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

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  #1  
Old 10-09-18, 01:27 PM
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Smart T-Stat installation: Low Voltage output on Burnham MPO IQ Boiler Control

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.

https://www.thermoflo.com/wp-content...e+-+MPO-IQ.pdf

Regards,

Ken
 
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Old 10-09-18, 05:56 PM
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Not an electrical guy to say the least, but are you checking T-T on the IQ with the thermostat disconnected at T-T? In other words, there is nothing connected to T-T.
 
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Old 10-09-18, 06:39 PM
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Terminals 1, 2 and 3 are one switching section and the TT is a second switching section. A smart thermostat requires a three wire connection as shown below. I believe you can leave the red and white as currently connected from your DHW control unit.

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If you try this setup...... I'd put a 2-3A fuse in the C line as a safety in case there is interaction.
You don't want to damage the IQ controller.
 
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Old 10-10-18, 07:44 AM
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Thank you for your responses.

zoesdad - I took the reading directly from the TT terminals on the IQ, i believe i disconnected the wires to the thermostat for that reading but can double check. Does it matter whether i take the readings from Rh-W and Rh-C with the wires connected or not?

Pete - I attempted the setup as you described above initially. However, i do not believe the IQ is giving me enough volts. With a multimeter, the Rh-C terminals 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 ideas of why i may get funny readings?
 
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Old 10-10-18, 09:52 AM
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I always disconnect any load connected to the source if I want to check source voltages. So in this case I would disconnect all thermostat wires from the IQ and then check voltages at the IQ. I don’t know if it makes sense, since I an electrical ignoramus(lol), but I think that eliminates any funny readings caused by faulty equipment if that’s possible (the ecobee in this case) or loose/bad wires etc.

But I would think you would measure 0 volts from Rh to C at the ecobee with the C wire disconnected from the ecobee. Not sure though. And even without the C wire connected to the ecobee I don’t see how you should get 14v between Rh-W. Seems to me it should be 24v as you say.

But …. is it possible that the ecobee does something funny if you hook up Rh and W but NOT C - and that’s why you see 14v? Seems to me that is an incomplete hookup.

Need someone more knowledgeable than me here!
 
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Old 10-10-18, 10:17 AM
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From 2 to 3 should be 24vAC
From 3 to left T should be 24vAC.
Do you have that at the controller ?
 
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Old 10-10-18, 11:06 AM
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PJ Max, that is the issue, i dont have the proper voltage at the boiler transformer. I am just wondering why that would be. I may need a pro to fix this issue.
 
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Old 10-11-18, 10:09 AM
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Well I said I was an electrical ignoramus – and I still am, but here is a diagram from page 44 of this manual -

https://www.simplyplumbing.com/media..._io_manual.pdf

and I don’t see how Left “T” is 24V AC positive. I can see how “2” is 24V AC positive, and “3” is 24V AC ground, as they state in the following applications guide:

https://www.thermoflo.com/wp-content...e+-+MPO-IQ.pdf


but it looks to me like Left “T” would be the same as “3”, that is, 24V AC ground, and NOT 24V AC positive. (referencing 1-2-3-T-T down at Option Panel)

Maybe I’m just lost on where Left and Right “T” are in the diagram, or maybe more likely how to read the darn thing in general. Just thought this diagram (and manual) might be helpful.
 
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