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Energy Solutions

Waiting for Hot Water

The D'Mand system, installed under a sink, allows the user to bring hot water to the sink very quickly without wasting the water.

Over the past several weeks, I’ve written about water conservation as a strategy for saving energy and examined a number of water heating options. This week, we’ll look at the issue of water waste while waiting for hot water and what to do about it.

Here’s the problem: Most of us waste a lot of water waiting for hot water to reach a sink or shower. In a large house, it’s not unusual for 50 feet of piping—or more—to extend from the water heater to the most distant fixtures. And with 50 feet of pipe, it can take a long time for hot water to reach the tap; while waiting all the water goes down the drain.

How long you have to wait for hot water depends on three factors: the distance from the water heater, the diameter of the piping, and the flow rate. The effect of distance is pretty obvious: the further hot water has to flow, the longer it will take to get there. With a new house, you can keep plumbing runs short by locating bathrooms and the kitchen near each other.

The effects of pipe diameter and flow rate aren’t quite so intuitive. The smaller the diameter, the faster hot water will reach the tap. That’s because smaller-diameter pipe holds less water. Fifty feet of 1/2″-diameter pipe holds 0.8 gallons, while the same length of 3/4″ pipe holds 1.4 gallons and 1″ pipe holds 2.3 gallons. When you’re waiting for hot water, all of the cooled-off water sitting in the pipe has to flow out before hot water from the water heater reaches the tap.

The flow-rate of a faucet or showerhead affects the wait for hot water because it governs how quickly the cooled water sitting in the pipes will be emptied out. If you have a water-conserving bathroom faucet that delivers only a half-gallon per minute (gpm) and most of the piping in the house has 3/4″ piping, a 50-foot run from the water heater requires almost three minutes to get hot water to your sink.

The irony is that even as the flow rates of faucets and showerheads has dropped, plumbing codes are increasingly mandating larger-diameter piping—so the wait times for hot water have increased—a fact that’s exacerbated by the larger houses we’re building.

In addition to the long wait for hot water, all of the cooled-off water sitting in the pipe goes down the drain. Nationally, there’s a huge amount of water wasted in this way.

Additional water (and energy) waste will occur if you tend to turn on the shower and then go do something else as you wait for hot water. This all-too-common practice can be extremely wasteful, especially if you leave the bathroom and get distracted after turning on the water.

So what can we do about this problem? The most common solution is a bad one: installing a continuous-circulation pump. These circulators, standard in most hotels, keep hot water flowing through the plumbing loop in a house all the time so that hot water is always available with almost no delay. The problem with this approach is that you waste energy. Your entire hot-water piping system acts like a radiator that operates 24/7. Insulation on the pipes helps, but there are still significant losses.

A much better solution is an “on-demand circulator.” With this system, a user in a remote bathroom or kitchen pushes a button to activate a small pump, usually under the sink. This pump pulls hot water from the water heater, and sends the cooled-off water that’s been sitting in the pipes back to the water heater, either through a separate piping return that has been installed (most common in new construction), or via the cold-water line (more common in retrofit applications). As soon as hot water reaches the tap, a temperature-controlled switch turns off the pump. The system is also available with an occupancy sensor—to automatically turn on the circulator pump when someone enters the bathroom, though this will lead to some waste since the circulator will operate whether or not hot water will be needed.

Several manufacturers offer the same basic on-demand recirculator, called the D’Mand system, which was developed by ACT Metlund. ACT Metlund sells the system online and also licenses the technology to pump manufacturer Taco and Swedish plumbing and heating system manufacturer Uponor, which was previously known as Wirsbo.

If you don’t want to go to the expense of buying an installing the D’Mand system (the unit costs over $300, plus installation), you can at least deal with the excessive waste that occurs if you often leave the shower on longer than needed while waiting for hot water—by installing a specialized showerhead valve. With the Lady Bug showerhead adapter made by Shower Start, LLC, or an integral showerhead that includes this adapter, you turn on the shower and when hot water reaches the valve, flow is reduced to a trickle. After stepping in the shower, you turn a knob or pull an attached cord to resume full flow.

In new construction, reducing waste while waiting for hot water can also be accomplished with a “structured” or “home run” plumbing system, in which separate lengths of cross-linked polyethylene (PEX) tubing extend from a manifold at the water heater to each water-using fixture or appliance. With this increasingly common approach, waste is reduced by matching the PEX tubing diameter to the flow rate of the fixture—so that small-diameter tubing that holds relatively little water is used with low-flow fixtures.


  1. homedesign | | #1

    Point of use Mini-Warmers
    Some diswashers have booster heaters for the hot water.
    What are the metrics of dropping the hot water supply all together and heat the water as needed right at the appliance?

    How much hot water do we really need at the bathroom sink?
    Would it save energy to use instant hot or "instant warm" heaters right below the sink?
    How much hot water do we really need just to rinse a dish or two?
    Maybe just a little instant warm water? Step on a foot pedal and get a little shot of warm water.

    I know that electric is not as efficient as gas..but think of the water saved and possibly energy.

    Provide tank or tankless gas water heaters for bathing..and use mini-water-warmers everywhere else?

  2. Alex Wilson | | #2

    Point-of-use vs. central water heating
    Great questions--that we need to be asking more regularly. I was recently consulting on a project (a church building) and point-of-use (tankless) electric water heaters in several remote lavatories is what made the most sense. These will go right under the sinks (one of these tankless water heaters will probably serve several sinks), so there will be very little delay in getting hot water and more significantly, the church will be able to turn off the boiler during the summer (a massive boiler has been kept operating during summers just to provide a small amount of hot water in remote sinks).

    Regarding dishwashers, this is a very interesting issue that I've been trying to understand. As far as I know, all American dishwashers (and dishwashers sold in America) include booster heaters that heat the water to about 140 degrees (the temperature needed to get the dishwasher detergent to function optimally). What is unclear is whether this booster heater goes on even if the incoming water is already at 140 degrees. Some anecdotal testing indicates that the booster heater on at least some dishwashers operates whether or not it's needed. If the booster heater on a dishwasher does not come on unless needed (we would hope this is the case), then in some situations (with inexpensive water heating Btus from natural gas or a mix of solar and natural gas and expensive electricity, for example) it might make sense to boost the output temperature of a central water heater to 140 or 145 degrees so that the electric booster heater doesn't come on in the dishwasher.

  3. zimmerdale | | #3

    International solutions
    On a recent visit to Brazil, I was surprised to find only cold water taps at most sinks. Every residential home that we visited used a small electric on-demand heater that doubled as a shower head. I know that a warmer climate helps damper people's affection for warm water, but is hot water really necessary for washing one's hands and dishes?

    While living in Germany, I discovered that the washing machines are only supplied with cold water. They include a small on-demand heater in the machine to heat water when the user deems it necessary.

    I'm also looking at the Hot2o solar add on heater, or something like it. I think a seasonal approach to solar hot water could also work for most attentive homeowners. Turn off the water heater in the summer, and take showers/wash dishes in the evening, after the sun has heated the water.

    Bosch also makes an on-demand gas water heater designed as a backup for a solar water heater. It only adds heat to the water if it isn't up to temperature as it passes through the heater. I think this is an elegant design.

  4. GBA Editor
    Martin Holladay | | #4

    Tropical water temperatures
    In tropical countries, "cold" water often enters a house at 70 or 80 degrees F -- a fairly pleasant shower when the temperatures are hot. But in Vermont, winter cold water temperatures might be 38 degrees. That's different, and it takes more electricity to raise the temperature of the water to a comfortable temp. It's hard to wash your hands in 38 degree water.

  5. pderas | | #5

    ACT Metlund D'MAND System
    I recently had this system installed in our farthest bathroom from the water heater, with remote controls for the kitchen and an additional bath.

    It is installed in our crawl space, which is a great solution for an unlovely appliance like this...Ours is a larger, and hunkier, model than the one you have pictured here.

    I am really excited about this solution to a big household water-wasting problem and have enthusiastically endorsed it on my blog.

    We use it any number of times during the day - Not only for hand washing, but also doing dishes and running the dishwasher.

    I also sent a pointed suggestion to ACT to develop an in-wall unit to solve the "unlovely" issue and retain storage in under-sink cabinetry, or allow installation where there is no cabinetry to hide it. The instructions suggest that the best installation location is on the floor of the sink cabinet, but we were able to install it under the house with a letter from ACT that satisfied our building inspector.

    We kitchen and bath designers like to reserve the bottoms of cabinets for roll-out shelves rather than cluttering them up with equipment. Even a wall hung model, mounted on the back wall below the sink, would resolve this issue and have us all specifying these units with every kitchen or bath remodel we design.

    Unfortunately, at present, one would have to build a sturdy shelf to mount the unit up where it would not be visible.

  6. more2do | | #6

    Endless Control, Custom Comfort and Flexible Installation
    RedyTemp hot water circulators have been utilizing TACO pumps in their systems since 1994. All RedyTemp circulators are engineered to satisfy most desired hot water lifestyles. These include wired or wireless push-button on demand hot water circulation, time-of-day schedule based hot water circulation, occuppancy/sensor based or a combination of all. This convenience is possible due to RedyTemps custom internal microchip controller. RedyTemp also features an easy tool-less adjustable temperature control dial on the face of the unit which allows the homeowner to calibrate/dial-in to their perfect instant hot water comfort setting. Optimizing temperature by the homeowner maximizes comfort and minimizes waste from waiting. Additionally, this allows the consumer to eliminate hot-cold-hot sandwiching or temperature fluctuations which can occur when water in the pipes cool down faster than at the sink a common complaint with some circulators on the market.

    Another distinctly unique engineered feature is RedyTemp's use of a normally-closed solenoid valve in conjunction with a checkvalve. Check valves prevent flow from occuring in a single direction only, ie. preventing cold line water from entering into the hot water line. However, this does little in preventing hot water line siphoning. Hot water line siphoning occurs when a homeowner opens a cold water tap expecting water exclusively from their cold water line. But, due to the drop in water pressure in the cold water line, the higher pressured water in the hot water line passes through these one-way check valves mixing with the cold water that's exiting out the then open cold water tap. Thereby, raising the demand on the hot water heater needlessly as cold water now enters the water heater to replace that which siphoned past the one-way check valve. RedyTemp patented the incorporation of a solenoid valve together with a check-valve eliminating the water, time and energy waste associated with water from the hot water line siphoning into the cold water line. RedyTemps more efficient flow control limits crossover between the hot and cold lines to only occur during a temperature based cyclic-pumping event. Resulting in homeowners receiving cooler cold water comfort without water siphoning from the hot water line during cold water use.

    RedyTemp systems work with or without a dedicated hot water return line. RedyTemps TL series systems allow relocation of the temperature sensor nearest the last hot water load on the loop while the pump can be installed at the water heater. This allows hot water to be delivered up to the last hot water usage point and not waste additional energy heating pipes past the last HW use point. Thereby extending the life of the water heater and the circulation system while minimizing energy consumption throughout the system.

    The additional components required in acheiving RedyTemps higher efficiency and comfort does add additional cost to the price but, it can more then make up for it in the first year of operation in most cases.

  7. Alex Wilson | | #7

    Question about RedyTemp
    This looks like a great product; thanks for bringing it to my attention. My only concern is whether it would most commonly be installed to circulate more often than needed (i.e., based on timer control instead on on-demand control). If this is typically set up for continuous-circulation or timer-circulation, I'm not sure I would recommend it--even though it offers an option for on-demand circulation. Can you comment on how this controller is typically configured? Thanks, -Alex

  8. sylvia Cates | | #8

    international solution
    In Germany all dishwashers and washmashines have their own heatingsystem build in, for that only cold water piping is needed. a really smart solution to the problem.

  9. Frank Richter | | #9

    Solar Domestic Hot Water (SDHW)
    In response to Jason Miller's comment about solar domestic hot water (SDHW) and timing the use of hot water for late afternoon or evening after the sun has had a chance to do it's work: a wise SDHW user can also take full advantage of the timer on his/her dishwasher, timing it to run early afternoon. Theoretically (the techies out there can refine this a bit, I'm sure), this serves two purposes. A well-insulated tank, on a hot sunny day, could easily reach 160 degrees F or more, so this would bring down the temperature a bit (a good thing) AND allow the owner of the SDHW system to take full advantage of the afternoon sun for recharge. I'm told by users of SDHW in the Northeast U.S. that this works quite well (at least during the summer months).

  10. rustyshackleford | | #10

    wasting water isn't my exact problem
    For people like me, who have a well and live in an area where water is plentiful, we could really care less about wasting water while waiting for the hot water to ge to the tap. But I AM concerned with a related problem, which is the hot water left in the pipes after I'm done using hot water. I've read somewhere, perhaps an article linked here, that it can waste up to 15% of one's water heating energy.

    I'm trying to analyze the best way to deal with this, other than common sense stuff like insulating pipes. My (electric resistance) water heater is quite close to my two bathrooms, but pretty far from the kitchen. I figure I could either put a second small water heater near the kitchen, or install a recirculating pump. The recirculating pump idea seems very wasteful at first, but then I did a back-of-the-envelope calculation of the surface area of heat loss for the two options. A 30 gallon Bradford White (22" diameter * 30" high) has a surface area of about 20 ft^2. Thirty feet of 1/2" water pipe has a surface area of only 4 ft^2. Even if the pipe is insulated to R4, whereas the water heater is R16, the pipe still has less heat loss. And if you throw in the fact that the pipe isn't always full of hot water - because of occupancy sensors or timers - it seems like the recirculating system is superior. Plus, better livability.

    I don't really need the kitchen water heater to be 30 gallons, but that seems to be the smallest size with 2" R16 insulation. I suppose I could install a smaller unit with an insulation blanket. Another option might be an on-demand propane unit. Propane is more expensive than electric-resistance here, believe it or not, but I don't use that much hot water in the kitchen anyhow.

  11. spinoza2 | | #11

    go smart tankless
    When I built my house I purposely designed it so the kitchen and bath were next to each other--all the water fixtures in the house are within ten or so feet from one another. I then installed the tankless water heater in the bath as well, next to the washer and the toilet. Even in the dead of a New England winter we only need to wait a couple of seconds before getting the hot water we need. Also, both our washer and dishwasher have built-in heaters. I feel I've pretty much created an optimal situation, at least as far as water waste is concerned.

  12. jcamposedson | | #12

    Good day to all and apologizes for my poor english.
    I don´t know whether you all use storage tanks for FW as we do here in Brazil.
    I have three of such tanks on the top of the higher ceiling, meaning that all water demanded comes by gravity.
    One tank (1m3)is for cold water, another tank for hot water (2m3), heated by three possibilities (solar vacum pipes, gas heater and additional serpentine on a fire place that eventually also helps to heat by normal convection, plus a third tank that storages 1m3 of rain water used for all toilets and for one laundry machine used only for dog´s cloths, floorcloths, etc.. Unfortunately on this one, I have to use an electrical pump to have the saved water running from a 25m3 concrete tank, placed on the lower part of the house, to the roof but obviously what I save in water compensates this small amount of energy used.
    I live in the highlands of the southern part of the Country and yes, it may be cold in the winter time, with temperatures reaching around minus 7 Celsius degrees or in another words, n average of a hundred nights per year with temperatures below zero.
    The house has 3 floors and on the most used one, the 1st floor, we are located far away from the above mentioned tanks, therefore until we get hot water on the taps it usually takes 2 or 3 minutes thus not exactly on the green side neither cost effective.
    Question is: Is there any kind of valve that by simple method of measuring running water temperature inside plumbs, recirculate it back to cold line until it reaches a desired temperature, meaning that even with open faucets there would be no water until it is hot? Something like a device with a spring that only opens valves when hot water is present and close by itself when no more water is running on the system. That would be cheap as would not be powered. (I guess).

  13. GBA Editor
    Martin Holladay | | #13

    Response to Edson Campos
    The short answer to your question is that there is no valve the does what you would like -- at least, I have never heard of a valve like that.

    There is a valve used for showers that monitors the temperature flowing through the shower head and reduces from full flow when the temperature is cold to a trickle when the temperature is hot enough to use. (When a person steps into the shower, he or she can adjust the valve to enjoy the full flow of the hot water.)

    These valves are called Evolve ShowerStart Thermostatic Shut-off Valves (TSV). Here is a link to the web site: Evolve shower head.

    The valve does waste the cool water that goes down the drain, but it limits the waste of hot water. If a person turns on the hot water valve and waits for the hot water to arrive, the valve limits the amount of hot water that goes down the drain before the person steps into the shower.

  14. jcamposedson | | #14

    Dear Martin, tks for the prompt answer.

  15. Dan Mbugua | | #15

    Waiting for Hot Water
    What if we install an electric water heater with a temperature sensor at the inlet soon as hot water comes in the sensor turns off the electric heater... this is just an idea... does this make sense please advice

  16. GBA Editor
    Martin Holladay | | #16

    Response to Daniel Njenga
    Your idea will work. Just purchase and install a small on-demand electric resistance water heater. Most of these heaters include a temperature sensor -- when hot water arrives at the inlet pipe, the heater will turn off.

    Here is a reputable manufacturer: Stiebel Eltron point-of-use water heaters.

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