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Is constant circulation the goal?

28W Member Posts: 141
If you do a precise heat loss calculation for a house, and you have a mod-con system with ODR, then could you tune the system to the point where its Btu output at the radiators exactly matches the house's heat loss (at design temp)?  In such a scenario, would the circulator for the radiators run continuously, with the boiler cycling on and off to maintain the correct water temp?  Is that the goal?


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,541
    Pretty much

    Need a room sensor to achieve that most of the time,also most boilers don't have a modulation range low enough to allow it in shoulder seasons,or in the case of last winter,the whole thing!
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  • CMadatMe
    CMadatMe Member Posts: 3,086
    From the Godfather


    I should clean off the dust and pull out the laptop and post all his writings. Have every one in chronological order in one pdf file..

    There was an error rendering this rich post.

  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I like to look at it a different way.

    First, I am a homeowner with a mod-con with outdoor reset. But I am not a heating professional. On the other hand, I read a lot, especially here, and one can learn a lot at this site, both on these threads, and in the other Resources here.

    If constant circulation were the goal, it would be easy: just wire the circulators to run all the time (or at least whenever at least one thermostat is calling for heat), and let the thermostats turn the various zones on and off as needed. Let the boiler fire when its aquastat shows the boiler too cold, and stop when it gets too hot. This would work, but would not be ideal.

    The way I look at it, and the way I set up my system, is that I want to set the reset curves for my boiler (I have two zones, and two reset curves, one for each zone) so that the heat produced boiler just matches the heat loss of the building. Now in practice, this cannot be done because the heat loss is not exactly proportional to the temperature difference between the inside temperature and the outside temperature. But that is a very good first approximation. What throws it off is that external wind speed varies, and that changes the infiltration of outside air and the heat loss. Also, if you have good windows, the amount of heat gain due to the sun varies as the sun goes up and down and around, and as clouds get in the way of the sun.

    So what I did was to set the heat output of the boiler to very slightly exceed the heat loss under conditions of no sun and no wind. In "normal" weather, the thermostat will call for heat 8 to 12 hours and shut off. If it is windy or especially cold, it can call for heat for 18 hours.

    The way the controller in my boiler works, the reset control adjusts the target supply water temperature, and it modifies the firing rate to maintain that temperature. The temperature it uses is determined by the reset curve and is a function of the outside temperature. I have two reset curves, one for each of my two heating zones. Their heat needs are quite different, hence the need for two reset curves. But my boiler's controller allows for this so it is no problem.

    If I could set that curve perfectly, it would run 18 hours a day all the time except for cloudy windy days. I actually have the curves a bit off because my boiler is too big (even though it is the smallest one in the product line), and it would cycle too fast in warm weather that way. So I run it a little too hot in warm weather to keep it from cycling too fast. But since it is too warm for the load, the thermostats do not call for heat continuously in those warm days. Unless I could get a custom designed boiler to match my house, I cannot do much better. (Another possibility is to get an insulated storage tank with about a one-hour capacity, and put it in my smaller heating zone to reduce its cycling rate. But I do not have room for such a tank, and do not choose to pay for it (mainly the installation cost).

    So constant circulation is not the goal, but it is a consequence of trying to match the reset curve to the varying load of the house.
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