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When Cast iron should and should not sweat

Fluidruid Member Posts: 2
I have a new 250K Weil Mclain CGa in this ancient gravity conversion house, with three inch mains and the lot. Many big radiators. The previous was a 270k input Janitrol. Scary gas works, w/non-safety pilot.  

Heat loss proves sizing is correct, and the system comes up with time.  However, I am nervous about flue gas condensation in the boiler due to prolonged cold return water.   

I have a bypass between the supply and return, though its only one inch.  Should bypass' be as large as the header/manifold size?

I had originally considered piping this thing with primary/secondary like a mod/con, but changed my mind when I weighed cost/time parameters. I kind of wish I had now.  W/M manual has an option for p/s piping arrangement, but strangely they also show a bypass below the closely spaced tees.  That threw me off. 

I now have a 0010 pushing the load, but I think I'll slow it down w/ a 007.

Anyone every install p/s piping on a standard, high mass setup? Would this help with avoiding flue gas condensation, since its more like injection?

I do see evidence of a small amount of moisture being produced, and the boiler has only been installed for a month.


  • Ironman
    Ironman Member Posts: 7,182
    edited December 2010
    Check the Thread on the Main Wall

    About thermostatic bypass valves.

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Glen
    Glen Member Posts: 855
    the culprit -

    is most likely low returning water temperatures; regardless of gas (NG or LPG) condensation is accelerated once return temps fall below 150 deg F. A bypass would work (adjusted manually) and can be set and left alone but probably not the best. I like the idea of PS; a hot loop injecting into an outdoor reset controlled secondary loop. It adds a pump but then you can take advantage of the mixing capabilities of the control you choose and maybe save a buck or two on fuel. tekmar is your best bet and the one I am most familiar with but there are others as well.
  • Fluidruid
    Fluidruid Member Posts: 2

    I think you're talking about the Tekmar 256. A friend turned me on to it, and I've been offering it to customers since I installed it  on my own boiler. Works great, but you need a second relay to run the pump if its a zone valve, single pump setup. Also gets tricky when you throw a prioritized indirect DHW in there. You then need a single master relay, and a slave that runs the heat zones. 

    Really wish I just did the P/S on that big boiler. I have a feeling I'll be back to make it right.
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