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Boiler Protection: Automated Bypass Valve Needed?

bdubya Member Posts: 16
Q: Should I install an automated bypass valve that monitors return water to the boiler? If so, what are my options i.e. 2 way or 3 way valve or another pump in the bypass?

I have an 82% efficiency cast iron SE140 slant-fin boiler piped for two zones with each zone having its own circulator piped on the return of boiler. My boiler is controlled by a Hydrolevel 3200 Plus with outdoor reset. The zones are controlled with a Taco zone control unit. This aquastat stops the circulators if the return water in the boiler drops to condensing temps and restarts circulators after the water temps rise. While this is better than no protection at all, I am realizing that this type of boiler protection starts and stops the circulators way too often during cold starts which takes forever for the radiators to start warming the home. I have a tee installed on the supply side leading to the return side with a manual valve on it but have never used it to manually throttle the bypass flow during cold starts. Is it better to have an automated bypass monitoring return water temps and tempering bypass valve to open and modulate to keep return water above condensing temp or is the Hydrolevel enough protection and my worries are needless? See pics - sorry if they seem sloppy but wanted to help you make sense out of the piping rat's nest. Thanks in advance.


  • mattmia2
    mattmia2 Member Posts: 6,592
    The bypass should be partially open, that would keep the circulators from short cycling. A thermostatic valve would be better but the aquastat and fixed bypass are acceptable.
  • bdubya
    bdubya Member Posts: 16
    Thought I should add that when I do open the manual bypass valve there is flow going from supply into return but I have no idea what the mix temperature is going into the boiler. Also should add that I am adding CI baseboard (replacing fin tube) in zone #2 (have upright column CI in zone #1) and I'd like to take advantage of the ODR function more and let the boiler fire down to the minimum return temperature without condensating to allow a lower CI temperature in shoulder seasons but protect the boiler at the same time. I feel that the Hydrolevel doesn't really do a god job of that and I worry that its circulator hold function doesn't protect the boiler enough. Just a worry I have - no proof it isn't protecting as that proof usually shows up when it's too late.
  • mattmia2
    mattmia2 Member Posts: 6,592
    You would need to make significant changes to the near boiler piping to do outdoor reset with a cast iron boiler, you would need somewhat separate systems to protect the boiler return water temp and to mix the system water based on an outdoor reset control. Primary-secondary piping with an electronic mixing valve that controls the heating water temp is the first thing that comes to mind but there are other ways to do it too especially with a CI boiler that doesn't have minimum flow requirements.
    EBEBRATT-Ed Member Posts: 12,622
    The only way to protect the boiler for sure is to have a sensor on the boiler return pipe. There are many was to pipe this. See the attached. You can download the whole series of Caleffi manuals there is about 25 of them
  • hot_rod
    hot_rod Member Posts: 17,470
    A couple of determining factors for adding or upgrading return protection methods.

    1 if the boiler can see a large slug of cold, room temperature water. Old gravity conversion systems, radiant concrete slabs as examples, lots of high water content cast radiators.

    2 if the boiler has to run more than 10 minutes or so to get the return above 130F. Same issue caused by high mass emitters or boiler sized close or below design load condition maybe.

    I suppose you could group the various methods in to good better best.

    The bypass pipe and valve or a bypass pump are certainly better than nothing. They get set at some arbitrary point and protect at that exact condition. So they may be over or under mixing most of the time. That could slow heat delivery to the radiators if they are blending down SWT.

    An aqua stat that holds or turns the circulator off as return temperature drops. Now you have added some intelligence into the decision making, a good thing. Depending on loads, flow, boiler output they can bang on and off every few minutes under a big cold return load, kind of annoying and not ideal for the circulator. It needs to be on a boiler that can safely operate without flow, cast iron, not for copper tube type boilers :) You still see it used on some of the price-point Euro cast boilers.

    Very best is a control, valve, or device that watches return and smoothly modulates the mix of supply and return. Think of it a slipping the clutch in your VW bug to start with a heavy load going uphill.

    The very simplest is a high Cv thermostatic mixing valve. Use one intended for that application. Most off the shelf 3 way thermostatic have low Cv for DHW mixing use.

    A variable speed circulator properly applied is another clean method.

    If you need to mix down temperature for loads a 4 way motorized can do both mixing and return protection with an actuator and control, tekmar, HBX, Taco I-valve for example.

    If you can observe the boiler from a cold start, maybe your first heat call this fall. That will be one typical condition. The other is first start on a design day where it is trying to catch a design load, and get its return up.

    Or coming out of a frozen home condition :(
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,869
    edited August 19
    Looks like you bypass is only for the one zone. Best would be a proper repipe, with proper supply/return manifold piping on the wall, with a 3 way bypass that @hot_rod mentioned. Then when it's time for a new boiler it's an easy swap out.
    Your not-to-code flue pipe on the water heater scares me.
    EBEBRATT-Ed Member Posts: 12,622
    If it was me, I would use a 3-way mixing valve driven by a Johnson 350 0-10 volt output but not cheap