Might they have crimped the pipe where it goes through the roof? Do you have a big snow accumulation on the roof? Generally, unless it is designed for it, you can't retrofit a power vent. The ridge cap vs box vents shouldn't make a difference although if your house does leak air into the attic, that would likely exhaust it better. If this happens regularly, you could try leaving a basement window cracked to provide makeup air as an experiment. Most houses are loose enough where that isn't a big factor unless you have a combination of events: the furnace is running, the dryer is running, the range hood is exhausting, and maybe bathrooms. All of these things are trying to exhaust air and it has to come in somehow - the flue for the WH might be the easiest place. As we tighten up homes, providing makeup air for these types of things becomes more important to engineer into the house. Many of the newer and more efficent units are closed combustion, they draw their own combustion air from outside. A power vent doesn't, it is typically used where you need to force the air further or out a different path than a conventional flue would support. An air-to-air heat exchanger has the ability to help balance the load and recover most of the energy you've paid for, but those that will work in super cold climates get much more challenging and expensive.