About Gas Water Heaters
Modern technology has significantly expanded the selection of storage type gas water heaters and allowed to combine various functions and advantages to produce more energy efficient, safe, reliable and installation-flexible products. Tank-type gas water heaters are generally categorized into (3) main types by type of venting they use: atmospheric vent, direct vent and power vent.
There are also several cross-breeds, such as "power direct vent", "fan assisted atmospheric vent" and even "combi" water heaters, most of which are described below.
Atmospheric vent water heaters
Flue venting: Vertical only - roof or chimney. Utilizes natural draft (non fan-forced) which occurs due to pressure and temperature differential in hot flue gases and colder outdoor air. Select models are fan-assisted.
Intake air supply: Indoor air (must be sufficiently abundant for proper combustion and diffusion).
Vent pipe: Type-B gas vent pipe.
Main advantages: Quiet. Lowest cost and lowest venting pipe costs, relatively easy installation. No electricity needed (unless it’s a fan-assisted model).
Main disadvantages: Uses conditioned (cooled or heated) indoor air, reducing overall system efficiency. Accidental hood displacement and backdraft (bath vent fan, clothes dryer or range hood ON) pose possible health hazards and operational issues.
Summary: Atmospheric vent water heater is a good, simple and inexpensive choice when it is installed in an area isolated from the rest of the house and given an abundant air supply with no backdraft.
Direct vent water heaters
Flue venting: Vertical or horizontal pipe run, but horizontal termination only. Must be installed next to and vented through an outdoor wall. Non fan-forced exhaust.
Intake air supply: Outdoor air.
Vent pipe: Most commonly, manufacturer-provided rigid or flex vent kits.
Main advantages: Safe (sealed combustion means no accidental gas leakages), quiet (no blower fan) and efficient (uses outdoor air instead of conditioned indoor air). No issues from insufficient indoor air supply. Some models require no electricity.
Main disadvantages: Expensive and limited venting options. Limited vent pipe length and low installation flexibility.
Summary: Direct vent option is a good and safe choice where water heater is installed inside a new home, with limited space or backdraft issues.
Power vent & power direct vent water heaters
Flue venting: Fan-assisted, horizontal or vertical.
Intake air supply: Outdoor air for "power direct vent" models and indoor air for regular "power vent" models. Note that air intake pipe for power direct vent models cannot be longer than exhaust pipe.
Vent pipe: PVC, CPVC, ABS or
PolyPropylene.
Main advantages: Power venting allows for highest installation and termination flexibility among the (3) types and lowers the flue gas temperature so that inexpensive plastic vent pipes can be used.
Main disadvantages: High unit cost. Blower fan makes noise, requires electricity and increases power consumption.
Summary: Power vent water heaters are a safer alternative to atmospheric vent units. Power direct vent models are a more installation-flexible alternative to direct vent units.
Combi gas water heaters
"Combi" or combination water heaters incorporate a built-in heat exchanger coil which heat up water for a low-temp heating system, such as radiant floor heat. In essence, they are the opposite of indirect water heaters, where the domestic water in the tank is heated by the heat system water via built-in heat exchanger.
Simple and cost effective ways to increase gas water heater efficiency
- Insulate the hot water pipes where possible using foam pipe insulation. It is also advisable to insulate first 3ft of cold water piping near the water heater. Be sure that there’s at least 6" of clearance from any metal flue pipe.
- Install a hot water recirculation pump. Various options, such as timer, bathroom occupancy sensor or Auto-Adapt (self-learning) will help to further increase savings. If a hot water heater has recirculation outlets (usually on the side), they should be used for piping.
- Lower the hot water temperature setting (3-5% energy cost reduction for every 10°F). Consider minimum manufacturer’s recommended settings for Legionella prevention.
- Install a drain water heat recovery pipe (heat exchanger). It captures heat from shower/tub drain water and pre-heats the cold water incoming to the water heater.
Frequently Asked Questions
Q: What’s the difference between standard atmospheric vent and "damper" atmospheric vent models?
A: Automatic damper on atmospheric vent models closes off the flue vent pipe when gas burner inside the water heater is Off, preventing the "chimney effect", or the natural draft which pulls the conditioned air from the building through the vent pipe. Standard dampers are electrically powered. Thermomechanical dampers use bimetallic switch which is activated by hot flue gases.
Q: Does direct vent water heater require electricity?
A: It’s model-specific, please check the product description and specifications.
Q: What’s the difference between power vent and power direct vent water heaters?
A: Power direct vent WH’s draw combustion air from outdoors through a separate pipe, while standard power vent heaters use indoor air, similarly to atmospheric vent models.
Q: What type and size expansion tank should I use with my water heater?
A: You’ll need to use a thermal expansion tank, such as
Amtrol Therm-X-Trol, B&G PT, Calefactio HGTE, Watts PLT series or similar. Sizing depends on factors such as tank volume, static supply pressure, maximum hot water temperature setting, incoming water temperature, etc. We recommend consulting the sizing chart provided by manufacturer (see “Literature” section in individual product description).
Q: Should I consider gas tankless WH as an alternative?
A: Yes. There are numerous benefits of tankless over storage. However, there are drawback as well. We suggest you read
more about tankless here.
Q: What’s the difference between dilution and combustion air?
A: Dilution air combines with flue gases to lower their temperature and concentration and under certain conditions, to facilitate venting. Combustion (intake/fresh) air is mixed with gas to facilitate burning (combustion process).
