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Smart T-Stat installation: Low Voltage output on Burnham MPO IQ Boiler Control

kbogden Member Posts: 2
I am a DIY'er with little experience with boilers attempting to install an ecobee 3 thermostat before the winter and am running into some difficulty. It appears the voltage output on the control system (see page 9 of wiring manual in the link below) is low with direct terminal readings on my multi-meter for Rh-W (Left-T and Right T in schematic) of 14 volts and Rh-C (C wire is Terminal 3 in schematic) of 2 volts. The output for Terminal 2 and 3 leading to the Indirect Water Heater read 24 volts. My meter readings are directly from the terminals not from the t-stat wires in the wall. According to the schematic in the link below, it is a 24 volt system, does anyone have any advice as to why the voltage output from my boiler is below 24 volts. The multi-meter readings with the indirect water heater disconnected are the same as above.

I very much appreciate any help with this issues in advance.





  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,564
    My experience with the Ecobee showed me it needed a lot more juice to power up the wireless functions, than my boiler transformer was able to provide, and so I just kept my Honeywell VisioPro.—NBC
    HVACNUT Member Posts: 4,594
    The left T' will go to R on the Ecobee.
    The right T will go to W on the Ecobee.
    Terminal 3 will go to C on the Ecobee.

    I've never used the Ecobee so if theres A/C involved, you must separate Rc,Rh by removing a jumper, program it to separate, or maybe the Ecobee recognizes there are 2 R power sources.

    If you have A/C, use its transformer to power the thermostat, R and C and you'll connect TT on the board to Rh and W on the Ecobee.

    Like @nicholas bonham-carter wrote, if your going to use the boilers transformer, make sure it can handle the load. At least 40va.
  • kbogden
    kbogden Member Posts: 2
    Thanks for the responses. The boiler is only rated for 24 Volts so that could be the issue.

    HVACNUT - I attempted the setup as you described above initially. However, with a multimeter, the Rh-C terminals (on the boiler) give me 2 volts while the Rh-W terminal gives me 14 volts. I believe both those tests should give me a reading of 24 volts, which is the voltage required for the ecobee. Any idea of why i would get these funny readings?

    Unfortunately i do not have central A/C to power the ecobee.
    Thanks again
  • GBart
    GBart Member Posts: 746
    edited October 2018
    They aren't talking 40volts, they mean 40va as in watts, transformers are rated in watts, usually a gas appliance may not have the power to run the units controls and a wifi t-stat, the transformer has to be 24VAC and 40VA (watts) or larger.