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Hydronic Separation - Is it working?

jdbs3 Member Posts: 32
Reference attached picture - Bosch Greenstar condensing boiler with system components. The boiler is attached to the tees making it the secondary. The primary loop is the rest of the system which includes radiant floor zones, forced hot water baseboard zones, and DHW.

When all thermostats are set to OFF, the DHW is not calling for heat, and the boiler high limit is taken off Summer Mode (where the heating zone pump and consequently central heating are switched off), the boiler will run continuously. Yes, this is an issue; an HVAC person is coming tomorrow morning to diagnose/fix this problem.

However, the supply and return side system piping between the boiler and the black Buderus mixing station heats up over time as the boiler continues to circulator how water just within the boiler. In ~40 minutes, the water in this piping went from 74° supply / 72° return to 102° supply / 98° return.

The only circulator pump that was running wa
s the boiler pump.

With hydronic separation of the secondary (boiler) loop and the primary loop, should hot water be migrating, over time, through the supply and return pipes to the Buderus mixing station.

Or is it natural, over time, for the temperature of the hot water in the boiler loop to 'leak' into the temperature of the water in the secondary loop. And, again over time, all the way back to a closed valve?

If this is not supposed to happen, how is it 'fixed'?


  • Zman
    Zman Member Posts: 7,569
    I can't quite follow the piping in your picture. Some temp increase at the mixing station is normal. If you are feeling it beyond the station you may have a piping or check valve issue.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 23,277
    The pipes are connected, no? Then you will -- inevitably -- get some mixing. If the pipes are metallic, you will get some heat conduction just in the metal. One can avoid the mixing problem, if one wants to, by using a heat exchanger. One can avoid the conduction problem by using pipe with high temperature insulating properties. Is it worth it? Um... no.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ironman
    Ironman Member Posts: 7,376
    The boiler circulator will run when it receives a call for heat. So, the question is: what’s calling the boiler to heat?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • jdbs3
    jdbs3 Member Posts: 32
    Thanks for the feedback. So the migration of heat up to the black Buderus mixing station is to be expected.

    the question is: what’s calling the boiler to heat?

    Yes, hopefully the HVAC person coming tomorrow morning will diagnose and fix that problem.

    And, hopefully, also the second major problem described below.

    As noted,, the pipes heating up going to and from the Buderus is to be expected. In the picture, there is a Danfoss ESBE ARA 663 floating point actuator on the left bottom side of the black Buderus mixing station.
    There is a Tekmar 360 controller mixing controller to the left of the black Buderus mixing station (see smaller blue box between the 2 Taco controllers)

    The Tekmar 360 show an arrow for 'Close'. So the 3-way mixing valve behind the Danfoss actuator should be closed. But evidently it is not. With all thermostats set to OFF, and the DHW not calling for heat, the fact that the temperature gauges (left = supply, right = return) on the Buderus show that over ~40 minutes, the water in this piping went from 74° supply / 72° return to 102° supply / 98° return says the valve is not completely closed.

    Bottom line: If there was a call for heat, the water temperature getting into the radiant floor zone would be too hot, staying in the range of 150° - 185° dependent on the Bosch Greenstar condensing boiler high limit setting. I've run these tests to verify this is happening.

    Another explanation for the radiant zone staying too hot could be gravity back-flow from the return supply of the boiler mixing with the supply side of the radiant floor loop. But I believe this is not the issue since it is 1) clear the mixing valve is letting hot water creep past a supposedly "Closed" valve, and 2) unlike the supply pipe from the boiler (below the Buderus), the return pipe to the boiler (below the Buderus) was somewhat hot, but could be held with your hand.

    The mixing valve itself may not be bad. It might just be that the closed position of the value is not aligned with the closed position for the Danfoss actuator. We will see.

    Hoping the HVAC technician can solve both major problems.

  • hot_rod
    hot_rod Member Posts: 22,121
    edited September 2022
    The spacing of the P/S tees is a bit wide, but if anything you get some flow through the boiler when a secondary circ is running

    Are the manifolds or Pex on the supply of that two port manifold warm. Temperature should not conduct into the Pex if that valve and circ is off. Possibly a check in that mixing circ also

    Turn the red and blue valves above the mixing device off, or the yellow handles below and see what cools
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream