Water Heaters - combustion air gas water heaters and HRV

I have 2 gas water heaters in the basement, 4" exhaust vents go up through the roof, which is a ways up as the house is 1.5 stories.
Installer/contractor put in an 8" round duct for combustion air that goes directly to outside between floor joists.
Makes me wonder, if I have an 8" hole thru the side of my house (at basement level), what is the use of the heat recovery ventilator I planned on using? And why did I pay extra for sealing the house better than most?
Also, wouldn't 6" for intake air be enough?

This is one of my most popular subjects. Combustion air in an unconfined space.

By your own admission, this will fall under tight construction which is different from ordinary construction.

Under tight construction, you are allowed 1 opening to outside if it is properly sized and freely communicates with the outside air. A grill or louver over the opening changes the actual air available by values of 75% of opening size for metal or 25 % for wood.

Now for sizing of the exterior opening, under tight construction. You must have 1 inch of opening per 1,000 BTU and it must be larger than the cross-sectional area of the vent connectors (not the vent). But there are other factors to consider, a horizontal duct opening to outside air is rated at 1 inch of opening for every 2,000 BTU and a direct opening without ducting is rated at 4,000 BTU per inch.

Now, you can do some math. The 2 4 inch vent connectors have a cross-sectional area of a little more than 25 inches. An 8" opening is just a little over 50 sq. inches. So the first requirement is met. the vent connector area is smaller than the exterior opening.

An 8" opening with no cover, louver, just a simple 8" hole in the wall will provide 200,000 BTU.

An 8" opening through a horizontal duct would be just a little over 100,000 BTU.

If there is any type of outer cover to keep bus or critters out of your basement, now is the time to factor that.

The 6 inch opening you referred to would cover the vent connectors as it is just over 28 sq. inches, but the BTU value at max would be 56,000 BTU ducted as you described. But you have 2 water heaters, which is how many BTU combined?


Hope this helps and if you have any more questions, post them in this thread and someone will answer them.

the water heaters are 40 gallon each, with 40K BTU/hr rating. Normally one of the outlet valves is shut and the temperature control is set low, only use both when we have house guests.
Haven't been outside to measure the actual exterior opening dimensions yet.

Notuboo is absolutely correct and don't forget that the vent must be sized for the maximum BTU capacity and not what is "usually" being fired.

Is there any practical way that you can enclose these water heaters, essentially make a small room with the duct to the outside? You could use insulated removable panels for access.

Outside vent at end of 8" duct is 32 square inches.
Yes, I can easily isolate the 2 water heaters. They are in a corner of the mechanical room, so I just need to build the walls, and will make them easily removeable for access, with one side on hinges. good idea, thanks...