Get Optimal Performance from a Tankless Water Heater
by Willem Maas, 01/17/06
Willem Maas is the Founder and Publisher of GreenHomeGuide.
Tankless water heaters have caught the eye of many homeowners lately—as a way to reduce the 19 percent of total home energy use consumed by water heating. According to research sponsored by the National Renewable Energy Laboratory (NREL), heating water with a tankless system is 12 to 34 percent more energy efficient than using a conventional storage tank system. And when tankless systems are installed at each water outlet, gains in efficiency can range from 28 to 50 percent.
Others are impressed by the ability of tankless systems to deliver hot water "endlessly" in a busy household, as well as the space saved because a storage tank is no longer needed.
These factors, combined with rising energy bills, a $300 federal tax credit legislated in the Energy Policy Act of 2005, and greater availability, make tankless a technology well worth considering.
But although tankless systems have been used in Europe, Japan, and elsewhere for decades, they’re a new technology in North America, and homeowners describe a wide variety of experiences with them.
Some are proudly satisfied while others feel "shnookered" by delays in getting hot water to the tap and changes in temperature during hot water use.
To help you increase the odds that your tankless experience is positive, we’ve examined six commonly reported problems and gathered solutions.
1. Get the right size heater for your home and uses.
Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. To ensure that your heater will deliver enough hot water, determine the flow rate and temperature rise you’ll need. Find the appropriate flow rate by listing all the hot water outlets to be connected to the heater and adding up their flow rates in gallons per minute. Make sure the heater you purchase can deliver the most common temperature rise of 70 degrees Fahrenheit (39 degrees Celsius) at the desired flow rate.
2. Consider the minimum flow rate.
To avoid overheating, tankless water heaters require a minimum flow of water—ranging from a half gallon per minute to two gallons per minute—through the heating system. Failure to meet this requirement is the source of complaints such as "I do not like that when I wash dishes I have to keep the flow of water higher just to keep hot water going" from the Berkeley Parents Network. To avoid this problem, make sure the flow rates at outlets you’ll connect to the heater exceed its minimum requirements.
3. Hire an experienced plumber.
Tankless heater installation is complex, requiring proper configuration and sizing of the unit to fit the home’s plumbing, gas or electric supply, and ventilation requirements. Ask the plumbing supply house or hardware store where you’re buying the heater to connect you with three factory-trained, experienced plumbing contractors, and get bids from all three. Most manufacturers also will refer you to local plumbing contractors.
For more tips on the ins and outs of tankless installation, read homeowner Gordon Gray's 9 Best Practices for Choosing and Installing a Tankless Water Heater.
4. Choose a point-of-use model if you’re concerned about wasting water.
Many tankless users believe they "waste a lot of water while waiting for it to turn hot." Water waste is caused by two factors. First is the number of feet of plumbing—and thus cool or cold water—between the tap and the water heater. Because of this, whole-house tankless systems do not offer a water-saving advantage over conventional storage tank systems. However, tankless systems can be installed in a point-of-use configuration in which a smaller, dedicated heater is located very close to the water outlet.
The second cause of water waste is the amount of time required for the heater to generate hot water. For a conventional tank system with hot water in the tank, that time is zero. Tankless systems are at a disadvantage here, as explained in the next tip.
5. Instantaneous isn’t quite as fast as it sounds.
While tankless water heaters are often called "instantaneous water heaters," unheated water inevitably flows through the unit before sensors detect it and water heating begins.
The two best strategies for dealing with this much-discussed weakness are preheating and buffering. Both solar hot water heaters ($2,500–$4,500) and drain water heat recovery technologies (about $500) increase the temperature of water entering the tankless heater, mitigating the impact of delayed heating. A 5- to 10-gallon storage tank plumbed right out of the tankless heater dilutes cold water with hot water in the tank and makes the delay less noticeable. This solution has a lower up-front cost but is less energy efficient.
6. Correctly ventilate the heater’s exhaust.
The tighter a building gets, the easier it is to create pressure differentials that can cause a combustion appliance to backdraft, bringing exhaust gases like carbon monoxide into the living space, according to Arnie Katz, senior building science consultant at Advanced Energy. Whether it’s a tankless or not, a gas-fired water heater should be sealed-combustion, direct-vent to prevent this situation.
For more information:
We found generally good reviews for the natural gas powered Takagi T-K2 and the Bosch AquaStar 2400E. These are large "whole house" water heaters.
For a smaller application (i.e., one bathroom) consider the Bosch AquaStar 125B or the electric Bosch PowerStar AE125.
Comments
On February 19, 2007, Robert wrote:
A typical storage based system is 78% thermal effecient and a tankless is 81% to 82% thermal effecient. I do not know how you can state that is 12-34% more effecient than todays storage based systems. In addition, the additional amount of gas used by tankless product and their users is never mentioned. Also, ever article says that the tankless product will handle a whole house. If you have a roman tub filler there is not an individual tankless product that can handle this demand. Most roman tub fillers produce 15GPM at 45PSI and higher volumes and higher pressures. At a 70/30 hot to cold ratio a roman tub filler will need 10.5 GPM of hot water. Plus no one every mentions the additional cost associated with installing a tankless product. Stainless vent and a larger gas line are additional costs that are hardly ever mentioned. I have yet to see an article that states the pay-back period for installing the tankless sytem over a traditional storage based system. There are better storage based options out there today that exceed the effeciencies of a tankless product. There are storage based gas products that are 90% thermal effecient and have a 50 gallon plus storage capacity for under $1500.00. The tankless companies are overselling the benifits of their systems.
On February 21, 2007, PatF wrote:
I live in an earthquake zone so did not want to use gas. I live alone, so did not want to store hot water all day from my morning shower to my evening dish & pot washing. I don’t have a Roman tub, and don’t want something that holds that much water. My old footed tub does quite well when i need to soak away aches. Essentially, my tankless heater only uses energy when i need it. The water that i “waste,” while waiting 30 seconds for the water to get hot, goes into my rain barrel for additional water. If there were more of us using the shower in the morning, i’d probably add a small tankless heater in the bathroom.
On May 4, 2007, Pete wrote:
Tankless heaters are more efficient than storage water heaters because not only do they usually burn more efficiently, but also you don’t end up keeping 40-60 gallons of water heated to 120 f or more 24-7. Even with a roman tub you’re using a lot of energy keeping a lot of water hot when actual demand is probably only several minutes per day.
On July 6, 2007, Patricia Denney wrote:
I used to have a tankless water heater and loved it. I had a smaller house. Now being the only house we could get at the time this house is too large for only one and would have to have two fitted. Until homeowners get used to the idea that huge oversized homes are too big a footprint on this earth in building materials and energy they will not use a tankless water heater to save energy. Why should they? They have the money and they will spend it regardless. I cannot wait to move once more into a smaller cozier home and have a tankless water heater fitted. Sustainable wood floors and cabinets. E windows Etc. We have all got to do something about climate change and start now! Who wants a massive roman tub anyway?
On July 26, 2007, R. Mosura wrote:
I’m currently investigating the benefits of either the electric tankless versus the gas tankless. Electric seems to winning out as I plan to convert to solar very soon anyway. Regarding the “roman tub” . We have it on order and comes with its own supplemental heater to keep it nice and warm....so no need to worry about the “endless” hot water question. (aprox. $250 to $350 for the extra tub heater )
Thanks for all the great info here.....now I can learn something useful on the internet.
On August 7, 2007, Jeremy wrote:
I am considering an electric tankless water heater in combination with solar thermal panels and an indirectly fired storage tank. I think this may be the perfect setup for our location in a sunny climate with chilly winters. (SW Ontario, not far from Detroit).
Does anyone have experience with this combined system. I’d love to hear someone’s trials and tribulations!
On August 26, 2007, Lori Perrault wrote:
While we definitely researched the purchase of a tankless water heater and also like Pat F above live in Earthquake country - should an earthquake occur - the water in your water heater is good backup water. So we opted for an automatic gas shutoff valve and a tank.
On August 27, 2007, PatF wrote:
<GRIN> My house is surrounded by earthquake-strapped rain barrels since i plan to “shelter in place”—along with 6 nearby family members. Purification filters are stored with the tents, sleeping bags, etc., outside of the house.
On September 5, 2007, Zoe wrote:
I am thinking of installing a tank-less hot water heater and I am living in earthquake country as well. I think I’ll model after Pat’s strategy. Having a tank of water is nice when a major earthquake knocks out water supply. But that is not a good enough reason to stick with the hot water tank. Suppose we got lucky and no major earthquake hits for the next 10 years, that tank will still be heated day in and day out. That doesn’t make much sense in terms of energy conservation. I’d opt for the rain barrels as emergency water supply. It takes a little more maintenance work, but it’s not rocket science.
On September 20, 2007, Joanne wrote:
A number of years ago my water heater started to leak. I’t was located in the house, in a cabinet in the kitchen. Because of the location of the water heater, I had to go with a small unit. I researched and the Takagi-Jr was the ONLY unit that would fit in this location. Otherwise I would have to pay a plumber to relocate it to another area.
Energy efficiency was more important to me than $$ savings, especially since the unit, cost of the special stainless steel vents, cost of the plumber, etc was significantly more. There are two drawbacks though. Since I’m in earthquake country-Southern Cal-I can no longer think the water in the tank could serve as an emergency water supply. Good think I’m a hiker and have a water purifier available.
The second it I installed a water-saver soap-up feature in the shower. Basically when you soap up, you shut off the water by pressing a buttun behind the shower head. Because of the lack of demand for hot water, this turns off the water heater. Luckily I can rinse off fast enough while there is still hot water in the line. By the time the cold water kicks in, I’m done.
Overall, I am very happy with it and would, and have, highly recommended going this route.
On November 28, 2007, c kennard wrote:
The only tankless water heaters I’ve used are in a private residence in Hong Kong. There was a tankless unit at each water source. I found the water pressure low, and the heat adjustment during use difficult. In my opinion, the heat adjustment was unsafe for children. This was some time ago, so perhaps there has been a lot of improvement. I’d like to know what current experiences with children using the units are like.
On November 29, 2007, Frank wrote:
About the minimum flow requirement. Many people leave the water running when they wash dishes. It is very much like letting the water run when you brush your teeth. It is wasteful and serves no purpose. Bad technique.
Better to fill the sink or a container of some sort at maximal flow and then use the water with detergent to dip in to clean the dishes. You can then rinse them all at once later.
Adjusting the temperature is easy with modern units. Technology evolves for the better.
On November 29, 2007, Pat wrote:
In general, most hot water heaters in the U.S. are set below scalding temps. In some places, it set that way by law—mostly for rental units. If you have children, i’d recommend using that setting, 120dF i *think*. Since my tankless HWHs are in insulated crawlspaces, i never change the setting on the heater itself. Just use the cold water faucet to mix it. My then-11 yr old granddaughter did not have a problem with keeping her bath at a good temp for her (and my water bill higher than i like). My 2-yr old GD doesn’t use household water unsupervised.
On January 11, 2008, Tori wrote:
When I lived in Japan, all residences had exclusively tankless water heaters. Let me tell you, no one loves hot baths and showers like Japanese people, and I was pretty delighted with the heaters myself. The water would be hot pretty much right away (most people have the tankless heater located in the bathroom so the pipes are short—it takes a little longer in the kitchen unless they had two heaters, one for kitchen and one for bath).
They are so much more efficient and so much better for the environment, I can’t wait to own my own home so I can get one. The money savings alone make it worth it, but then to know you get amazing hot water and you’re helping out the planet feels better.
On January 11, 2008, Rick Dill wrote:
There are two sides to efficiency with hot water. One is heating efficiency and the instant heaters are excellent for that. The other is water usage. The instant heaters require a minimum flow (approximately 10% of the maximum) or they shut down. In a house-sized heater, this means that you must use a large flow if you want to get any hot water at all.
The net is that the instant system is worse from a water usage standpoint which counters its being good on energy.
I am currently looking for a small hot water storage solution to buffer the instant heater start-up and also to allow small flows of hot water to be used.
On February 14, 2008, John W wrote:
In 1984, while living in Tallahasse, FL, the city utilities division made an energy savings offer to all homeowners that had total electric homes, with an offer to convert to natural gas for heating, cooking, and water heating. This was done to avoid building another electrical generating plant, due to the rapid growth of the city. Even though the generating plants were powered by gas, they decided that to convert 50% of total electrical homes, the cost would actually allow them to not make a power plant building for further growth for at least 25 years, since this would free up enough power for commercial expansion, the primary reason for the increase in generation capacity.
The homeowners were offered this package, with a low-cost interest loan for 50% of the total cost, paid through the customer’s utility bill. There were some federal funding involved, which made this possible. I took advantage of the program, and had the largest Bosch tankless heater installed in my 3br 1 1/2 bath home, with a washer and dryer, and dishwasher. The results were astounding!! At times, with a wife and 2 kids, showering, getting dressed, cooking, washing, and running the kitchen sink, many at the same time, (some of you remember what it’s like, with both parents working full-time professional jobs,) WE NEVER RAN OUT OF HOT WATER, NEVER!!!
I’m a life-long advocate of GAS tankless hot water heaters, so much so, that having recently retired, and relocated in the Philippines, I’m in the process of “moving Heaven and Earth,” to have one installed in our little home here. It’s very rare here, so I’m looking to Hong Kong, Taiwan, Japan, for a unit, to avoid the costs of shipping from Germany.
If anyone does the proper research, and get the unit that suits YOUR situation, it’s an investment in your home that will quickly repay the expense involved, by having hot water at your disposal, for as long as you need it. By the way, it also came in very handy , during the times of power outages from storms, hurricanes, heavy thunderstorms, and tornadoes. Many days and nights, the comfort of cooking, and bathing, when almost everyone else on my block was sitting in the dark, gives one a sense of being able to provide for one’s family, even in disasters.
Just some words from someone that’s been there, that’s all
On February 14, 2008, Carol wrote:
As an addendum to Tori’s comment, Japanese wash and rinse themselves in an area near the tub, using a small container of warm water. They only get into the tub AFTER they are clean. That way more than one family member can use the same heated tub.
On March 5, 2008, Jules wrote:
FYI, the article refers to “the most common temperature rise of 70 degrees Fahrenheit (39 degrees Celsius)” - actually, 70F is 21C. 39C would be about 102 degrees F. Big difference.
Would love to have one of these heaters, maybe this summer we’ll switch from our old clunky tank.
On April 28, 2008, Fred wrote:
We switched last year from 50 gal oil-fired (New England) hot water tank that never ran out of water (the oil burner could reheat the water faster than we could use it) to a gas whole house instantaneous heater. We were converting to a gas furnace; the hot water heating switch was a by-product of that decision.
The only downside - and one that I understand many wives (or husbands who live in the kitchen) complain about - is the delay to get small quantities of hot water. We’re installing a small 2 gal electric point of use hot water tank under the kitchen sink to fix that problem.
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