Tankless Water Heaters
(I wrote this for a friend who owns a three-story building and was replacing the water heater. She had never heard of tankless water heaters so I wrote this up for her. Posting it here so I can link to it.)
A tankless hot water heater is a device that, instead of keeping multiple gallons of water constantly hot in a tank, heats it by passing the water through a small heating unit, like a stove burner, that only goes on when someone turns on the hot tap. This saves energy and money because it doesn't run at all unless it's being used. If no one is home, water heating cost is zero.
You also never run out of hot water. This is why tankless water heaters are also called "demand" or "on demand" water heaters. They are also included in the "point of use" or "POU" category because it's become common to install small ones (which cost between a fifth and a tenth the price of a normal whole-house tank heater) at each sink, shower, tub, dishwasher, and washing machine. (POU also includes mini-tank heaters that are for delivering hot water in short, intense bursts such as in commercial dishwashers.) The advantage of point of use water heaters is that they eliminate the need to pipe hot water up from the basement, so there won't be water heated and then left in the pipes, cooling and wasting its heat, after the user turns off the tap. For people on upper floors, this also greatly cuts down the wait for the water coming out of the hot tap to actually get hot. (There will still be some wait time because the heating unit won't go on until the water is flowing.)
Tankless water heaters also tend to have longer working lives.
There are a few drawbacks to tankless water heaters. The first is that they are prone to mineral build-up if the water is at all hard, which can actually destroy the unit, so you will need to descale the pipes anywhere from a couple of times a year to once a month, depending on how hard the water is. Yeah, that requires a special tool. Neglecting this chore can void your warranty.
The other big one is that they need to be installed by a plumber who is trained on tankless water heaters. The advice of ordinary plumbers regarding what size of units to buy and where to put them might not be good. A unit that's too small may get overloaded and shut down under conditions of high demand. One that's too big may not go on if you only want a gentle stream of warm water (for instance if you need to wash a cat). (I remember a campground that had one big demand hot water heater for all ten of its showers on the assumption that groups would all be showering together. For just one person to take a shower, at least three showers had to be running with nobody in them or the hot water wouldn't go on.) They might want to install it inside a wall where it's hard to get at when it's time to descale the pipes. (Not to be too paranoid, but plumbers who are not trained on tankless water heaters usually don't like them and may install them badly on purpose to get you to switch back to a conventional one.)
Demand hot water heaters also generally have internal computer chips, so they need electricity. Some just plug in, but some need to be wired in. Also, if the power goes out for a few minutes while you're taking a shower, if the water heater is tankless, your shower will be cold almost instantly. With a big old tank water heater, you may not even notice.
Finally, some manufacturers will void the warranty if their heater is worked on by anyone other than one of their certified technicians.
Now, I'm assuming that any tankless hot water heater you install will be electric. Gas hot water heaters, like other gas appliances, need air flow and ventilation, which adds a whole extra layer of complexity. If you google "tankless hot water heater problems", about half of what you find will be about problems related to the gas.
I've also heard that, in some situations, a combination of a tankless water heater turned up to its hottest possible outflow temperature with a cheap single-handle shower controller can result in wild temperature fluctuations. Something about the two appliances not playing well together. This was at least twenty years ago so that technical problem may have been solved.
That's the basics of what I know about tankless hot water heaters. At https://blog.totalhomesupply.com/point-of-use-water-heaters-guide/ there's a chart about halfway down the page that tells you how to estimate your needs.
A tankless hot water heater is a device that, instead of keeping multiple gallons of water constantly hot in a tank, heats it by passing the water through a small heating unit, like a stove burner, that only goes on when someone turns on the hot tap. This saves energy and money because it doesn't run at all unless it's being used. If no one is home, water heating cost is zero.
You also never run out of hot water. This is why tankless water heaters are also called "demand" or "on demand" water heaters. They are also included in the "point of use" or "POU" category because it's become common to install small ones (which cost between a fifth and a tenth the price of a normal whole-house tank heater) at each sink, shower, tub, dishwasher, and washing machine. (POU also includes mini-tank heaters that are for delivering hot water in short, intense bursts such as in commercial dishwashers.) The advantage of point of use water heaters is that they eliminate the need to pipe hot water up from the basement, so there won't be water heated and then left in the pipes, cooling and wasting its heat, after the user turns off the tap. For people on upper floors, this also greatly cuts down the wait for the water coming out of the hot tap to actually get hot. (There will still be some wait time because the heating unit won't go on until the water is flowing.)
Tankless water heaters also tend to have longer working lives.
There are a few drawbacks to tankless water heaters. The first is that they are prone to mineral build-up if the water is at all hard, which can actually destroy the unit, so you will need to descale the pipes anywhere from a couple of times a year to once a month, depending on how hard the water is. Yeah, that requires a special tool. Neglecting this chore can void your warranty.
The other big one is that they need to be installed by a plumber who is trained on tankless water heaters. The advice of ordinary plumbers regarding what size of units to buy and where to put them might not be good. A unit that's too small may get overloaded and shut down under conditions of high demand. One that's too big may not go on if you only want a gentle stream of warm water (for instance if you need to wash a cat). (I remember a campground that had one big demand hot water heater for all ten of its showers on the assumption that groups would all be showering together. For just one person to take a shower, at least three showers had to be running with nobody in them or the hot water wouldn't go on.) They might want to install it inside a wall where it's hard to get at when it's time to descale the pipes. (Not to be too paranoid, but plumbers who are not trained on tankless water heaters usually don't like them and may install them badly on purpose to get you to switch back to a conventional one.)
Demand hot water heaters also generally have internal computer chips, so they need electricity. Some just plug in, but some need to be wired in. Also, if the power goes out for a few minutes while you're taking a shower, if the water heater is tankless, your shower will be cold almost instantly. With a big old tank water heater, you may not even notice.
Finally, some manufacturers will void the warranty if their heater is worked on by anyone other than one of their certified technicians.
Now, I'm assuming that any tankless hot water heater you install will be electric. Gas hot water heaters, like other gas appliances, need air flow and ventilation, which adds a whole extra layer of complexity. If you google "tankless hot water heater problems", about half of what you find will be about problems related to the gas.
I've also heard that, in some situations, a combination of a tankless water heater turned up to its hottest possible outflow temperature with a cheap single-handle shower controller can result in wild temperature fluctuations. Something about the two appliances not playing well together. This was at least twenty years ago so that technical problem may have been solved.
That's the basics of what I know about tankless hot water heaters. At https://blog.totalhomesupply.com/point-of-use-water-heaters-guide/ there's a chart about halfway down the page that tells you how to estimate your needs.