Sort by the content of the post. .......... If the question includes:
 
* A person identifying his level of knowledge .........(not reserving expertise for a later one-up)
* Make
* Model
* The complaint or problem observed
* Identification of how it is installed
* Geographic area to know the temperature extremes and moisture
* The symptoms
 
It might be a valid request, - otherwise, be alerted. Clueless innocence: This might be someone on the first step of learning. A child is more prone to destroy something; an adult takes responsibility to maintain or care for something. This might be someone with no experience. The clueless confuse game conventions, like 'personality avatars' and 'user names' with a need to remain anonymous on the web.

Because a post is antagonistic doesn't mean the person is a flamer. There is a difference between a flame - which is a diversion from the subject or a deliberate misinterpretation to divert
VS
an expressed disagreement. A disagreement, with a response recognizing the disagreement and why he disagrees, is a reasonable response in a discussion.

A flame-prone personality will settle for the cat-fight instead of recognizing an interest in an answer to the subject. When a stated point of disagreement is made, it is time to go on - but usually the flame-prone will try to win attention to the mean-ness of someone disagreeing with them.

Noticeable techniques:

He or a partner will insult you, then they will accuse you of their insult at a later post.

They will insist that you are not 'qualified' to talk about your post, taking on the role of approval agency - a 'one-up.'

Clueless maliciousness: There are others that take on the appearance of someone who needs education to bait (trolling) the well-intentioned with clueless questions. The flamer has more interest in social points than in fixing a mechanical problem so hasn't made the step toward adult responsibility.
 
A subcategory of this is the clueless deception. Someone comes on as unknowing, then switches to tell you they are in the profession after they receive an answer, then they 'correct' you with inadequate or dangerous advice - or try to convince other readers of your inadequacy. An HVAC pro will respect their specialized knowledge and not flame in their territory. Answering without giving dangerous advice is done by EXPERIENCED writers from their own viewpoint, not as an attack.

An ordinary professional consult is done face-to-face in an office or job site as a business transaction. Flamers take advantage of the anonymity of the web to turn a consult into an insult.

Sometimes, if you are willing to risk going outside your profession into one that you've seen often, like construction (it is difficult not to see how a house is built while you are running HVAC inside it) you might be stepping on another's area of expertise. I've had one flamer tell me it is absolutely IMPOSSIBLE that a house would sag toward a masonry chimney because code building practice doesn't allow fastening the structure to a chimney (true - but that doesn't account for construction before codes, soft ground, nor inexperienced folks building homes a half-century ago,) so he began a rant on the DIY site using character assassination techniques shown on the other pages (any teen girl knows them.)
 
Often, flamers are (or claim to be) in a 'sister' profession - like stationary engineering (large commercial boiler operator.) There is competitive gain by showing they are more informed (DIY's don't know there are usually completely difference experiences between high-pressure commercial steam equipment vs low pressure steam, hot water heating, and ducted systems.) Other professional skills, like masonry, sheetmetal, electrician, plumber are part of the repertoire of HVAC.