Question about aftereffects of fireballs cast inside

[huginn]

Based on an article in Dragon magazine 123 a fireball can produce heat over 2000 which is well above what the point that wood catches fire.
If cast in a room then that room for an instant it is over 2000 degrees but then what? Does it instantly go back down to room temperature or does it take time?
If it takes time then is there some basic guidelines that can be used?

I am bringing this up as I am in a group that is about to enter sewers and the idea of casting a fireball into the sewer drain before we go in has been mentioned and I am wondering what may go wrong with this. I can think of 2 things pockets of methane gas or some of the fireball comes out a manhole cover

The sewer drain entrance is where everything comes out and into the river

[LibraryOgre]

Generally, I play it as the heat going down to reasonably tolerable levels soon after the fireball explodes... only flavortext remains, as it were, unless something catches on fire.

As for what can happen? Well, the fireball will fill up space. It will expand equally in every available direction until it fills its necessary area, so, unless the sewers are REALLY BIG, or the shaft is REALLY LONG, it's going to shoot fire out of at least one manhole cover... the one they are in front of.

Pockets of methane may lead to explosions underground. Or, possibly, fires.

This is a really bad, really FUN idea.

[Jay R]

The sewer fire in Omaha blew flames out of multiple manholes.

Just sayin'.

[hamlet]

It should also be pointed out that the fireball acts exactly as the DM says it does. So physics, as in actual real world physics, don't necessarily apply. If you want to have it's volume expended largely by shooting out of a manhole cover, then that's what it does. Which would certainly attract a lot of attention from the locals.

The fireball is, as I understand it, a very low pressure thing, so it's not going to break down doors or anything. In fact, I'd honestly say it probably wouldn't go shooting out of manhole covers unless those covers specifically have holes in them. There's just not enough pressure to pop a cover. The shaft up, though, might eat up some of the expansion room.

Also, it's not just methane pockets that can ignite. Detritus in the sewer that isn't we enough can catch fire and continue to burn for a while. Not only that, it would fill the entire region with a nasty smoke, too. Fun for a complication to the entire crawl.

Save yourself some work, though, and calculate the volume of the sphere that the fireball creates, then when it's cast, you have to figure out how many "squares" on the map it fills. We figured this one out a long time ago and realized that one fireball filled virtually the entirety of the dungeon corridor going round corners and into areas that hadn't been explored yet. That player got himself a reputation after that.

[Jay R]

For the record, a 20-foot-radius fireball fills a 335-foot-long 10x10 corridor, or 33 1/2 10x1x10 squares in any configuration.

[Of course, if the scum on top catches fire, that can spread far beyond that. And methane gas adds to the overall area. But if there's a lot of methane in an enclosed area, they couldn't breathe.]

[jojo]

[...And methane gas adds to the overall area. But if there's a lot of methane in an enclosed area, they couldn't breathe.]

This. A good rule of thumb is that if there is enough flammable gas to ignite then there is not enough oxygen to breath safely. As a corollary to this rule: save versus poison.

[jk7275]

For the record, a 20-foot-radius fireball fills a 335-foot-long 10x10 corridor, or 33 1/2 10x1x10 squares in any configuration.

[Of course, if the scum on top catches fire, that can spread far beyond that. And methane gas adds to the overall area. But if there's a lot of methane in an enclosed area, they couldn't breathe.]

Not all monsters needs to breath and the OP mentioned casting fireball into sewers before the party goes in

[Knaight]

This. A good rule of thumb is that if there is enough flammable gas to ignite then there is not enough oxygen to breath safely. As a corollary to this rule: save versus poison.

Sounds reasonable to me. I've used methane poisoning before. With that said, there's also the matter of local concentration differences, most notably in terms of water solubility - the water might still have enough dissolved oxygen for creatures with gills long after the air above it is dangerously high in methane content if the water is flowing fast enough from another region which doesn't have such high content.

[Lord Torath]

Sounds reasonable to me. I've used methane poisoning before. With that said, there's also the matter of local concentration differences, most notably in terms of water solubility - the water might still have enough dissolved oxygen for creatures with gills long after the air above it is dangerously high in methane content if the water is flowing fast enough from another region which doesn't have such high content.

That brings up the question of where the methane is coming from. Generally, it results from the anaerobic breakdown of organic matter. If there's enough dissolved oxygen in the water, you get nitrogen and CO₂. After all the oxygen is used up, you start getting methane. If there's a lot of methane in the air, there's probably not a lot of O₂ in the water. Depending, of course, on the currents - both air and liquid - in your sewer system. You could have slow cul-de-sacs where liquid stagnates and generates methane that bubbles up and then gets blown around by faster-moving air.

[Telok]

Sparks from a metal grinder are a couple thousand degrees if I recall correctly. They don't hurt you because their size is miniscule and the duration of the contact is close to zero. While Fireball's contact area is pretty much every exposed surface the duration of it is a relatively undefined amount of time that is generally assumed to be less than ten seconds.

That's not a really good answer to to the original question, but it's a starting point if you want to do research. Since heat transfer depends on material, area, thermal difference, and duration, and the duration is the unknown variable, you could search for experiments focused on that.

[FreddyNoNose]

Based on an article in Dragon magazine 123 a fireball can produce heat over 2000 which is well above what the point that wood catches fire.
If cast in a room then that room for an instant it is over 2000 degrees but then what? Does it instantly go back down to room temperature or does it take time?
If it takes time then is there some basic guidelines that can be used?

I am bringing this up as I am in a group that is about to enter sewers and the idea of casting a fireball into the sewer drain before we go in has been mentioned and I am wondering what may go wrong with this. I can think of 2 things pockets of methane gas or some of the fireball comes out a manhole cover

The sewer drain entrance is where everything comes out and into the river

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