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Mod / con boilers & p/s piping

Timco Member Posts: 3,040
So I was getting an error code on a mod con I installed. Code was boiler reaching it's high limit. I piped in direct return to get colder returns but one zone is just 30' of 3/4"fin bb and a small kick heater. Apparently that zone is too small, and the boiler was overshooting. I had the ODR unhooked because it sets back soo far. Even with a-stat set to 170* setpoint, it went over. Factory wants a p/s piping, but wouldn't that (primary loop) be just like a small zone? Wouldn't the boiler condense less? Wouldn't making the degree day setpoint 190* or higher raise the setback temps? Re-hooked ORD sensor. On a 20* day here it was making 128* water, which seems really cool. Other issue is that the bb needs warmer water than that.

Thanks,  Tim
Just a guy running some pipes.


  • eluv8
    eluv8 Member Posts: 174
    Modcon overshoot

    Which mod con are you using, what is the Supply/Return Delta T?

    A couple of thoughts: Buffer Tank, or Disconnect the end switch off the small zone so it cant turn on the boiler zone this way it would be supplied when other zones are calling(Could lead to underheating if heat loss is more per sq ft of other zones) This should work well if you run an aggressive outdoor reset curve and the boiler is sized well extending boiler run times. The other option would be wiring in a make on delay relay or a differential control to not allow the boiler to come on until the load is greater.

    Ideally I would pull the load from a buffer or a secondary coil on the indirect tank or not have such a small zone.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Flow Rate

    You could use a hydro seperator in place of the pri/sec piping. In your case you have to little flow thus the pri/sec advice. What is the heat loss of that zone that you are trying to overcome? You can then compare that to the footage of board and run a 30 degree delta-t to give you a better shot at condensing. You must use the 160 water rating for the board. Also, I would highley suggest you find out if they make a low temp aquastat for that kick. I do know that Beacon Morris does for theirs. If not you may have some problems with it.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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