Tankless Hot Water Heaters

Posted September 27, 2008

A tankless hot water heater will save you 10-20 percent on your heating bill and give you a virtually endless supply of hot water.  This is possible because the tankless hot water heaters only heat the water when it is needed.  A normal tank based hot water heater will keep a set amount of water at the requested temperature at all times whether it is being used or not, so when you are not using hot water, say at night, the tank is still keeping the water hot.  Since heat always travels from hot to cold, the tank will radiate some heat into the air surrounding it, causing what are called standby losses.  Since the tankless hot water heaters only heat water when it is needed the standby losses are eliminated.

The tankless hot water heaters also use less fuel to heat the water.  A conventional gas fired hot water heater will have an efficiency of about 60%, meaning that 60% of the heat from burning the gas will be used to heat the water, the rest of the heat is lost up the chimney.  For a tankless hot water heater the efficiency is usually about 80-85%, meaning at least 20% more of the heat generated goes into the water and is not lost up the chimney.

On the downside, since the tankless hot water heater has no stored hot water, when you turn on the tap, it will take about 10 seconds longer for the system to kick in and get up to temperature than a conventional hot water heater.  This can result in more water use as you wait for the water to heat up.  If you are on a marginal well this should be a consideration.  On the other hand the water that is used while waiting for it to heat up can be captured in a bucket and used for other things such as watering the garden.  The other disadvantage is that if you have hard water, there is more maintenance involved.  The tankless hot water heater works by having a the water flow through a series of small pipes that go back and forth over a burner.  In a hard water area these small pipes will build up scale and if left untreated will eventually plug up the heater.  To avoid this you have to flush the system with a weak acid solution to dissolve the scale.  There are kits available that have a pump and an acid solution and when installed, you need to put in some extra valves and connectors to allow you to connect to the unit.  I flush my unit twice a year, once in the spring and once in the fall.

Another advantage of the tankless hot water heaters is the size.  The unit I have (Takagi TKD20) is only about 20.5″x14″x8.5″ and hangs on the wall out of the way.  In my house the same unit is used for both domestic hot water and for generating hot water for theinfloor heating.  This saves a lot of room in the utility room, as there is not tank and no furnace.  The unit I have can generate about 7 gallons of hot water per minute, so with low flow fixtures, you could have two people using hot water at the same time and still not run out.

Posted under Devices

9 Comments so far

  1. alyssa October 2, 2008 8:25 pm

    ummmmm this didnt really tell me how its energy efficient :\

  2. Ward Edwards October 3, 2008 10:45 am

    It is energy efficient because you don’t have the standby losses (about 20% loss) and the burner is more efficient, 80-85% vs 60-65% for the conventional gas hot water heater. Both these factors combine to produce more hot water with less gas burned. The standby losses increase in significance when less hot water is used, such as a 1 or 2 person household or intermittent use, such as a cottage.

  3. Landon December 2, 2008 2:05 pm

    How much does it cost??

  4. Ward Edwards December 2, 2008 2:30 pm

    I bought mine from http://www.tanklesswaterheaters.ca and at the time it was about $1500.

  5. paul anderson January 5, 2009 1:26 am

    ive put in a few gas tankless units but not electric does any one know how well any of the electric ones work ?

  6. Ward Edwards January 5, 2009 1:00 pm

    I haven’t any direct experience with the electric units, but from what I have heard they work just as well. The only real problem I have heard is that they need a 60 amp service, so if it is far from the panel, wiring could be an issue.

  7. Jim Green January 21, 2009 1:46 pm

    We installed an electric tankless hot water heater in the spring, upgrading our electrical panel at a substantial extra cost. Six months later there is so much scale buildup that pieces of scale continue to block the pipes causing either a total lack of hot water or lack of pressure so that the cycle does not kick in.(We have well water) I have cleaned scale out of the unit weekly since Christmas and never know if we will have hot water or not. The company we bought the heater from has said we should install a water softener…now they tell us. One of the reasons we put in the unit was to allow us to use the space for other purposes. I have recently read that you have to flush the system with a weak acid solution and there are kits available but we should have installed extra valves and connectors when we first put in the unit. Can anyone help us with some suggestions?

  8. Ward Edwards January 21, 2009 5:39 pm

    I have fairly hard water, but have not had any significant problems with scaling. I do have a new house and the pipes are primarily PEX, which supposedly is not subject to scaling. I do flush the system once a year, but I may move that up to twice a year just to make sure. I suspect that the scaling you are getting is in the feed line to the heater and is due to the accumulation of scale from the time the house was built. You might want to look into replacing that line from the pump to the heater.

    In order to flush the unit you need a spigot on the incoming and outgoing lines that can accept a garden hose type connector. I got my kit from Low Energy Systems

  9. priusmaniac August 13, 2009 8:54 am

    Instant heaters are more interesting in the electric form. In that case, you replace an electric boiler by an instant electric heater. For the same reason of no standby losses, you gain 40% of energy consumption despite the fact that the instant electric heater is actually 4 or 5 times more powerful then a boiler heather. In the same time, this can be the disadvantage of the instant electric heather because you really need a high power supply otherwise it won’t work. Well at least that was up to the time I passed along the place and revisited the system. The trick is as follows; there are instant electric heaters made to supply hot water for a toilet hand wash, in this case, the power is at a similar level then for a conventional boiler. Normally the problem would be that your temperature would be to low if you increase the water flow, like what is needed for something like a shower. Of course, you can already partially solve the problem by using a low flow shower apple but the real solution comes from combining the instant electric heater with a wastewater heat recovery system. Indeed if the water usually comes to the heather at 10°C, it comes at close to 30°C when it is first passed through a heat exchanger on the 37°C waste water exhaust of your shower. In that case, the instant electric heater will start at a lower temperature but as the inlet temperature starts to increase so will the outlet temperature, until you finally have a stable situation where the power is sufficient to compensate for the losses. Obviously, the better the recuperator and the higher the heather power, the sooner you will get to the equilibrium point. With a good match, you can even get there sooner then what would be the case with a boiler especially if the boiler is distant from the shower. Of course, your instant heater must be at your shower or very close to it, which is possible thanks to his very small size. The extra advantage of this system is not only that you use less energy but also that you use electricity instead of fossil fuel and that your electricity can be made in a green way, which is by definition never possible with a fossil fuel like gas, fuel or coal. Sometimes people say it is better to use a solar water boiler, but when you do the math, the base investment is 600 $ for a waste heat recovery exchanger and 300 $ for an instant electric heater, while it is 5000 $ for a solar boiler installation. In more, your solar boiler will leave you with cold water after a while or when there is no sun in the winter. Actually for the same 5000 $ you are better off buying photovoltaic cells to make electricity for sale on the grid. You can also use it for your instant heater and enjoy the summer time money when you are away from home on vacation especially when considering that a solar water boiler, when in vacation, is equal to a pure loss, because you can’t sell hot water back to the grid.

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