Pop one under your sink and save on hot water
A point of use water heater can save you energy if you frequently use small amounts of hot water at a particular outlet in your home. These units come in either tank or tankless form.
They are pretty much always heated with electricity in the standard voltage for your area. For instance, in North America, a point of use water heater is almost always a lower amperage,120V unit, unlike higher capacity whole-house electric tankless water heaters, which are usually 240V because of their very high amperage.
You can typically install a point of use water heater yourself if you have some basic plumbing experience. They plug in directly to a wall socket, so if you do not already have one underneath the sink you plan to install the heater for, you’ll need to run a line there and install an outlet. Hire an electrician for this, or do it yourself if you are experienced at wiring.
When does a point of use water heater make sense?
Point of use hot water may be a good energy saving option where you need a small amount of hot water at one time, and where the distance from your main water heater (tank or tankless) is significant. It can also make sense for homes with no central hot water heating, such as mobile homes.
Washing your hands with a point of use water heater
Consider the act of washing your hands with hot water in an upstairs bathroom sink. In my home in Toronto, the plumbing run from the water heater to the upstairs bathroom is 32 feet through half-inch copper pipe, which works out to 0.3 US gallons or 1.2 liters of water in the pipes. So when you turn on the hot water tap, run it until the water heats up, then wash your hands with, say, 250 ml or 1 cup of hot water, you have drawn almost five times more hot water than that into the pipes, where it will sit and lose its heat over time.
Kitchen uses of a point of use water heater
You might equally wash your hands in a kitchen using an under sink point of use water heater, but if you live in a 2+ storey house, chances are the pipe runs from your kitchen sink to the water heater are shorter. If you’re the type who likes to wash one or two dishes at a time using hot water, you might save energy with one of these heaters, over drawing bursts of hot water from the main supply every five minutes, then leaving much of that hot water to cool down in the pipes.
Of course, if this is how you wash your dishes, you should read my page on Energy efficient dishwashers, where I provide compelling research to show that washing dishes by hand is almost always a waste of energy.
One benefit of point of use kitchen water heaters is that you can often use the hot water from them directly to brew a cup of coffee or tea, as these heaters, like many on-demand hot water heaters, generally provide potable hot water (unlike most home hot water tanks, which should not be used for drinking). However you can’t really use these point of use heaters as a major source of pre-heated water – for instance to get a head start on a pot of water for cooking pasta – because while they do have a tank, their capacity is quite small. Some can only provide 3-4 gallons of hot water per hour.
If you are thinking of switching to point of use hot water partly to be able to brew tea or coffee directly from it, check out my Energy saving kettles page, where I provide another way to get instant hot water for your coffee or tea, that may save you more energy.
Finally, if you live in a mobile home or trailer and just don’t have the space for a water heater of any type, consider the point-of-use water heater pictured at left, which is built into a kitchen sink faucet. If you live in a relatively warm climate where the water coming into your house is already above 70F, this will add enough heat to make the water feel hot.
It’s not so great for cold climates, since it’s tankless and just heats the water as it is passing through, and it’s not at a high enough wattage to heat it to the almost scalding temperatures we’re used to. You’ll either get just lukewarm water, or very low flow, or both, depending on how cold the input water is, but if your only other option is cold water, it’s an improvement.
Wasted hot water?
With point of use hot water, you won’t experience all this ‘wasted’ hot water. Now, is it really wasted? That depends on where your pipes run, and whether you’re heating your house, and what your main source of hot water is.
If you’re not heating your house, then the hot water from your downstairs water heater, which stays inside the pipes after you wash your hands, is just wasted. If you’re running the air conditioning it’s even worse, because that heat will eventually warm the airspace in your home, so you’ll not only waste the energy used to heat that water, but you’ll use more energy removing that heat from the house through your air conditioning system.
If you’re heating your house, then the heat isn’t wasted (assuming the pipes mainly run through interior walls or on the interior side of well-insulated outside walls). That heat will work its way into the home, and since you’re heating anyway it will mean less work for your home heating system. (That’s why energy efficiency buffs like me always leave the hot water sitting in the tub overnight after a bath – instead of dumping that perfectly good heat down the drain.)
What is your main source of hot water?
The other factor that determines whether point of use hot water makes sense from a cost perspective is the energy source for your main hot water heater.
If you have a heat pump hot water heater, or a fossil gas tank or tankless water heater for the whole house, you are probably paying half to a third as much to heat a gallon of water in that heater, than you’ll be paying to heat a gallon in a point of use water heater with electricity. The 1-cup hand wash means 3 cups of wasted heat in the pipes coming from your main hot water supply – so 4 cups of relatively cheap hot water from the heat pump or fossil gas heater. The 1-cup hand wash from a point of use water heater uses pretty much exactly 1 cup, but using heat from electricity which is typically 2-3 times more expensive. So while you might use only a quarter as much hot water to wash your hands, the financial savings work out to somewhere between 33-50%, assuming you don’t derive any benefit from the hot water left in the pipes.
On the other hand, if you do have a fossil gas heater and want to cut your greenhouse gas emissions, you’re better off to minimize use of the hot water from the gas heater, and use a point of use water heater in areas where you need occasional small amounts of hot water.
If your main hot water source is heated with electricity or propane, both of which cost much more than fossil gas, you will definitely save using point of use hot water in this kind of scenario. And for an electric hot water tank, you may be able to survive with a smaller tank for your main hot water supply if you also use point of use water heaters in places like bathroom sinks, because those small but frequent draws of hot water won’t dilute the heat in your tank when it comes time to do a load of laundry or take a shower.
If you don’t have a main hot water source, then a point of use water heater may be a great way to avoid having to get one!
And the answer is….
I went into considerable detail here because I want to make sure you don’t buy a unit expecting big energy savings, and then discover your energy bills haven’t really budged. To me the potential energy savings from a point of use water heater only add up if (A) you don’t actually have a central hot water heater, and you are looking at a point of use water heater as a way to avoid installing a central one, or (B) there really is a compelling reason to have many short bursts of hot water at a sink that is far from the central hot water heater.
What about convenience?
At this point I hope I’ve convinced you that a point of use water heater, from an energy efficiency point of view, might save you energy under particular circumstances. But many people choose a point of use water heater for convenience as well, and this is where they shine.
It really is convenient, when you’re ready to wash a couple of saucepans in the middle of throwing together a gourmet meal, to be able to turn on the kitchen tap and get instant hot water. With a point of use water heater that’s just what happens; with a central water heater (tank or tankless), there is that annoying delay.
Likewise, in a bathroom far from the main hot water supply, if you need to wash your hands or face in hot or warm water, it’s nice not to have to wait a minute or more for hot water to come up from the tank.
If your main hot water source is an electric tankless water heater, you may also have to deal with the startup delay on these heaters. Some are truly instantaneous, while others may take several seconds of water flow for the heating to kick in. A point of use water heater will remove both the start-up delay and the running-through-the-pipe delay.
Point of use shower water heaters
There are point of use water heaters designed specifically for showers. These are common in most places other than North America, in various forms. Many of the same energy saving considerations I identify above apply to these as well. I cover these units in more detail on my electric shower head page.
Insinkerator point of use water heaters
I have avoided placing any Insinkerator point of use water heater ads on this page. I do not like to recommend products that are going to disappoint my site visitors, and although there are many great features about these water heaters, I have read too many customer reviews of these heaters identifying problems with early breakdown such as cracked tanks, failing electrical components, and hassles getting warranty service. Some people are very happy with these heaters (including some of the reviewers who have repeatedly replaced a failed Insinkerator point of use water heater with yet another of the same manufacturer), but don’t forget that when you buy an energy efficient appliance that breaks down quickly, you also need to factor in the energy used to manufacture the replacement appliance.