natural gas requirement for 3 ton RTU

[dallas]

What is the typical natural gas flow rate required for a 3 ton rooftop unit. Do these units usually have the capability of regulating flow (not pressure) on their own or does one need to supply a flow regulator

 

[hvacman]

You need to know the MBH input for the heater, the tonnage is associated with the cooling. Then you need to determine the incoming gas pressure to size your gas pipe. Gas pipe sizing tables are in the International Mechanical Code book or the National Fuel Gas Code Handbook. You can run higher pressure gas through smaller pipe and you'll probably need a gas regulator that can handle the MBH input @ incoming pressure, and an outlet pressure per the equipment data sheet usually between 7-14" w.c.
Hope this gets you started.

 

[dallas]

Thanks for your help. So i see I need a gas regulator to handle 8 psi supply line pressure and 3.5 W.C. outlet pressure. Do need a flow regulator as well or will the unit handle that function.

 

[imac]

You need a pounds in, inches out pressure regulator. The outlet pressure will typically be set a 7.0" w.c. You must design a gas supply system that will deliver the appropriate volume of fuel to the appliance from the pressure regulator so that the gas pressure does not drop more than 1.0" w.c. across the entire supply line. This is where your sizing charts are handy. You should be able to rely on the trade to design and install the appropriate gas line.

Once the fuel reaches your heating unit, the appliance regulator will further drop the gas pressure to the manifold pressure which is typically 3.5" w.c. Most gas valves cannot tolerate an upstream pressure greater than 14.0" w.c. This is why you typically find the delivery pressure set at about 7.0" w.c. Except propane which is delivered at a higher pressure, usually 11.0" - 11.5" w.c. Propane is delivered into the manifold at 10.5" w.c. in most cases. Minimun upstream pressure is about 5.0" w.c. for natural gas and 11.0" w.c. for propane.

You do not need to regulate the flow of gas. The gas flow rate is controlled by the number and type of orifices installed in the unit. Thus, gas pressure is critical and must be checked and verified by a licenced technician on start-up of the appliance.

Hope this helps.