A traditional water heater stores your water in a tank, keeping it hot until you need it. This standby power loss can cost you a bundle of money; in fact, it can account for up to a third of your water bill. In comparison, a tankless water heater only produces hot water when you need it. Though a tankless heater costs more initially, its on-demand technology will cut the cost of your utility bill.
Benefits of Tankless Water Heaters
As anyone who has been the last in the house to shower can tell you, traditional storage water heaters can run out of water. This is because hot water is stored in a giant tank and heated slowly, then kept warm until you use it. When all the pre-heated water in that giant tank is used up, you are forced to wait for the tank to fill back up and slowly re-heat before your hot water is restored.
There are two clear issues with this model: first, you are forced to wait on hot water, and second, you are paying for the energy the storage model expends keeping your water hot.
Tankless water heaters offer an alternative to this system, using electricity or gas to heat your water quickly as you need it. Not only are these heaters smaller, they also offer a range of benefits including zero risk of water damage from a tank rupture, significant utility savings, and, most importantly, unlimited hot water.
Because some of these units are so small, they can be installed in many more places than storage water heaters, opening up opportunities to mount them under sinks in remote locations or near appliances that need an extra burst of hot water.
For a more in-depth comparison of tankless water heaters, see
EcoDirect’s guide to tankless water heaters.
Sizing Your Tankless Water Heater
When investing in a tankless water heater, it is very important to size it properly or it will not be effective. This isn’t difficult to do; all tankless water heaters are rated by temperature rise at a given flow rate, which is very simple once you understand it.
A natural gas model that can deliver 6.4 gallons per minute of hot water when raising the temperature 35° F or 5.6 gallons per minute of hot water when raising the temperature 45° F.
Since it takes more time to heat water to a higher temperature using the same power, it makes sense that the amount of water the tankless water heater could deliver at a 45° F rise would drop. This is important fact to keep in mind when sizing a tankless water heater, as this model would be unable to meet the demands of an application requiring 6 gallons per minute to be heated 45° F, while it would be perfect for an application that only required 6 gallons per minute heated 35° F.
This is getting a little bit technical, so lets take a step back and look at where these numbers are coming from.
Maximum Gallons per Minute
Gallons per minute is a measure of the number of gallons of water an appliance or fixture uses in a single minute. When describing a tankless water heater, this is specifically referring to the gallons of hot water that can be delivered.
To find the gallons per minute for your application, make a list of all the fixtures (faucets, showers, etc) and appliances (washing machine, for example) that could at any one time be running simultaneously. Then, calculate the gallons per minute each uses.
For fixtures, this is as easy: just put a bucket underneath the fixture to be measured, turn it on, and time it for about 15 second of runtime. Measure the amount of water in the bucket in gallons, multiply this number by 4, and you have your gallons per minute.
For appliances such as dishwashers and washing machines, you will need to consult your manual, but they can run around 2 gallons per minute. Some dishwashers can even heat their own water.
Once you have the gallons per minute for each appliance that could be run simultaneously, add these numbers up to find the gallons per minute that your tankless water heater will supply.
Temperature Rise
The temperature rise is a measure of how much you want to heat your water. You will need two numbers for this: the temperature you want your hot water and the temperature of the incoming water in your area. For most households, 120° F should be sufficient for hot water needs (though some appliances, such as dishwashers, may need even hotter water). Incoming water temperature depends on your location.
To determine the temperature rise you need, just subtract the lower temperature from the higher.
