Water heater not drafting correctly

[waltabillion]

I had two 40 gal gas hot water heaters. I replaced one with a 50 gal heater. The specs for the new heater say a 3 inch vent pipe is adequate.

The directions from the new heater say to test the exhaust after "15 minutes of normal operation" holding a match near the hood. The match flame should be drawn into the hood, should not flutter or go out.

I assume "15 minutes of normal operation" means the unit should be heating for 15 continual minutes. When I test the exhaust , the match flame flutters and/or goes out.

When I try the older unit, the match does not flutter or go out, but it isn't drawn into the hood.

Both heaters vent to a near 90 degree turn then the two vents Y together a couple feet from the tanks. The Y is a 3 inch pipe. After the Y, the pipe expands to 4 inches. The 4 inch goes about 5 feet to where it merges with the furnace vent. The slope of the vent pipe up to where it merges with the furnace vent is about 1 inch per foot, probably more.

I had my home inpected 3-4 years ago, and at that time both heaters were exhausting correctly.

I suspect one of 3 things: I am not testing correctly; the vent configuration is not adequate given a slightly larger heater; the vent from the furnace to the roof is obstructed. (I visually inspected the roof vent and didn't see anything obvious.)

Any ideas?

[HayZee518]

as you know heat rises, so do the exhaust gases from your hot water tank. for the gases to try to accomodate a horizontal run [from your ninety degree turn] is seemingly impossible. to vent the horizontal runs should be inclined to at least a 45 degree slope towards the chimney. that bonnet on the hot water tank top draws ambient air upwards to form a draft with the rising exhaust.

[waltabillion]

as you know heat rises, so do the exhaust gases from your hot water tank. for the gases to try to accomodate a horizontal run [from your ninety degree turn] is seemingly impossible. to vent the horizontal runs should be inclined to at least a 45 degree slope towards the chimney. that bonnet on the hot water tank top draws ambient air upwards to form a draft with the rising exhaust.

Thanks for your reply.

I didn't change the vent design. And the one tank I replaced drafted properly.

The specs say a rise of one quarter inch per foot at minimum so I think the one inch per foot that I have should be fine. This is the same rise as the old tanks.

The 90 degree turns are on both the old and new tank. This is the same turn that was on the both old tanks.

My thinking now is, I am quite certain that both old tanks drafted correctly and now the old tank that I did not replace is not drafting quite like it should. That makes me think something is obstructing the vent between the basement and roof, probably at the roof.