Design

Published on April 30th, 2008 | by Low Impact Living

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Low Impact Living: The Low Down on Tankless Hot Water Heaters

tanklesshotwaterheater.jpgEditor’s note: As part of our new content partnership with Low Impact Living, we’ll be bringing you tips and tricks from their massive and growing archive of products and projects that can help you save energy, water and money around the house. Today, LIL blogger Jason Pelletier takes a look at tankless hot water heaters. This post was originally published on Monday, April 28, 2008.

Tankless hot water heaters are becoming a must-have appliance in homes these days. They purport to offer many benefits – instant and endless hot water, less wasted space, and a smaller carbon footprint as well. They can even be used to power radiant underfloor heating that is a luxury found in many green homes.

But are they truly the best option? What can you expect from one, and what do you have to worry about with installation? Finally, are there other options available that offer the same benefits without some of the challenges? We’ll try to cover most of that in this piece.

First off, some basics about how tankless hot water heaters work. Standard non-electric storage hot water heaters typically are about 60% efficient, meaning that 60% of the energy in whatever fuel they burn is converted into heat for your water. The other 40% of energy is wasted, some due to inefficient burners and some due to the gradual cooling of water stored in the tank through time. Tankless or on-demand hot water heaters address both of these issues: they eliminate the storage-related losses, and they employ sophisticated burners and controls to improve on the combustion efficiency. Top-of-the-line models can be 80-85% efficient, which results in a 30+% savings on your hot water energy bills and your carbon footprint compared to your tank in the corner. (electric unit savings will be lower, since storage electric HW heaters already have efficient heating mechanisms).

There are some important caveats associated with tankless hot water heaters, though. If you’re considering one, make sure you take into account the following issues when you’re weighing the pros and cons:

  1. Instantaneous hot water means it will be hot at the outlet of the water heater right away, not necessarily at your tap. If you have a home where your shower or sink is far away from the hot water heater, replacing a storage with a tankless hot water heater might not solve your cold start problems. All of that water sitting in your hot water pipes will lose heat quickly, and you’ll have to clear it out when you turn on the shower before you get too hot. Many modern green homes employ new plumbing techniques where the hot water heater is located in the center of the house and each fixture is individually plumbed with smaller and less expensive plastic tubing to solve this problem and reduce hot water pipe losses.
  2. Tankless hot water heaters consume more fuel per unit of time than do standard hot water heaters, since they can’t rely on the stored hot water in the tank. This means that you might have to replace your gas line with a larger diameter one, all the way from the gas meter to your hot water heater. This can be expensive depending on your layout. Most tankless HW heaters need a 3/4″ line, while most older homes have 1/2″ lines by default.
  3. Proper sizing is critical. The required size of a tankless HW unit depends on two main factors: the temperature of the water coming in, and the maximum demand you expect at any one time. When people are disappointed by their new heater’s performance, it is often because they under-sized the unit, either because they under-estimated how frequently they need multiple hot water uses or they underestimated how cold their water is coming into their house. Most manufacturers provide charts, tables or calculators to help with proper sizing – make sure you are honest about your uses, and make sure you follow the manufacturer recommendations. And, be sure to plan based on your winter water temperature – it would be a real bummer to wake up on the coldest day of the winter to find out that your new water heater underperforms!
  4. You might have to improve your venting. Tankless HW heaters burn alot of fuel, so your existing venting might not be sufficient. You may have to install larger, straighter ducting, or perhaps install the unit outside. Both can add to the cost, so be prepared.

bosch-pos-hw.jpgThe tankless hot water heater segment is increasingly crowded. All major manufacturers make models that will suffice for an entire single family home and that meet the upcoming Energy Star standards for tankless hot water heaters. Top manufacturers such as GE, Bosch, Noritz, Rinnai, and Takagi all make models that comply with the performance characteristics of Energy Star, but we found it interesting that the warranties for current whole-house models from only three of the five companies met the 10 year heat exchanger / five year parts levels set by Energy Star: the new GE 9.4 GPM model (10/5), the Rinnai R75LSi (12/5) and Takagi Mobius T-M199 (10/5). The Noritz 0751 and the Bosch 2700ES met the performance characteristics, but the warranties fell short of what will be required for Energy Star labeling in the future. We suspect that all will soon upgrade their warranty programs to meet Energy Star standards.

What about other ways to get the same benefits? Today, you have many options.

  • First off, solar hot water heaters are the most environmentally friendly units you can buy, and they’re much more affordable than a solar electric system. You still get attractive rebates and tax incentives in many areas to offset the cost. They’re still 2x as expensive as a tankless hot water heater, but if you’re really trying to cut your footprint this is the way to go.
  • There are several new ultra-efficient storage hot water heaters that are even more efficient than the best tankess versions – up to 95% efficient! How do they do that? They employ specialized burners, heat exchangers and insulation to get the absolute most heat out of your fuel. AO Smith’s Vertex hot water heater is good example. You’ll probably pay about the same or perhaps a bit more as compared to a tankless version, and they are large, but they are even more efficient.
  • Consider either a point-of-use tankless hot water system or a recirculating pump if you have a decent hot water, but are tired of waiting a minute or two for hot water each time you take a shower. These units fit right under your sink and will provide almost instant hot water. The point-of-use hot water heaters (such as the Bosch PowerStar) are just smaller, less complicated versions of electric tankless heaters, so they have the same benefits. A recirculating pump will redirect the cold water that has accumulated in your hot water pipe back to your heater, thereby saving water and some energy in the process. You can connect them to switches, timers or motion sensors so that there’s hot water waiting before you even get to the bathroom. These pumps can be HUGE wasters of energy, though, if they are used for a long time each day. If you are eco-minded, make sure you buy controls that only run the pump when you truly need hot water. Versions that allow this kind of customization include the RedyTemp and the Metlund D’MAND system.

If you are wondering how much these various options will save in your home, create a profile in Low Impact Living’s Environmental Impact Calculator and go to the Projects page. There you can review how much you can save by installing a tankless or high-efficiency storage (such as the Vertex above) hot water heater versus just insulating your current version.

Efficient hot water heating is a true green no-brainer. Not only are they light on the planet, but they will also increase the comfort of your home and the effectiveness of your appliances. Fortunately, you now have many great options to choose from!

See also:

Cleantechnica: Water Heaters Get ENERGY STAR and Reduce Bills, Emissions

Green Options: Greens in the Shower — Some Like it Cold

Eco Child’s Play: Water, Water Everywhere

Green Building Elements: Elements of Building — Water

Image credits: GE and Bosch




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  • Quinne

    I stayed away from tankless water heaters because of what you said. They burn more fuel. So, in order for me to reduce my carbon footprint, I just switched out my oil with B5 blend. It’s biodegradable, burns cleaner, reduces emissions, aaaaaaaaand it costs about the same as regular oil. Pretty neat. Working at NORA helped me a lot. I was able to research greener alternatives and ran across this site: http://oilheatamerica.com/index.mv?screen=bioheat

    It helped me make the switch much easier. Thanks for the info though. I’ll email your link to all my friends.

  • Quinne

    I stayed away from tankless water heaters because of what you said. They burn more fuel. So, in order for me to reduce my carbon footprint, I just switched out my oil with B5 blend. It’s biodegradable, burns cleaner, reduces emissions, aaaaaaaaand it costs about the same as regular oil. Pretty neat. Working at NORA helped me a lot. I was able to research greener alternatives and ran across this site: http://oilheatamerica.com/index.mv?screen=bioheat

    It helped me make the switch much easier. Thanks for the info though. I’ll email your link to all my friends.

  • EBONY

    I have been considereing purchasing a more efficient and earth happier water heater. How do these above options stack up with radiators? I live in Maryalnd and have radiators through out the home and yes, it get pretty cold in the winter. Are some of these options better than others for both hot water and heat?

  • EBONY

    I have been considereing purchasing a more efficient and earth happier water heater. How do these above options stack up with radiators? I live in Maryalnd and have radiators through out the home and yes, it get pretty cold in the winter. Are some of these options better than others for both hot water and heat?

  • Uncle B

    Good article, opened my eyes to some problems the ads don’t address. Thank you for the Solar endorsement, I think that this is the easier, faster way for me to save a few bucks on hot water.

  • Uncle B

    Good article, opened my eyes to some problems the ads don’t address. Thank you for the Solar endorsement, I think that this is the easier, faster way for me to save a few bucks on hot water.

  • http://www.ecochildsplay.com Jennifer Lance

    Not happy with my new Bosch tankless. It burns way more propane than the old Paloma, and it is a pain to start when it gets finicky. Really want to put a solar hot water heater in to prewarm the water before it reaches the Bosch.

  • http://www.ecochildsplay.com Jennifer Lance

    Not happy with my new Bosch tankless. It burns way more propane than the old Paloma, and it is a pain to start when it gets finicky. Really want to put a solar hot water heater in to prewarm the water before it reaches the Bosch.

  • GreenAndGrowing

    We have a tankless hot water heater with a recirculating system. The Renai is the best upgrade we installed in our new house.

  • GreenAndGrowing

    We have a tankless hot water heater with a recirculating system. The Renai is the best upgrade we installed in our new house.

  • Toby

    Are people using more water with an “endless supply”?

  • Toby

    Are people using more water with an “endless supply”?

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  • Jerry Kahn

    I have electric hot water and wish to stay that way: there is no greenhouse gas issues (except for the coal that is burnt to make the power, that will go away partially when I have solar panels on my roof)and the heater is mechanically simpler and more efficient than natural gas heaters.

    Soon, I am going to replace my plumbing in my house and I was considering adding a tank, maybe 5-20 gallons, that would be plumbed ahead of my hot water heater. This tank would allow the colder water coming out of the ground to slowly heat up and not add so much heating demand on my existing hot water heater. I would have the tank in the house, so that it would slowly warm to the ambient air temp in the house, say 60-65 farenheight, rather than the 40 or 50 that comes out of the ground. Has this been done before that you know of? And if not, does this sound feasible?

  • Jerry Kahn

    I have electric hot water and wish to stay that way: there is no greenhouse gas issues (except for the coal that is burnt to make the power, that will go away partially when I have solar panels on my roof)and the heater is mechanically simpler and more efficient than natural gas heaters.

    Soon, I am going to replace my plumbing in my house and I was considering adding a tank, maybe 5-20 gallons, that would be plumbed ahead of my hot water heater. This tank would allow the colder water coming out of the ground to slowly heat up and not add so much heating demand on my existing hot water heater. I would have the tank in the house, so that it would slowly warm to the ambient air temp in the house, say 60-65 farenheight, rather than the 40 or 50 that comes out of the ground. Has this been done before that you know of? And if not, does this sound feasible?

  • http://greenbuildingelements.com Philip Proefrock

    Jerry,

    While an electric water heater is theoretically more efficient than a natural gas heater, it is a very inefficient process to convert heat (from coal burning) into electricity (and lose more than 2/3 of that energy to plant inefficiency and transmission loss) and then convert it back into heat to warm your water.

    Similarly, using solar PV to create DC electricity, and then convert that to AC and then use that to heat water is vastly more inefficient than a direct solar hot water system.

    As far as your tempering tank idea goes, the heat that it absorbs from inside your house has to come from somewhere, and in this case, that is basically your home heating system. So while you save a bit on running the water heater, you add a bit more to your home heating system.

    In any case, I don’t think the cost of the added plumbing and the tank would pay back in energy savings for an extremely long time.

  • http://greenbuildingelements.com Philip Proefrock

    Jerry,

    While an electric water heater is theoretically more efficient than a natural gas heater, it is a very inefficient process to convert heat (from coal burning) into electricity (and lose more than 2/3 of that energy to plant inefficiency and transmission loss) and then convert it back into heat to warm your water.

    Similarly, using solar PV to create DC electricity, and then convert that to AC and then use that to heat water is vastly more inefficient than a direct solar hot water system.

    As far as your tempering tank idea goes, the heat that it absorbs from inside your house has to come from somewhere, and in this case, that is basically your home heating system. So while you save a bit on running the water heater, you add a bit more to your home heating system.

    In any case, I don’t think the cost of the added plumbing and the tank would pay back in energy savings for an extremely long time.

  • kent

    excellent article, redoing the master bath and looking to put a tankless in the master bath area because of the long run from existing water heater.

    I think you should find a more enviro friendly photo the water softener will dump more salt laden water than you will save with the tankless.

  • kent

    excellent article, redoing the master bath and looking to put a tankless in the master bath area because of the long run from existing water heater.

    I think you should find a more enviro friendly photo the water softener will dump more salt laden water than you will save with the tankless.

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  • Anthony Airhart

    American Tankless has refused to refund us for their pretty much worthless Adtec Tankless One unit. Nothing but lies and BS…no money. I would stay far away from them!

  • Anthony Airhart

    American Tankless has refused to refund us for their pretty much worthless Adtec Tankless One unit. Nothing but lies and BS…no money. I would stay far away from them!

  • Steve

    I have a long run from the hot water heater to the kitchen sink and in the morning, it takes at least two minutes for the water to start getting warm. I’d like to use an electric demand heater to warm the initial water flow until the hot water from the heater arrives. I’d also like it to have a thermostat that would turn off the demand heater once the hot water from the tank arrives. Is there such a beast on the market? Thanks for any help!

  • Steve

    I have a long run from the hot water heater to the kitchen sink and in the morning, it takes at least two minutes for the water to start getting warm. I’d like to use an electric demand heater to warm the initial water flow until the hot water from the heater arrives. I’d also like it to have a thermostat that would turn off the demand heater once the hot water from the tank arrives. Is there such a beast on the market? Thanks for any help!

  • http://marissaaf.wordpress.com/2013/04/12/the-difference-between-gas-and-also-electric-water-heaters/ marissaaf.wordpress.com

    That’s a lot of money that we’d enjoy being able to
    spend elsewhere without giving up our current standards of living.
    A thermostat inside the heater tells the heater when to use
    the ignition system to add more heat to the stored water.
    The problem with traditional water heaters in the home, however,
    is that there has been only so much room to store
    heated water.

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