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Natural Draft Installation Tips

Tips for proper installations of a natural draft tankless water heater for indoor use.


PLEASE READ: This page and understand that back-draft of exhaust is not only a potential risk for Carbon Monoxide Poisoning, but can ruin your tankless water heater.

IMPORTANT NOTE: Few installations of natural draft tankless heaters made by any manufacturer meet combustion air requirements. They simply require mounting in a large open room. Few homes and cabins have a "unconfined" combustion air space that is large enough. "Unconfined" means an open room with no closable doorways or openings. (Even the largest garages and basements seldom meet these minimum cubic feet interior space requirements) There are standard codes that require minimum cubic feet of room space relating to the btu capacity of gas fired appliances.

For example: Our EZ-101 model is rated at 12kw/41,000 btu. At the standard recognized code for gas fired appliances being a minimum of 50 cubic feet of combustion air supply per 1,000 btu. This means that our EZ101 requires an open room "unconfined" combustion air space of 2050 cubic feet. (Our 81,891 btu EZ-202 requires 4050 cubic feet of combustion air space if mounted in a building).

In comparison, A standard 66/72 passenger school bus interior is 8 foot wide and 6 feet tall and about 35 feet long. Standard exterior length is 39.5 feet (class "B" CDL). Therefore, the interior of a typical school bus is 1680 cubic feet which is still 20% too small for our EZ101 heater to meet the minimum indoor mounting combustion air space code. A typical 26X24X8 two-car garage has almost exactly 5000 cubic feet of combustion air.

NOTE: If more than one gas fired appliance is in the same room, the btu ratings of all appliances must be added together and the room combustion air space must not be less than the local, state, or Federal code requirements. This is the reason that ALL manufacturers of tankless heaters recommend that indoor installations be made using only a direct vent model. We consider the natural draft models to be used only for uninhabited large buildings, barns, large storage sheds, lean-to, and outdoor type applications. We do not recommend that natural draft tankless heaters be installed indoors for any reason.


A natural draft tankless heater has no blower fan and does not connect to an A/C power source. A natural draft type tankless heater takes it's "intake" combustion air from inside the room in which the heater is mounted. It then uses a natural draft to pull the exhaust out through a flue pipe exhaust.

NOTE: This page and the information on this page does not apply to any EZTANKLESS brand tankless heater that is direct vent (connects to A/C 110V power supply).


The tankless heater freezes internally because cold air is drawn into the exhaust while the heater is not turned-on. This especially can happen during the night in cold climates.

The CAUSES and the components explained:

If a natural draft tankless water heater is installed indoors and vented through the roof or a wall, when your furnace turns on during the cold winter months, there is a possibility that the furnace can draw cold air from outside through your tankless water heater exhaust.

THIS APPLIES TO: Any gas-fired appliance that uses room air for combustion and exhausts to the exterior of the home, cabin or building including exhaust hoods over ranges can cause your tankless heater to freeze.

THIS INCLUDES USERS OF FIREPLACES, WASTE OIL FURNACES AND WOOD OR COAL BURNING STOVES. The "back-draft" caused by the suction of other devices will bring cold air into the tankless heater through the exhaust pipe (flue) and this cold air will travel over the heat exchanger. Over a short period of time this can cause tubing inside the heat exchanger to freeze. The subsequent expansion from frozen water inside the heat exchanger will cause cracks and breakage in the interior of the heater. This will lead to a substantial water leak when the heater thaws and usually the tankless heater itself is irreparable.

THIS CAN BE AVOIDED! Please follow the simple step below to protect your water heater from unwanted freezing.


Install your natural draft tankless water heater exhaust so that it is drawn into the draft of the main chimney. This way you are eliminating the risk of cold air being pulled down your exhaust pipe when the tankless heater is not running. As well as the risk of exhaust gas back-draft when the tankless heater is running at the same time as another "fired" and exhausting device. Basically, this is all about the fact that; fire of any kind consumes air and the air flow of this process of combustion needs to be understood and controlled.

CAUTION: We do not claim to be the authority on this type of technology. In fact our company recommends that indoor installations of any tankless heater be done using only a DIRECT VENT model.     ( EZ-Ultra  or  EZ-Deluxe )


Information for "Remote" Users: Cabin, Camping, Barn, Tool-Shed, or Workshop.

A lake or deep-woods cabin, campsite, workshop or outbuilding is perfect for the installation of a tankless type water heater. In rural America it is common for a homeowner to have a barn, workshop or tool shed that is some distance from the home. If cold water is plumbed to this building, a tankless unit can supply hot water for showering, cooking, washing vehicles, hand washing, and general cleaning needs. You may wish to install the unit near the sink and in this case a small unit will be perfect and economical to purchase. You may wish to use one that is fired by LP gas and a small tank can be easily installed for this purpose and may easily last the whole season or longer. Just like your outdoor barbeque, you must remember to turn off the gas valve after each use to insure that the LP does not leak away. Some people have large LP tanks for the furnace in the workshop and/or rural home. We have models for this type of gas. Additionally, in a workshop or garage installation there is no open pilot light to worry about.

In fact, we have customers with remote mountain cabins and campsites who are using our EZ-101 model with a small LP tank. Some are getting their water from a gravity feed system via a holding tank or down hill gravity fed delivery system. Some are using gasoline powered pumps to pick-up mountain stream or spring water and deliver it to the cabin via hose or plastic pipe. Our customers never cease to amaze us with their diverse and innovative ways to supply their remote locations with easy hot water for cooking, cleaning, and showering.

Disclaimer: We cannot be held responsible for freeze damage as this is a common problem with the draft type tankless heaters from all manufacturers. We make every effort to explain these scenarios on our web site at the appropriate locations for every one of our products that are of this type of construction. If you live in a cold climate, most experts agree that a direct vent is the best type of heater to use. The reason manufacturers are switching to direct vent (See our "Deluxe" model) or (See our "Ultra" model) is because of several reasons which include the possibility of back draft freeze damage.


If installing a natural draft tankless water heater inside any enclosed farm or industrial building or workshop, if the natural draft tankless heater shares the same space as any combustion device that has an exhaust pipe of any kind, there is the potential for a back-draft.

-The EZ Tankless Team