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As others have mentioned, you need a qualified engineer to assist you and you won't find this in a forum. It worries me that you are basically asking how to size a duct for a given flow, one of the most basic hvac design skills. You might want to start determining which applicable codes and...

As long as you have enough throw, 19 ft should not be an issue.

@moideen. Sorry, but I am not sure you really understand psychrometrics and if this is a real building and not "millennial homework" assistance, you are about to make a mess. Plot the two points on a psychrometric chart, does 68 F @ 55% RH follows a sensitive heating process off 58 F sat...

The screenshots of your software show a drop in water content which means that to achieve your design point you need to reduce your water content, which you will not be able to do with a sensible heating process. I think need you to consult your application with a qualified mechanical engineer...

How about trying to negotiate a balance? Small pipe diameters to 1.5 m/s as per his request, larger branches to 2.4 m/s simply because they will not fit in the walls. Then you show him you are doing things "his way" but also doing your due diligence...that usually goes a long way with ol guys ;-)

Here in Canada, the code allows you to design down to 1.2 m/s. I would, advice the client that with 1.5 m/s itwould make the pipes bigger, increased capital cost, keep pressure higher than needed in many locations of the system, etc. If they would still want to proceed, then so be it. Usually...

Most of your questions have been answered, but just out of curiosity, why an open tank? Can't think of any advantages

It seems as if your calculation is theoretically correct. 300 MBH might be a bit oversized, I would go for 2 x 100 MBH for some back-up and redundancy.

For smaller core zones, set it to a very low minimum. We have done VAV only without heat (cost saving) without significant issues. Just communicate to the owner the limitations of the system as the other poster indicated.

Hopefully this is not homework, but here is a hint: knowing the refrigerant type, find the compressors working pressures. The rest should be easy if you know your thermodynamics.

"logical reason" sometimes there isn't. It was likely well intentioned by the designer, but as others have pointed out, there are better ways to do whatever the designer was trying to achieve.

SMACNA standards are designed to simplify structural design work for HVAC engineers. As a structural engineer, you should be qualified to design this over and beyond what SMANCA does in a prescriptive manner. You are asking for free design assistance on a discipline that should be your domain...

The manufacturer is likely correct. It will likely not work exactly how you are planning. Heat transfer to the coil is directly related to turbulence, by reducing the air speed, you are likely reducing the heat transfer on the coil. As others have mentioned, this would also have an impact on...

The definition of whether ACH is 100% outside air or just air movement depends on the standard. Some NFPA standards for petroleum facilities and wastewater treatment facilities define ACH as air changes per hour of 100% outside air. It is also the same for some healthcare standards. Most other...

Depends on the authority having jurisdiction, in my area, there is no "grandfathering" of these systems and the latest standard applies most of the time.

A rough approximation of the comparative savings could be modeling a properly designed and upgraded system and manually calculating separately having the pumps running all the time.

Why are you using such an outdated version of the standard? Around here, the AHJ identifies which version of a standard is to be used, 07 has been out of commission for awhile. Not familiar with that clause (a bit different from the latest versions). How big is your parking garage (certainly...

If you cool down the air to 7 C (which is very cold, you might want to check your air distribution) and throw the air into the space, assuming the space load analysis is correct, you will end up colder and dryer than your estimated return air temperature conditions, so this point is not correct...

May I suggest that you type all in lower case instead? A sketch will definitely help, having a hard time figuring out your scenario, all I can say that trying to cool a transformer your size with 3" hole (if that is correct), does not sound right.

Check your combustion calcs, you might only need to make up 1/3 or 1/4 of flue gas flow.