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Setting Outdoor Reset Curve

Chester
Chester Member Posts: 83
Any suggestions on a good starting point for setting the curve on my new Greenstar boiler? My design day outside temperature is 5 degrees F, my Manual J design day heat loss is approx. 30,000 Btu/hr and I have enough radiation capacity (cast iron radiators) that I should be able to meet that heat load with 130 degree F water.

According to the FW200 instructions I can either program a "traditional base point/end point" curve or I can select my heating type (radiators) and fine tune the default curve.

To start with, I thought I'd keep the default warm weather shutdown (68F) and the factory default basepoint (76F), and then set my design water temp at 140F.

Comments

  • Ironman
    Ironman Member Posts: 6,868
    76* SWT may be a little too low, but there's no way to know for sure without knowing your EDR and comparing that to the output the rads will give at selected SWT. There are performance charts that you can find online (maybe even on site) to give you those numbers.

    One of the nice things about the FW200 is that it functions on a learning curve and after a couple of weeks, it will begin to cycle the boiler based upon that. In other words, it will begin to compensate for MINOR errors in the settings of the reset curve.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Chester
    Chester Member Posts: 83
    Good advice. thanks!
  • unclejohn
    unclejohn Member Posts: 1,757
    Is this a building or a house? I would think of a building as having multiple tenants. If you want to be comfortable I would not even set a WWSD your room thermostat will take care of that. I would set the minimum water temp. at 100*f and max at 140*f for starters and adjust from there.
    Jean-David Beyer
  • Chester
    Chester Member Posts: 83
    It's a house. Thanks for the response!
  • Rich_49
    Rich_49 Member Posts: 2,692
    Regarding WWSD and what temp to use . You really have to look at the occupants needs . There is no set temp for this . An older person may require or sense the need for heat at a higher temp than the setpoint referenced by HatterasGuy . He and I are robust creatures whereas a 60- 80 yo person may due to health reasons need that heat on at 68* ODT .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Chester
    Chester Member Posts: 83
    Ironman said:

    76* SWT may be a little too low, but there's no way to know for sure without knowing your EDR and comparing that to the output the rads will give at selected SWT. There are performance charts that you can find online (maybe even on site) to give you those numbers.

    One of the nice things about the FW200 is that it functions on a learning curve and after a couple of weeks, it will begin to cycle the boiler based upon that. In other words, it will begin to compensate for MINOR errors in the settings of the reset curve.

    Can you explain the FW200 learning curve a bit more? One of the things I'm noticing about the newly installed boiler is that it will shut down even though the thermostat is still calling for heat. For example, it'll be 50F outside and the boiler will be sending out 90F water, but then the boiler and secondary circ pump will shut down even though the zone valve is open and the t-stat is still calling for heat. Is this part of the learning curve you mention? Otherwise I can't figure out why the boiler wouldn't just run until the t-stat shuts it off. It's not like it's bumping against the warm weather shutdown or the boiler high limit. My current ODR curve is set for 80F/130F and the house stays warm.
  • aircooled81
    aircooled81 Member Posts: 197
    maybe this chart i configured for a job i just did might help illustrate. I used peerless as a refrence to configure the chart for my needs locally.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    My W-M Ultra 3 has the Warm Weather Shut-down feature. It comes set to the default temperature, which is 70F. Since my indoor thermostats are set to 69F, the WWSD never takes effect, so it does not matter if I have it or not.

    Of course, I could set it lower, but what for? If I wanted heat even when it is 70F or greater outside, I would have to raise the WWSD set point, but I am not that old yet. And even if I were, I would have to raise my reset curves since at the current settings of the reset curves, I could not get the house any warmer than about 71F.

    My downstairs zone in radiant in a slab at grade, and the boiler is a little oversize. I set the minimum temperature of the reset curve to 80F and it stays there until the outdoor temperature goes below 50F. The reasons I do not let the reset curve to go any lower are:

    1.) If lower, the slab does not reject heat into the house as fast as the boiler the boiler generates it at minimum firing rate and it cycles too rapidly.

    2.) The hysteresis of the control watching the reset curve is +|- 5F. If the reset curve were set to something like 74F, it would never shut off because the supply water could not get much lower than the temperature of the slab, and if the slab were warmer that 5F below the set point of the reset curve, it would never shut off.

    At the high end, I originally set the temperature to 120F as that seemed to be the most I would wish to deliver to the slab. And that worked well for several years. But last winter, the outside temperature went down to about 2F even though design temperature around here is 14F and I could not get enough heat, so now it is set to 130F when it gets to 0F outside. It rarely gets that hot, though. Your mileage will surely vary, since your boiler, your environment, and your house are surely different.