On Demand Tankless Gas Hot Water Home Heaters
How to Choose the Best Domestic Instant Heater Systems
Jun 29, 2009
Tankless water heaters are "on demand", heating water only when required. This saves energy, compared to a conventional 40 gallon "always on" heater. They cost about $500, excluding piping and other costs.
Popular brands include Bosch (Aquastar) and Paloma.
Advantages of Tankless Gas Water Heaters
Advantages compared to a conventional 40 gallon heater are:
- Eco-friendly: saves energy.
- Unlimited hot water. Can deliver hot water for hours, not limited by tank capacity.
- Constant temperature, doesn't get cooler as hot water runs out of the tank.
- Small. Can be mounted on wall, freeing up floor space.
- May qualify for tax credits.
Disadvantages of Tankless Gas Water Heaters
Many of the points below can be found in the instruction manuals of tankless gas heaters. Homeowners should search the Internet for instruction manuals before buying, to avoid unpleasant surprises.
- Longer delay (a few seconds) before hot water starts flowing. This can make short uses (washing hands) impractical. Water is also wasted, waiting for the hot water to flow. Placing the heater close to the point of use will reduce this problem.
- Heater can switch off if flow rate is too low. A heater's minimum flow rate specification is therefore important. Low-flow shower heads may not draw enough water to trigger the heater. The same goes for trickle applications (shaving). Homeowners can end up using more water just to keep the heater on.
- Limited heating capacity. Might not heat water to high enough temperature in winter, depending on required flow rate of water. Some models have a specified minimum inlet water temperature (example: 60 degrees Fahrenheit).
- Needs more maintenance than tank water heaters. Valves may need cleaning or lubrication every year, replacement every few years.
- May need larger hard line (not flexible) gas pipes (3/4 inch), water pipes (3/4 inch) or vent pipes (5 inch).
- Some models require stainless steel venting (not galvanized steel), which can cost hundreds of dollars.
- Needs high gas pressure. Gas pressure in some locations can be too low. This needs to be checked with a manometer.
- High gas flow rate can be noisy.
- Warranty terms can be strict. Warranties can be voided by installing without licensed plumbers, installing the heater in the bathroom, not doing annual maintenance.
Tankless Gas Heater Sizing and Options
Maximum water flow rate for tankless gas heaters range from 2 to 7 gallons per minute (gpm). This is with a temperature rise of about 45 degrees Fahrenheit in the outlet water. Flow rate can be increased at the expense of lower temperature rise.
2 gpm is considered too low for most home use (except maybe in summer), with 4 gpm the minimum recommended by many manufacturers for a single application (shower or dish washer or clothes washing machine, one on at one time). 6 to 7 gpm is recommended for two simultaneous uses.
Gas ignition systems can be powered by:
- Electrical mains. Simple and reliable but will not work if the mains electrical power is cut.
- Battery (D cells). The batteries need to be replaced but the system is otherwise reliable.
- Gas pilot light. The pilot light can be blown out by high wind if the tank is located outdoors. Even indoor pilot lights can be blown out if there is no automatic damper on the vent.
- Water turbine ignition. Water flow turns a turbine which powers the electrical ignition. The turbine can jam if the inlet water is not filtered.
Computer controlled heaters allow thermostat control of water temperature, providing constant temperature water even if the water flow is turned up or down. However they can be less reliable, requiring good operating conditions (water pressure, gas pressure). They also run off mains electricity supply, and will not operate if there is a power blackout.
Simple manual control heaters are more reliable but the gas throttle needs to be turned up or down manually to regulate the temperature (shut off is automatic). This is less of a problem if the faucets and showers allow mixing of hot and cold water to control temperature.
The Best Tankless Gas Water Heater
Choosing the right tankless gas water heater can be complicated, especially when piping and venting requirements are considered. A qualified plumber should be consulted before any purchasing decision is made.
The plumber will also be able to advise which brands he is able to service (since many heaters need annual maintenance), which brands have a good reputation for reliability. Buying a heater that can't be serviced would be a waste of money.
Small (2 to 8 gallon) mini-tank electric water heaters can be installed near the point of use (either in line with the tankless heater or on a separate line, depending on the requirements). This leverages on the strong points of both types of heaters. The mini-tank heaters can deliver instant hot water for washing hands and shaving, while using less energy on standby compared to a 40 gallon heater.
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