In an earlier article, I wrote about the newest offering in electric water heating, heat pump or hybrid water heaters. Elsewhere in this issue, I cover solar water heater options. So, what is left to consider this day and age?
An alternative is on-demand or tankless water heaters. This option is exactly what the name implies, a water heating system without a tank that only operates when hot water is needed. It uses either gas or electricity to “flash” water hot (as I like to phrase it), producing a certain number of gallons per minute (gpm) of hot water.
Fans of on-demand heaters claim better efficiency because water is only heated when needed. No heat loss from a tank of water. They are correct that storage water heaters, the most common water heater in the US, do lose heat as they sit. But are on-demand systems that much more efficient and practical?
Let’s take a look at some of the things to consider when contemplating the on-demand option. The first factor is to size it for the number of people/activities likely to use hot water at the same time. Proponents may dismiss this, saying you simply adjust your schedules and habits to live within the capacity of the unit. Practicality says that isn’t likely to happen especially with teenagers in the house, if work schedules change, or you have a home full of company.
Sizing involves picking a unit that can produce the gpm you need. Small units produce less than 2 gpm while bigger ones can be 6 gpm or more. Let’s give it a go with the following example:
Two people need to shower at the same time while another is washing up in the sink. Using the data below:
Water needed: 2.5 + 2.5 + .75 = 5.75 gpm
Next you use the incoming water temperature to calculate the heat rise needed to reach your desired water temperature. Many sites recommend using 50oF but if you want to be conservative, go colder. Common temperatures in Virginia range between 47 and 57oF. Using 50o, you will need a unit that can produce nearly 6 gpm at a 50o rise.
While researching this article I came upon another criteria in sizing, the minimum flow rate. Some on-demand models need a minimum flow rate before they fire up, say between .5 – .6 gpm. This is useful to avoid the expense of feeding a slow leak but some low flow rate faucets and even some high efficiency appliances will not use this much. Low flow may equal no hot water. Just keep it in mind.
You also need to account for your water condition. If your water has a high mineral content, an on-demand heater may require using a water conditioner or the warranty is void. This is due to the extremely fast rate of heating, what I called “flashing” earlier. The minerals in the water fall out of solution and rapidly clog the workings of your unit. If you already have a water conditioner, skip this step.
What about maintenance and repair? Can you get anyone to come work on it when it acts up? These are very sophisticated appliances and not every contractor in the area will be able or, want to, work on them.
So now, fossil fuel or electric? A fossil fuel unit has faster heating capability but needs lots of input air. Be sure to locate it close to a good source of outside air so it doesn’t starve the home in cold weather.
At this point you are saying to yourself, “Self, when is the writer going to address the electric option?” Well, right now as a matter of fact. When looking at on-demand heaters, do not, under any circumstances, select an electric one. Why? They require LOTS of juice to produce hot water and provide substantially fewer gpm than gas alternatives.
What do I mean by saying the electric models use a lot of juice? I mean they are energy hogs. These units demand so much energy at one time, they really should have their own electric service. A typical home has an electric service rated for 200 amps. A relatively small electric tankless unit can use 116 amps! Your co-op sizes the transformers outside to serve normal household loads and frequently connect more than one home to a transformer. Installing a tankless water heater could overload that transformer leading to power quality issues for you and anyone else connected to the transformer.
Are on-demand heaters a good option? That depends. If you have a small home with just a couple of people, perhaps a gas-fired unit will meet your needs. If you have a vacation home, a gas -fired unit might be ideal as you can get hot water the minute you walk in the door, something not possible with a tradition storage heater.
The bottom line for this option is you really need to do your homework before taking the plunge. And under no circumstances, select an electric one.