Calculating CFM per room based off heating/cooling loads - Need Guidence

[Jholt55]

I am new to the HVAC industry, and my boss is wanting me to design an HVAC system for a two floor office area as best as I can, and then he will review what I come up with. I have calculated my sensible heating/cooling loads for each room, as well as the outside air ventilation loads and latent loads. I totaled the sensible, ventilation, and latent together to get a total coil load of 3.5 tons of cooling, and 72,000 btu/h of heating. From this I sized a furnace that was rated for 80,000 btu/h heating and 4 tons of cooling.

What I am struggling with is how to determine the CFM required for supply and return for each room. The rooms are all to be 75 F summer, and 72 F winter. Do I assume a supply air temperature of 55 F for cooling and 90 F for heating and figure out the temperature difference between the supply temp and room temp, and use that to get the CFM? Then do you choose the max CFM between heating and cooling and size the ducts based off that value? Or do you determine CFM based off room volume and required air changes?

Here is an example calc I did for one room:
- Cooling sensible = 4,580 btu/h, heating sensible = 6,310 btu/h, cooling supply temp = 55 F, heating supply temp = 80 F
- Cooling CFM = 4580/(1.08*(75-55)) = 212 CFM, Heating CFM = 6310/(1.08*(90-72)) = 325 CFM
- I would then size the supply ducts for the largest of the two which is 325 CFM

What am I doing wrong, and where do air changes come into the calculation? If someone can walk me through the steps to designing the CFM and duct system, I would greatly appreciate the help!

 

[Drazen]

Ventilation air changes are not present at all in your calculation.

You came to the point where you realize that issues are much more complex than your boss believes.

It's necessary calculate ventilation air changes separately, than based on all calculations decide on type of system, and only than you can go into practicality of choosing duct size.

Normally, cfm for cooling is much larger than one of heating, which points me to the idea that your heating supply temp is very low, and yes, such low heating supply temperature is not used, it is deemed to create feeling of unpleasant draft. You should go to 35 deg. C or more (switch to F).