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Steam Boiler - ML6984 / ML7984 Zone Issue

We are stuck.

A residential customer with a gas fired steam boiler that has 2 separate zones. They have ML6984 actuator heads and unknown valve body models. One of the zones failed. We special ordered the ML6984 and valve body from our specialty supplier that's familiar with this type of system.

The replacement ML6984 actuator and valve body were installed. However, the valve would open in close in reverse. If you turn the thermostat down then the valve would open and vice-versa. Looking at the next best option, the ML7984 has a configuration where you can reverse the open / close.

Installed the ML7894 with the appropriate configuration. Turn the thermostat up and the zone opens. Turn the thermostat down and the zone closes. Great!

However, the boiler will not fire when the thermostat is turned up and the zone opens. The other zone is operating fine. The technicians in our network are stumped.

Anyone familiar with this problem here?

Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,168

    Really…. What thermostat are you using. You know that if you connect the R and the Y on a thermostat it will work in reverse to the R and W terminals.

    I'm going to look at your valve literature and see what can be done. BRB

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • kperry1513
    kperry1513 Member Posts: 2

    Its a Honeywell home thermostat - I don't have the exact model. I'm going to get some more info on the problem.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,168
    edited January 8

    OK first question Is the auxiliary switch from the old valve still in working condition? Can is be used on the replacement valve actuator?

    Next,

    I read this in the instruction sheet from 2008. ( you posted the 1999 version)

    To Replace ML6874:

    1. The ML6984 is a direct replacement for the oldML6874 in single actuator hook-up.

    2. For parallel, multiple ML6874 actuators applications, an isolation relay must be used. The isolation relay contacts should be placed in the new ML6984 actuator signal input "W" and "B" circuit.

    To Replace Line Voltage Auxiliary Switches:

    These accessories are no longer available. Replace the old actuator with new model and 272630D position feedback/auxiliary switch module. Operate an external relay, such as R8845U, using the adjustable SPDT pilot duty output on the 272630D. If a second, independent output switch is needed, wire the 2-10 Vdc position feedback signal to an Approved, adjustable, voltage-controlled relay

    I take this to mean that you need to purchase the 272630D position feedback/auxiliary switch module and possibly a relay in order to get the boiler to turn on when the valve is open.

    Look closely at the working zone valve and see what the old valve has that the new valve does not have. that is probably the part or switch that is turning on the boiler.

    ML6984/ML7984 printed 2008

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • pedmec
    pedmec Member Posts: 1,171

    most of these types of actuators require an auxiliary end switch.