Sam's distrust of fireplaces could be traced pretty directly to a 13-year-old Dean's rendition of The Three Little Pigs, which had rapidly turned into a thorough and gruesome lesson on siege warfare and how every weakness can be turned into a tactical advantage if you're sneaky and inventive enough. As an adult, Sam could look back and be impressed by Dean's innovative pedagogy, but at the age of 9, he'd been so utterly horrified by the idea of unexpectedly boiling to death that he'd taken cold showers for almost a month and refused to eat or drink anything hot enough to produce steam.
That laid the foundation for his distrust, but all the times they'd had to squat in older abandoned buildings had built a pretty expansive edifice on top of it.
Take their current situation as an example: they were in between credit cards, so a motel was out of the question, but the late November temperatures in Pennsylvania meant they needed some source of heat. They'd found a place old enough to have a massive fireplace, but couldn't start burning anything until making sure the flue was clear. Usually they'd rock-paper-scissors it--and Dean always lost--to pick who' got stuck with that bear of a job, but the last time there'd been raccoons and bats and Dean wound up having to get rabies shots, which Sam still felt guilty over laughing about for so long before he'd realized Dean was in actual trouble.
Which is why Sam found himself crouching in a pile of ashes, flashlight in hand, peering up the flue with some trepidation while Dean was out with the comparatively pleasant job of collecting enough combustible material to keep them from freezing overnight. He'd pretty much gotten over his guilt by now, and was mentally kicking himself for volunteering just because Dean had absent-mindedly rubbed the spot where he'd been bitten the worst.
As if his perfectly understandable apprehension wasn't enough, it was also dawning on him that he didn't really know what he was supposed to be doing. There were shots fired last time, but that could've been Dean attempting to either clear the flue or scare off the raccoon fastened to his collarbone. Usually Sam got to go scrounge things to burn, which meant he couldn't see what Dean did to make sure they didn't die of smoke inhalation.
He'd just about decided to simply take the shotgun approach and damn the consequences when Dean staggered into the room, arms so full of wood he obviously couldn't really see where he's going. "We're in luck," he crowed. "The woodshed's in better condition than the house, and packed pretty full." Heedless of the once-luxurious parquet floor, he dropped his load with a crash and brushed the splinters and bits of bark off his coat. "Any luck here?"
Without waiting for an answer, he squeezed in beside Sam, knees tangling and shoulders jammed uncomfortably together.
"Maybe?" Sam guessed, not wanting to admit he had no clue. "I don't see anything...." Though really the flashlight wasn't strong enough to properly illuminate the length of the flue.
"Huh." Dean dug out his own light, which--unlike Sam's--could be focused, and did his own examination. "Well, no bird nests, so that's a good sign. And the chimney cap looked okay when we pulled up." He flashed a smile at Sam, grotesque from the harsh lighting and close quarters. "And if there's any raccoons up there, I just dare them to come down once we've got the fire going and the soup's on."
"I thought we were doing potatoes," Sam protested, because he still felt a little stoked about finding both them and real butter when they'd last stopped for gas. (Under pressure, he might've admitted to preferring the east coast just for the existence of Wawa. Fresh fruit that wasn't bananas! Salad that wasn't just iceberg lettuce and soggy croutons! And all located conveniently enough that he could be in and out by the time Dean finished filling the Impala.)
"And soup." Spoken in that tone of voice it had the force of law. "Do we have any tomato?"
"Just Manhattan clam chowder," Sam lied, because tomato was strictly for grilled cheese sandwiches. "Does it matter?"
"Too chunky. Thought I taught you that way back when--needs to be thick but homogenous so it'll cling properly." He dug through the shopping bags and emerged with a victorious cry. "Ha! This'll teach 'em."
"Right," Sam agreed somewhat mournfully, recognizing belatedly that the whole thing had somehow become a vendetta while he wasn't looking. In most ways, Dean was still that 13-year-old boy who secretly longed to defend a castle against an army. Apparently probably-nonexistent raccoons would do in a pinch. Ah, well--they'd be sticking around in the area for at least another week. Plenty of time to restock.
With that cheerful thought, Sam leaned into the fireplace to start laying the wood--just in time for a very irate squirrel to land on his head.
In the end they did wind up going the shotgun rout to clear the flue, and wound up eating squirrel along with potatoes, even though Dean complained (as always) about the bones.