Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

newbie - modcon boiler control, and pump questions

hi everyone,

this is my first post, but i've been trolling for a few months now - thanks in advance for any advice you can offer. i'm in the middle of a major home remodel where i’m doing a multi-zone radiant install, myself. i’ve done a ton of homework (including studying Seigenthaler’s book, building my own software to validate my design, and generally obsessing about the details for months), but there are still a few fundamental questions for which i could use some advice.

first of all, i’m seriously leaning towards going with two triangle tube boilers, with tekmar controls and thermostats. my current thinking is to use grundfos pumps.

one question i can’t seem to really find a clear answer to relates to temperature mixing with modulating boilers. in standard fashion, i have two boiler output temp requirements - “hotter” for the primary DHW needs (indirect tank), and “120ish” for the PEX-in-concrete loops (8 zones). i don’t really have radiators in this house, so i don’t have a need to pump “hotter” water to heating zones.

given that i’m going to configure the controller to consider the DHW as a primary, turning off demand to secondary when primary is called for, my question is, do i need to use a mixing valve configuration, or will the controller just tell the boiler to ramp up the output temperature based on primary loop demand? i understand that with a non-modulating boiler, i would need mixing loops to get the radiant zone temps down to the 120, but with a modulating boiler, i don’t believe i do. from what i can tell, and i admit i’m poorly informed on this particular point, i think the controller will instruct the boilers to fire the primary temp up to 180 but throttle back the boilers to 120 as needed if the demand is from the secondary floor loops. correct theory? or, is the boiler water temp output generally fixed and i have to “mix it down?"

question number two relates to the circulator of the primary boiler loop. in my design, i can “see” the volume and flow requirements for each zone based on my heat loss calcs and such, and therefore can get pump curves and choose circulators… that’s pretty straightforward. but for the main circulator loop where the closely-spaced Ts are circulating off of, i really have no idea how to size that circulator. my gut tells me that a delta-P pump isn’t required on that loop, whereas i do plan on using the smart variable speed grundfos’ pumps to feed the zone manifolds. does anyone have any advice, or can point me in the right direction to understand the math, rules or theory behind sizing the “main loop” circulator?

again - i know i’m not a professional, but i’ve decided to do this job myself and i’m confident in my ability to learn, and i’m willing to make mistakes. the end results cannot possibly be worse than the house pre-construction, and i’m dedicated to learning.

appreciate any guidance.



  • Zman
    Zman Member Posts: 7,588

    What you are talking about is called domestic priority. The boiler will turn off the heat circulator turn on the indirect circulator and ramp up  the boiler to satisfy a domestic call. You can set up a "Time out" on this feature so the house won't freeze during a malfunction.

    Tekmar makes great products. they are especially handy if you are looking for indoor feedback or if you like having scene and remote capabilities.

    The triangle tube boiler can do all the controlling you have described onboard with no additional controls required.The boilers will manage each other using the "cascade" feature. The outdoor sensor will tell the boiler what temp to fire at based on the outdoor reset curve you select.

    I am  not sure what you have in mind for piping. you should end up with a circ for each boiler, the 110 comes with a 15-58 you just need a check valve.

    with most indirects the 15-58 works well also.

    On the radiant, it really just depends. If you keep your loops in the 1/2" x200' range the grunfos alpha generally works well unless you have an enormous number of loops (somewhere around 20 loops)

    You don't need a main circulator loop. The radiant circ will pull water through the hydrolic separator.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein