This recently came up in a different thread, and although I already posted some thoughts there, it was clearly a threadjack and worthy of a new thread.
In my (non-legal) opinion, if I own a piece of equipment and that equipment had a manual at one point, I should have access to a replacement manual if needed.
Picture my universal remote control- I lost the manual for it before I bought my last tv and I found myself needing to look up new codes for it. I can accept that the manufacturer might sell me one if I was to ask for them to print and ship me a new copy. But I can also generally find the manual on the web for free, which allows me to reprogram the hardware (in this case remote control) so that I can still use it. Isn't it the same thing with HVAC controls manuals?
Someone on the other thread suggested that there are HVAC controls manufacturers/dealers who will issue cease and desist letters if someone is in possession of a manual for their own equipment. Is that even possible? Is there a chance I misread you Matrix?
(I posted parts of the following in the other thread, sorry for the repeat)
If you want/need a manual, it is presumably because you have some of their hardware and you need to modify it in some way. Presumably, when the product itself was installed or sold to you, you were given or had the opportunity to get the manual. The fact that you don't have the manual can cause only one thing- frustration at the company for not being able to use the product.
So perhaps from the legality side, a case could be made for obtaining a manual if you are in [legal] possession of the device itself. But a much easier case could be made in the court of public opinion if a company turns to the dark side and pesters people who try to use their products.