There will be better answers from MEP people...

...but the "automated" outputs you're seeing are products of the OOTB-applied standards based on the pre-loaded energy settings (look under project information) for building use/operation etc.

These definitions, along with the defined spaces, are the aspects of the .rvt file that the .gbxml outputs fetch.

But before acting on the data - confirm if your project/practice works with the standards defined by Revit (a bit like the BOMA rules of area measurement not applying this side of the pond) - otherwise you'll first (as I understand it) need to acquire new table data for your Revit installs....

I have some words of warning first. We have not been able to get accurate heating and cooling loads directly from Revit no matter what we try. We've had marginally better success exporting the gbxml info from Revit into Trane Trace, but the space information can still be inaccurate. Be wary.

"When I click a space it gives me information such as specified airflow l/s. Where has this information came from?"

Specified airflow is just a number you can type in. Actual airflow comes directly from the air terminals placed in the space.

"Then I also have Dimensions information: Area, Perimeter, Volume, Unbounded Height and Computation Height. What is the difference of Unbounded Height & Computation Height?"

Unbounded height is kind of a strange one. It's supposed to display the "largest potential height of the room based on room height parameters". The actual height of the space will be determined by a room-bounding element like a ceiling though. Computation height is used to compute the perimeter and area of a room. You shouldn't have to worry about this unless you have a sloped wall in your room.

"But where has the number of people information came from?"

The number of people per room/space is determined by your space type settings, or it can be specified per room. If you look at the properties of the space and click on "People", you'll see that you can leave it at default or you can change it to a specified value.

"Does the area information stop at ceiling level or is this from floor to slab? "

The space will stop at a room-bounding object. The ceilings in the linked model should be set to room-bounding. If there is no ceiling in the room, the space will continue until it finds another room-bounding object, usually the floor above or the roof.
"Finally the calculations in the report are reliant on U values (I think) so does the architect input this info into his model and my report picks up on this?"

The architect should be inputting U-values into their model, but I wouldn't count on it. In your heating and cooling loads options, you can set your own U values under "Building Construction".

Good luck!

I'm not an engineer so I can only go by what engineers I've worked with have told me. The general response after their own careful comparison has been that Revit is more conservative when comparing apples to apples (as much as can be) using their other tools (like Trane Trace) and within a few percentage points. Now I've also been told it is totally unreliable. I suspect that too often the subtle settings involved are either overlooked or their importance is minimized and ultimately it affects the results. The classic garbage in garbage out. I've been party to a few conversations challenging results that, after revising some settings, end up backtracking and admitting that Revit is reasonably close to the expected values.

Regarding the responses so far:

Revit MEP until this release (2014) could really only provide heating and cooling loads based on the data supplied via the building information and individual space settings (HVAC zone info too). With 2014 it became possible to read information from the model elements instead, when Revit began calculating the "u" values of assemblies. The work in 2013 and 2014 (adding thermal properties to materials) paved the way to this.

It is still common practice to ignore the "actual" model elements and focus on the conceptual form (spaces) of the model. The 30,000 foot view can focus on the total volume or drill down into individual space settings. The more accurate the space settings are the closer to a reliable result we are supposed to be able to achieve. The million dollar question is how do "we" calculate and define the right result, then how close is that process to the one that Autodesk engineers have applied.

Space volume only extends to the top of the settings for each space. If one is set to 10 FT and a ceiling above is at 12 FT then there is two vertical feet of space volume not accounted for. A space needs to be at least as high as the highest boundary element. If it is a vaulted ceiling then the space should be a bit higher than the highest point in the vault to allow Revit to calculate the whole volume.

If you don't elect to let Revit evaluate the model elements then the results depend entirely on the space model, all their settings and modeling techniques. For example, if we don't model spaces like unoccupied chases and plenum then we are glossing over subtle design conditions that can affect the outcome.

A chase can be perceived as exterior surfaces without a space in it. In other words a wall that has a space on either side is considered interior while a wall that has a space on one side causes Revit to think the space side is interior and the other is exterior. These subtle differences knock results out of whack and enough of these conditions can throw the results further off track.

It is important to resolve warnings (they appear during the initial stage of the H & C report) associated with spaces in the preparation for a heating and cooling loads report so a "tight" space model exists.

Autodesk also expanded the API (with 2014) to allow engineers to craft their own calculation rules and apply them instead. If you can't reconcile the results after exhausting all of the settings and features that Revit has as well as creating a "tight" space model then the API may be the route to take. One session at Autodesk University said that was their choice and it's proved effective for them so far.

Ultimately a fair comparison should make sure that the settings in Revit and another tool are based on the same assumptions and rules (as much as possible) so any differences in the results are easier to identify, hopefully.

have the heating and cooling loads tools got any better, and has anyone used insight?

I use gbXML to take the Space data out to Carrier HAP. There's a lot of data entry that you can do in Revit before the export, though. For instance, you can use Space Separator lines to carve up large spaces into interior and exterior zones. You can also enter in occupant counts, lighting watts/sf and equipment wattage. All of that data will carry though the gbXML export, and then all you have to do is enter in the wall and window data. Revit still sucks at this one, but given the other time savings I don't mind having to hand enter the envelope data. It's so critical that I would be checking every room even if it could be exported correctly.