I am a mechanical design engineer with a degree, and several years experience in both railway rolling stock and heat exchangers/pressure vessels. No experience of building services.

How easy would it be to change industries and become a HVAC engineer? Is there additional learning I would need to complete? Most job adverts seem to suggest my degree would be ok, but that I would need experience. Wondering how easy it would be to convince someone to give me a go and let me get that experience?

Most are termed mechanical design engineers rather than HVAC as they will do CWS, DHW and a few other bits.
Not sure where you’d go with it, I’d try the big boys like Arup, Buro Happold and Parsons Brinkerhoff. You might be able to get onto a grad programme part way through or taken on as a junior engineer as you already have industry experience.
I have a LOT of documents including ebooks, CIBSE guides, BSRIA docs (VERY helpful) and regs which I would suggest you would find very helpful. Not sure how I’d get them to you (perhaps a dropbox thing if you can set one up?).
Brush up on heat load calks, find out which software is popular (hevacomp, IES), perhaps do a BIM course?, parts L & F, BREEAM.
Mail in profile, it may take me a few days to put the stuff in (I have a LOT of stuff).

I’m a Project Manager for a company who designs/builds/maintains Chilled Water and HVAC systems for Navy Ships and Submarines.
The last 3 Design Engineers we’ve employed had no prior experience – we’ve accepted that because what we do is so specialised, we’ll need to invest in the people we employ.

Sounds promising. Thanks for the help.
Wrecker, I will try and set up a drop box and send you a mail.