Gas field in specified area, alternate...

But wait, there's more!
Another approach I've found on the webs, is to make a Gas Match Field, which makes a new field based on some other value.. eg. Density or Temperature.
You can actually specify a Density that is not the Pyro-sim's density, by using a Source Volume (turns out they don't always have to go into the last input). Point that to your custom volume
Finally, set the control mask to your new field.

All three of these nodes should be plugged into a single merge, which then goes into the Velocity Update of the pyro solver. (or the advect...)

I think this approach might be more flexible, if you are using it to refer to an internal pyro-value like Density or Temp?

Another approach I've seen skips the external source volume and copies the Temperature from the pyro solver itself. They use Gas Match to make a field which is "equal" in size/type to Temperature,
Next they use a gas calculate node to copy the Temperature to the custom field,

Finally, They  use a gas field wrangle to fiddle with the newly created field. Normalise values, or do whatever..

They use the custom field as a mask for dissipate & turb.
Quite a neat way to mess around with the temperature.

Gas field only in specified area. Stolen from the Sidefx site

1.  In the DOP network, add a SOP Scalar Field node after the Pyro object.
   
   • Turn on Use SOP Dimensions.
   • Set the SOP path to the path of the fog volume object (for example /obj/mask_object). Make sure Use Object Transform is on.
   • Set the Data Name to something indicating the purpose of this field, for example NoiseMask.
   This attaches the fog volume as field data on the Pyro object with the given name.
   
2. Create a Gas Turbulence node and connect it to the fourth ("Advection") input of the Pyro solver.
   
   • On the Bindings tab, set the Density Field to the name of the mask data, for example NoiseMask. The effect of the node is multiplied by the value of this field.
   • Use the controls on the Turbulence Settings tab to control the amount of turbulence added.
     
     
     
     
     Important thing to note - by default the control field will not be 100% in control.. You'll see some "leakage" of turbulence. This is because the Control Influence is set to 0.5 by default.. Whack it up to 1, and maybe tweak the remap-graph to fine tune the effect of the turb/whatever.