Mechanicals-Atmospherically Vented

I’ve been on a couple code compliant blower door tests recently and was amazed to find atmospherically vented gas water heaters installed in these new homes, just like the one in the photo below.

Installing one of these water heaters adds a bunch of code requirements to the building.  First, because these water heaters use inside air for combustion, the room the water heater is located is required to have a specific volume of air for combustion.  This volume is calculated by the input BTU of the water heater divided by 1000 times 50.  Many gas water heaters have a BTU rating of around 40,000 BTU’s, (40,000/1,000) x 50, which equals 2000 cubic feet.  This water heater need to be in a room that is 10′ x 25′ x 8 feet tall.  That’s a big room for just the water heater!  Chances are, the mechanical room or closet the heater is located won’t be that big.  If not, additional combustion air will need to be supplied to the room for combustion.  There are a few ways this can be done.  A transfer duct that leads to an adjacent room, the room cannot be a bedroom, bath or attached garage.  The formula for the transfer grill size is input BTU’s of water heater/1000 = the net free vent area of the grill.  You can take credit for the volume of air existing in the room where the water heater is located.  So, if you have an 8′ x 8′ x 8′ mechanical room with that 40,000 BTU water heater, you’ve got 512 cubic feet of combustion air.  You still need an additional 1488 cubic feet of equivalent.  Using the formula for net free air, you’d still need an additional 300 square inches of vent between the water heater room and adjacent areas.  That’s more than two 12″ x 12″ vents.

Another option is to add a combustion air duct to the outside.  This is usually the option chosen.  The photo below shows a combustion air duct at the new home I recently tested.

This is a six inch hole that is open to the outside, there is usually no damper on the duct.  Every time the wind blows on the side of the home where this duct vent is located, outside air enters the house.  Every time the water heater is heating, outside air is being drawn in.  Every time a bath fan, kitchen fan, or dryer is in operation, air is coming in through this duct.  It needs to be there because of the atmospherically vented water heater or possibly some other venting combustion appliance.  I’ve been in homes where the end of the duct is turned up, like in the photo.  I’ve also seen these vents dumped into a five gallon bucket.  These methods, often unsuccessfully, try to limit the amount of air that is naturally pulled through these vents.  I’ve also seen these vent plugged or completely closed off, defeating the purpose they were installed for.  This can lead to spillage or back drafting of the appliance, which can increase carbon monoxide levels inside the home.  A potentially deadly situation.

When I blower door test a home with one of these ducts, I am not allowed to seal the duct for the test.  The duct is normally open, so I have to test with the duct open.  The photo below shows what this duct looks like in thermal imaging during cold outdoor temperatures.  The air entering the home is near freezing.

A better choice to an atmospherically vented gas water heater is one that uses sealed combustion. Another choice is to use an electric resistance water heater. The unit below is a Marathon water heater by Rheem, arguably the best electric resistance water heater on the market.

Still another option, which is growing in popularity is an air source heat pump water heater. I have personally yet to install one, but when the 1990’s electric water heater in my home eventually stops working or springs a leak, it will be replaced by an ASHP unit. One thing to be aware with this type of water heater, most units will use the heat from inside the home to heat the water. Not a problem in the spring, summer or fall, but during the winter, having a water heater robbing heat from inside the home might be an issue. You also don’t want to locate this unit inside a small space. On the plus side, (or during certain times of the year, could be a problem) there is some dehumidification happening while it’s heating the water. ASHP water heaters are the most efficient water heater that can be installed, also one of the most expensive.

One of the tricks in building a new home, limit the amount of holes that need to be punched in the envelope. Every one is a potential water and air leak. I also recommend avoiding heating equipment that uses inside air for combustion or requires an open vent to supply combustion air to an appliance, this includes water heating, space heating and solid fuel appliances like wood stoves and fireplaces. Do your research when choosing to install any of these appliances, don’t be afraid to ask questions of an installing contractor, preferably before the equipment is purchased and is being installed. What is required by codes and manufacturers may surprise you.

Leave a Reply

Your email address will not be published. Required fields are marked *