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Assume the surface of the planet is experiencing ice age conditions and the facility was designed to keep the occupants alive for 6 months.

An emergency forces the permanent closure of all Heating, Ventilation, and Air Conditioning (HVAC) vents for immediate survival. Going to the surface isn't an option.

I had read, though I can't find the source at the moment, that any underground habitat requires cooling or the human's body heat will turn the place into an oven.

There seems to be some doubt that the human body will cause the temperature to rise in a closed underground chamber. If I could access this study on thermal engineering I wouldn't have asked because it would have my answer. However, the link proves that in fact, the temperature will rise if there is no venting. Natural thermal convection based on human body heat. https://www.sciencedirect.com/science/article/abs/pii/S1359431118332563?via%3Dihub

As for size of the facility, using the rpg's construction rules, 400 people would fit in a 240,000 square feet building.

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    $\begingroup$ Hi Pentallion, we really encourage users to ask one question at a time to get the best results. I’d recommend asking just the first question for now, then linking back to it once you have some good answers. Not only will that bring more attention to both of your questions, but you may be able to expand the answer for humans alone to one that includes the second. I’ve voted to place your question on hold until you get a chance to edit it - feel free to ping me with the @ symbol when it’s ready! $\endgroup$ – Dubukay Aug 2 at 1:08
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    $\begingroup$ Even been in a cave? Noticed the average temperature underground? The stink of the livestock will kill them first. $\endgroup$ – user535733 Aug 2 at 1:10
  • $\begingroup$ Hello and Welcome to World building Pentallion. I would recommend adding a question back into the body of your... well question. You seemed to have removed it. Your also missing some details like, how big is this facility. How much open air there is. And what are the heat vents doing? (heating the facility, or pumping heat out of the facility). $\endgroup$ – Shadowzee Aug 2 at 2:05
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There is a lot of unknowns in your question and this is one of those questions where the devil is likely in the detail, but the key point is that unless you're VERY deep underground, you don't need heat vents in the first place. Being underground is actually good for your populace as it acts as natural thermal insulation. In point of fact, in a town called Lightning Ridge in Australia (a mining town inland where temperatures can soar) there are some homes that are effectively excavated out of the ground, underground homes if you will, because of the thermal insulation provided. Given that it doesn't often rain out there you can protect your entrance from rain pretty effectively so it works. But, I digress.

The point is, that if you live in a cave that is fairly well insulated, you have a natural heat source with you; body heat. In point of fact, in Stockholm, the heat generated from people in Central Station is redirected to heat office buildings, reducing their heating costs by up to 25%. In an ice age, you probably don't want to vent away any of that heat away at all.

Your bigger problem is in fact oxygen. Your heat vents are going to double as air vents, allowing air to circulate through natural fissures in the rock formation of your underground habitat and back through the vents, allowing for fresh air on a constant basis. Closing those vents is more likely to cause the air to go stale, leading to a buildup of CO2 but at the very least thinner oxygen levels over an extended period.

Personally, if you're going to shut the vents for an extended period, try to get some plants growing down there; I don't know the tech level of your colony but if they have artificial lights I'd be growing as many plants as I can down there to freshen the air and re-use the detritus all those people and animals are going to generate, if you get my meaning.

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Look at what happens when miners or speleologists get trapped underground with no supplies: lack of oxygen is the main risk for their survival.

Death by starvation happens in weeks.

Death by dehydration happens in days.

Death from lack of oxygen can happen in hours or even minutes, depending on the amount of air contained in the environment when the trapping happens.

The temperature of a closed environment where human metabolism is the only heat source cannot get higher than the body temperature, (as soon as the environmental temperature is higher than the body temperature the energy flow will revert) so the environment is not going to turn into an oven.

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    $\begingroup$ "As soon as the environmental temperature is higher than the body temperature the energy flow will revert": that is exactly the problem. The human body produces waste heat continuuously. If the human body cannot dissipate waste heat it will overheat and die. $\endgroup$ – AlexP Aug 2 at 8:57
  • $\begingroup$ @AlexP, the OP states that "human in a closed environment make it a hoven". $\endgroup$ – L.Dutch - Reinstate Monica Aug 2 at 8:58
  • $\begingroup$ The human body emits on avg. the energy of a 100 watt light bulb per hour. The people in question won't die of starvation in weeks nor dehydration in hours, so they most certainly will live long enough to die of the heat their bodies generate, which is why mining companies have heat vents and underground facilities vent their heat. They are enclosed, unlike cold caves. $\endgroup$ – Pentallion Aug 3 at 8:03
  • $\begingroup$ @Pentallion, nowhere in my answer I am comparing them to people working in operating mines. $\endgroup$ – L.Dutch - Reinstate Monica Aug 3 at 8:52

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