## Space cfm calculation in a residential house

## Space cfm calculation in a residential house

(OP)

Hello,

A house heating load as following (the numbers are only as example):

- Winter outside/inside temperature design : 0F/70F

- Supply air temperature: 120F

- space load (envelop + infiltration): 60000 Btu/hr

- OA ventilation requirement 100 cfm, (7560 Btu/hr)

the question is about air flow calculation:

1- cfm = 60000/(1.08*(120-70)) = 1111.11 cfm

2- cfm = 67560/(1.08*(120-70)) = 1251.11 cfm

which one is much correct.?

Do we consider ventilation load in space cfm calculation.?

A house heating load as following (the numbers are only as example):

- Winter outside/inside temperature design : 0F/70F

- Supply air temperature: 120F

- space load (envelop + infiltration): 60000 Btu/hr

- OA ventilation requirement 100 cfm, (7560 Btu/hr)

the question is about air flow calculation:

1- cfm = 60000/(1.08*(120-70)) = 1111.11 cfm

2- cfm = 67560/(1.08*(120-70)) = 1251.11 cfm

which one is much correct.?

Do we consider ventilation load in space cfm calculation.?

## RE: Space cfm calculation in a residential house

Hope that helps.

## RE: Space cfm calculation in a residential house

## RE: Space cfm calculation in a residential house

Trashcanman is correct in that for residential use the cooling cfm determines the required fan capacity of about 400 CFM/ton. The fan rpm is reduced during heating so discharge air temperature is about 105 to 120F.

But for your equation question you have to solve for CFM SA Htg as follows:

Q = 60,000 + 7560 = (120 - T mix) x 1.08 x CFM SA Htg

Where T mix = ((100 x 0) + (CFM SA - 100) x 70)/CFM SA Htg

Solving both equation simultaneously, you get CFM SA Htg = 1,111.11

## RE: Space cfm calculation in a residential house

## RE: Space cfm calculation in a residential house

## RE: Space cfm calculation in a residential house

To answer your question, outdoor air ducted to the furnace mixes before entering and the resulting mixed volume must be heated to offset space heat losses. Outdoor air at the space level is infiltration leaking through windows and doors and the heat loss must be added to the space transmission loads to determine temperature/volume of supply required to maintain comfort conditions.

I suggest you find a circa 1985 ASHRAE fundamentals or similar manual load calculation manual via google search and perform a complete pencil and paper load calculation. Yea, it is tedious, but you will be able to junk in and not live with junk out of the programs thereafter.

## RE: Space cfm calculation in a residential house