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CI baseboard and CI rads with 95F vs. 180F water

We have 2 zones, one with CI radiators, one with CI baseboard. The thermostats tally usage (i.e. time during which there is a call for heat).

When we used 180F or 140F water from the old boiler, the two zones ran roughly the same amount of time. Now we have a condensing boiler, and it has just gotten slightly cool here (around 5 degree-days each day) and I am running the boiler at a fixed (no reset) 95F supply temperature.

The radiator zone calls for heat maybe 1 hr 30 min per day. The baseboard zone calls for heat 10 to 11 hours a day.

Is this reasonable? I figure there is some difference in the heat output bevavior at low temps between baseboard and radiators, but that much??


  • Tony_8
    Tony_8 Member Posts: 608

    Is the EDR of the emitters. Bbd is quite a bit smaller than rads, needing more time at the same temp. Running at that temp and it's corresponding firing rate won't hurt you to run longer as long as it keeps up with demand. You could boost your water temp 5F and try it if it makes you feel better to have shorter zone run times. The rads should still run shorter.
  • Chuckles_2
    Chuckles_2 Member Posts: 52

    I don't really want shorter run times, the run times should be close to 24 hours in both zones. It seems that this will be impossible, and we'll have to use higher water temperatures than really needed by the radiator zone.

    But they were balanced when we had 180F water, so I don't understand about EDR. Certainly it depends on how much baseboard we have, but it shouldn't get so unbalanced with colder water, or should it?.
  • S Ebels
    S Ebels Member Posts: 2,322
    All other things being equal......

    What you're seeing is an excellent example of how a high mass heat emitter is able to transfer BTU's at a lower temp than a low mass (BB) type emitter. The rads are able to produce heat with the lower water temps to a much greater degree than the baseboard. Another plus for big iron is the fact that the high mass is able to store and release the BTU's over a longer period of time. There isn't much you can do to balance the system other than install a second temp zone for the BB and run it higher temp than the Rads if you want to keep run times down. If you want to be able to decrease water temp to the BB zone you'll have to add some additional length to that zone.
  • Plumbob
    Plumbob Member Posts: 183
    OH, OK

    > Another plus for big iron is

    > the fact that the high mass is able to store and

    > release the BTU's over a longer period of time.

    Yes, that makes sense. But it implies that things will get better when I am running close to 24 hours /day, because the business of "holding heat" won't be relevant. So during colder weather, when the issue of whether it's condensing or not is relevant, I shouldn't have the same situation I'm having now.

    Essentially. even though the water is so cool (95F), I am using "bang bang" heating (since the radiator zone is running only a couple of hours). But the radiators are holding the water and stretching out the effective heating time, while the baseboards are not doing that so they need more run time. That's how I interpret what you are saying, anyway.
  • Mike T., Swampeast MO
    Here's a simple experiment

    After running for couple minutes, measure the delta-t across one of your standing radiators and the delta-t across one of the baseboards. (Measure at the inlet/outlet connections.)

    Provided both are reasonably balanced at design output, you will likely find a significant difference.

    At such a low supply temperature your iron baseboards likely have VERY little convective output. But since their surface area is quite limited compared to the standing iron, their average temperature must stay higher (delta-t lower). With the supply temperature fixed they simply need flow for a significantly longer period of time.

    The standing iron rads on the other hand have a large surface area for radiation--their average temperature will be lower (delta-t higher), plus once the circulation stops they continue to give off the heat of the water inside.

    As the weather gets colder and your supply temperature higher you will probably notice that the two grow comparitively closer in run time. In all honesty, I'd try to establish a reset curve that results in the longest possible run time in the baseboard.

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