Latest Discussions : HVAC


06:00AM | 05/12/08
Member Since: 05/11/08
10 lifetime posts
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.



07:07AM | 05/12/08
Member Since: 05/11/08
10 lifetime posts
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.



01:10PM | 05/12/08
Member Since: 05/11/08
10 lifetime posts
OK so I ran calculations for heat dividing the three areas of the master suite into bed, bath and sitting room. The results were:


Bthu 3775

2 - 6" runs

59 sq in

189 cfm


Bthu 6463

4 - 6" runs

100 sq in

323 cfm


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:


2 - 6" runs feeding 2 4x10 floor supply registers

2 - 6" runs connect to 2 6x10 wall mounted returns


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


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.


05:27PM | 05/12/08
Member Since: 05/11/08
10 lifetime posts
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.


08:45AM | 05/13/08
Member Since: 05/11/08
10 lifetime posts
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?


11:11AM | 12/29/08
Member Since: 05/11/08
10 lifetime posts
Well its 7.5 months since we first discussed this topic. Since then I have had the bonus room floor insulated from the garage by removing the garage ceiling drywall and old fiberglass insulation and had spray foam installed into the joist bays and air seal behind the knee-wall. While I had the area opened I did run an 8" round duct to this area for a floor mounted 10"x10" return grill running to the basement cold air plenum (to correct the supply/return imbalance).

Winter is here now and the results are much better, but not perfect. The former 7-9 degree drop from the bonus room to the rest of the upstairs it is now only 3-4 degrees cooler.

Based on Heating Pro calculations I believe I have sufficient registers and returns for the entire space. I did learn a few things while doing this.

1) When I was tying back into the basement return plenum I noticed other returns, 6” round pipe going through wall cavities to upstairs. I saw the original installer of the system chose to just cut 6” round holes on top of return plenum and dropped one end of a 6” 90” through this hole (didn’t use a takeoff) and have gravity hold it in place (he didn’t even bother to make slit cuts and bend last inch of 90 to partially seal the hole). The problem I see is the hole was cut intentionally large (I guess to be sure 6” round 90 fit in easily) so with say ¼” gap all around the pipe entering the plenum that results in say a 4.7 sqin leak for each cold air take off . I think there are about 6-7 of these so total leakage could be 30 sqin or so. I always wondered why the door to the basement slams shut when the 2nd floor furnace turns on. It also explains some of my gas water heater and furnace backdrafting…. Anyway I am wonder if such leakage right on the plenum means than any grills another 20-30’ away via 6” duct will have seriously reduced draw and in effect not resulting in effective cold air return.

2) The supply duct that feeds the last the few floor registers in the cold bonus room is a very long from the furnace (probably 60-70’). It is rectangular through basement (supply plenum) reduces to smaller and smaller rectangular and then eventually square (8x8) duct as you pass each 6” round register takeoff. The very last register has a noticeably lower airflow through it when the furnace is running. I did tape all the duct (cleat) seams when I pulled the drywall down but some seams were not accessible so I couldn’t get all (only got to about ½ of them). I am wondering of there are credible duct booster fans I could use to boost the last couple of registers airflow and increase BTUs to room.

So I my list of options seem to be:

A. Live with it and do nothing. Not my nature to leave it half solved.

B. Find a way to fix the return plenum leaks so the return is truly coming from the second floor and not undermined by basement plenum leaks. Don’t have an easy way to do this as basement ceiling is finished drywall and would hate to go to all the trouble only to find it makes a minor improvement.

C. Find a way to boost airflow on the last supply register with some type of booster fan. Might add noise and not sure if these really help anyway.

D. Move thermostat to coolest (bonus) room and then play with dampers to reduce airflow to other rooms so they don’t overheat. (concern is this might put stress on furnace blower motor if overdone).

E. Other ideas?

Anyway I am at a bit of a loss on what is the best next step. Any suggestions would be great.



07:56AM | 01/22/09
Member Since: 05/11/08
10 lifetime posts
I have ran a HEATPRO1 model of my second floor room by room and computed a heating load of 37,246 btuh, the furnace serving this area is 79k btuh net (100k mid efficiency). Is such a gap considered a large overdesign?

This might explain short cycling and unevenness of heat across various rooms. Also hard to change now. Are there any trade secrets about living with this (turning down gas pressure on gas valve to lower BTHU, fan speed changes, can you remove a burner (has 4 today, 25k each), othr ideas?). Just wondering if there are remedial ideas short of installing a new furnace.

A couple of additional questions.

A couple of the rooms have sloped ceilings where the wall is only 4 feet high then the ceiling runs 45 degrees until it hits a 4' flat length at the top. I wasn't sure how to model this in HEATPRO1 so I just said it was a full height wall (8’) and a flat ceiling across to the other wall. Does this create significant btuh errors?

I have some skylight windows in these sloped ceilings and I just handled them as orinary glass openings (R=1.5) but wonder if a glass opening in a ceiling, even at 45 degrees is a much larger heat loss than the same opening on a wall. Should I model skylights differently?

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