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Relay Control Question for Hot Water Loop off Steam Boiler

Bobby D.
Bobby D. Member Posts: 36
I created a hot water zone loop off my old steam boiler to heat my finished basement, as per Dan’s advice and Mad Dog. It has a transfer tank to circulate the steam boiler water and heat the radiant baseboard loop. Two TACO circulators run together to circulate water from the steam boiler to the tank, and run the baseboard loop at the same time.

I want to connect a MARS FD7 Relay to allow the TACO circulators to run only when the burner is not running. This will prevent the radiant baseboard loop from taking energy away from the steam boiler while it is running, otherwise the steam boiler will continuously run to try to make steam.

A service tech gave me a rough sketch on how to wire it (I have a stack relay on the stack to the chimney) which I misplaced. I have a general idea, but need some expert advice again.

So my question is this, how and where do I wire the FD7 to control the TACO circulators to only run, when the burner is off.

Many thanks in advance to all at the Wall.


  • Ron Jr._3
    Ron Jr._3 Member Posts: 603
    edited September 2010
    Make sure the coil on the FD7 is for low voltage

    Some of them are built for 24 volts , some are 120 , others are 220 . Since you want to prioritize the steam thermostat you'll need it low voltage .

    We use something similar to the FD7 nowadays . It's called a RIB relay . Can be used for low and line voltage zones , and a combo of both . Pretty cheap control too and can be found at most supply houses ( on Long Island anyway ) .


    Tell us which relay you get and we can walk you through the wiring .

    Is there an aquastat in the boiler ?
  • Bobby D.
    Bobby D. Member Posts: 36
    Follow Up

    There is no aquastat on the boiler, a seperate gas hot water heater supplies hot water.

    Can I not use the Mars FD7 relay ? I do have this version of the FD7 and it is 120 volt,

    My understanding of these controls is limited, but the way it was explained to me, is that the relay would know when the burner motor was running and only activate Taco circulators when not running.

    Here is a link to the one I have


    If it cannot be used for this I will purchase the Functional Devices unit, but since I have this one already can it be used ?

    Thanks for you input.
  • Ron Jr._3
    Ron Jr._3 Member Posts: 603
    That would work ............................. sort of .

    A few problems though .

    First , without an aquastat in the boiler , you won't be able to control the heat if the hot water zone is on and the steam zone is off . You'll want the boiler to maintain a certain amount of temperature to heat the hot water zone . You'll also want to shut the boiler off before it starts to steam if the hot water zone is on alone . If it's too much work to add a well and aquastat into the boiler , you can always use a strap-on aquastat to the supply pipe going into the heat exchanger from the boiler .

    The only way I can see using that FD7 ( if the coil is 110 volts ) is to wire it directly to the power supply of the oil burner . Doing it this way the hot water zone will never turn the boiler on if that zone calls on it's own . With that Taco relay , you wouldn't even need the FD7 to control it this way . There's plenty of dry contacts in that Taco box .  

    I'd bite the bullet and spend the extra $20 and get the right relay . Even an extra Taco single zone relay would do the trick ( with an aquastat , even better ) .
  • Bobby D.
    Bobby D. Member Posts: 36
    The Right One

    Ron, I will by the correct relay and will post again when I have it.

    Thank you again.
  • Bobby D.
    Bobby D. Member Posts: 36
    Correct Relay in Hand

    Ron Jr. I have the following relay in hand, please advise on how to set up.

    The relay you reccomended and the one I purchased.


    Thanks in advance, any any wall eys want to chime in feel free.

    Bobby D.
This discussion has been closed.