Tankless Water Heater Venting

Quite frequently we receive questions from home owners asking why their current hot water heater venting will not work with a new tankless water heater. When you are reading tankless water heater reviews you will want to know if it’s a non-direct vent or a direct vent. This will have an impact on which unit and where you can have the unit installed.

Before we go any further we must share this with you:


The proper ventilation of a tankless water heater is imperative for your safety and those in your house. If the venting for your new unit is not properly installed it could lead to carbon monoxide poisoning. Carbon monoxide poisoning can cause series injury and possibly lead to death. This article is a broad over view and should NOT be used in replacement of the manufactures’ insulation guidelines. Always refer to manufacture’s manual for proper guidelines when installing a tankless water heater.

 Since this is such an important issue we are going to try and cover as much as we can. Any sort of appliance which burns, wood, natural gas, liquid propane or any other fuel will produce carbon monoxide. As stated before, carbon monoxide is a very series mater. You want to get the carbon monoxide gas out of your home in a same manner.

Your old water heater most likely has an energy factor of 0.80. Basically this means for every $1 of gas you use, $0.80 goes to heating water and the other $0.20 goes out the vent. As the unit ages the energy factor will decrease over time. So if your water heater is 10 years old and has never been serviced it might be operating at 0.70 energy factor. So $0.30 worth of heating fuel is going out the vent. The main point here is the temperature of the exhaust gas is higher then 212º.

A new tankless water heater or a tank water heater is going to have much higher energy factor, as high as 0.96. Typically, in tankless water heaters, how this higher energy factor is achieved is by recapturing the exhaust gas created from heating the first exchanger. The captured exhaust gas is then used in a second heat exchanger to preheat the incoming water. Since most of the heat from the exhaust gas is passed to the water, the exhaust gas has a very low temperature. “How low?” you might ask. Below 212º

Why 212º? 212º is the boiling point of water. In an old water heater the exhaust gas exceed 212º and there is no condensation created in the vent stack and all the elements are vented outside. In a new efficient tankless water heater there is a very small amount of condensation created in the vents. This small amount of condensation contains highly corrosive elements which will over time eat through a standard venting, causing carbon monoxide to leak back into the living space.

Older water heaters typically use a vent pipe called Type B. Type B is made out of galvanized or aluminum. While aluminum will not rust it will corrode when exposed to acidic condensation. High Efficiency tankless water heaters will either need a Category III stainless steel vent or PVC, depending on the manufacture’s recommendations.

An additional reason why you will most likely need to upgrade your vent is most tankless water heaters use a powered fan to vent the unit. The fan creates a “positive pressure” and the vent system needs to be UL listed as positive pressure and sealed to prevent carbon monoxide from leaking into the home.

Gas or LP tankless water heaters can be classified into 2 vent categories: Power Vent and Direct Vent. Power Vent uses the indoor air for combustion. The room in which the unit is installed will need to be larger enough so that there is enough make up air for the unit to operate correctly. You can not install power vents in a closet or under a sink. Direct vent uses air from outside for combustion. On a direct vent there are two vents, on is the intake and the other is the exhaust. Some manufactures offer concentric venting. This is basically 2 pipes in one. The pipe has an inner pipe used for exhaust and the outer pipe is used for combustion air. Concentric venting pipes will speed up the installation process, and in our opinion looks better. However, concentric piping is very costly when comparing to regular PVC.  These units can be placed in smaller locations such a closet.

As we said before, venting your tankless water is the most important aspect of the install. Always refer to the manufacture’s installation guidelines.