B-:Vent stack above roof icing up and tripping 2nd stage pressure switch on furnace

greybeered

Am looking for helpful suggestions on how to prevent or reduce icing up on b-vent stack above roof during v. cold spells in Alberta winter, when overnight low can drop to -35 degrees Celsius (-31 degrees Fahrenheit).

Context is 2 mid-efficiency natural gas furnaces plus a 75 gallon conventional gas water heater that vent through single 8" b-vent running up through insulated external chase on 2-storey house that includes about 4 feet of b-vent above roof (up to the rain cap). The order of connection in furnace room is first 60,000 btu main furnace, second 40,000 btu furnace, and third conventional HWT, all connecting to 8" b-vent that runs up through the external chase.

Recent experience is that after a few days of very low temperature overnight, the rain cap on 4' b-vent stack (8" diameter) above second storey roof iced up, and I'm guessing that ice build-up on exterior ( and possibly interior) of that 4 feet of 8-inch diameter b-vent (above the roof) caused constriction such than 2nd stage pressure switch on the main furnace did not prove closed, leading to the main furnaces going into soft lockout, then hard lockout, during v. cold night. Of note is that the HWT is conventional, without induced draft.

Other than checking (and if need be replacing) the pressure switch assembly on the main Ruud UGPR natural gas furnace, my initial thoughts are to (1) insulate exterior of the existing 4 feet of 8-inch diameter b-vent above chase with rockwool, and surround that with 10-inch diameter single wall galvanized pipe, or (2) install post-purge delay switch on the main furnace with perhaps 2-minute or so post purge delay on the induced draft motor to reduce extent to which any combustion gases (from the main furnace) remain in the b-vent.

As I'm not overly familiar with the natural gas code, I certainly don't wish to take any measures that would contravene code; and I would prefer not to incur the cost now of redoing the b-vent from bottom to top (if I can avoid it), as all three devices are now near 10-15 years old. Hence any helpful suggestions on interim measures to reduce icing up w/b appreciated. Thanks.

mattmia2

i'm guessing the screen on the cap is icing. perhaps someone local will know what designs don't ice.