Air Handler duct sizing

Jon_blaney

I am planning to replace an oversized oil furnace with an air handler. As I plan the duct system I note that furnaces have fixed cfm output for a given capacity while air handlers have a variable capacity. The btu output is a function of fan speed and water temperature. Should you design the duct system for max output or something closer to you btu requirement? I am in a heat dominated climate.

HVACNUT

The ductwork only needs to be sized to accommodate the CFM of the system its attached to. BTU's have nothing to do with it.

Jon_blaney

I asked the question because unlike a furnace, a particular fan coil can have a range of CFM outputs, different fan speeds. Lower cfm produced lower btu output if the water temperature is constant.

HVACNUT

> @Jon_blaney said:
> I asked the question because unlike a furnace, a particular fan coil can have a range of CFM outputs, different fan speeds. Lower cfm produced lower btu output if the water temperature is constant.

OK. Say max 20K BTU's at 180° through the coil at 1,200 CFM on High speed. The ductwork needs to be sized to handle 1,200 CFM at High speed. A lower fan speed actually has the capability to extract more BTU's from the coil.
Ever notice in your car, with the heat on, the high speed fan temperature is cooler than low speed?

pecmsg

Air handlers do not know what size coil is going on there so must cover a large range of sizes.

The furnace is a fixed CFM + or - a small %.

You do need to know how many CFM's are required per room to know what size ducts to run.

Are you using metal or flex?

Jon_blaney

OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.

HVACNUT

> @Jon_blaney said:
> OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.

Correct. Keep in mind, the main supply trunk duct needs to reduce in size as it goes down the line.
For example, the supply starts out sized for 1,200 CFM. After takeoffs to say, two branch ducts, the main trunk must be transitioned down to keep up the static pressure. So if the first two branch ducts need 150 CFM each, then the trunk duct gets reduced just downstream of those branch ducts to accommodate 900 CFM, and so forth. And never tap a supply branch duct off the end cap at the end of the truck duct.

ChrisJ

HVACNUT said:

> @Jon_blaney said:

> OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.



Correct. Keep in mind, the main supply trunk duct needs to reduce in size as it goes down the line.

For example, the supply starts out sized for 1,200 CFM. After takeoffs to say, two branch ducts, the main trunk must be transitioned down to keep up the static pressure. So if the first two branch ducts need 150 CFM each, then the trunk duct gets reduced just downstream of those branch ducts to accommodate 900 CFM, and so forth. And never tap a supply branch duct off the end cap at the end of the truck duct.

That's one way to do it but not the only way.
When I did my ductwork I didn't go that way.

I also sized everything for 0.05"WC drop @100'.

pecmsg

Jon_blaney said:

OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.

What you need is a accurate room by room load / loss (Manual "J") done, then a Manual "D" Duct Design.

Do not try this yourself, it rarely works out well!