Taco RMB not to use Nest thermostat?

Billiambob · January 18

I just watched the Taco training module for their Radiant Mixing Block (RMB-1) and they say not to use an electronic thermostat like Nest or Eccobee. They don't explain why not to use them. Is this because the RMB demand heat signal is to be 24VAC or a closed switch (zero volts).

The voltage from Nest is 24VAC for no demand and drops to 1.6VAC when demanding heat. My Nest connection (heat only) are two R&W wires to a Taco zone controller input (Red & White) and Demand input on the RMB. My RMB does not run even when there is 24VAC on the two wires. The RMB also does not run during calls for heat, (voltage across Red and White wires drops to 1.6VAC). The RMB does run continuously if a jumper is placed on the Red and White wires.

Does the RMB need a short to work or should 1.6VAC be low enough (not Zero or short)?

Also why wouldn't the RMB be running continuously when 24VAC is present with no demand and 1.6VAC during demand?

Thanks in Advance

pecmsg · January 18

as far as the NEST there known to have issues when Power steeling.

Billiambob · January 18

How do I make it work?

Add a relay? or only use a mechanical thermostat?

109A_5 · January 19

Hello Billiambob,

The Nest and other similar thermostats use solid state switching, solid state switching often has an inherent voltage drop across it (and other switching anomalies) in the closed or On state. By comparison mechanical switches and relay contacts normally have very little voltage drop across their closed contacts and only brief contact bounce when initially closed.

To put a relay inside this type of thermostat would be an engineering problem since a relay inside the thermostat needs its coil energized to operate. The Nest often relies on a parasitic method to acquire power to operate and keep its battery charged, so they use a solid state switching method for activating connected equipment. Old school thermostats used a simple mechanical method to sense the heat and close the switch to operate the connected equipment.

Your RMB was probably engineered to ignore voltage in the gray zone (a voltage above a certain threshold, but not at the nominal 24 VAC). So with its logical states (active or inactive) it behaves like 1.6 VAC is the same as 24 VAC.

The probable fix is to install a buffer relay. Since a common relay coil will probably tolerate the 1.6 VAC voltage drop of the solid state switching and the relay contacts will provide the RMB with the low voltage drop contact closure it needs.

IMO it is kind of sad to have to add another point of failure into a system to make it work correctly. I suspect in this case there are no highly defined standards shared between heating equipment manufactures and the heating control manufactures trying to gain a market share.