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ODR Starting Temperature

NYplumber Member Posts: 503
Good evening wallies.

I am well practiced (well sort of anyway) in finding the lowest supply water temperature on the coldest day based off of the installed radiation. What I need some guidance on is finding the starting supply water temperature for when the boiler goes out of wwsd and into heating mode as well as the wwsd temperature (65*f or 62*f?). Installing a boiler next week as part of a case study and the HO doesnt want to change his recessed cast iron wall rads to trvs (too many and cost will be way up). Will use the thermostats as a high limit (like most mod cons are unfortunately ran) but for the time being will wire the tt with a jumper to see how it runs. Wont heat the home evenly, but that is a given.


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,665
    guidance on is finding the starting supply water temperature for when the boiler goes out of wwsd

    Conceptually, do a heat loss for when it is 65F or 62F outside, just as you did for design day. This will be a lot less, obviously. And use that temperature. If you used a computer program to compute your heat loss, doing it over with just a different outside temperature is easy. Just change the outside temperature and print out the report again.You should not have to re-enter all the data.

    For my system, my WWSD is set at 70F because that is the default setting on my boiler and I have seen no need to change it. I could probably lower it a few degrees.  I run my house thermostats at 69F.

    For my radiant slab, the supply temperature when it is over 50F outside is 76F. For my baseboard zone (I have a lot of baseboard for the heat load there), the supply temperature when it is over 50F outside, is 110F. These temperatures are a little high because if they are lower, there are control problems. In the baseboard zone, any lower temperatures results in rapid cycling because that zone just does not require much heat. For the radiant zone, the difference between the supply temperature and the desired room temperature is just too low and the differential in the boiler is +|- 5F and it would just never shut off until it gets much colder outside.
  • CMadatMe
    CMadatMe Member Posts: 3,066

    WWSD should always be the set point the homeowner sets his thermostats to. You want your curve to provided the water temp needed to satisfy the thermostat. If you start your heating curve lower then the thermostat set point you will struggle to reach it. May not be an issue in the swing months but Jan,Feb could cause an uncomfortable home. Gets worse if the homeowner sets back his therms at night..
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • NYplumber
    NYplumber Member Posts: 503

    Doing the division, required BTU / EDR an artificially low number results, so this method doesn't work. Set it to 120*f as a starting point.

    Ideas welcome.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,665
    If you start your heating curve lower then the thermostat set point you will struggle to reach it.

    For my system with radiant slab heat, I set my WWSD to 70F because that is the default and saw no compelling need to change it. I set my thermostats to 69F. That allows me to raise the thermostat a little when I am sick (rare).

    Now my minimum supply temperature to the slab is set to 76F.  This allows the boiler to still be hot enough to supply enough heat to let the thermostat shut off the circulator to that zone. I could set the supply temperature a little lower, but then in warm weather the slab would not radiate enough heat even at minimum firing rate to use up all the boiler could supply, modulated all the way down, so it would cycle too often. As it is, my reset curve for that zone is flat at outdoor temperatures all the way down to 50F where it starts up and gets to 120F by the time it gets down to 6F outdoors.

    I tried calculating the inflection points on the reset curve, but quickly found out my heat load calculations are not accurate enough. The math was done accurately, but the R values just are not good enough because I do not really know what is in the walls and what the construction really is, and I do not really know the air leakage rates. When I used those calculated values, the reset curve was too high; i.e., supplying too much heat. I spent a long time adjusting that to more suitable values. It is still a little high when it is quite warm out.

    I do not use setback, because the slab downstairs has too long a time constant for that to work. And both upstairs and downstairs, the reset curve is so close to the actual load that it would take much more than 4 hours to recover from setback. So why bother.
  • Gordy
    Gordy Member Posts: 9,531

    Knock 10-15% off your calculated heat loss then try your math.  Programs are padded.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,665
    Knock 10-15% off your calculated heat loss

    I am sure you are right, as far as these things go. But I imagine my assumptions about the construction inside my walls are wrong too. I have insulation inside those walls, and it includes thin "builder's batts" and blown in urea-formaldehyde foam. But those walls are just two-by-fours. Over the top floor rooms is about 14 inches of blown in fibreglass.  But I do not know the construction at the corners or around the windows. I do not even know if the radiant slab is insulated or not (I am pretty sure it is not.) Except ..., ..., ... .

    Now I do not need to calculate it anymore because I took a couple of years adjusting the reset curves (each zone has its own curve) so it works just fine all the time except when it is quite warm outside. Then the curve is a little too high so that the boiler does not cycle on its limits. It is clear that I need a smaller boiler (about half the size I have), but mine was the smallest in the product line (80,000 BTU/hour input) at the time. From that manufacturer, it is still the smallest.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    start temp depends on

    your boiler firing differential and your emitters.

    Your absolute minimum start temp must be greater than the boiler firing differential plus room temp. so if the boiler is set to start at 80, with a 20 degree firing differential, and you have a 70 degree room temp, it won't fire until the water comes back at 60... unless it's a "split differential" where with an 80 target it fires at 70 up to 90 for an 80 average. Gotta know how your control works there.

    for convective baseboard I usually bump it up to a 90-95 minimum regardless
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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