Toilets, Fridges, Heat

The other day Housemate came to me and said, “You aren’t doing that ‘if it’s yellow, let it mellow’ thing again, are you?” So I see it’s time for my annual diatribe against the heat-wasting effects of winter toilet-flushing (here’s last year’s).

I thought this year it might be instructive to compare the (unknown-to-most) evil of winter toilet-flushing to the (extremely well-known) evil of “letting all the cold air out of the refrigerator.” I am sure almost all of you can remember standing in front of an open fridge looking for a particular item, while a parent, a spouse, or maybe even the voice inside your own head says “Hey! Don’t let all the cold air out of the fridge! You’re wasting energy!”

So let’s look at this with hard data. Which wastes more heat– swapping out the fridge air for room-temp air, or flushing the toilet? Write your answer here before you read on: ________________

Okay. Here are the assumptions for this thought-experiment. To give all possible advantage to conventional wisdom, we will assume you have a super-efficient, ultra-low-flush toilet model that uses only 1.6 gallons per flush (our actual toilets use 3 gpf; really old ones use 5), and that you have a huge, cavernous fridge, such as this one. In fact, we’ll assume that you’re not only letting the cold air out of the fridge compartment (15.6 cu ft), but also out of the freezer (6.3). We’ll assume that the fridge is kept at 36ºF, and the freezer at 0ºF. Assume that the cold tap water to the house comes in at 50ºF. And, lastly, we’ll assume that you have not been reading this blog and so you still keep your house at 68ºF in the winter.

So now we need to find the heat difference between a fridge(+freezer) full of cold air, compared to room-temp air, then do the same for a flush worth of toilet water. Here it is:

1) The fridge: 15.6 cu ft of air x .081 lbs/cu ft = 1.26 lbs of air. Temperature differential between fridge & room = (68º – 36º) = 32º. Heat capacity of air is 0.24 BTU per pound per ºF. So the heat loss = (1.26 lbs x 32º x 0.24) = 9.7 BTU

2) The freezer: 6.3 cu ft of air x 0.87 lbs/cu ft = 0.55 lbs of air. Temperature differential = (68º – 0º) = 68º. So the heat loss = (0.55lbs x 68º x 0.24) = 9.0BTU

3) The toilet: 1.6 gal of water x 8 lbs/gal = 12.8 lbs of water. Temperature differential = (68º – 50º) = 18º. Heat capacity of water is 1.0 BTU per pound per ºF. So the heat loss = (12.8 lbs x 18º x 1.0) = 230.4 BTU

So, we find that the total heat loss for replacing all the fridge plus freezer air with warm air is 18.7 BTU, while the heat loss from one toilet flush is 230.4 BTU. In other words, you could “let all the cold air out of the fridge” a dozen times before you have committed a heat-crime as great as flushing the toilet once.

What can you do about this? I left suggestions in last year’s post. And don’t fret about the fridge. And you might consider joining the Brazilians, who are being encouraged to pee in the shower (in their case, to save water– but same idea.)

(P.S. For those really interested: if you pee a pint into the toilet and let it cool down to room temp without flushing, you get back 1.6 BTU of free heat!)

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20 Responses to “Toilets, Fridges, Heat”

  1. bc Says:

    Yay, a new Cold House season. For drying indoors we set up one bathroom (center room in the apartment) as a dehumidifier closet. We hang the clothes over the tub pinned to wire shelving resting on two shower curtain bars. Start up the dehumidifier and turn on a box fan with the door closed.My watt-meter measurements are:Dehumidifier, compressor running: 450 watts (assume it runs continuously until the load is done)Box fan on high: 92wattsA full load of towels or jeans that hold a lot of water takes 4 hours to dry versus 1 hour in the dryer. So it's (.45kW + .092kW)*4 ~= 2.2kWh or half of what the dryer would take (4.4kW * 1 hour)The result is a toasty *dry* room in the center of the house, with no heat dumped outside.You can cut the total energy use down to about 1/4 of the dryer usage by doing several loads serially and periodically swapping out clothes as they become dry. Remove the faster-drying artificial fibers and hang up clothes from the next load. Also, turn clothes that have dried on the side facing the fan.

  2. Turboglacier Says:

    Oh, that's a great idea, BC. I'm definitely going to investigate a similar setup when we get to the new house next month. Housemate's jeans don't go in the dryer anyway, and she has complained that air-drying the Cold House they start to mildew before they get dry. Last winter I set up a similar "drying closet" in an actual closet, with a miniature space heater keeping it about 100ºF in there. But your scheme is better.

  3. Green Grrl Says:

    And I thought all I was saving was water by letting it mellow. Nice!

  4. brushfiremedia Says:

    Why flush at all? Why not use a kitty box type of arrangement for solids and a bottle for liquids. When each has cooled to ambient, thereby returning as many free BTUs as possible, you scoop, or dump outdoors.

  5. Turboglacier Says:

    Brushfire– two good reasons: neither local health codes nor Housemate allow it.

  6. brushfiremedia Says:

    But Turbo, you must at least admit that my plan is superior to the "let it mellow" technique in all other aspects, no?

  7. The MSILF Says:

    Could you use some of the bathwater in the tub with a bucket for some of those flushes?

  8. Turboglacier Says:

    MSILF– yes, I was doing this for a while last winter, though it's a little sloppy. I'm also using the dishwasher waste water (after thorough cooling) as flush-water. Two good flushes out of a dishwasher run…

  9. Benno Says:

    But wait! If your house/bathroom temp is _below_ the temp of incoming tap water (which seems to be the case at least some of the time, looking at the data in the Graphical Interface entry), shouldn’t flushing result in a heat gain for the house?

    • coldhousejournal Says:

      Haa! Yes, on occasion that would be true! Our incoming tap water in winter is about 43º. That post was written back in the days when I still considered mid-60’s to be “normal” indoors! (Since, it has once or twice crossed my mind to figure out, if I just wanted to heat my house to 40º, whether using tap water would be the cheapest way to do it! : )

      • Benno Says:

        Oh yes, I can just picture a Cold House Geothermal Heating/Cooling System based entirely on tap water! Offhand I can’t endorse it (I tend to be a water miser – probably read Dune when I was too young and impressionable), but it is entertaining to think about.

        p.s. one possibly more housemate-acceptable “let it mellow” approach is to pee into a bottle. You can buy items specifically designed for this purpose, though some brands of fruit juices come in a jug with a nice wide mouth and convenient handle… Anyways, the lid contains the odor (as well as preventing unfortunate messes if careless kitties knock it over) and it can be stored discreetly so your crackpot nature isn’t quite so obvious to every guest.

  10. More History « Cold House Journal Says:

    […] Fascinating.  I have mentioned Thoreau, but I don’t think I have used the word “potty” in any post, so I’m not sure how my blog got linked with this important question.  But since it has arrived at my doorstep, I’ll take a stab at it.  My first inclination is to answer, half tongue-in-cheek, “At Emerson’s house”– and this was likely often true.  While at Walden Pond I imagine he might have had an outhouse, but probably just went in the woods– I don’t recall any mention of outhouse-building in Walden, and it would seem unlike him to devote labor and materials to building a little palace to “go potty” in.  On thing’s for sure– he didn’t waste any heat with a flushing toilet. […]

  11. John Says:

    A few things:

    1. If you have food in the frig/freezer, you lose a lot less heat, because that food takes up space and basically stays the same temp while the door is open. This makes the frig even less of a problem.
    2. The process of cooling down the air inside the frig is far from 100% efficient. This makes the frig more of a problem.
    3. I’ll bet the water in your toilet doesn’t reach thermal equilibrium with the air (hence the sweaty toilets in summer when the temp differential is even higher). This makes the toilet less of a problem. (This is easily testable, but make sure the clean the thermometer afterwards.)

  12. John Says:

    Sorry, of course the most important thing is that the frig must put out more heat than it removes from the things inside it. That is, if you open the frig, you end up heating up the house.

  13. PablitoRun Says:

    Have you considered converting the toilet to a dual flush set up?

    They have kits for ~$20 although I don’t know the details on the water savings.

    Alternatively you could just open and close the valve when you use the “potty” so that you are only flushing with fresh tap water.

    A huge inconvenience sure, but less so than using left over bath water. You could also only fill it up 1/2 way for the mellow yellow.

    BTW I love your site. I can’t quite get my house down that cold due to the wife, but she is a trooper. My thermostat is usually ~64.

    • Cold House Journal Says:

      “Have you considered converting the toilet to a dual flush set up? They have kits for ~$20 ”
      Yeah, I bought one of those at my last house– was not impressed. Could never get it to really work right, then it quickly just broke altogether. Real dual-flush toilets are quite nice and a great idea.

      Anyway, we’ve pretty much settled into our own version of “dual-flush”– “big flush” and “no flush” : )

  14. Efficiency & Bathroom Humidity « Cold House Journal Says:

    […] The dehumidifier (running) uses about 400 watts to power its fan and compressor.  All of that winds up as heat in the bathroom.  Moreover, it claims to condense 1.6 liters of water for each 1 kWh of electricity it uses– which about agrees with what I’ve observed.  Now, each liter of steam condensed back to liquid releases a further 3,600 BTU of heat, which equals roughly 1 kWh also.  So, for each kWh of electricity used to run the dehumidifier, we get back 2 kWh of heat in the bathroom.  It is, effectively, like a 200% efficient space heater.  Plus, we get a quart of distilled water every day  (it’s enough to “small flush” the toilet, saving yet more energy!) […]

  15. Was it a good idea to buy a composting toilet? | Eat Wholegrain Says:

    […] then absorbs heat from the room before you flush it back outside again, taking the heat with it.  Check out this analysis in Cold House Journal comparing the (unknown-to-most) evil of winter toilet-flushing to the (extremely well-known) evil […]

  16. Was it a good idea to buy a composting toilet? - Live Wholesome Says:

    […] then absorbs heat from the room before you flush it back outside again, taking the heat with it.  Check out this analysis in Cold House Journal comparing the (unknown-to-most) evil of winter toilet-flushing to the (extremely well-known) evil […]

  17. Edward Says:

    I’ve never thought about calculating the heat of fridge, freezer, and toilet before. Thanks for the post, my mind has opened.

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