Bonus Room AC problem

[rotts4u]

First of all I want to say thanks for this site. I have been searching and reading all morning on various topics but have not yet found my specific problem so here goes.
First I live in Chattanooga and recent temps are hovering around 95-98 during the heat of the summer.My house was built in 1997 and was built to the local power companys ENERGY RIGHT program. Low E windows, house wrap, insulation between each floor etc.

I have a 2000 sq foot basement (un heated but dehumidified and since it is a lot under ground it stays 65F year around and I keep humidity at 50% to keep tools from rusting etc.

The main floor of my house is 2100 feet and runs off a bryant 3 ton Heat pump split unit. It keep the house perfectly cool and all the duct work is in the basement so of course it is cool too. Discharge air at any vent in the house is 49-51F and my total all electric power bill runs under $200 year around.

Upstairs I have a bonus room(S) of about 700 feet total. The roof rafters are 10" and they have R30 in them with the foam inserts for venting, I have a full ridge vent and full vented soffit all the way around the house. I have two gables and one contains a power fan of 1620CFM. The 10-12 pitch roof faces full west exposure and the BLACK shingles result in attic temps of 125F in peak times even with all the soffits open etc.

The unfinished part of the attic has loads of blown insulation and none is blocking the soffits. And the main problem as you may have guessed is the bonus area stays HOT. In the peak I see 76-80 degrees from the 1.5 ton 12 seer Bryant Heat pump split unit. Basically the same unit type as the main floor except smaller in size.

IT has frozen up once or twice over the last 8-9 years but it is rare. I did check the discharge vents and noted that the airflow from them on my annometer is quite a bit higher than the main floor. 550 ft/min vs 350 ft/min downstairs. Could that indicate insufficient ducting?

The freon lines have a long run to get upstairs because they run up through a chase in the basement. Also the air handler is in the attic so I have been spending the last few years double wrapping the flexi return duct with another larger flexi duct as well as wraping the main trunk (hard line) to keep what btus I have in the ducts and not in the attic.

I have had the local power company out to evaluate insulation and the units output. They did some wetbulb tests and confirmed the unit is putting out about 16-18kbtu. Heating in the winter is never a problem at all.

I have been checking air in vs air out temps at the air handler and the farthest vent and I see the air is still picking up some heat in the attic but due to the shape and layout of the attic all the ducts have to run on two sides of the attic so at least one of the flexi runs is long and a requirement due to space restrictions and layout.

At night after the attic cools off in the attic the unit does low 50sF discharge. But in the day the farthest output duct is only 68F into a room that is 78.

I have spent days and weeks sealing up duct leaks, over insulating everything, venting the attic etc.

I even added a portable AC unit of 10K BTUs and while it helps it is still not enough (it does have outside water drain and air duct to exhaust air.

At this point I want to install a new unit but I assume I need something like 2.5-3 tons. I have called three different places and none do real load calcs. They just say " 1.5 ton should be enough for 700 square feet. But yet in my SE USA location nearly every bonus room I go into is WARM in the 95 degree days. I also assume my current freon lines and air handler are not big enough nor will the main air trunk. So I am basically having to start over.

I need to get the room down to 72 min for a new home theater/office so I am trying to get some tips on what to do. Basically I am going to just hire someone and tell them what I want as they dont seem to have any qualified people around here to properly size it. Or at least I cant find them if so and I have two close relatives in the HVAC business and this includes them.

And humidity is pretty good up there with it staying in the 50% range =,- 5%.



Anyone have any tips or sizing questions?


Thanks in advance for your help


Don


[Edited by rotts4u on 07-03-2006 at 09:57 AM]

[re2ell]

rotts, since no one is willing to run a load calc for you, be safe and do it yourself. sizing by square footage is hideous, it simply does not work. you appearently have done a lot of evaluating, the small investment of load calc will save you much grief, and since the design conditions are changing, load calc is almost the only way to go.

[llskywalker]

You have done all kinds of things to the house and duct systems it sounds like. Go to the home page of this site and on the HVAC Calc button at the top to the right and pay the 49.00 to perform the load calc yourself.

[brent]

Does your main house unit run continuously during hot weather? Do you have high return grilles for your main unit? Maybe if your main unit could draw some more hot air before it drifts up to your bonus room you could improve things. How many and what size flex runs do you have on your upstairs unit?
Post a picture.

Brent

[psychometric]

You'll have a hard time with a 125* attic. Soffits are open, ok, where is that air going. Please don't say ridge vent...you need a power vent. 115 volt, 2000 cfm job to cool that attic. That may not fix your issue, too many other variables, but you need a power vent.

[rotts4u]

Originally posted by brent
Does your main house unit run continuously during hot weather? Do you have high return grilles for your main unit? Maybe if your main unit could draw some more hot air before it drifts up to your bonus room you could improve things. How many and what size flex runs do you have on your upstairs unit?
Post a picture.

Brent

No the main floor unit does not run all day like the bonus room. It cycles on and off and seems to keep the 2100 feet (9-10 foot celings too) cool no problem.

The main floor and upstairs dont share any ducts or returns because the main floor ducts and return pipes are all in the basement two floors down from the bonus room.

The upstairs unit has a 12" fleix trunk with a 16" around it for insulation. There are 6 outlets ( all 6" flex lines that come off the main hard trunk) to handle the 600 cfm fan output into the bonus room. And the return air grill is 20-25" filter size.

[rotts4u]

Originally posted by psychometric
You'll have a hard time with a 125* attic. Soffits are open, ok, where is that air going. Please don't say ridge vent...you need a power vent. 115 volt, 2000 cfm job to cool that attic. That may not fix your issue, too many other variables, but you need a power vent.

I already have one power fan in the attic of 1620 cfm and to be honest the temp inside the unfinished attic which is not that big with a 700 foot room and the angled roof line etc. Anyway the temps didnt really go down with or without the fan running. It is mounted in one of the two gable vents of approx 15" diameter. It is completeld sealed around the fan shroud with great stuff foam like the manufacturer called for and I have tried leaving the other gable vent open and closing it which they claimed would force the air in through the soffits instead of the other gable vent and just coming in and back out without cooling the attic. Neither way makes a huge difference.



The amount of sun makes more difference than anything that I can find due to the black roof and the west face of the roof line.

[brent]

Do you have high return grilles for your main unit?
Does the bonus room have a door that you can close or is it open to the rest of the house. I think the hot air is drifting up to the bonus room and putting a lot more load on it.

Brent

[docholiday]

Bonus rooms are difficult at best to properly size. The ehaust fan is only complicating matters. Odds are the unit is simply not large enough. 700 sq feet of bonus room can be a fairly hefty load. My guess is they used the rest of the home as a guidline to size that. You may find that you need as much as 3 tons (which means all new ductwork).

Many times the knee walls are considered "unconditioned" space when in fact they are much worse with temps as high has 140 on the other side. Insulated Roof rafters are fine too but do not have the same insulating ability as an actual full attic. Depending on whats under the room, a garage or something can be pretty warm all summer too, unlike a "unconditioned space" such as a basement.

My office is in a bonus room and while it's only 120 square feet, it takes almost a ton to keep it cool on the hottest days. (of course mine is not insulated as well as yours is).

[Gunnery Sergeant Hartman]

Supply ductwork is undersized for a 1 1/2 ton system.

[cn]

[QUOTE]Originally posted by rotts4u
[B]First of all I want to say thanks for this site. I have been searching and reading all morning on various topics but have not yet found my specific problem so here goes.
First I live in Chattanooga and recent temps are hovering around 95-98 during the heat of the summer.My house was built in 1997 and was built to the local power companys ENERGY RIGHT program. Low E windows, house wrap, insulation between each floor etc.

[Edited by cn on 07-07-2006 at 03:32 PM]

[biscuitnt]

I think docholiday gave you some good insight. Bonus rooms are a proverbial "b**ch". You have a heavy load from the roof line, and the knee walls as mentioned.

I would be willing to help you out with a load calc. if you have a plan that you can send via email.

If you do this, please also send the elevations and orientation of the structure.

Shooting from the hip, I bet you will need at least two tons for the climate in TN.

Regards,

[dash]

We have found many second floor bonus rooms ,with floor "trusses",where they installed the insulation at the ceiling of the first floor .This allowed hot attic air to circulate above the insulation ,and below the bonus room floor,warming the floor to the point the system could not cool the room.

If yours is like that ,the attic fan, is likely making it worse.




Find a lot where the "knee wall" insulation has fallen down as well.

[Sean Burke]

Welcome, Don.

I was wondering if you'd turn up here.

[rotts4u]

I never posted the final outcome but I ended up having a 3 ton Carrier unit with Puron installed. I dont have the exact model but it is a variable speed fan and 2 speed compressor with the Infinity theromstat and now everything is great. All summer long it had no problem getting the temps as low as I wanted and even though the unit is bigger than the old one the power bill is the same and the room is 6-7 degrees cooler too. I guess because the 15 SEER vs the old 10.

They did install new duct work with higher R value flex lines too which helped as well.

Thanks everyone for all the tips. I like the new super quite unit so much I am thinking about upgrading my main unit (12 years old) to the same model and size.

[dash]

I never posted the final outcome but I ended up having a 3 ton Carrier unit with Puron installed. I dont have the exact model but it is a variable speed fan and 2 speed compressor with the Infinity theromstat and now everything is great. All summer long it had no problem getting the temps as low as I wanted and even though the unit is bigger than the old one the power bill is the same and the room is 6-7 degrees cooler too. I guess because the 15 SEER vs the old 10.

They did install new duct work with higher R value flex lines too which helped as well.

Thanks everyone for all the tips. I like the new super quite unit so much I am thinking about upgrading my main unit (12 years old) to the same model and size.

Two speed Carrier ,nice equipment.Upgrade the other system and watch the powe bill drop!

[rotts4u]

I know this is an old thread but while I am here I looking for server room advice for work I thought I would update this as well. After the new unit was installed in the bonus room and temps dropped as they should have I started to notice a bit of mold forming on the walls around a few electrical outlets, switchplates and the canister lights in the ceiling.

So I removed all the covers and carefully sealed them up with caulk or greatstuff foam. It seems that a small amount of warm,moist attic air was seeping in around these openings and as the moist air hit the cold room air the moisture caused mold to form. Now that I have sealed this up no more mold, temps are great and all is well.

And as Dash stated above I have also upgraded the main floor HVAC heatpump as well to a newer more efficient unit. It was an old 3 ton split with 10-12 seer but lately the coil which is in the basement had started showing sings of age and allowing water to drip on the basement floor. So I knew it was leaking. A recharge would fix it for a few months then it started again. A new coil was going to cost too much for me to spend on a 14 year old unit so I put in a newer one including a new compressor and air handler.

It is a 2 stage compressor 16 seer with a fully programmable thermostat and it has been in approx 9 months. Thus far in that 9 months my power bill has been consistently 20-40$ lower per month than last years same month. So between the higher SEER unit and the programmable thermostat it is making a real difference.

The installer for the main floor unit also insisted on installing a second return vent approx 25' away from the exiting hallway return that was there already. It was only a 20-25" grill which he said was too small for the 3.5 ton air handler he installed. I am not sure why he used the mismatched units with a 3 ton heatpump and 3.5 ton rated air handler but that was his suggestion.

Now I have second return grill that is 12x16 in the den and I have already noticed that the return duct work doesnt make a noise like it use to when it came on. It used to sound as if the sheetmetal trunk was going to cave in sorta. The old air handler was rated for 3.5 ton capacity also.

Anyway power bill is down. air flow seems improved and the trunk is not gonna cave in everytime the ac comes on.

Thanks for all the advice for the house. Now if I can cool off our 25 square foot IT server closet at work our AC problems are over for a while.