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What are your thoughts here?

user-4053553 | Posted in General Questions on

This does not really matter ultimately but its an interesting thought exercise.
Last winter my furnace stopped running (old age and a squirrel built a nest in the chimney), i have since replaced the furnace with high efficiency so the chimney is not an issue, but being a meticulous pragmatist (and a little measurement crazy) i noticed the water ended up colder then an empty tub. When the gas person was red tagging the gas lines he suggested filling the tub with hot water for some heat till the chimney was cleared, so i measured the temperature of the tub with my IR temperature gun before doing anything, after the water was filled and the next day. The tub was a bit below room temperature, it was nice and hot with the hot water and was cooler then the empty tub 24 hours later.

I can’t explain this so i recently repeated this (with a working furnace and hot water tank).
Before filling the tub was 17.5ºC, i half filled it with hot water (40G tank) which was something like 65ºC, and the next day it had cooled to 12ºC. The water again ended up cooler then the empty tub. The outdoor temp was about the same during the 24 hours (which i can’t remember). The IR reader is not able to be calibrated but makes very repeatable measurements, i have tested this.

This is the setup, the washroom is 6 feet deep (from the door) and 5 ft wide (as wide as the tub) The tub is on the only exterior wall in front of you when you walk in. When you walk in the toilet is on the immediate left, sink is beside the toilet (both on the left wall) and the tub is in front of you. The taps for the tub are on the right hand side. I hope this description makes sense with no picture.
I am pretty sure the wall is drywall, 2×4 with fiberglass, wood siding, very thin blanket and vinyl siding. The other 3 walls are interior walls at room temp.
Starting underground there is crawlspace, washroom, attic (being on the exterior wall there is very little attic left as it slopes this way, above the tub there may only be 6 inches of attic left, over the washroom sliding door there may be 2ft of attic height). Attic insulation is fiberglass in the studs, 2×4 or 2×6 at best. The washroom door is always open unless someone is using it.
Starting inside you have hallway, washroom, slab on grade outside the washroom at the same level as the washroom floor give or take a couple inches, its a covered porch area facing north. I don’t know deep the slab is.

So i can’t figure out why the water does not end up at the same temp as an empty tub, is the slab more conductive to heat then air or dirt would be? Is there another possibility?
The house has 14.85ACH50, with a high percentage of the leakage coming from the crawlspace, and more information that might be helpful is on another question i just posted
But keep in mind the lower tub water temp is happening at about the same temperature over the 24 hour test period. I am also hesitant to repeat this test a third time because of the high cost of water here.

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  1. davidmeiland | | #1

    Not sure I'm reading the question correctly, but I think what you're observing is the fact that the process of evaporation is cooling the top surface of the water slightly below room temperature. Evaporation takes energy--when a liquid evaporates, it takes heat with it.

  2. charlie_sullivan | | #2

    I agree with David. Same as how sweating cools the body on a hot day.

    If you want to test that theory you could try the experiment again, but put a plastic sheet over the top of the tub. Or I suppose a film of vegetable oil over the top of the water could accomplish the same thing.

  3. user-4053553 | | #3

    I can't agree with the evaporation theory, any heat lost would be regained from the surrounding material, and be ultimately replaced by the furnace otherwise we could cool water by putting it in a glass and letting it sit (and no need for a fridge)

  4. GBA Editor
    Martin Holladay | | #4

    You might be right about the bathtub. But you should know that "letting water sit" is a perfectly good method of refrigeration. When I was growing up on the Mediterranean, lots of people used this method: you get a big terra-cotta pot that is somewhat porous, and you put some water in the pot or keep the exterior damp. Install a wooden cover. Leave it outdoors in the shade. Evaporation lowers the temperature of the air in the pot. If you hang a bag of food in the pot, the food will stay cool.

    Later edit: it turns out that there is a Wikipedia page about this method of refrigeration:

  5. davidmeiland | | #5

    Here are two virtually identical drinking glasses, sitting on the countertop in my kitchen for several hours undisturbed. The one on the left is full of water.

  6. user-4053553 | | #6

    Interesting, i assume the glasses are the same colour, whats the relative humidity, did they start out at the same temperature and whats the temperature difference in the photo?
    I wish i had an IR camera :)

  7. user-4053553 | | #7

    That is clever, but it does need a specially designed vessel, and i assume pottery has some R value and the pores are actively accelerating evaporation.
    Maybe what i should do is repeat the experiment on a hot summer day, see if i get the same results (a low humidity one and again on a high humidity one)

    I wondered if different materials facilitate heat transfer, perhaps the concrete has lower R value then air and a lot of thermal mass to "suck out" heat from the warmer house that its in physical contact with), or the water has lower R value then air which gets colder more quickly form the concrete nearby then the heating effect of the air in the room.

  8. davidmeiland | | #8

    The glasses are both clear glass, the RH is probably in the 40% range, and there's about a 1.5 degree difference in the temps. The glass with water started out somewhat warmer, because the water I drew from the tap was warmer than room temp by about 15 degrees. It took a while for it to cool down, and then a while later it was cooler than the room temperature glass. Overall about 5 hours.

    Notice my reflection in the glasses. Hard to see that with an IR thermometer, and it makes it hard to get a reading you can use. Having the visual of the entire scene is a lot easier, obviously.

    An interesting thing happened overnight. The woodstove went out and the room cooled down several degrees. Right now the water glass appears slightly warmer than the empty, because it is taking it longer to follow the air temperature drop.

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