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  1. #1

    Confused Boiler or system bypass

    Hi, can someone explain how these bypass's work, what happens when there is no call for heat and the circulator is not running how is the minmum temp maintained? Why not use a low limit control to maintain the minmum temp,or do the bypass's work on convection?


  2. #2
    Join Date
    Jan 2010
    Vancouver, BC
    A system bypass is created when a portion of the boiler supply water is diverted to the return; therefore, bypassing the heat emitters and sent directly to the boiler. Usually, this will be done to (a) mix some of this hot water with colder return water before it enters to boiler, boosting return water temperature, and avoiding condensation forming on the heat exchanger or (b) to satisfy minimum flow through boiler when one or more zones in a system by themselves, when calling for heat, do not produce adequate flow.

    A boiler bypass is created when a portion of the return water is diverted away before reaching the boiler return inlet. The water is usually diverted to mix with supply water; therefore, bypassing the boiler and sent directly to the heat emitters. This may be done to lower the water temperature supplying, for example, a radiant floor zone.

    Among other equipment, a thermostatic mixing (diverting) valve may be employed to vary the volume of water that is mixed (diverted). When there is no call for heat, both the gas valve and circulators (typically) are de-energized. There is no combustion, and no return water entering the boiler; therefore, there is no minimum return water temperature to maintain.
    We do it right... after lunch!

  3. #3

    Smile Bypass's

    Thanks, that was a good explaination,my concern was with a cold start boiler,I thought there maybe a problem at that point, but it appears that the condensation forms when the return water is cool, I am using a Taco PC700-2 reset control which has a minimum temp setting which only functions when there is a call for heat. It's all starting to make sense now.

  4. #4
    Join Date
    Jan 2010
    Harrisburg, PA
    Almost all the equipment to day wants a boiler bypass installed with large water volume systems. The idea is to protect the boiler and not affect system flow use a boiler bypass diverting water around the boiler. A system bypass will reduce the flow in the system and give less heat. Increase system flow and increase heat output at any given water temperature.
    If adding an OD rest control to a cast iron boiler use it as system reset not a boiler reset. Install the sensor on the system piping in a turbulent flow area not a laminar flow are for best sensing. The boiler will get above the condensing range quickly and still give longer run times. The second thing you could do is p/s with a valve between the boiler and the closely spaced tees to slow the water down through the boiler and not affect system flow.
    See link about boiler protection for other ideas.
    In the left column click on Cast Iron Boiler Protection

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