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!”
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!)