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Can Switching to a Dual-Flush Toilet Save Heat?

The water that sits in your toilet tank robs space heat from your home during the winter — but is it a little bit of heat or a lot of heat?

During the winter, the cold water sitting in your toilet tank draws heat from the air in your bathroom. The net result: your furnace or boiler has to work a little harder.
Image Credit: Erik North

First off, my wife just joked that I used a photo of a “male bathroom”: seat up and two rolls of toilet paper.

Regarding the heat savings mentioned in the headline, we’ll see… I haven’t done the math yet. But it is a minor claim occasionally made alongside the claim that these toilets save water.

Some manufacturers of dual-flush or low-flow toilets make extravagant claims on water savings; and tacked onto the end of the marketing copy there is sometimes a blurb on heat savings:

  • “Saves water”
  • “Saves lots of water”
  • “Saves water”
  • “Saves heat”

Wait… what? One of these things is not like the other. If this is true, how does reducing your toilet water use affect heat loss?

The water that fills your toilet tank is usually cold

First off, admit it… you didn’t think that a toilet would have anything to do with energy efficiency. Saving water, sure — just not energy savings. But the heating dynamics of a house are ridiculously complicated and interconnected.

One of those dynamics is internal net heat gain. Anything that radiates heat within the enclosure will add BTUs to the building. Remember your last indoor concert (Dropkick Murphy’s, St. Patrick’s Day 2011… I should get out more)? Remember how sweltering hot it was? That’s because all those hundreds of bodies are adding heat into the building enclosure.

In the home, every incandescent light bulb, television, pet, and person adds BTUs to the house’s internal heat load. Conversely, there are some items which absorb heat and cool the house. Cold water pipes are one; and another is the water sitting in your toilet tank.

Every time someone flushes the toilet, the water in the toilet tank is replaced with ground water. In Maine (and most northern climates), the ground water is colder than room temperature. The cold toilet tank will absorb heat until it reaches room temperature.

Time for some boring math

Fortunately, the math is super-straightforward.

According to the EPA, toilets account for 110 gallons of water use per day on average. Three quarters of this is for liquid waste and one quarter is… not. (Distasteful, I know, but difficult to avoid mentioning).

I’ll be using info about my house, since… why not? Our dual-flush toilet uses 3.5 gallons for a full flush and 1.5 gallon for a partial flush. We live in Portland, Maine, and our handy USGS groundwater map shows that the ground water temperature ranges from around 48°F to 50°F.

So let’s review all the data:

  • Gallons for full flush: 3.5
  • Gallons for partial flush: 1.5
  • Flushes per day (EPA data): 31
  • Number of full flushes per day: approximately 8
  • Number of partial flushes per day: approximately 23
  • Pounds of water per gallon: 8.34 (I’m not going to muck around with variations based on pressure and temperature changes)
  • Temperature of entering water: 50°F
  • Room temperature: 70°F

Important Note: Let’s not quibble about the numbers. These numbers could be wildly off from real-world heat loss measurements, but we’re painting in broad strokes here. The idea is to illustrate some thermodynamic calculations, not to use experimental precision and accuracy.

  • Water saved: 46 gallons per day [23 flushes * (3.5-1.5)]
  • Pounds of water saved: 383.6

Just pause for a moment and think about that: nearly 400 pounds of water. OK, time’s up.

  • BTUs to heat water: 7672 (383.6 lbs @ 50°F * [70°F – 50°F] * 1 BTU/lb./F°)

A BTU is the amount of heat needed to raise the temperature of 1 pound of water by 1 F°. (See this article in which I talk about BTUs in a bit more detail.)

Knowing that there are approximately 138,000 BTU in a gallon of #2 heating oil, we get 0.055 gallons of oil per day, or a bit over 20 gallons of oil per year.

It takes energy to warm up the water in your toilet tank

Holy crap! That’s nearly $80 a year just heating your cold, cold toilet water. That’s ridiculous. We should mandate a Federal “swappin’ out your inefficient toilet” Task Force, right?

Well… hold your toilet-fixing horses, mister. There are many, many caveats here.

First, will you use the partial-flush feature of your new toilet? Ceiling fans, programmable thermostats, zoned heating systems… there’s a long list of mechanical systems in your house which could (emphasis on could) save you money but largely don’t. And the why in this equation rests squarely on the fact that people don’t use them properly. In our house, we make a conscientious effort to use our dual-flush system, but the results depend on use.

Second, I pulled a wee sleight-of-hand in using my house. There are only a small handful of states as cold as Maine, and only one (Alaska) that is significantly colder. Most states have warmer climates and warmer groundwater. Instead of warming from 50°F to 70°F, it may be 62°F to 70°F, with a corresponding reduction in heat loss.

Third, dual-flush systems are much more mechanically complex than classic plunger systems. So they’re more apt to have issues than a simpler set-up. There’s not much savings of any kind if a gasket continually leaks water.

Fourth is the question of overall water use. Households with only one or two members won’t use anywhere near as many gallons for toilet flushing as are shown in the calculations above. Conversely, a very large family is apt to slack off or have some members who ignore the dual-flush feature. They’ve forgotten the first rule of the Dual-Flush Club: Don’t forget to use the dual flush.

Fifth, fossil fuels won’t be needed to produce space heat for 365 days a year. The toilet tank may be warming up to room temperature, but for big chunks of the year it will be heated by the warm summer sun, not by your oil boiler.

Last, my math makes one rather silly assumption: that trips to the bathroom will be sufficiently spaced out to allow the cold water to fully reach room temperature. Not bloody likely.

Will a dual-flush toilet lower your energy bill?

So is there any benefit? The minor point is that, yes, in our neck of the woods there is some small heat savings benefit to dual-flush toilets or toilets with reduced flush volumes.

The broader point is that everything that is hot or cold is a component of your home’s internal heat load. Once you start thinking about your house this way, the cooling benefit of a heavy window shade on a sunny day is more obvious, and uninsulated cold water pipes are not quite as innocuous. They’re all part of how well your house’s enclosure controls temperature.

Erik North, the owner of Free Energy Maine, is an energy auditor and home performance specialist in Westbrook, Maine. He is also the author of the Energy Auditing Blog.


  1. gusfhb | | #1

    Time to get a new toilet, 3 1/2 gallons, my dual flush toto uses 1.6/.8 and I have used the plunger once in 2 1/2 years! Haven't been able to buy a 3 1/2 gallon toilet here in the people's republic for well over a decade[edit 23 years apparently]

  2. wjrobinson | | #2

    Dual flush, another overkill
    Dual flush, another overkill contraption

    Standard modern low cost units are simple and already dual flush. #1, tap handle. #2 hold handle down during flush, takes one extra second.

    Save a few hundred and simple. Only one handle,

    Would love to be the "simple green" blog meister and clean this place up a bit.

    Green can be simple and less is always closer to green than more.

    Smaller homes, smaller families, smaller cities, and simpler plumbing to start just the short list...

  3. Erik North | | #3


    Couldn't agree more except that my house was built in the 30s and the sewer line is touchy. I've experimented with ratcheting the flush flow down and...well, we'll say I've used the plunger and snake more than once in 2 1/2 years. One day we'll rip up the basement floor and replace it.

  4. user-984364 | | #4

    The obvious solution ...
    ... is to route your drainwater heat recovery system into the lines feeding your toilet. ;)

    As an aside, I've found the dual flush adapters for existing toilets work well, are simple to install, don't cost much, and don't waste your current perfectly functional toilet. Much better than a trying to twiddle the handle just right, AJ, IMHO.

  5. davidmeiland | | #5

    I can get around this problem by feeding my toilets from the water heater. Presto, no more cold water in the toilet to need heating.

  6. GBA Editor
    Martin Holladay | | #6

    Response to AJ
    I'm surprised, AJ! I thought you hated "overkill contraptions"!

    So when did you stop using an outhouse?

  7. wjrobinson | | #7

    Eric, no twiddle. Tap option
    Eric, no twiddle. Tap option 1. Hold option 2.

    Simple no twiddle. No twiddle for you.

    Martin, me simple like simple. Tap or hold simple.

    Outhouse is a unneeded contraption in much of the world. Family I visit outside of Merida Mexico just use back of lot. Aj use tree yesterday. Aj not afraid of simple. Aj like gadgets too. Aj has love hate relationship with way too many tools gadgets and toys. Aj like modern bathroom too.

    Now let's talk bidets.

  8. Expert Member
    Dana Dorsett | | #8

    Quibbles on the math, (why not!? :-) )
    But in the cooling season you're getting "free" sensible air conditioning rather than burning oil to bring that water up to room temp.

    But there's also no 100% efficient oil burner or 100% efficient heat distribution, so the 20 gallons/year should actually be bumped to 25, to reflect 80% average heating system efficiency.

    So if the heating season is but 2/3 of the year, call it ~17 gallons of oil per year, if your heating season is 3/4 of the year call it 19 gallons, but do the math on the sensible cooling too, eh?

    And why use oil as the space heating paradigm? NOBODY uses heating oil any more 'ceptin' a distinct minority of flinty Mainers who live off the gas grid. Even in New England far more homes are heated with natural gas than with oil (like 2x as many). For the broader audience in the US it would be more appropriate to use natural gas at ~80% efficiency or heat pumps at an average HSPF of ~8 for the calculations, and only 8 months of heating season. The net financial cost is may be a coupla 10s of dollars annually, even in areas with high-priced natural gas.

    And what family flushes 23-31 x/day? (If ya gotta go that often, go outside- the dog does! :-) If you're flushing that often, most of that water isn't in the tank long enough to come fully up to room temp or even close to it, (but the overnight tankful probably will).

    Short of better modeling, MEASURING and logging the net heat outflow during both the heating & cooling seasons would get you closer to the real average BTU/flush on heat/cooling loss you get out of it, but it's not big money either way.

  9. GBA Editor
    Martin Holladay | | #9

    Repsonse to Dana Dorsett
    Thanks for your comments. Your points are all valid -- at least for homeowners outside of northern New England.

    Believe it or not, however, the majority of homeowners in northern New England burn fuel oil (and a little firewood, of course).

    And the reason we don't get any energy benefit from our toilet tank water in the summer is simple: we don't have air conditioning.

  10. Erik North | | #10

    Yeesh, after I said not


    Yeesh, after I said not to quibble with the math (joke). No, good points and I did forget to include heating system efficiency.

    Expanding a bit on natural gas, at least Martin's and my neck of the woods: I live in Portland, Maine. If I lean a bit to see out my office window, I can see where the DPW marked the Allen Ave. NG lines during some spring construction last year.

    When Efficiency Maine was offering some massive weatherization incentives a couple years back, I researched getting NG installed. Per the utility co, I had two options: convince enough neighbors to commit to converting that they would install the line down our entire street or they would install the first 100 feet for free and I could pay $100/ft to extend it to my house. The NG line is 285 feet away.

    So, we're not JUST being flinty though there is that.

  11. davidmeiland | | #11

    Don't you live in the NW somewhere? Folks up in my neck use heating oil, those that haven't converted to propane or just gone all-electric. My house had an oil tank when I moved in, it was the first thing to go. Still, the truck makes its rounds all winter and my heating tech seems to do plenty of oil boiler repairs, and even a few replacements.

  12. Expert Member
    Dana Dorsett | | #12

    Response to David
    If "NW somewhere" means "... a north west neighborhood of Worcester, MA..." - guilty as-charged, but that feels pretty much like New England to me. :-)

    EIA data suggests that in New England fewer than 1/3 of homes are currently heated with oil, more than half by natural gas. The graphic on this page tells the home-heating fuel story by regions fairly succinctly:

    While there are three houses ( of 17) on my block still heating with oil, the economics of converting are now compelling given the gas main out front, and buck a therm gas & $4 oil, and my neighbors have been following their pocketbooks there. There is evidence in my (circa 1923) home of there once having been an oil tank (a capped off pipe through the foundation that smells of oil when the cap is off) as well as a coal bin (stained & corroded 4x6' section of slab & wall at the back of the basement below what is now a basement window), but when I moved in it had a circa 1968 gas-burner heating the place (which has since replaced by more correctly sized & efficient gas-burner.)

    For those off the gas-grid facing record propane & oil pricing, the economics of retrofitting ductless air source heat pumps is also pretty compelling, even in Maine, and even if it's not a total solution to the home heating. In my native Pacific Northwest (haven't lived there in 3 decades, but visit often enough) ductless makes even more sense. (I have multiple relatives in the PNW who have made that investment in the past 2 years.) You can do a lot with a 1.5-2 ton mini-split, even in Maine, and when displacing $4 oil the after tax ROI is downright CRAZY-GOOD. The exceptions to the cost effectiveness of ductless in Maine would be island communities that are also off the power grid relying on diesel power for the local grid facing extremely high power rates as well as high fossil-fuel prices.

  13. CanAmSteve | | #13

    Another option
    We have a saying - "if it's yellow, let it mellow. If it's brown, flush it down."

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