Replacing your old energy hog of a hot water heater is on your to-do list with a new Energy Star Rated unit but you’re uncertain whether to go with a tankless hot water heater or a conventional hot water heater. You’ve read how the tankless heaters save the most energy and get you the biggest tax credit but you’ve also read how tankless heaters aren’t right for use in every situation and you’re wondering how you can tell if one is right for you.
Tankless hot water heaters are the most energy efficient because they don’t consume any energy except when hot water is being used. Standard hot water heaters maintain the water at a specified temperature even when it’s not being used and this standby energy consumption can be significant. One of the biggest advantages of a tank style hot water heater is that they can heat water to a higher temperature and maintain that temperature over long periods of time. Tankless hot water heaters come in two basic types-Points of Use and Whole House units. The requirements for each are quite different. The Point of Use tankless heaters simply have to supply hot water to a single point of consumption, say a kitchen sink and dishwasher while a Whole House tankless hot water heater has to supply water to where ever it’s needed throughout the whole house.
Tankless hot water heaters can qualify you for a $300 Federal Home Energy Tax Credit but installing one could get you into hot water if you’ll excuse the pun. Will a tankless hot water heater solve your problems or just create new ones for you? Let’s take a closer look at what you need to consider before investing $800 to $2,000 in a tankless unit. By the way that’s the average cost of a single unit itself if you install it yourself.
There are three factors to consider when choosing a tankless hot water heater.
· The volume of water measured in GPM (Gallons per Minute) that the tankless heater will be required to heat
· The temperature of the feed water and
· The temperature that the water must be heated to.
These three factors will determine the capacity of the unit and whether you will have to install one unit or gang several units together. Let’s take a closer look at each of these factors.
Feed water or ground water temperature impacts the size of the unit
Simply put the colder the water coming into your home the higher the rating of the tankless heater will have to be. For example the ground water in the state of Michigan is 30°F colder than the ground water in the state of Florida (42°F as opposed to 72°F) and a unit of a given GPM rating would have to be sized 33 to 50 percent larger. This brings us to the specification called Temperature rise.
Temperature Rise
Temperature rise may be defined as the difference in temperature of the water exiting the heater and the temperature of the water entering the heater. For example if you like to take really hot showers with the water at 130°F and you live in Florida where the ground water is 72°F the required temperature rise would be 130°F - 72°F or a temperature rise of 58°F. If you lived in Michigan, on the other hand, where the ground water temperature averages 42°F the required temperature rise would be 130°F - 42°F or 88°F.
Tankless water heaters are rated by their temperature rise at a specified GPM, For example a given unit might be rated at 33°F temperature rise@3.0GPM. The same unit would produce a 99°F temperature rise@1.0GPM. As you can see the temperature rise varies inversely with the demanded flow rate. This leads us to the next factor, the required flow rate in GPM.
Calculating flow rate
The Federal Energy Policy Act of 1992 required all faucet / shower fixtures made the USA to have a flow rate of no more than 2.2 GPM at 60 PSI (Pounds per Square Inch). To calculate the required GPM rating of a tankless water heater add up the GPM ratings of all the faucets, showerheads, etc.
To calculate the individual flow rate
· Turn the fixture on as you normally would. If it’s the bathroom sink set the taps so the water temperature is what you normally use.
· Place a graduated container, such as a kitchen measuring cup, under the fixture and collect the water for 10 seconds
· Measure the water in the container and convert the measurement to gallons. Here’s a handy calculator for making conversions.
· Multiply the measured amount by 6 to get the GPM flow rate.
Calculate the total required flow rate in GPM by adding up the individual flow rates that you calculated above.
You now know the required temperature rise and the required total flow rate in GPM for the whole house tankless heater. For the sake of this instructional let’s say that we need a temperature rise of 38°F @ 11 GPM. That’s well beyond the range of any one whole house tankless water heater so what do we do? We install multiple units in tandem, for our example her we could use two Tempra 29 series, 240 Volt units in tandem.
Warning! Here’s where you can run into real trouble.
Double check the electrical specifications for the tankless hot water heater that you’re thinking of buying and installing. The average home doesn’t have a large enough Ampacity service to handle a whole house electric powered tankless hot water heater. For example The Steibel Tempra 29 requires 120 A (Amperes) @ 240 V. To supply this kind of current a house needs to be wired with a minimum 200 A service. If you have to run two tankless heaters to get the temperature rise and GPM flow that you need you will also double the electrical load requirements. Very few homes have that kind of power available.
Gas fired tankless hot water heaters
For whole house residential installations you are better off going with a gas fired unit which will eliminate the problems presented by such a large electrical load. Gas fired units are available for use with both natural gas and LP (Propane) gas. Gas fired units are also available as vented units and vent less units, the latter being the best style for the do-it-yourselfer because it eliminates the need to install a vent pipe to the outside of the home.
Electric supply lines or gas supply lines
If you go electrical the actual electrical connections to the service panel is best left to the professional unless you are really confident in your electrical skills. The same applies to running gas lines to a gas fired unit. If you do feel confident that you can install this wiring or gas lines yourself, be sure to apply for and get the proper permits from the building department and have all the required inspections performed. If you install an LP gas fired unit the LP gas supplier who installs the supply tank will also inspect the system for leaks.
