Tankless Water Heaters: Not sure if I'd Demand them

Tankless Water Heater Diagram

Tankless Water Heater Diagram

Tankless water heaters, also known as instantaneous or on-demand water heaters, have become pretty popular lately.  I mean, who can resist the allure of "never ending hot water"!  But while they may be more efficient than a standard tank-style water heater, do some careful analysis before you go ripping out your current tank to install this solution.

Chances are your current method of heating water is a big insulated tank that either has an electric or natural gas burner.  The tank has a thermostat and when the water in the tank gets below a certain temperature, the boiler kicks on and heats up the water.  As with any system, heat is constantly lost through the walls of the tank (remember, heat flows from hot --> cold).  Although if you have good insulation this heat loss should be minimal.  But it is this "stand-by" heat loss that lowers the efficiency of a tank heater compared to a tankless style water heater.

An "on-demand" water heater does just that: heats up the water  at the instant it is demanded by you when you turn on the "hot" faucet.  The picture above is a great diagram of an instantaneous hot water heater.  When you turn on your hot water, the water comes straight out of the pipes in the ground and flows through coils that are in a heat exchanger that surrounds the flame.

This is more efficient than a tank because you don't have the stand-by losses that occur when you have a whole bunch of  hot water just hanging around waiting to be used in your shower or dirty dishes.  The water is heated only when it is needed, and never any other time.  Energy Savers (sponsored by the Department of Energy) has a great analysis of how tankless water heaters work seen here.

In Fall of 2008 Consumer Reports did an analysis on tankless water heaters with the summary: "They're efficient but but not necessarily economical".  Some of their drawbacks and a few others are listed below:

  • In order to activate the burner, demand water heaters need good pressure at a minimum flow rate.  This means that if you just need a tiny bit of hot water running while doing dishes you will need to keep it on a higher flow rate.
  • Sometimes water can run hot, then cold, then hot (cold-water sandwich)
  • High up-front costs
  • Because the burner has a maximum amount of heat it can supply at once, demand water heaters have a maximum flow rate through the burner.  Meaning if one person in your house needed to take a shower while the dishwasher was going, the load would be too big on the instantaneous water heater and it would not be able to supply hot water to both!  So while technically you may never run out of hot water, you wont be able to use too much of it at any one instant

But doesn't the advertisement "never ending hot-water" really defeat the whole concept of energy efficiency?  While there are only two people living in my household (my wife and I) we have only ran out of hot water once.  And that was when I experimented with turning down the heat on our standard hot water tank, and resulted in a cold shower for an unhappy wife.  We do have a 50 gallon tank, which may be a tad too big for just two people?

This guy has some good recommendations on his blog, A Concord Carpenter, about his experience with his Rinnai tankless water heater:

I have a RINNAI brand on demand heater in my house and properly used these heaters are great. Improperly used they are disappointing and expensive.

To avoid disappointment, make sure to size the unit correctly for your house water usage. The size and number of whole house tankless water heaters you need will be largely driven by flow rate and that is determined by the number and types of fixtures you may have running at one time. My unit can only handle one shower and one faucet or the dishwasher at one time.

The key point to take away from that quote is that if you do decide to go with a tankless water heater, SIZE THE UNIT CORRECTLY!

While tankless water heaters do have a niche, don't assume they will work for every application just because you see them being marketed everywhere.  You may be better off with a standard tank with solar thermal back up! You would definitely want to consider a tankless heater if you have a guest house or a cabin that is rarely used.

In later blogs I will do more cost/benefit analysis comparing the different methods of heating water and which one (tank, solar thermal, tankless, hybrid) really works out best in the long run taking into account energy prices, up-front costs, efficiencies, etc.

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Thanks for the insights. I also think that tankless water heaters are great for space and energy savings. Especially now, when you can get a federal tax credit of $300 (tax year 2011), might also qualify for state rebate, and even rebates from local gas companies available is some areas. It's just not wise to keep a full tank of hot water if nobody using it during a day.
With a tankless water heater, it heats water as it passes through the unit, so you will never have to use hot water that has been stored in an old rusty tank. As the years pass by, traditional tank-type water heaters start to rust and build-up scale inside the tank, unlike this tankless water heater. You will never have to worry about getting a rust filled water flowing through your faucets.
@Dewey... you've obviously never used a tankless water heater, because if you had you'd know that there is no way to set the water at a specific temperature as you advised. There is a knob to set the flow rate for the gas, and a knob to set the "sensitivity" of the flow detector. The temperature of the water that comes out depends on 1.) the setting of the gas control knob, and 2.) the flow rate of the water. Here's how it works... Let's say you've adjusted the gas knob on the heater to get 105 deg. water at full flow, and then you turn on the hot water tap full blast to get the hot water quickly from the heater in the garage to the tub upstairs. A minute later, you get hot water, same as with a tanked type heater. It's a bit too hot for a shower, so you turn the tap to "warm" and start to shower... At full flow, the water moved quickly through the heat exchanger, but when you turn the tap to "warm" it slows down the flow of hot water, so the water passes through the heat exchanger more slowly, and it gets MUCH hotter!!! so about 2 minutes later this super-heated water arrives at the tap (it takes longer to get there at the lower flow rate) and you're back with 105 deg. water again, even though the tap is set to warm.... so you turn the tap a little more to the "cool" side and continue showering. Back at the heater, the cooler tap setting has reduced flow below the threshold of the flow detector, so the heater shuts off and simply passes cold water through. a couple minutes later, the cold water arrives upstairs and you're having a cold shower... so you turn the tap back to full hot and start over, wasting more water as you wait for the hot water again. If you want a good shower, you have to adjust the gas knob so that the full-flow water temp is exactly right for showering, then always use full-hot. Unfortunately, that means it's not really hot enough for washing clothes, dishes, or mopping the floor. As far as "selecting the right size unit", forget about it! If you adjust the gas knob to where the temperature is 105 with one tap open at full flow, and then you have more than one tap running, the water won't be very hot because of the faster flow rate going to two taps. If you turn up the gas to heat the water sufficiently for two taps and then only use one, you'll be scalded! ...then when you turn the tap down cooler, the flow sensor drops out and you get cold water again. The "drop out" problem can be reduced somewhat by adjusting the "flow sensor sensitivity" knob, but there's a catch - the "flow sensor" is actually a differential pressure switch, and the way you make it more "sensitive" is to gradually close a valve that restricts the cold water entering the heater... if you adjust it to allow your hot water to flow freely, then your bathroom faucet won't get hot water, even on "full hot" - it just doesn't flow enough water to trigger the flow sensor. If you adjust the sensitivity knob so the bathroom faucet flow is sufficient to kick the heater on, then the flow is restricted so that it takes f o r e v e r to fill a mop bucket at the kitchen sink... you've basically converted your "whole house" heater to an RV sized unit - AND the bathroom sink water will be scalding, because it flows slower than the kitchen sink or shower. If you adjust the gas knob to give good hand washing temperature in the bathroom sink, then you will only get lukewarm water at the higher-flowing taps. adding an external temperature control valve seems like a cure, but i dunno... the shower head or faucet strainer limits the max flow, so I think adding cold water at the TCV will reduce flow through the heater, causing the flow sensor to drop out, same as if you reduced flow at the tap. I would hope that some of the super-expensive models have a true analog flow sensor, plus an analog outlet temp sensor, and a computer-controlled variable-opening gas valve. Otherwise, you will only get "good" hot water when using whatever amount of water you've adjusted it for. the rest of the time it will either be too hot, not hot enough, or drop off and give you cold water. As for being more compact, that depends. They're smaller, but require a lot of "free space" on both sides around the air inlets.
Hi it's a great post. you have really described well for thankless water heater. please tell me that if we install a solar hot water system then tank is associated with the water heater or we have to purchase it individually.
Absolutely, the most important part of the equation in terms of user satisfaction is correct sizing. I've talked with plenty of people that complain that their tankless water heaters don't provide enough hot water, only to find out that their heaters are completely undersized for their needs.
I had a small reem scorching water heater for beneath the sink that lasted 4 years. This one was easier to put in and heats water faster and takes up less room (each have been 2 half of gal). It stays cooler below the sink (it is enclosed) than the reem.
I am thinking of using a tankless waterheater in a vacation home in the Sierra's. I've heard that if you don't drain the water, religiously, the tankless waterheater will break (due to smaller than usual pipes). I'm worried that we are caught by an early freeze or a guest forgets to drain the water and it's a disaster. Any experience/comments? Thanks.
please send me the details suppliers name, which i could be able to contact.
most chinese families use tankless gas heater. but those rich like to use tank ones to show their lifestyle difference from ordinary families. and rich ones like to have same lifestyle like western cooutries. the first reason Chinese use tankless is we don't have enough room space for a tank, though we own larger houses than before. House is still very expensive. the second reason is tank ones are more expensive than tankless. i use Noritz in Shanghai with 11litre/min. for 1 bathroom and 1 kitchen. I notice that the manufacture site give larger unit for USA user at the same demand
md, I cant remember where I found the picture. Are you looking at this for your home or business?


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