[hearthman]

Ansi Z21.88

The standard your Fp is listed to provides you enough info. The ANSI Std., section 2.32 Joints in Direct Vent Systems, tests to 25 Pascals positive pressure in the combustion chamber with the vent collar plugged. They decided to use positive pressure over negative because the test had to be done with the unit cold and yet replicate as closely as possible burning conditions. The unit must have passed at least one delayed ignition test before this test to insure it wouldn't flunk this leakage test after a severe delayed ignition. This delayed ignition test will scare the hell out of you!
With the unit plugged up and pressure taps in the combustion chamber and vent, it is pressurized to 0.1 wci (24.9 Pa) The allowable leakage rate for the combustion chamber is 4% of the input BTU rating in thousands times 15 cubic feet of flue products based on 10 cu ft. of wet products and 5 cu ft. of excess air per 1000 btus. The vent can leak up to 8% under these same conditions.

Since the unit expands as it gets heated, this test more accurately reflects true operating conditions than sticking it into a box and sucking it down. However, most mfrs. have conducted their own pressure house studies to know the depressurization limits of each unit and where the critical points are through IR thermography, tracer gas, and smoke puffers. This info. is proprietary and closely guarded. Beware of claims someone has "tested to a certain std.". Did it pass? Says who--the mfr. or a recognized testing lab? Biiiig difference.

Hope this helps.