Calculating Heat Load/Air Volumn Requirements?

ddawg16

I'm adding a 2-story addition to my house. Existing house is 1026 sq'...the addition is 1050 sq' more.

I already have Lennox 60K BTU forced air heater (6 years old).

I know 60K btu is borderline for a 2000 sq' house...but....I live in California...and temp swings are not that much. In the 6 years we have been using it...on even the coldest nights, it would run maybe 15 min. I sized the unit knowing I was doing the addition and insulating the house better.

Prior to my start of the addtion, the house had no insulation except in the attic and Kitchen (12 years ago I tore out the walls and insulated it).

The addition will have lots of insulation....My largest wall is 2x6 construction so I can put in R19. Sub-floor is insulated...and the house is getting all new windows and all the existing walls (except for one short one) are getting (already) insulated.

I came across a web site some time ago that had guidlines for the CFM each room should have....I can't find it now.

From my forced air unit to the addition, I have one spot...and one spot only to bring in the air...the largest duct I can fit is a 12" round one....I will have to make a plenum to split the air and duct to the 2 floors...from that plenum, I'll run the flexible ducts as needed.

I just need to know what sizes to make them....hence, the CFM for each room.

I'm not overly worried about the upstairs master bath and bedroom....it will have a gas fireplace...so if it's a bit cool...I have a second heat option...

Links?

Advice?

I want to be able to show some somewhat accurate calculations to the inspector if he questions my setup. This is the only part of the drawing package that is not covered in detail by the architect (something about me telling him....don't worry....I got it)

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beenthere

I thought in Cali for heat you either lite a couple candles, or threw a twig in the fire place. LOL

If your going to zone the addition from the existing, a bad rule of thumb that will work for you is .6CFM per sq ft of room area. If its going to be all one zone, might want to drop down to .45 or .5 CFM per sq ft of room area. This is for heat only. But will also cover A/C if your not in one of the hotter zones.

If your using a sheet metal duculator, then what ever it says, go up to the next size for flex duct.

ddawg16

Thank you sir.....it's heat only....

Interesting thing....some of the load calcs I'm trying to run show that I only need about 25K BTU for the whole house....and my unit is 60K BTU....considering how much insulation I'm adding to the house, I don't expect my FAU to work very hard...

I'm 4 miles from the beach....No AC needed....ceiling fans do quite nicely....in the winter, about the coldest it will get is maybe 35 deg...but by noon it's usually back into the 60's.....

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beenthere

Using the .5 CFM per sq ft area should get you the lower air flow the addition needs. And keep both ends close to the same temp.

Now with the addition, your furnace might have to run 25 minutes on those coldest of nights.

ddawg16

Only for the kids rooms.....I have a hot wife (in more ways than one)....we will be just fine upstairs.....away from the prying ears of the little ones.....

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beenthere

LOL... So no door undercut on your bedroom.

ddawg16

Nope.....actually, I plan to put in a return duct.....I have a spot that is pretty much mid point between the bathroom and bed room..I was going to put a 6" flex duct there...grills on either end....with the output dumping into the stairway.....I'm hopeing the flexible duct will do a good job of damping any noise.

This is the upstairs floor plan.....if you look where the toilet is...though not shown, I have a false wall there that gives me an area that is about 14" deep....I have my vent stacks running up through there......that is also where I am brining in the 12" duct from my FAU....half goes down through a 12x10 hole in the floor...the rest is for the bed room and bathroom....the return duct will be just to the side of the toilet and exit out into the stairwell.

Am I overlooking anything?

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ddawg16

Oh....one other thing beenthere.....based on your input....0.5 CFM....if I'm using a 12" circular duct to feed my addition...that works out to 'about' 2000 CFM.....so, if my addition is around 1000 sq'....then I'm in the ball park....right?

I'm going to make a penlum to feed a majority of the air downstairs....so by adjusting the grills....I should be able to balance the load....

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beenthere

Your gonna need at least an 8" flex for the return for that master suite. Plus you'll want the return to favor the bedroom side. Looks like the suite is roughly about 500 sq ft. So it needs roughly 250 CFM.

12" round sheet metal is only about 600 to 700 CFM.