# Abbreviations on school bus / truck fan switches

**URL:** <https://boards.straightdope.com/t/abbreviations-on-school-bus-truck-fan-switches/939334>\
**Category:** Factual Questions\
**Created:** [April 3, 2021, 6:08pm UTC](https://boards.straightdope.com/t/abbreviations-on-school-bus-truck-fan-switches/939334 "2021-04-03T18:08:17Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![themapleleaf](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/themapleleaf/32/14349_2.png) [@themapleleaf](https://boards.straightdope.com/u/themapleleaf)\
**Post date:** [April 3, 2021, 6:08pm UTC](https://boards.straightdope.com/t/abbreviations-on-school-bus-truck-fan-switches/939334/1 "2021-04-03T18:08:17Z")

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This is going to be a really useless thread, but I just want to satisfy my curiosity. As a kid on the school bus, I would look at the driver’s command panel. I noticed that there were switches marked “FAN AUX DEF” and “FAN RR HTR”. I tried to decipher these abbreviations. What exactly do they mean?

Is the first one for some kind of auxiliary defogging device? And if so, what is is it auxiliary to?

I assume the second one is for a rear heater? In [this](https://doczz.net/doc/642986/bus-catalog---chalk-s-truck-parts) catalog of school bus parts, there is also (p. 260) besides the switches already mentioned, a “fan aux[iliary?] h[eate?]r” and a ''fan r[igh?]t h[eate?]r" .

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**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\
**Post date:** [April 3, 2021, 7:34pm UTC](https://boards.straightdope.com/t/abbreviations-on-school-bus-truck-fan-switches/939334/2 "2021-04-03T19:34:21Z")

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Auxiliary defrost (or defog) and rear heater. The defrost would be a fan mounted on a stand aimed at the windshield. The windshield is too large for the normal hot air nozzle to reach all of it.

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<div class="post-metadata">

**Author:** ![themapleleaf](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/themapleleaf/32/14349_2.png) [@themapleleaf](https://boards.straightdope.com/u/themapleleaf)\
**Post date:** [April 3, 2021, 8:51pm UTC](https://boards.straightdope.com/t/abbreviations-on-school-bus-truck-fan-switches/939334/3 "2021-04-03T20:51:52Z")

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> [@mixdenny](#):
>
> Auxiliary defrost (or defog) and rear heater. The defrost would be a fan mounted on a stand aimed at the windshield. The windshield is too large for the normal hot air nozzle to reach all of it

Cool. So far, I think all the questions I’ve asked on these forums have received a satisfactory answer - no matter how nitty-gritty.