Storage tanks use energy at a relatively slow rate, storing the heat for later use. The disadvantage is that over time, the water inside the tank will cool down causing the heating system to activate to heat the water back up. Additionally, once the tank’s supply of hot water has been exhausted, there is a significant delay before hot water is available again. Larger tanks tend provide hot water with less temperature fluctuation due to this greater storage.
There are also tiny electric storage water heaters with capacities ranging from 8 to 32 litres (2 to 6 gallons) made for installation in kitchen and bath cabinets or on the wall above a sink. They can provide hot water long enough for hand washing, or, if plumbed into an existing hot water line, until hot water arrives from a remote high capacity water heater.
High-efficiency condensing units are available with or without a storage tank and can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cooled and are mechanically ventilated either through the roof or through an exterior wall. At high combustion efficiencies a drain must be supplied to handle the water condensed out of the combustion products, which are primarily carbon dioxide and water vapour. A hybrid unit has a small storage tank. A condensing storage tank has a full sized storage tank.
Sizing is the technique that matches the capacity of the hot-water source to the needs of the homeowners.
- For tank water heaters, the key criterion is hot water storage capacity
- For tankless water heaters, the key criterion is hot water flow rate
Generally there are peak usage times for water use. Storage tanks have a first hour rating guide and comparing this to the peak usage is a good way to size the unit. Here are some typical flow rates for household appliances:
Typical Shower | 2.5 GPM |
Typical Bath Tub Faucet | 2.0 to 3.0 GPM |
Bathroom Vanity Sink Faucet | 0.5 to 1.5 GPM |
Kitchen Sink Faucet | 1.0 to 2.2 GPM |
Clothes Washer | 1.5 to 3.0 GPM |
On demand tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you’ll need.
First, list the number of hot water devices you expect to use at any one time. Then, add up their flow rates (gallons per minute). This is the desired flow rate you’ll want for the demand water heater.
To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50ºF (10ºC). For most uses, you’ll want your water heated to 120ºF (49ºC). In this example, you’d need a demand water heater that produces a temperature rise of 70ºF (39ºC) for most uses. For dishwashers without internal heaters and other such applications, you might want your water heated at 140ºF (60ºC). In that case, you’ll need a temperature rise of 90ºF.
Rebates are available thru FortisBC and thru the BC Government (Some conditions apply). Thru FortisBC a storage type heater over 67% efficient is eligible for $200. A tankless unit and a hybrid unit over 90% efficient is eligible for $500. A condensing storage unit over 90% is eligible for $1000.