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Part 2:

Tankless water heaters have revolutionized the water heating business. They produce an endless supply of hot water cheaper, cleaner, and faster than ever before. These heaters have offered home and business owners a substantial energy savings, as well as, a smaller carbon foot print. As with most new technology, tankless water heaters continue to evolve. The average direct vent tankless water heater will usually carry a low to mid 80's energy factor. These heaters have been around since the late 1980s. The condensing tankless water heaters were introduced around the mid-1990s. These condensing heating units have proven to be even more energy efficient while reducing the heaters carbon footprint even more. The condensing tankless water heaters today now boast a 10% to 15% higher energy factor over the conventional tankless water heaters. It is not unusual to see condensing water heaters have an energy factor of 92% to 97%.

Venting Costs

In addition to the operational energy savings, the condensing water heater is less expensive to install. The installation of the heaters exhaust venting carries a substantial part of the overall cost of the installation. The conventional tankless water heater venting, usually exceeds $30.00 to $45.00 per foot. This high cost is due to the cost of the special vent pipe, the time it takes to engineer, and the time to install the special venting.

Venting a condensing water heater is substantially less expensive. If the outlet water temperature is 150F or less, PVC pipe can usually be used to vent the heater. If the water temperature is greater than 150F the manufacturer will recommend a specific type of pipe to be used. Installing PVC pipe usually costs between, $2.50 to $4.50 per foot to install.

Virtually every service tech I know knows about PVC pipe and how to install it.

How Condensing Tankless Water Heaters Work

A condensing water heater utilizes the burnt exhaust gases to preheat the cold incoming water. This process also cools the exhaust gas temperature. Exhaust temperatures in conventional heaters usually run 310F to 360F. Condensing water heater exhausts usually run between 95F to 130F. This temperature reduction allows the use of PVC rather than the more expensive Category III vent pipe.

This article refers to the non-condensing tankless water heaterVenting is extremely important to gas fired tankless water heaters. When products are not vented properly, bad things can and will happen. A few of these are: 

1. Complete unit failure due to condensate being allowed to enter the heating unit.
2. Corrosion will damage the heater's heat exchanger. 
3. Carbon Monoxide Poisoning can occur. 

The proper venting of burnt fossil fuels such as propane gas, natural gas, gasoline, diesel fuel, etc., should always be installed by a licensed, task certified professional. Short cuts should never be taken in regards to the venting system. All manufacturers' instructions and state codes must be followed.

A typical tankless water heater's exhaust venting should travel horizontally through an outside wall or vertically up through the roof. Keeping the vent piping as short as possible will help keep costs down. Always consider your tankless installation location, venting labor and vent pipe are a large part of the installation cost.

Most tankless water heaters require a special vent piping material. This material is known as "Category III" and is typically stainless steel. Category III is required because the combustion efficiency of tankless heaters make it very probable that condensate will be formed within the venting system. This condensate is highly acidic and will destroy standard vent material in a short period of time. Type "B" gas vent is typically used when venting standard tank type water heaters. This type venting material cannot be used on tankless water heater installations. Another reason Category III vent is required is because the vent systems are under pressure from a fan within the heater. This is known as "Positive Pressure" venting and requires that the vent system be UL listed and sealed to prevent carbon monoxide from leaking out into the occupied space. All vented heaters as well as other products have maximum lengths that the venting can be run. The number of elbows required in the system will shorten installation distances. Always consult the manufacturer instructions and never exceed these distances and recommendations!

Florida codes and manufacturer instructions do not allow common venting to vent together or tie into an existing masonry chimney. The condensing gasses quickly begin to cause damage to the masonry and result in structural damage to the property as well as a very unsafe condition with carbon monoxide. If you can run the proper vent product up through an existing chimney to the outlet and provide the proper condensate drain to protect the unit, you can use the existing chimney as a "chase" to run your new vent pipe in. (Read the equipment's manufacturer installation instructions!)

To the best of my knowledge, all vented type tankless manufacturers require that you either slope horizontal venting away from the heater, or provide some type of condensate drain within 3' of the vent connection. This is to protect the unit from damage caused by condensate. The condensate in the venting will destroy the heat exchanger if allowed to run back to the unit. A unit's heat exchanger will be ruined within a few years if not vented properly. All Category III vent manufacturers have the ability to provide a drain tee even when venting is installed straight up to properly protect the unit.

Our next blog will cover venting condensing tankless water heaters.