How to Vent a Tankless Water Heater with Furnace

When it comes to venting a tankless water heater, you are most likely to be confused by the two main venting approaches practiced right now. The situation gets even more confusing when it comes to venting a tankless water heater with furnace. It’s always advisable to talk to a qualified plumber to help you decide the best venting system based on the type of heater and furnace you have in your home.

Venting a Tankless Water Heater with Furnace

There are two ways to vent a tankless water heater with a furnace. The first is a power vent that uses indoor air for combustion and an outside vent to exhaust gases. This type of venting system doesn’t need an intake valve, but there has to be enough air inside the room. The second option is a direct vent that uses outdoor air for combustion and requires two vents: one for the air inlet and one for the gas outlet.

Since venting a tankless water heater with a furnace becomes a complicated subject when you think of the two situations above, we have compiled this article to shed more light on the matter. Be sure to read between the lines when deciding between the standard and modern tankless water heaters.

What Are the Venting Requirements for a Tankless Water Heater?

The requirements for venting a tankless water heater depend on the type of venting system you use, which we will discuss in the next section. However, every venting system should ensure the heater acquires enough combustion air.

The required combustion air may be from the outdoor atmosphere for outdoor tankless water heaters or drawn from indoor air for heaters installed inside the house.

While outside water heaters freely and easily get air supply, you might require extra air ducts and inlets for an indoor tankless water heater.

When you opt for a horizontal installation system, you’ll need a downward gradient with various components, including:

  • A vent adapter
  • A stainless steel vent hose
  • A small plastic or metal cap as a vent connector between parts inside the house and those outside it
  • Two ninety-degree elbows, one for the termination point of the vent

For a vertical system, which requires an upward gradient, you will need the above components and a vertical condensation trap. However, you’ll be exempted from adding the trap if the tankless water heater you buy cones with a built-in condensing unit.

With the vertical system, you’ll also require a tight-fitting rain cap for the termination and a storm collar. You can also make the termination with a ninety-degree elbow to keep out rain and debris.

Where Do You Vent a Tankless Water Heater?

Tankless water heater venting is possible in three ways. The venting location and method you choose depends on the type of tankless unit you have and how much you want to preserve the beauty of the home walls or roof.

Ways to Vent a Tankless Water Heater

A tankless heater can be vented in two ways: direct vent or power vent. The venting can be through a side wall or the roof, with a traditional gas-fired water heater with a storage tank requiring roof venting.

Power Vent

A power vent water heater uses indoor air for combustion and relies on an outside vent to exhaust the corrosive combustion gases. No inlet or intake vent is required for this type. There has to be enough room and louvers or vents supplying air inside the room.

A gas tankless water heater using a power vent can be terminated on a side wall since the exhaust air is blown horizontally by the combustion fan instead of vertically. The setup here allows for more flexibility in the position of the hot water heater inside the house.

Direct Vent

A direct vent water heater uses outdoor air for combustion and thus has two vents, one for the air inlet and the second one as an exhaust gas outlet.

Some tankless water heater manufacturers make units with concentric venting, a single vent piping system containing an outer inlet vent, and an inner outlet vent for gases. For example, you can now find a Rinnai tankless water heater with concentric venting.

A concentric venting method means that only one penetration is made through the ceiling or wall. However, the holes made for concentric vents are larger since these vents usually have a diameter of five inches compared to the three inches of power venting exhaust pipe.

Do All Tankless Water Heaters Require Venting?

As mentioned, water heater venting depends on the type of heater you have. Not all tankless water heaters require venting.

Tankless Water Heaters That Require Venting

A gas water heater will always require venting to take the corrosive gases out of the home.

The corrosive nature of the combustion gases necessitates a separate vent and not the chimney flue since the gases will easily corrode the masonry chimney liner and the standard flue piping.

Gas Tankless Water Heater

A standard gas furnace with an efficiency of 80% or lower can share a vent with a gas-fired water heater, but you can also run the exhaust pipe for the heater inside the existing vent pipe for the furnace to reduce the chances of rusting the furnace vent.

High-efficiency tankless water heaters with rates as high as 95% or more cannot use the existing vents or share venting with the furnace since they produce highly acidic combustion gases that eat away standard galvanized pipes. You need a steel vent for gas tankless heaters.

Indirect water heaters, which use heat from a heat pump, boiler, or furnace, also require safe venting for the exhaust gases.

Tankless Water Heaters That Do not Require Venting

An electric tankless water heater does not require venting, making installation cheaper than a gas water heater. An electric water heater uses electric burners instead of propane or natural gas, thus eliminating venting.

Electric Water Heater White Color

The lack of venting makes electric water heaters efficient up to 99% since no heat is lost through vented air.

Outdoor tankless water heaters installed in warmer climates do not require venting because they enjoy unrestricted airflow and can withstand temperatures below freezing point.

Note that a condensing tankless water heater does not require metal venting because it has an efficiency rate of up to 95%, making the temperature of the exhaust gases lower. As such, you can vent such units with plastic, polypropylene, or PVC piping.

Bottom Line

When installing a tankless water heater, you have to consider the type of heater to guide you on what kind of venting you will need.

Since a high-efficiency gas water heater produces corrosive combustion gases, you should avoid connecting it with the furnace vent or other existing vents for other appliances.

If your home has a standard water heater that shares venting with a gas furnace, you can reduce the effects of the combustion gases from the heater by adding a separate exhaust pipe for it to run through the furnace vent. However, this is not necessary.

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