ecobee3. Both where not charged. The Nest came from a 12 thermostat house one year ago where someone installed them for the sale of the house and the buyer called soon after to find out why they where all dead. No C wire.
The Ecobee had a slightly higher draw and it peaked very slightly with the Wifi connection events. I tested with the heating relay closed. Peak of 58 milliamps ac or 0.058 amps.
Confirmed. Ridiculously low draw. Nest was 0.048 amps ac
Most controls have the capacity to handle a single smart thermostat load without issue.
Your smart thermostat may draw more.
Central Services guys strongly advise against hacking to find a C connection on controls without a C terminal.
A rib relay is safer for your currently working controls, your thermostat and you.
However; the fears of over loading a transformer, that I've heard repeated for years,
are way over blown.


  • Jamie Hall
    Jamie Hall Member Posts: 22,147
    Useful. Thank you! And as you note, the usual problem is finding a usable constant power source at all...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ChrisMcc
    ChrisMcc Member Posts: 24
    What's funny (to me) about this is how far these numbers are from what - in my case - Ecobee publish. I went through an entire ordeal of upgrading the transformers that power my Taco pump controllers to make sure they'd handle the book loads. Like you, I've never measured anything above about 8-9mA. But I'm ready if they start sucking down current for some reason.
  • Teemok
    Teemok Member, Email Confirmation Posts: 358
    edited October 17
    Oh geez, those are a decent size to start with. No harm being oversized. I think 8-9ma is after the capacitor/battery is fully charged. Ecobee is careful to not publish much as far as I could find. They give a watts per hour or something. Meant to address total power consumption / parasitic concerns. They must have some R and W switch load handling specs. People quote those.
    Don't put multiple actuators loads through them. The switches in the thermostats do burn easy. I was testing a rats nest of wires and my meter leads shorted a wire for just a glance of a connection.(as if it happens another way) Lucky it was a fresh unset up install and I had that ecobee on the truck. It took having to replace just one smart stat for me to create a new rule. Smart t-stats come off the wall while diagnosing. Once the problem is discovered and it all works with a jumper, then I'll remount it and final test.
    EBEBRATT-Ed Member Posts: 14,534
    The Nest problem is that some think you get 24v on a T &T control. And with the thermostat open you might get that due to a backfeed in the control. The theory was that the Nest would charge on the off cycle. It doesn't work in my opinion..
  • Teemok
    Teemok Member, Email Confirmation Posts: 358
    Yeah, I've never seen that work.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,534
    Are you trying to hook a Nest to a millivolt system ? You will need a transformer to run the nest and a relay at the system ...
    I have enough experience to know , that I dont know it all
  • Teemok
    Teemok Member, Email Confirmation Posts: 358
    @Big Ed_4
    My lack of god no. No, I'm not trying to do that. LOL. I don't have a problem finding or making a C connection. I was just posting pointless info. Thank you for your kind offer to help.
  • hot_rod
    hot_rod Member Posts: 20,790
    They seem to consume a lot of battery if they are always searching for Wifi signal. So the ones that don’t have C or access to a common burn through batteries m.

    I thought early versions went into a fast charge if the battery dropped to some  low voltage, 3.6 maybe. That seemed to be when they overloaded transformers 

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Teemok
    Teemok Member, Email Confirmation Posts: 358
    @hot_rod Not with these examples. The nest was dead when acquired a year ago. The Ecobee is 6 months off the wall. I measured both the whole time while connecting to WiFi before and after. No significant amp draw.
    The point is moot.