Trying to understand outdoor reset

Discussion in 'Boiler Forum' started by philtrap, Sep 27, 2013.

  1. philtrap

    philtrap Member

    Jun 19, 2013
    Civil Engineer
    Long Island, NY

    I'm trying to understand the boiler outdoor reset curve and it's workings... I'm specifically curious about the circulator pumps running and the length of time it takes to heat a home. Heres my question in kind of the form of an example:

    The outdoor reset curve is set so that at 65 degrees F outdoor the boilers output is at 120 degrees F. The inside of the house is 65 (and outside) and the call for heat is set to 75. The house heats up eventually so heres my question... I feel like it will take a long time to heat the house since the radiators (fintube) are only at 120. Without the reset, the boiler would throw out 180 to the radiators and I would think it would heat the house faster. The problem with taking longer to heat the house at 120 vs 180 I feel is the circulators would run longer using more electricity. I'm thinking once the system is stable and it gets colder outside the circulators won't run that much.

    I hope you understand my question, I just want to learn more and understand this...
  2. Dana

    Dana In the trades

    Jan 14, 2009
    With outdoor reset you can't/shouldn't use a deep setback strategy. If you let the house cool to 10F below the setpoint you have to then raise the outdoor reset curve higher to have reasonable recovery rates, and with modulating condensing boiler that means a lower average combustion efficiency.

    Ideally with the reset curve tweaked to the minimum (for max combustion efficiency) the circulation pumps (and burner) will run nearly continously, with VERY steady room temps. Yes, this DOES use more electricity- there's a trade off, but in most systems the significantly higher combustion efficiency more than makes up for the extra power use from an operating cost point of view.

    From a comfort point of view there's no comparison- running constant room temps means the temperature of the walls are never super-cold, making it less comfortable even after the setback recovery ramp has brought the room-air up to temp. You can usually be as comfortable at a couple degrees lower temp using outdoor reset than when using a deep setback strategy.

    Some reset controls have a programmable "boost" function for faster recovery time to be able to take advantage of setbacks, while still maintaining a low reset curve.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    IT can be a big deal on comfort - it depends a lot on the mass of what you're trying to heat up. I know that the air temperature doesn't mean a lot when you sit on a couch that has been cold-soaked from a deep setback, or the walls are cold, sucking the heat out of you! Much nicer to keep things pretty constant.

    The outdoor reset tries to keep the boiler temp just hot enough to keep the house at the thermostat setting while running will ramp up that boiler temp based on it being colder outside (and thus increasing the heat load) to keep things running at 'just enough' to maintain the temperature you've set. More time at lower outputs is more efficient, but can be a bear trying to recover from a deep setback.
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