Does running a fridge in a thermostat-electric-heated room cost energy?
October 16, 2020 8:05 AM   Subscribe

I was in a Zoom call with my colleagues who were, unusually, at the office. I was going to ask them to unplug our mini-fridge while they were there. But then I thought, the office is going to be heated according to a thermostat all this winter. The refrigerator is doing work which will dissipate into the room as heat, and so the work done by the heating system will be reduced. So is there actually a net increase in energy usage?
posted by East Manitoba Regional Junior Kabaddi Champion '94 to Science & Nature (17 answers total) 1 user marked this as a favorite
If heat is electric energy use is exactly the same. If the heat is via heat pump then the heating is significantly more efficient. If the heat is via gas/oil or other carbon fuel than it'll depend on the electricity source.
posted by Mitheral at 8:10 AM on October 16 [4 favorites]

You're correct that every watt coming out of the wall is going to turn into heat. The only other thing energy can turn into is matter and it's not easy.

Of course, the fridge and the office are contributing to the eventual heat death of the universe, so... *stern look*.
posted by ftm at 8:10 AM on October 16 [5 favorites]

I guess it will cause wear-and-tear to the fridge. That's a thing.
posted by East Manitoba Regional Junior Kabaddi Champion '94 at 8:13 AM on October 16

Does this also mean that leaving the lights on in a windowless room at the office similarly doesn't have a net energy cost?
posted by East Manitoba Regional Junior Kabaddi Champion '94 at 8:14 AM on October 16

And running a server farm mining Bitcoins all winter... not that I would abuse company resources in such a manner
posted by East Manitoba Regional Junior Kabaddi Champion '94 at 8:16 AM on October 16

Be aware than many buildings have set back heat control. If the building's set point is lower than the outside temperature than any electricity use in the building is going to result in additional energy use. (Usually not a concern in East Manitoba).

Also a server farm cranking bitcoin is going to have an order of magnitude or two or three more energy usage than a single fridge. And the rooms they are in are usually cooled (heating is rarely required) separate from the rest of the building (and held at a lower temperature). So you could have the situation where the A/C for the server room is running full blast to support the bitcoin farm and the heating unit for the area on the other side of the wall is running to heat the wall that has been cooled by the Server A/C.
posted by Mitheral at 8:25 AM on October 16 [1 favorite]

Electricity is inefficient, so there's net loss in transmission. In addition to removing heat, the fridge runs a compressor, and there is wasted energy/electricity in removing heat.
posted by theora55 at 8:32 AM on October 16 [4 favorites]

Does this also mean that leaving the lights on in a windowless room at the office similarly doesn't have a net energy cost?

In a building heated by resistive electric heat (and only resisitive) any electricity use is exactly the same as far as heating efficiency goes. Computers, lights, cooking, fans and motors of all types, etc. it doesn't matter; ultimately all the electricity used ends up as heat.
posted by Mitheral at 8:33 AM on October 16 [1 favorite]

I guess the office probably uses gas heating. So we should unplug the mini-fridge. Thank you for saving the planet. And, more importantly, our corporate profits.
posted by East Manitoba Regional Junior Kabaddi Champion '94 at 8:39 AM on October 16 [2 favorites]

I think your intuition is right, as others have said, if the heating is electric.

The two complicating factors are how much power is lost heating electrical components outside of the living space and the impact of scheduling. If the HVAC runs at a higher voltage (say 240 or 480V) than the fridge (often 120V in the US), there's a good chance you're spending a little more power heating the lines and transformers outdoors and inside the walls when using your fridge as a heater. It's probably a few percent difference at most for a building that isn't very old or unusual. Scheduling is also important for both cost and environmental impact, though how much and in which direction depends on where you live. For example, if the HVAC runs more at night and the fridge runs equally at all times, they may be using power largely from different sources.
posted by eotvos at 8:48 AM on October 16 [1 favorite]

Yeah, exactly every resistive heating element (space heater, baseboard heat) is exactly as efficient* as every other electric device entirely within the environment to be heated when it comes to generating heat, namely 100% efficient. A cellphone, a lightbulb (in a windowless room), a bitcoin miner (without any outdoor cooling infrastructure), a fridge, a window A/C unit set down in the middle of the floor, doesn't matter; they all convert their energy usage in the end to pure heat.

Of curse, even "electric" heat these days is mostly non-resistive. A standard for window-mount heaters, built-in heaters, and water heaters is that they're heat pumps, not just resistive heating elements. A heat pump is the principle behind an air-conditioning unit; it spends energy not only generating heat (which all expenditure of energy does) but also moving heat against a thermal gradient. For an air-conditioning unit, the heat pump extracts heat from the interior of a building and pushes it into the hotter air outside; for an electric heater, it does just the reverse, extracting heat from cold air outside and pushing it into a warm building. This means that the localized efficiency of many electric heaters is actually larger than 100%, usually closer to 300-400%, in that they produce more thermal energy inside the building than they expend electrical energy, as much as 4 times as much energy. This doesn't violate conservation of energy because most of that energy is actually coming from the exterior: the heater is making the cold air outside slightly colder by wringing thermal energy out of it.

*So if every device is equally efficient at producing heat, you might ask, why is a resistive heater a distinct thing from a cellphone, a lightbulb, or whatnot? A "good" resistive heater has a high wattage, probably has a thermostat, and has some number of fans to disperse the generated heat into the room. Given enough time, a lightbulb (preferably incandescent) can generate as much thermal energy as a heater; that's how the old EZ-Bake ovens worked, but they will be considerably less effective at bringing a large room to a constant desired temperature, which is a comfort issue above and beyond merely increasing the total thermal energy in an environment. This footnote is actually relevant to your question, too: a fridge will produce a localized hotspot, and while eventually that thermal energy will disperse throughout the building, there'll be significant lag between that energy being generated and being detected by the thermostat, which means the building's heating system might hit a setpoint slightly higher than desired. OTOH, if you set the fridge directly in front of a thermostat, it might well save energy since the localized hotspot will trick the thermostat into thinking the building is warmer than it is (and it will piss off anyone working in the building, who will shiver in 40-degree rooms as the thermostat blissfully reports 70-degree interior temperature).
posted by jackbishop at 9:06 AM on October 16 [3 favorites]

Footnote on the mini fridge - ask them not only to unplug it but also to prop the door open somehow (in a way that won't mess with the door hinges.) Closed fridges that are off, even mini fridges, can develop a nasty smell.
posted by gudrun at 9:20 AM on October 16 [4 favorites]

Ummmm, If you put a mini-fridge in a small perfectly insulated room it would still use electricity. It would pump heat from the fridge into the box which would then be warmer and raising the temperature difference between the outside and inside of the fridge thus increasing the rate at which the inside of the fridge warms back up and has to turn on again to pump the heat from inside to outside again.

As long as the office is not kept as cool as the inside of the fridge... it will be wasting energy trying to keep that few cubic feet of space at a lower temperature.

If you unplug it, it will become a room temperature box and the building will have another few cubic feet of air to keep warm, but the fridge won't be wasting energy playing the little game of "let's use energy to move a bit of heat from one place to another over and over".

You usually take this as ok when you're trying to heat a place for human comfort and the utility of the other thing that is providing some heat in addition to it's function. Computers, TVs, Lights, Ovens, yadda yadda. But a fridge is just using energy to move heat in a circle so is actually always a loss.
posted by zengargoyle at 9:54 AM on October 16

But a fridge is just using energy to move heat in a circle so is actually always a loss.

When compared to a resistive heater (EG: a base board heater) the effect is exactly the same. Long term run a 100W fridge or a 100W heater for 6 hours a day and the effect on the temperature of the room is exactly the same. From a climate control/energy efficiency point of view there is no difference between a simple electric element and the motor in a fridge. All the electricity used eventually heats the room.

If the HVAC runs at a higher voltage (say 240 or 480V) than the fridge (often 120V in the US), there's a good chance you're spending a little more power heating the lines and transformers outdoors and inside the walls when using your fridge as a heater.

This is really getting out into the weeds but most businesses are going to be getting grid power as three phase 600V (Canada) or 480V (USA) or higher. If the HVAC equipment is running at 600/480V then there might be 1-2% efficiencies from not transforming down to 208/120. However building transformers in all but the largest plants are usually inside the building so those transformer losses manifest as our good friend heat inside the building envelope.

Either way the transmission losses for both point of use voltages are the same because the voltage isn't differentiated until the lines enter the building.
posted by Mitheral at 10:01 AM on October 16

Our 'green team' went round our offices last Christmas and turned off all the fridges (among other things). On return in the new year, we had a problem with the electrics tripping. Long story short, it took multiple trips from maintenance and a great deal of suffering to trace the problem to the fridge. The sum of the time, grief, and the cost of a new fridge, far outweighed any energy saved.

Bottom line: leave your fridge on.
posted by StephenB at 11:02 AM on October 16 [2 favorites]

Running the fridge will reduce the need to heat the room, thereby reducing the energy expended by the heating system.

But maintaining the temperature differential between the room and the fridge will consume some energy. Heat pumps are efficient, but they do not have 100% efficiency.
posted by Winnie the Proust at 11:15 AM on October 16

I agree in general principal. But it sounds like Maxwell's demon. While you can call it heat in a universal sense some of the work that the fridge is doing is in effect wasted on localized entropy manipulation. There is work done that is not heat but merely separating cold from hot and letting entropy remix them and doing that work over again. That bit of energy into work to give entropy something to do is inefficiency as a heat source. The inefficiency of doing that entropy shuffle is the wasted heat that warms the room. Part of the energy in your computer is not turned into heat but also is turned into a sort of entropy reversal in the computations that it does. A perfect computer would generate no heat yet still take energy to run (I think Feynman had a lecture on this).
posted by zengargoyle at 11:27 AM on October 16 [2 favorites]

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