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Hydronic heating and line voltage thermostats

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  • Hydronic heating and line voltage thermostats

    Hi, I am completely new here, but found this site by searching with Google and there appears to be some very knowledgeable peolple out here....

    So I have a residence with a Rinnai heater and six individual "heater cores" that have fans controlled by the thermostat. The thermostats are manual double pole "King" brand. Very basic and frankly work only as an on off switch. I want to use a Honeywell double pole line voltage programmable thermostat to replace each current mechanical thermostat. One pole will operate the Grundfos pump to circulate the water and the other pole switches the fan at the heater core, just like it is currently set up. Here's the catch: Both poles are on the same leg....Will the electronic portion of the thermostat operate from both poles on the same leg with no common or do I have to find a way to switch one of the poles to another leg to give the electronics 220v? The thermostat has no common, only four wires; two load, two line and it is intended for two legs, not two lines of the same leg.

    I have no common in the J boxes, but even if I did, the thermostats have only hots, no common.

    If anyone knows anything about this, I would be forever thankful!!


    Last edited by notbob1; 07-04-2009, 12:47 AM. Reason: Mis-spelling in title.

  • #2
    Re: Hydronic heating and line voltage thermostats

    It seems to me that the electronic part of the T stat would be internally connected to the line side of the T stat, the line side should have one of each leg present there that will satisfy the internal need for voltage. The line side of a T stat is always HOT the load side switches each individual leg and is basically independant of the line side. There is no common in a 220v line. The problem is, if both legs are the same line, then you don't have 220v there at all and the supply for the internal voltage for the T stat would not be there. You might be able to rewire the line side from another location and switch the 2 different legs for the heater and fan so that you have 220v at the T stat. I assume that the fan and pump run off the same 220v supply line? You could parrallel the pump and fan on the sam 220v switch from the T stat, but I assume there is a delay to let the water get hot before the fan goes on, you can still do this, but you will get cold air blowing for a couple minutes.


    • #3
      Re: Hydronic heating and line voltage thermostats

      what are the model #'s of honeywell & grundfos? breid