I have a bonus room over a triple car garage in a 15 year old house. The room was finished with the original build and had HVAC and insulation throughout. I am in the process now of removing my garage ceiling and the batt insulation to have foam insulation sprayed into the floor joist cavities for air seal and Rvalue. I'll also be beefing up the knee walls with spray foam.

Before I do this though I want to be sure HVAC is proper. The total area is a master bedroom, ensuite and sitting area. The sitting area and 1/2 of the ensuite are over the garage. The three areas are supplied by nine 4inx10in floor ducts. There are two 4x10 wall retun grills in the BR and one 4x10 return in the bonus.

I am wondering of there is insuffiecient return air relative to supply air and this is adding to the comfort problem in the bonusroom. Since I have the garage ceiling open I am wondering if I need to modify return air before I foam and redrywall it as it would be a bear to do this later.

Add to this is the fact the bonus room is at the extreme far end of the house from the furnace/AC which is in the basement as well.

Any ideas on the HVAC design here?

Comments appreciated.

b

Nine supplies divided by three returns equals insufficient returned air, unless you have enough air leaks to dump 2/3rd the air outdoors through leaks in the walls.

http://www.heatpro.us/duct/squareft.htm

4 x 10 x 9 = 3600 sq inches of supply.
4 x 10 x 3 = 1200 sq inches of return.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.

Thanks HK,

One of the returns in the bonus room runs from the cold air (CA) plenum in the basement through 6in round duct (insulated). Extending additional cold air duct from the basement CA plenum is going to be very difficult (finished walls and ceilings everywhere). I am wondering if the 6" round return duct could support additional return lines/grill(s) or is it a dimishing situation where the 6in duct is the limiting factor on return air flow and not the 4 by 10 grill. I estimate the 6in run is about 30 feet long with four 90s in it.

How about a duct booster fan used in reverse on 6 in round CA return duct to boost performance? Crazy maybe but the alterntive to get to the plenum is really ugly.

b

First find out what heat and cooling is needed in the rooms. Then figure out what size and number of supplies and returns are needed to have enough air to do the job.

After that, sticking ducts off the sides of ducts and other fun jimcracky guesses is entertaining.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.

OK so I ran calculations for heat dividing the three areas of the master suite into bed, bath and sitting room. The results were:

Bed
Bthu 3775
2 - 6" runs
59 sq in
189 cfm

Bath
Bthu 6463
4 - 6" runs
100 sq in
323 cfm

Sitting
Bthu 6208
3 - 6" runs
76 sq in
310 cfm

The rooms are all connected but have no doors separating the areas.

The existing ducts are:

Bed
2 - 6" runs feeding 2 4x10 floor supply registers
2 - 6" runs connect to 2 6x10 wall mounted returns

Bath
3 - 6" runs feeding 3 4x10 floor supply registers
1 - 6" run serving a 4x10 wall mounted supply 1' above floor (this is a wall register on both sides of a shared wall between bath and sitting area

There are no returns in the bath


Sitting

3 - 6" runs serving 3 4x10 floor supply resisters
plus the other half of the wall mounted register I mentioned for the bath.

There is one return 6x10 wall mounted with a 6" line connected through 30' of 6" round to the return plenum in the basement.

I didn't mention before but the house has two HVAC systems one for basement/main floor and the other for the second floor. We are talking about the second floor system.



So I conclude I am pretty close on the supply side of the equation. You could maybe argue for an additional supply register in the bath area but it looks OK. The question is the lack of return grills. Is this resulting in pressurization of these areas and consequently causing underperformance of the supply side. (ie the room pressurizes but without the retrun the duct performance is lowered and the cfms aren't accomplished)

Comments please.

" Is this resulting in pressurization of these areas and consequently causing underperformance of the supply side. (ie the room pressurizes but without the retrun the duct performance is lowered and the cfms aren't accomplished)"

Yes, whatever air supplies you need to deliver the right amount of heat, you need the equivalent cross-section in returns as well. Many houses undercut doors to act as returns from rooms to cut costs. In your situation, there is a door to the rooms over the garage that can't be undercut enough to return all the air to a central return in the other part of the house, so you need good returns equal to the supplies.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.

Well now my dilema. How to add returns 15 years after the fact without ripping open a lot of finished rooms. My house is new enough that all returns are ducted the whole run. Not like the old days where returns were carved out of open stud bays in interior walls.

That may be a way to get the airflow to work if I could hijack some used bays to convert to return air pathways. I assume though the usage of metal ducting for returns is for some good reason. Is it a bad idea to think of adding returns without metal ducting the whole way back to the cold air plenum?

Any other secrets of the trade to add return air paths?

I take it you didn't see my thought of a duct booster fan in the return path as a useful way to get an increased return out of the 6" round return I have.

A 6-inch duct is good for about 150 cfm, slightly more with a fan.

Ducts are open holes for fire and smoke to pass through, so metal lining is there for safety and a hard surface to make it possible to clean. Giving that up has been done to get return air done.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.

Thanks for the continued guidance HK.

If 150 cfm is 6" then if I work backwards to achieve similar flow (FPS) I get 270 cfm for an 8" round and 350 cfm for an 8" square duct.

Does that sound about right?

I am trying to see if I add one 8" round return duct from the basement return plenum to second floor with a large enough grill can I fix the return deficit with one additional return. My cfm deficit on return is about 372 cfm for the space. I am thinking if 270 cfm for an 8" round duct closes the gap (not quite enough but 50% better than what I have today). My sense is round duct might be easier to work into some of the angles and nooks I'll have to work around.

From a grill area square inches perspective I have a “budget” of 235 sqin for the three rooms which today has about 90 sqin of grills today. That leaves a 145 sqin shortfall.

My idea is to run an 8” round duct up from basement and carve out two interior wall stud bays and put a 10”x32” grill in place of the baseboard at this spot. The grill is slightly larger than I need but the duct is slightly smaller.

Does this sound logical to you?

"If 150 cfm is 6" then if I work backwards to achieve similar flow (FPS) I get 270 cfm for an 8" round and 350 cfm for an 8" square duct."
Good numbers for usual blower pressures.

Grills for boots to ducts often restrict the sq ft cross-section free area of the grill to half, so having oversized grills is a benefit.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.