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primary/secondary

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JL_2
JL_2 Member Posts: 17
On a primary/secondary system with a cast iron boiler the first set of closley spaced tees are for the sidearm. The secondary pump is bigger than your primary pump. Doesn't this create a mixing valve out of the first tee? So technically you don't have 180F degree water running through your sidearm heat exchanger? What if you had a secondary load of five or six high temp. baseboard zones all grouped together on one manifold with one pump, bigger than the primary pump?

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  • ALH_4
    ALH_4 Member Posts: 1,790
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    a < b

    You mention that the pumps are different sizes, but it all depends on the flow rates. The primary circuit should have a flow rate greater than the secondary circuit. otherwise, as you have pointed out, you are tempering the supply to the zones. Additionally, if the system circuits dump cold return water directly into the boiler without being tempered by the primary pump, you could end up with flue gas condensation problems in the boiler.

    I would suggest using a Thermic Valve to protect the boiler from flue gas condensation. Also, the pump sizing and flow rates should be probably be checked.
  • JL_2
    JL_2 Member Posts: 17
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    flow rates

    If the primary loop flow rate is 8gpm and the sidearm flow rate is 10 gpm. What is the solution? Should you parallel pipe the sidearm? Keep in mind if this is a conventional boiler the primary pump is going to run with the boiler.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    sizing

    There is no point in running the sidearm at 10gpm if the primary flow is limited to 8gpm. The extra 2gpm just recirculates through the sidearm and lowers the supply temperature. The flow rates were determined for a particular temperature rise/drop in order to achieve rated performance of each piece of equipment. It sounds like the boiler might have less capacity than the sidearm can accept. You might drop the flow rate of the sidearm to 8gpm and use the thermic valve on the primary loop, or another thermal protection method for the boiler.
  • JL_2
    JL_2 Member Posts: 17
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    Primary Flow Rate

    I appreciate your responses. I guess I am confused. What determines your primary pump size?
  • Brad White_2
    Brad White_2 Member Posts: 188
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    If by primary

    you mean the boiler circuit, it is a function of total heat to be delivered and your desired temperature drop.

    In a perfect world, your boiler would match the load, say 100,000 BTU's per hour (or put another way, 100 MBH). Say it is supplying 180 degree F. water.

    At a 20 degree temperature drop this would be 10 GPM. 180F out, 160F back. Average = 170F

    At 30 degrees temperature drop this would be 7.5 GPM. 180F out, 150F back. Average = 165F

    At 40 degrees temperature drop this would be 5 GPM. 180F out, 140F back. Average = 160F


    From each of these combinations you can bleed off X amount of 180F water to mix down to any lower temperature if even desired.

    You can also see how wider temperature drops create a need for more precise flow management, can't you? A missing 1.0 GPM at higher temperature drops becomes a larger percentage of the whole.

    I would like to reiterate the other excellent advice you got above, particularly Andrew's point about protecting the boiler. May I also add that, the way you describe the setup, the DHW heater is in series and indeed will form a T-mix at that tee and dilute the downstream available temperature, perhaps below what you will need. A solution would be to run that DHW heater in parallel, not in series, so that it gets 100% of the flow it needs (at a different temperature drop perhaps). Either way you are limited to your total boiler output.

    If the baseboard zones are all high temperature and the same theoretical temperature, they should be piped in parallel also, so that nothing upstream dilutes their supply temperature.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    P/S

    This article by John Siegenthaler might help explain.
  • JL_2
    JL_2 Member Posts: 17
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    Problems

    I read the article in April's PE and this is why I have questions. I am used to piping modcon boilers and am having a problem with multi-load systems. I guess the first problem is I am used to parallel pipeing the sidearm and the boiler turns off the primary pump, (essentially having two primary loops in one system). I suppose my question is what is the benefit of using a primary/secondary piping system in a high temperature (baseboard or Ultra-Fin) system with a cast iron boiler?
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