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How to size the secondary loop circulator when the primary loop is driven by a modulating boiler?

Chris_NH Member Posts: 12
edited April 2019 in THE MAIN WALL
Here is my understanding: In a hydronic (home heating) system with a modulating boiler that has an internal variable-speed circulator, outdoor temperature reset feature, and that is connected to a primary/secondary piping layout, the flow rate in the primary loop is set by the boiler itself to maintain a given temperature gain within the primary loop, for example 20 deg. F. On relatively warm days, when the boiler modulates its firing rate down, the boiler reduces its circulator pump speed, and thus the primary loop flow, to maintain the 20 deg. F temperature gain. On relatively cold days, when the boiler modulates its firing rate up, the boiler increases its circulator speed to maintain the 20 deg. F gain. Is my understanding correct?

Then, assuming it is correct, how do you specify (select) the size of the circulator in the secondary loop, which I assume has a fixed speed. My guess is that the secondary loop circulator should be selected to achieve a secondary loop flow rate equal to the max flow rate of the primary loop, which occurs when the boiler is producing maximum heat.

And finally, what’s happening on warm days when (if my assumptions are correct) the primary loop flow rate would be significantly lower than the secondary loop flow rate?


  • hot_rod
    hot_rod Member Posts: 19,256
    I'm yet to be convinced running that boiler circuit on a fixed ∆ is a good idea? For all the same reasons you would not want to constrain the distribution side ∆. Explained in I-23 attached.

    How are you piping the P/S? The "Harveys horseshoe" method should work with one less circulator being needed.

    The circulators need to be sized to move the maximum load.

    Piping size also.

    That "conveyer belt' type of primary loop often was hard to size a circ for. It would often be a high gpm requirement with very low head requirement, not a common range for most circs. Add the series temperature drops and the "loop" method is not always a workable piping method.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Brewbeer
    Brewbeer Member Posts: 616
    My system has a fixed speed circ on the boiler (primary) loop, and a variable speed circ on the system (secondary) loop, so neither, either or both circs can be variable speed.

    My understanding is that you want the flow through both loops to be approximately the same, which is easier said than done.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • Chris_NH
    Chris_NH Member Posts: 12
    edited April 2019
    @Brewbeer: I also have read that primary and secondary flow rates should be the same. And this makes sense. If primary flow is greater than secondary you get short-cycling. If primary flow is less than secondary than the temperature of the supply water reaching the secondary loop radiators will always be lower than the supply temperature measured by the boiler. BUT the piping examples shown in the installation manual that came with my new modulating boiler illustrate either a single loop system (so the boiler's circulator is the only circulator) or a primary/secondary loop system with one (or multiple) fixed rate circulators in the secondary loop. In both example applications, since primary flow rate is variable, the primary and secondary flow rates will in general be different, and often quite different.
  • Chris_NH
    Chris_NH Member Posts: 12
    edited April 2019
    @hot_rod: Thank you for the idronics reference. I'll read it and comment if (which I almost surely will) have questions.
  • Ron R
    Ron R Member Posts: 16
    A modulating circulator inside the boiler is interesting and I dont think I have seen that yet. From the modulating boilers with Outdoor re-set I am familiar with, the boilers circulator remains a constant speed during a heat or domestic demand, and instead the burner is what modulates to either maintain a constant TD during low demand or lower the supply temperature during milder weather.
    The relationship between Primary and secondary flow is really interesting--I have been reading Dan's book "Primary-Secondary Pumping Made Easy"---if I understand this right, Equal flow between the Primary and Secondary would mean that Boiler supply water temp equals System supply water temp, and I think if you had Higher Flow in the Secondary (System loop) it would mean cooler water (less than Boiler Supply temperature) delivered to the system.
  • Jamie Hall
    Jamie Hall Member Posts: 21,090
    The hydraulics of primary/secondary piping and pumping are horribly complex. It depends on how the separation is made -- low loss header? Closely spaced T's? Pixie dust? -- and the head loss through the circuits and the pump curves and... and so on.

    If the two pumps are pumping at the same rate, then almost all of the boiler water flow will also pass through the heating loop. If the loop pump has more flow, there will be some recirculation in the loop. If it's less, some boiler water will be diverted through the primary loop connection.

    The idea is to let the boiler do its thing to the water -- correct flow rate through the boiler, and it will add whatever BTUs it's fired for. And then let the loop do its thing; enough flow to deliver the heat needed at a reasonable delta T.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 19,256
    Of the 3 conditions it would be least likely that primary and secondary flows are always exactly the same. I guess no need for a P/S if that were always the case?
    In any type of zoned system, most are, the load flow rate is always different or changing.
    The boiler circulator modulating along with burner modulation, moving the exact same amount of energy to the system as the boiler is adding would make sense. As long as minimum boiler flow rate is present. Isn't that how some of the Viessmann have been operating for years?

    Hydraulic separations intent from my understanding was to prevent pumped circuits from interfering with one another, un-wanted flow conditions, when flow rates change or stop.

    I'm not so sure condensing boilers were part of the thought process back in the days that Gil developed the concept. In some instances the separation function will raise return temperature to the boiler, possibly moving it out of condensing operation.

    But the separation concept did allow for high pressure drop type of boilers to get along with miss matched distribution flows as the boilers often need a much higher head type circulator.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream