Can Switching to a Dual Flush Toilet Save Heat?

by Erik North on August 10, 2012

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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, we’ll see…I haven’t done the math yet. But it is a minor claim occasionally made alongside those of saving toilet water.

Dual flush or low flow toilets make extravagant claims on water savings but sometimes tacked onto the end of the marketing copy is 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 (if!) this is true, how does reducing your toilet water use affect heat loss?

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 toward 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 tank from your toilet.

Every time one flushes the toilet, the vacated water 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, this is super straight forward.

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. We live in Portland, Maine and our handy USGS groundwater map shows that the temperature ranges around 48-50 F. So let’s review all the data:

Our dual flush toilet uses 3.5 gallons for a full flush and 1.5 gallons for a partial flush.

All The Data – My House Version
Gallons for full flush: 3.5 gallons per use
Gallons for partial flush: 1.5 gallons
Flushes per day (Using EPA data): 31
Full: Approx. 8
Partial: Approx. 23
Pounds of water per gallon: 8.34 pounds per gallon (I’m not going to muck around with variations based on pressure and temp changes)
Temp start: 50 F
Temp end: 70 F

Important Note – Don’t quibble about the numbers. These could be wildly off real world heat loss but we’re painting in broad strokes to illustrate some thermodynamics, not produce experimental precision and accuracy.

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

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

BTUs to heat water: 7672 BTUs (383.6 lbs @ 50 F * (70 F -50 F) * 1 BTU/LB/Degree F)

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

Knowing there are approximately 138000 BTUs in a gallon of #2 heating oil, we get 0.055 Gallons/day or a bit over 20 gallons of oil per year.

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…right!?!

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

First, will you use it? 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 equations rests squarely on ‘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 slight 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 ground water. Instead of warming from 50 to 70, it may be 62 to 70 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 overall water use. Households with one or two members won’t use anywhere near those numbers. Conversely, a very large family is apt to slack off or have individual members who ignore it. They’ve forgotten the first rule of Dual Flush Club: Don’t forget to use the dual flush.

Fifth, fossil fuels won’t be producing the heat 365 days a year. The tank may be warming up to run temperature but big chucks of the year it will be because of the warm summer sun, not 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.

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 or otherwise reduced uses of toilet water.

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 while uninsulated cold water pipes are not quite as innocuous. They’re all part of how well your house’s enclosure controls temperature.

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