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Water Heating Is a System, Not Just a Water Heater

Water Heating Is A System

What’s the first image that pops into your when you see the term “water heating”?  It’s probably a water heater tank.  Depending on your interests or what you have in your home, perhaps it’s a tankless or solar water heater.  For most people, hot water is all about the device that heats water.  Water heating is a system, however, and the water heater is merely one component.

Water heating inputs

The water heater has two inputs:  water and energy.  The hot water that comes out of the water heater has to travel through a distribution system to reach the various fixtures—faucets, showers, dishwasher, etc.—where the hot water is used.  The fixtures have controls.  And then you have the people, an important part of any water heating system.

On the input side, the water varies from place to place and from season to season.  The mineral content or water softening chemicals can affect the life span and maintenance of a water heater.  The temperature of the water supply affects the energy use.  In cold climates, the water coming into a water heater is colder than the water being heated in warm climates.  The incoming water temperature is close to the average air temperature for a location, but it varies with location of the pipes and with the seasons.  In winter, the water coming into a water heater is colder and needs more heating.

The type of energy input is an important choice.  It could be electricity, natural gas, propane, wood, solar radiation, or some other type.  The fuel you choose narrows your options for a type of water heater.  Within types, though, you can choose low, medium, or high efficiency.  You also may get to choose from models that store hot water in a tank or those that heat water on demand.  I say “may get to choose” because a tankless water heater isn’t always an option.  You can’t do solar water heating on demand, for example.

Water heating outputs

The output side of the water heater is the part that’s gotten short shrift.  When you choose combustion of a fuel to heat your water, there are two outputs:  exhaust gases and hot water.  Removing the exhaust gases is a critical health and safety issue.  Sealed combustion or direct vent water heaters are the safest, but the majority of gas water heaters use a natural draft exhaust gas flue, making it the appliance most likely to put carbon monoxide into your home’s air.  Consider carefully when buying a water heater.  And remember that the exhaust gases are part of your water heating system.  Choosing an electric water heater avoids the exhaust gas problem and uses a fuel that keeps getting cleaner.

Pay attention to the hot water distribution

For electric and solar thermal water heaters, the sole output is hot water.  Sadly, the process of delivering that hot water is greatly flawed in most homes.  The hot water distribution systems still being installed today are based on out-of-date ideas and technology.  Since the 1992 Energy Policy Act lowered the upper limits on water flow rates for different types of plumbing fixtures, hot and cold water lines have been oversized.  The result is longer wait times for hot water, which wastes water and time.  It also wastes energy because it strands more hot water in the pipes when the tap is turned off than would be stranded with right-sized pipes.  Gary Klein is the guy who opened my eyes on this subject, and I encourage you to read his articles and watch his videos.

Also of great importance for the hot water distribution is the location of the water heater and fixtures.  The greater the distance between the source of hot water and where it gets used, the greater will be the wasted water and energy.  The most efficient hot water distribution system will be the one with the shortest runs in addition to having right-sized pipes.  That setup in the vintage Rheem water heater ad shows an efficient placement of the water heater for laundry room hot water.

The convenience problem, of course, can be solved with a recirculating pump.  A continuous recirculating pump that keeps hot water close to every fixture 24/7 will waste a lot of energy.  Putting it on a timer so that it runs only during the high-use times can reduce the extra energy usage.  A demand-type recirculating system is better.  Just push a button, and your shower will be hot in a few minutes without wasting water or energy.

Down the drain doesn’t have to mean lost forever

Drainwater heat recovery improves water heating efficiency
Drainwater heat recovery improves water heating efficiency.

But wait!  There’s more.  The water heating system also includes the drains at each fixture.  When you heat water, use that heat briefly while showering or washing dishes, and then send it on its merry way down the drain, you’re sending energy that you paid for down the drain, too.  It’s possible to recover some of that heat in a clever device called—even more cleverly—a drain water heater recovery system.  It’s simply a copper pipe wrapped around a drain, absorbing heat from the drain water.   Cold water runs through the copper pipe and gets pre-heated on its way to the water heater.  Showers are the hot water fixture that can best take advantage of this energy-saving device because the hot water runs down the drain for longer periods.

That’s a 10,000 meter look at hot water.  Water heating is a system, not simply an appliance that heats water.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.


Related Articles

The 3 Types of Energy Efficiency Losses in Water Heating

Down the Drain! — Heat Recovery for Your Hot Water

Heat Pump Water Heaters – A Better Way to Heat Water with Electricity?


Vintage water heater ad from Don O’Brien on, used under a Creative Commons license.


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This Post Has 31 Comments

  1. So what is considered best practice now for water distribution in a house: conventional mains and branches, or a central manifold with separate lines to each appliance?

    1. Roy, Gary discusses that in a Journal of Light Construction article from 2013, Efficient Hot Water Piping (pdf). In the example he gave there, the zoned trunk and branch layout was the most efficient and provided the quickest hot water to the tap. I spoke with Larry Weingarten last week, and he said home run manifold systems can work well in some situations.

      1. Allison, I just read that JLC article and it was quite good. However, I am not sure if it considered labor costs. When I was working for one of my former employers about 30 years ago, I proposed a residential water distribution system where the mixing valves for each sink, shower, and bathtub are located near the water heater with a temperature sensor and controller located at the end device that sends a control signal (perhaps wireless) back to the mixing valve. This would then only require one water line to each of these devices which could save on installation costs. It would also result in cooler water (compared to a separate hot water supply) being supplied to the end device, thus possibly saving energy. It was considered too complex an expensive back then, but the required electronics and actuator technology have come a long way since then.

  2. Sierra Vista required timer-controlled hot water circulator loops to reduce water waste, as water conservation is a big deal in Arizona. Oak Ridge National Labs published a study showing that timer-based circulators waste a lot of water heating energy, even in cases when the pump is programmed to only operate a couple of hours a day. Gary helped get us that changed. The code beefed up insulation requirements on hot water distribution lines and now requires either an on-demand pump or clustered hot water fixtures with structured plumbing. That’s having one’s cake and eating it, too!

    1. We find the time set incorrectly on the majority of timers we find on existing recirc systems. This is likely due to time changes and homeowners not aware of the need to adjust the clock on the recirc system timer. This issue motivated one GC we work with to use thermistors rather than timers for controls.

  3. I did some research on DWHR coils a number of years ago. They’re very pricey and thus only pencil out on very high-use showers. They don’t work at all on batch appliances like bath tubs, clothes washers or dishwashers, or on kitchen sinks where the bowl is filled with hot water and drained later. Hot water draws must be simultaneous with drainage or there’s no benefit. Moreover, it’s not economical to install a DWHR coil on lavatories or low-use showers unless the drain lines are closely connected to a high-use shower. Lastly, DWHR coils are obviously only practicable in basement or multi-level homes where the high use shower is on an upper floor.

    1. Thanks for all the detail on drain water heat recovery, David. I knew showers were the primary place they’d be used because of the longer drain time and simultaneous hot water draw, but I’ve never looked at pricing or the economics of using one. Seems like it would be great for a college dorm or a locker room but hard to do cost effectively in most homes.

    2. A previous home in Charlotte (and current home in Arizona) have a basement with a high-use shower on the main level. In both cases I rejected a DWHR coil due to the installed cost compared to our relatively low hot water usage (2 occupants, low flow shower head) and low energy rates.

      1. But they do make a nice piece of interior decor when you leave them exposed like the house in Toronto pictured above. Maybe if you factor in the value of that, it becomes cost effective.

  4. I just wanted to point out for anyone who might be on the fence and not aware: If you have geothermal heat pumps you can connect them (or one of them) to your hot water heater and get “free” hot water in the summer. We have three Climate Master geothermal pumps and a 105 gallon Marathon water heater and overall it’s a pretty good setup. Of course, if I had it to do over again I’d change a million things, but those are some decisions with our home we’ve been greatly pleased with!

    1. Megan, thanks for mentioning that. Yes, ground source heat pumps with what’s called a desuperheater can take that heat pumped out of your indoor air and put it into your domestic hot water. As you mentioned, you get to do that only when the heat pump operates in cooling mode.

      1. Yes! The name sounds a wee bit doofy, but it’s a clever system. We are in the Arkansas River Valley and while we do have four seasons, we probably run A/C a little more than we do the heat across the course of a year. The geo is worth it on its own in my opinion, but the desuperheater is a nice little perk!

  5. In western upstate NY we have frequent power outages. With a natural gas water heater, at least we have hot water. Would never consider an electric HWH here.

    1. Framistat, how long do your power outages last? Long outages do happen, but they’re rare in the US. And modern water heater tanks are well insulated, so it would be extremely rare to run out of hot water during a power outage.

      1. My kids in Texas learned last winter that demand gas water heaters don’t work when the electricity is out, regardless of the duration of the outage.

        1. Roy, yes, tankless gas water heaters use electricity for the controls. And since they don’t store hot water, you don’t even have that.

          1. Yea, and in this case, it didn’t work after the electricity was back on until they figured out how to reset it. A water heater with a tank may seem old school compared to these new tankless devices, but it is clearly more resilient in terms of electrical outages.

      2. Good point. Frequency and duration depends on where you are. The more frequent
        outages here average half a day, but they have been as long as two to three days.
        Given that the grid is aging, climate is changing, old trees are falling… I would not
        characterize longer outages as “extremely rare” as in general outages are becoming
        more frequent. The HWH here is 14 years old.

        1. Half a day isn’t a big deal. You can certainly have enough water with an electric water heater for a few hours. Two to three days without electricity would be a problem, and a gas water heater (as long as it’s not tankless or power-vented) would keep you in hot water for the duration.

          But living without electricity for all the other things you need it for would be a little dicey. You wouldn’t be able to heat the house even with a gas furnace because they use electricity for the blower. The solution for that would be either electrical storage or a generator, which could then power an electric water heater, too.

          1. Allison, you can operate a gas furnace with at lot smaller generator than is required for any type of electric heat. It takes a lot more generator capacity to operate an electric water heater compared to a gas water heater of any type too. Don’t get me wrong, I am a supporter of long-term electrification and have lived in several houses already that were fully electrified simply due to the unavailability of natural gas. However, going all electric puts all of your eggs in one basket and thus makes your home less resilient to electric power outages.

          1. You’re most welcome. BTW I have rigged my 2015 Prius with a deep cycle 12V AGM battery and an inverter to address the longer outage problem. A Prius is a very efficient generator, although in this configuration it only supplies 115V not 230V. Generation of 230V off the hybrid battery is possible but much more expensive.

        2. Great chart! Keep in mind that those numbers are cumulative (annual), not per event.

          @Roy wrote:
          > going all electric puts all of your eggs in one basket and thus makes your home less resilient to electric power outages.

          When I lived in Charlotte (lots of trees = ice storm outages), I had no problem powering my 1999 12 SEER heat pump and submersible well pump with a portable 5.6 kW generator (not at the same time, although not at all hard to manage). I had natural gas service so water heating and cooking wasn’t an issue during a couple of extended outages we experienced.

          Today’s high-seer right-sized heat pumps draw a lot less current and the inverter drive compressors virtually eliminate inrush current (a big deal when sizing a genset). And as Curt noted in his comment, today’s HPWH’s only draw a couple of amps. Likewise with today’s energy efficient refrigerators and LED lights. Lastly, a microwave oven plus a propane grille comfortably suffice for cooking.

  6. We’ve had a Rheem 80 gallon heat pump water heater for a couple years. I like the free cooling and dehumidification of the laundry room / pantry it lives in, as does my daughter who will perch in their to read when it is the coolest room of the house. My wife does NOT always appreciate the chilly room during what passes for winter in north Florida.

    Our next house has a large mechanical room off the second floor owners suite of all places, so the Rheem will make the move with us and can easily be ducted into the nearby air handler return if local chilling becomes an issue.

    Our next next house (as yet unbuilt) will allow for the water heater to be fairly centrally located, and I plan to pay attention to an efficient distribution system.

    The low current draw (~1.5 Amps at 230V) of heat pump water heater compressor is an excellent characteristic particularly helpful during power outages – it is much more easily operated by a standby genny than a conventional electric resistance tank element.

    1. I have the common layout of water heater in the garage which is on the opposite side of the house from our master bathroom, so I get to spend a minute or two waiting for a hot shower each morning. I gave up on washing my hands with hot water. The irony is that I also have an encapsulated attic with a 15-foot peak height, so the water heater could have easily been put in the attic and shortened the runs to all of the fixtures. It would be a great place for a heat pump water heater too, especially if I needed the additional summer dehumidification, which I don’t.

  7. What do you suggest ? Site- Greenville SC, water heater is 60 to 70 feet from bathroom. Pipes are mostly plastic. Would like instant hot water in the bathrooms. Local advisor said to use recirculating system ,I do not like the idea, too much pipe!
    Circuit breaker box at same location as the water heater.

    1. @ronald, an on-demand pump doesn’t require a return loop (can use cold water line for the return since it only operates for under a minute in a typical installation). The pump costs more than a circulator — in addition to the control circuit, it has a much higher flow rate for fast response — but it avoids the losses incumbent in a recirc loop.

  8. Allison,

    I was frankly disappointed that your otherwise excellent article only mentioned hard water issues in passing: “The mineral content or water softening chemicals can affect the life span and maintenance of a water heater.” As you are no doubt aware, regular flushing of a gas water heater will dramatically extend the life of the unit by minimizing precipitate buildup at the bottom of the tank. And be sure to replace the factory drain valve with a proper ball cock device for better agitation during flushing . Also, don’t forget about replacing the anode rod as needed (mine came out at 15 years and it looked like Swiss Cheese!) 20 years on my Rheem DHW heater and it’s going strong.

  9. The mandatory restrictors in low flow fixtures also mean longer wait times for hot water. Putting the water heater in the garage on one end of the house, with the owner’s suite on the other end means a really long wait for hot water at the shower. I prefer the Taco Genie Hot Water Recirculating System with Smart Plug. It’s a demand system with a computer that memorizes your hot water usage for the last 7 days and automatically recirculates hot water at those times. It’s not a continuous recirculation like a timer, but shuts off when hot water returns to the water heater. The pump and Smart plug are located at the bottom drain of the water heater. I used extra thick 1″ wall insulation on my recirculation pipe to minimize heat loss in the loop. Works really well in connection with my Rheem Heat Pump Water Heater.

  10. Probably unusual, but you _can_ do solar water heating on demand. My solar system heats a huge tank of static water via an input heat exchanger. That circuit can also be directed through my hydronic floors, and heated by an outdoor wood boiler or gas, so it remains hot in winter even with no sun. Domestic hot water is instantly heated when needed by a second exchanger in the tank – powerful enough to get the shower within one degree of tank temperature. (Yes, I had to write a lot of code to make all this human-friendly.)

  11. Great article. It’s all too frequent that a builder, or homeowner, tells me they don’t like a certain type of water heater because it takes too long to get hot water. What’s worse, most of them don’t seem to want to accept that the problem likely has more to do with their distribution system than the water heater itself.

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