How reset the UFactorTimesAreaValue of Heating Coil Water loop?

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I believe the problem is that your variable UFactorTimeArea begins with a capital letter and ruby treats that as a constant. Ruby does not allow you to make assignments to constants inside of methods, only within the class declaration. If you make the variable begin with a lower case letter I think you will be ok.

The very next line is troubling though. CoilHeatingWater::isUFactorTimesAreaValueAutoSized returns a bool so UFactortimeArea is going to be a bool, however you are trying to call the method isUFactorTimesAreaValueAutoSized on it. This method is going to be undefined for bool. Instead just say if( uFactortimeArea ) and then call heating_coil.setUFactorTimesAreaValue(foo).

Finally there is a simpler method of grabbing the hot water coils off the loop. Just do this...

air_loop.supply_components(OpenStudio::Model::CoilHeatingWater::iddObjectType) Thats going to give you only the hot water coils as generic ModelObjects. You will still need to cast them into heating coils, but it will save you a check to is_initialized. The full thing will probably look something like this....

air_loops.each do |air_loop|
  supply_components = air_loop.supplyComponents(OpenStudio::Model::CoilHeatingWater::iddObjectType)

  supply_components.each do |supply_component|
    heating_coil = supply_component.to_CoilHeatingWater.get # <-- this is safe because of how we called supplComps...
    uFactorTimeArea= heating_coil.isUFactorTimesAreaValueAutoSized

    if (uFactorTimeArea)
      heating_coil.setUFactorTimesAreaValue(user_input) # Or just leave autosized
    else
       # Or just leave the hardsized value that was already there
       heating_coil.setUFactorTimesAreaValue(some_other_user_input)
    end
  end

end

Alternatively if you wanted to get all of the heating coils in the model you could just do...

hot_water_coils = model.getCoilHeatingWaters

If you wanted to then go back and retrieve the air system you could do hotwatercoil.airLoopHVAC. I suspect you already know about this though since you probably had to do model.getAirLoopHVACs to fill your air_loops variable.