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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The Key to happiness with a tankless water heater is purchasing the proper unit for your needs. Most negative comments and horror stories about tankless water heaters can be attributed to the tankless water heater selected not fitting the needs of the user. All of the people that make correct choices are very happy with their tankless water heater. Most people will wonder why they took so long to install their tankless water heater after they have found the right fit for their needs. Some of the things you should look at closely are: Water supply - is it from a municipal water supplier or water well? What are the seasonal water supply temperatures in your area? Are the water purveyors constantly adding or repairing the water supply lines in your area? If yes you may need additional pre-filters to catch foreign materials so they don't collect in the heating chambers or fowl the flow switches. In some cases the shower mixing valves can contribute to the reported problems of temperature fluctuations with tankless water heaters. If you have pressure balancing or temperature balancing shower valves they can compete with the tankless water heater for dominance over the control of the hot water delivery. Also booster pumps on the hot water side used in recirculating hot water systems can be a problem if not properly installed. Some people have these items installed in their home and they worked fine with the storage tank type water heater. Then they installed their tankless water heater - experienced water temperature problems and started blaming the tankless water heater when in reality the shower valves were causing the real problem when a tankless water heater is installed. The fuel or power requirements of the tankless water heater. Gas tankless water heaters may require a larger gas line to adequately deliver the correct CFM and gas pressure to all appliances in the home. Electric tankless water heaters may need a larger electrical service panel and/or larger circuit breaker with larger wires to the unit.As you can see purchasing a tankless water heater requires some research to find the proper size and type for your situational needs. I recommend purchasing a tankless water heater from someone who can assist you with these answers and guide you through your installation not someone who sells water heaters and lots of other things unrelated to tankless water heaters. Unfortunatly not all plumbers and electricians are familiar with all tankless water heaters and the proper sizing and installation requirements for each manufacturer. So please check with the distributor if you have questions and concerns, they should be able to answer brand specific questions and how it relates to your installation. If not don't buy from them. If they can't help you before the sell and during installation they certainly will not be able to help you with problems after the installation should they occur. The best customer service I have seen is from Real people answering real questions for their customers. Check out my blog at Water Heater Guy
I work for a new company called Unplugged Power Systems. We do alot of alternative energy stuff like windmills and solar power. From my experience installing solar hot water heaters, it may be the solution you are looking for. As long as the outside temp is above -40 they will generate hot water. I live in Canada and it works here even in the winter. Basically it uses a holding tank and circulates water through a solar heater. In times that there isn't any sun, the on-demand heater makes up the difference. Its by far the cheapest way I have found. The solar arrays go for about $800 for evacuated tube systems and you can install them yourself. Don't pay more than $1k, some people offer them for up to $8k per array but don't fall for it :) Just my 2 cents worth.
Brooke, Thanks for the comment, but I dont know if I would encourage people to install solar thermal systems themselves. It's possible, but there is a whole lot of plumbing involved, and if done wrong, could be a major headache. What about drainback tanks, expansion valves, etc.? It's not that simple. Installing solar thermal or PV systems should probably be done by someone who knows what they are doing. Let me rephrase: Any time you are a) getting on a roof, b) hooking into an electrical system), or c) both, you need to have experience. But I do think solar thermal is an excellent way to get hot water!
Note that tankless water heaters may be a good alternative for businesses as well as homes. Pubs, Restaurants, Hotels, Motels, and more, all use TWH's for the very large cost savings, being able to expense or depreciate the up-front costs. So if you have an in-home business, or you just want to make points with your boss by cutting expenses, keep that in mind.
OK...I have had a tankless for 6 years, And I have now gone back to an 80 Gallon Storage Type. This is why. Tankless is great for about a 3 person family...but I have 4 kids. That's 6 people in my family. I noticed my utility bill skyrocketing. After doing the math I realized why. An 80 gallon storage model uses about 5kw. The tankless that I was using to support my family and useage requirements would use 24kw while in use. If you have a large family especially with teenagers, your savings go out the window. Infact, I am estimating about a $50 reduction in monthly utilities by going back to a storage tank. Just my 2 cents worth.
Kristine, If you fill up your sink with hot water to do your dishes why would you have to leave your water running ? If you set your heater to 90-105 degrees that is all you need and any hotter you will not be able to put your hands in the water. If you put water in your sink from a tank heater on one side and from a tankless on the other side am I to assume that the tankless side will be cold while the tank side will remain hot almost forever? The above statements about your dislike of the tankless heater simply are not true. They may be true in your way of thinking but the real facts that there is no way one side will react different because it came from a tankless heater. I have had one for to many years now and I know that my electric bills are $30-$55 less per month now than when I had a tank heater and every month I save that money. At the start it may be a little different getting used to where to set your faucet because you had a favorite place to set it and now the change has you off balance. For what it is worth; we as Americans dispose of 7.5 million tanks a year in the landfill while the third world countries have had the tankless forever and you would be hard pressed to find a tank in those countries except for special applications. Every month that you spend $30 more to just keep your water hot on standby I save the same amount to do something else with. I am in your corner and whatever makes you happy makes me happy. Everyone is allowed their opinion and thank God for that and the fact we STILL have that option but maybe not for long. I respect your opinion but I don't agree with it.
Dewey is right on with the savings but a little low with the $30 savings. Tankless is the way to go but be sure you get the right unit for the part of the country you are in. The unit that works in the south will not keep you in hot water in cold weather. The heater that works in the cold weather will work anywhere.
We lived with a tankless heater for 6 months and I hated it. I had to run the water a lot longer and harder to get hot water. Our water is very expensive, so I don't know about cost savings. Plus the water doesn't stay hot, you have to leave it running the whole time, like if you are doing dishes.


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