Some mfrs. have now begun putting diagrams in their fireplace manuals to address this issue. The typical form is an 18 inch min. lateral separation with the heater flue staggered at least 18 inches above the fireplace termination. If the other way around, noxious gases could get entrained down the cooling air concentric ring of the fireplace flue when the Fp is at standby. Normally, when in heating mode, outdoor air is entrained down around the firestops into the chase where it enters the appliance collar and vents up as the inner flue warms up. However, when the inner flue is cold, outdoor air, being cold and heavy, will tend to fall down this space and could enter the living compartment. If there is negative pressure in the home, this would tend to exacerbate the problem. Old triple walled pipe was a thermosyphon: outdoor air down the lower outer ring to the appliance, get heated then rise up through the middle section. Still, this is snap lock pipe and not tight fitting.

I've seen cases with oil flues where the fumes drove the homeowners out of the house. We found the chase top flashing rusted out as well as the top of the fireplace chimney and termination. All these had to be replaced then extend the heater flue at least 18-16 inches with the same make and model heater chimney.

HTH,
Bob

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Keep the fire inside the fireplace!
IAFCI Regional Director & Certified Fireplace Inspector
Certified Master Hearth Professional & HPBA BoD