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# figuring CFM required

Member Posts: 43
Hi guys, I am attempting to figure out the CFM required to heat a 300 sq ft room using the formula CFM=btu / (1.08 x delta T)

What is the typical delta T used in this calculation?

Thanks !

• Member Posts: 2,144
CFM

1st you need the btu load of the area,then compare the options for the btu heat/cool needed,as compared to a hot water coil(cfm)/furnace/ ductless split, ptac.
• Member Posts: 43
edited May 2014
have the btu

Hi  Techman,

I already have the btu of the room. The home is already heated by a furnace, but I need to install a duct into a currently unheated room. Want to make sure it is sized properly. I have all the parameters except for the temp differential typically used in this formula.
• Member Posts: 2,144
edited May 2014
CFM.

The btu/cfm for the 300s.f. room is being tapped off from the duct system for the main house? Does  the main house have AC? Flapping jaws here, that's about 6000-9000btu of cooling and that's about 1/2-3/4 ton of cooling , depending! Can the main house afford both h/ac cfm?
• Member Posts: 316
Delta T

Your temperature drop is based on the sensible heat ratio from your load calculation. A high SHR allows for a warmer coil, while a low SHR is the opposite. Once you have the CFM value, you have to determine if your blower can meet this requirement at an acceptable ESP using the performance chart from your unit.
• Member Posts: 43
no AC

The system in question is a forced air, oil-fired furnace. Definetly no AC plans for the future, main system is designed for heating only.
• Member Posts: 53
edited May 2014
CFM?

Forget the math, You got to check the manufactures Blower data for that specific furnace and then got to figure out the resistance of the ducts (a.k.a. manual D)
• Member Posts: 316
CFM

This is a wonderful practice; one of which our industry should have more followers. However, it tells you what you have yet doesn't address what you need (load). With these two bits together, we learn how closely the CFM required by the load and the field conditions (as your point states) can meet. The more narrow that gap is means everything to the occupants and the equipment.
• Member Posts: 349
Delta T

The Delta T value is the temperature rise across the appliance. With the furnace running, you take temperature readings of the return air coming into the appliance and the temperature of the supply/heated air coming from the appliance. The difference between the two is your delta T.

Now, if the area to be conditioned is far from the furnace, or the ductwork is not adequately insulated, the cfm value may very well need to be increased to take the duct losses into consideration.

You also have to have reliable numbers with regards to the actual heat loss of the room, so be sure that you are using a Manual J, or similar, method to determine the needs of the conditioned space.
Eugene
• Member Posts: 43
Thanks Eugene!

n/t