Cooling a bonus room

[gmahler]

My downstairs unit that I need to replace is a 2.5 ton unit that heats/cools about 1100 sq.ft and a room above the garage that is about 300 sq.ft. This bonus room has the longest supply lines from the air handler which is located central to the downstairs. It appears to have smaller ducts and a return that is sized the same as the supply (at least the return register is the same size as the supply - for a long time I thought they were both supply registers since they look alike).

Anyway, everyone who has looked at this arrangement has said that the only way to do this is to have a third heat pump for the 300 sq. ft bonus room. It is currently tied in to the downstairs and I'm wondering if that small return in this room is causing inefficiencies. Is this the way to go, or can I have a design that handles the downstairs and this one upstairs room that gets very low air flow currently??

[dash]

Does the bonus room have floor trusses ,as opposed to conventional floor joists?

What city/state are you in?Year built?

[gmahler]

The house is in SC near Charlotte NC. Built I would say between 1980-1985. I guess it is conventional. It's a separate section, with its own gable roof. This section contains the garage with the room above it. The garage has central metal supporting post. A common two story design with the main section and the garage connected by a dining room and utility.

[dash]

First you need a Man J load calculation to determine the correct size,you may already have the correct size,and have air flow problems to the bonus room.You can do it yourself at the bullseye above.


If the system is the correct size,it's a duct design or installation,problem.



If their are floor trusses,they are open and hot air can circulate from the attic under the floor,the insulation is usually at the ceiling ,so the truss openings to the attic need to be sealed off.


Could be no insulation in the floor,need to check,small hole in garage ceiling,or maybe pull a light fixture down.

Add insulation to the bonus room attic ,and vent with ridge vent.


Wall(s) that are common to bonus room and attic(kneewalls),need R11 ,plus high R foam sheeting,sealed tightly over the wall.



These are just some of the things we do ,to correct bonus room problems.

[pstu]

What would the pros think of adding a radiant barrier to the attic roof? It's become the norm where I live for new construction in S.Texas. I have seen some research suggesting it can save 8-12% in cooling bills (and some research saying more, which I am less sure about). Presumably it would reduce the cooling load in summer and the heating load in winter.

Have been doing a long and slow job of putting the foil type in my own attic, when time and weather permit. Can testify that you can really feel the difference when standing under a protected area, for what that's worth.

Hope this helps -- P.Student

[gmahler]

Thanks for the suggestions. Did have blown in insulation added to the roof I think it's R-11 (qualified for the electric rate from utility) Not sure about the floor being insulated, probably not, so there is the hot air from the garage below. What about the small return in the bonus room, it's the same size of the supply register (and only filtered by a piece of foam)? You'd have two air masses -one from the bonus room mixing with the cooler air downstairs. We currently use a window A/C during hot days - don't run it all day only as needed.

[dash]

Small return?

Try running it with the door open to downstairs,if it cools fine, it's a return problem ,if not you need more supply air.

If it needs more supply,you'll need more return or be forced to keep the door open.


Hopefully you added R11 to R19,to get R30.

Radiant barrier will help as well,as suggested above.

[coco]

being as the room is entirely surrounded by unconditioned spaces (attic above garage below) lot of heat loss and gain. you will definetly HAVE to have sufficient supply and return . if i cant get a high and low return in depending on how large, i will use 2 frames 1 high and 1 low in the same joist space w/ 1 return that has a manual damper. close off the bottom return in cooling to draw in the warmest air and in heating shut down the top return to draw in the coolest air.

[mojoman]

i would prefer a seperate thermostat in the bonus room--a mitsubishi ductless split heat pump--about 12000 btus would be the best answer--the existing system only reacts to the main tstat--youll never get the climate right in the bonus room--go for it and do it once and right

[docholiday]

Bonus rooms like these have a considerably different load than the other areas and trying to overcome them with the same system, even if zoned, will leave you with at least one uncomfotable zone or room. My home is no exception. My house stays constant through out except for my office which is in a bonus room over the garage. Kneewalls, roof, over garage, West arched window and end wall. Its a huge load of nearly 8000 btuhs for 110 square feet. Currently I supliment it with a window unit but the noise requires me to shut it off everytime I use the phone. A ductless mini split would be the answer and can be shut off when I am gone for a few days each week.

[uktra]

I have been involved in a court case over this very problem. Most people won't go to court over a bonus room, but this idiot builder chose a floor plan that put the master bedroom and bath over a 3 car garage! The builder won in court because he built it to code, but he still has a pissed off client and spent a lot of lawyer money to get a pissed off client, instead of fixing the problem. The problem is too much air flow from garage below and knee wall areas. You can zone off exsisting system, or put a small or mnini-split system for this room, but chances are it still will have too much fluctuation in temperature for good comfort. The best way to improve this room is to dense pack cellulose into the garage ceiling, as well as use foam chutes and net and dense pack the rafters between the eve and knee wall rafter intersection. This will increase comfort dramatically and lower utility bills.

[RoBoTeq]

The last three posts have the situation covered. With the differences in heat gain and heat loss in that room, a conventional system will never be able to be sized proper for all circumstances.

Consider this; during a sunny day that room is getting beat up on from three sides, the ceiling and not much better on the floor. If you calculate the needs for that room to stay cool while the system properly cools the rest of the house which does not have the same heat load, what happens when the sun goes down? Yes...that room gets too much conditioning.

I have been successful in zoning bonus rooms from conventional systems but the best bet is for that room to have seperate conditioning. I too prefer a ductless minisplit for bonus room conditioning.

[dpatty]

I would agree a ductless mini slit would be ideal. When the roomis not in use you would save alot of money by setting back the stat or even just turning it off. Just hope your main area has enough load to make proper use of the current system.

[airworx]

install a zone kit and do it right.