Are electric Tankless water heaters really energy efficient?

4 Comments

1. Lee says:

   January 29, 2010 at 10:15 am

   Yes tankless heaters are more energy efficient. It comes from the tankless heating only what you use, versus the tank having to keep the 28 gallons at a constant temp, 24/7. If you use more than 28 gallons at a time then you will not have hot water, cool at the very least; compared to the hot that you will still have with the tankless.
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2. bushmaster8 says:

   January 29, 2010 at 10:20 am

   I’m no expert on this. I, like you have been studying the issue online as I’ve been considering a change to the tank-less water heater(s).

   Based on my research I have come to these conclusions:

   1. Electricity is MUCH more expensive than gas for the equivalent heat from gas.
   2. Therefore electric tank less heaters are not going to cut your water heating bill.
   3. Gas storage type water heaters work by keeping 30 or more gallons of water at a high temperature all the time, therefore causing a higher total gas bill.

   My conclusion is that under normal circumstances, the total fuel cost turns out about the same for gas or electric.

   My negative concern regarding going to tank-less [whether electric or gas (yes, there are gas fired tank-less water heaters)] is the exorbitant up front "capitol" cost which is well over the much less expensive gas storage tank heaters.

   The key is "longevity" and maintenance and repair costs, as the tank-less technology is relatively new, and most of the manufacturers haven’t had many years to develop a well-known reputation for the long-term dependability of their products.

   It literally scares me to shell out that kind of money on a product that I don’t KNOW up front whether it will last long enough to make the fuel cost break-even point versus staying with gas storage heaters.

   So, at this point, I’m going to stick with my natural gas fired, storage tank type water heater [with a double layer of glass fiber insulation that I properly applied].
   References :
3. sensible_man says:

   January 29, 2010 at 10:36 am

   Yes, tank less heaters are more energy efficient. They use water run through small tubing that heats quickly. They only use energy when in use, unlike keeping water hot during non-use periods. Initial outlay of money is high and you need to choose brands carefully. Rinnai is one brand I hear many good things about. Even then, you want to make sure the installer does maintenance work on it. Many plumbers will install them, but since they are "high tech", they do not do repairs. If you take a 15 minute shower, with a low flow shower head, you are using 45 gallons of water. If even half of that amount is hot water, you used 22-1/2 gallons from your 28 gallon tank. After the first couple gallons, your tank is becoming cooler due to the replacement of the water from the tank with cold water. Your water heater now needs to heat all of the water in the tank. With tank less heaters, this is not true.
   References :
4. Robin the energy saver says:

   January 29, 2010 at 10:54 am

   Electric tankless water heaters will save you money in the short term but are not as good for the environment, and are not as good for the efforts of electrical utilities managing the supply of power, and for this reason they are falling out of favor in countries such as Japan where until recently they were very popular.

   The main reason tankless water heaters save money is that in a tank water heater, you produce heat throughout the day for use later, and some of that heat escapes from the tank into the surrounding living space (or furnace room or wherever your hot water heater is located).

   You can alleviate this somewhat by properly insulating your hot water tank (special insulating blankets are available for this purpose – get the thickest one you can find), putting pipe insulation on your hot water pipes, and by turning the thermostat on the heater down from the factory default of 140F to 120F, which is both more efficient and safer. But you will still probably find that an on-demand heater costs less to operate, at least in the short term, because there is so little energy lost to heat escaping from the device, since there’s no heat storage reservoir.

   The reason I say on demand hot water heaters are not as good for the environment is that they produce random spikes in electrical demand, which makes it harder for electrical utilities to manage generation load. An on-demand heater draws a huge amount of current while it’s in use. A tank heater, on the other hand, draws a smaller amount of current for a longer time, and stores that energy as heat. Imagine if in a city of a million people, everyone had on-demand hot water heaters, and everyone showered when they woke up. The utility would have to find a way to rapidly ramp up electrical production in time for the on-demand heaters to fire up and heat water for showers, then to rapidly ramp down electrical production once the showering period (say, 8:30 am) is more or less over. Since they can’t ramp up this fast with things like coal or nuclear plants, they wind up ramping up much earlier (say, 3am), and ramping down much later (say, 10am), so that although you have saved energy, you have indirectly caused the utility to produce a bunch of energy that isn’t actually needed, and may be wasted.

   For this reason, in Japan for example, there is an aggressive effort underway to get people to switch AWAY from tankless electric water heaters, and instead use a tank electric heater with some intelligence built into it so that the heater heats water to a hotter temperature when there is excess electricity on the grid, and then blends that water with colder water if necessary to supply a steady temperature.

   Note that in some areas you pay for electricity based on the time of day it is used, and in some areas you even pay based on the peak amount you use. So if you shower at 9am you are showering during ‘prime time’ for electricity prices, which means an on-demand electric heater will cost you a lot of the expensive electricity, whereas a tank water heater, if it has the smarts built into it to know that no hot water is required while you’re away at work, can give you heat stored up at night, then not start reheating the tank until the evening when electricity prices have gone down.

   The other thing to continue is the lifestyle changes required when you go to an on-demand electric heater. The house I live in now (in Costa Rica) has one, and I have to say it does not work that well. The water flow is much slower than you get from a tank (because the heater only lets out as much hot water as it can produce at that time), and for something like washing dishes, where you might turn the tap on, then off, then on, then off, it just doesn’t work the way a tank heater does, because there is a lag between when the water starts flowing and when the tank starts heating. So your pipe is like a sandwich of cold and hot sections moving towards the tap.

   The other on-demand tankless option is an electric shower head – assuming a shower is the only thing you need hot water for, which if you’re really into saving energy is probably a sensible option. We wash our dishes and laundry in cold water so in Costa Rica electric shower heads are a common thing in most houses. (But in our area the water is so warm that we can shower with cold water!) You can buy electric showerhead heaters in the US and Canada – but they do have that same problem of very high current draw while they are in use.
   References :
   http://www.green-energy-efficient-homes.com/best-hot-water-temperature.html
   http://www.green-energy-efficient-homes.com/hot-water-heaters.html
   http://www.green-energy-efficient-homes.com/electric-shower-head.html