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Byron Trantham
08-10-2007, 1:57 PM
I just had a new AC unit (uses Peron) installed. The old one was 17 years old ready for retirement. Worked great until the day before yesterday and yesterday. The AC just couldn't keep up with the heat. Here is the situation. The coil is installed in the attic. The compressor is out side (of course). This is a dual zone house - two AC units, one for upstairs and one for downstairs and the basement. The temperature was about 104 degrees outside. The unit would not get the upstairs any colder than 75 degrees. Today its about 88 degrees and the temp upstairs is 71 degrees - perfect. My questions is, is it reasonable to expect the unit to cool the house as well in the this extreme heat as it does on cooler days? We have a ridge vent, no fans in the attic. Would a fan solve this problem? I just talked with the tech who installed the unit and he was reluctant to recommend a fan for fear I would hold him to the results. I wouldn't. This is a crap shoot. Just thought that someone out there had enough experience with AC units to know what is reasonable to expect from them when its really hot.

Joe Pelonio
08-10-2007, 2:29 PM
I'm no guru, but I was cooling a 1,900 SF shop with 12,000 BTU heat pump and it was fine up to 100 which is the most it ever got there. A/C units cool better than heat pumps normally.

How many SF is that part of the house, and how many BTUs or tons is the
new unit? It's 12,000 BTU's to the ton.

If you have 2,500SF then you'd need a minimum of 20,000 btu's. You might have them test that there or no refrigerant leaks from the installation.

Lee Schierer
08-10-2007, 2:31 PM
The heat load on the house is going to be greater on hotter days. Whatever you can do to lessen that should help.

If you don't have adequate insulation in teh attic area you can be heating the house through the ceilings. You should have R-22 in the horizontal areas of the attic. Also insulating the duct work if it is in the attic will help. Many A/C installers put minimal insulation on the duct work. This insulation will also help you in the winter when you are heating.

Inadequate air flow in the house will limit the cooling ability. Floor mounted registers may not adequately cool the upper parts of rooms as heat rises and cool air sinks. Ceiling fans will mix the air within a room, but also add a small heat load from the motor.

Planting trees so the house is shaded, ventilating the attic, changing to a lighter color shingle and installing thermally efficient windows and doors will all reduce the heat load on the house.

Just a note on the attic ventilation. Whether or not you decide to install a fan, make sure there is a place low on the roof for air to get in that is equal or greater in square inches to the area where air will exit near the high spots. If you install a fan you will need even more inlet area so as to not starve the fan.

Jim O'Dell
08-10-2007, 3:53 PM
Don't know if this still holds true or not, but during the heatwave we had back in the 80's of 100+ days over 100, we were told that a properly working a/c could only get the inside temp about 20 degrees cooler than outside. More effecient units today may do better. Of course, they may have told us that because we were sweltering inside a stereo shop! Of course, all that equipment running didn't help any! Jim.

Al Willits
08-10-2007, 4:35 PM
Here in Minn you size an AC unit to run continually at a certain temp, but its a battle between cooling and moisture removal.
To much unit, gives cool wet air, not good, not enough unit, leaves dry warm/hot air, not good either, so if your system was installed in that vein, it would be nice to know what that continuous run temp is, anything above that and the unit will not keep up.

Make sure all the filters are clean and the condensing unit get washed a couple times at least a season, both are big causes of poor performance

Al

Dennis Peacock
08-10-2007, 5:25 PM
Around here, the rule of thumb is 600 SF per required ton of A/C for cooling. A 2,500 SF house would require at least a 4 ton unit. At least that's the way it's been done around here for as long as I can remember. The hotter it is outside, the harder the unit has to work. Keeping the outside unit in a shaded area will also help reduce the workload on the A/C unit.

Jeffrey Makiel
08-10-2007, 6:54 PM
I'm no A/C expert. However, I'd like to bloviate...

Rule #1: Never size an A/C unit based on square foot. The tech should have done a 'manual J' calculation on your whole house.

Rule #2: Never oversize an unit (as Al stated) or dehumidification, which is absolutely required in VA, is reduced...plus the house becomes drafty. It also costs more to install.

However, if your unit maintained 75 degrees on a 104 degree day, it's probably sized correctly and operating correctly. A 104 degree day is simply overwhelming and not what a normal A/C unit is designed to in VA. Also, 71 degree thermo setting is getting on the chilly side of the norm especially in these outdoor conditions.

An attic fan is excellent to rid your attic of heat and help extend the life of your roof. Ridge vents are fine, but your eaves should also be vented as well as your gable wall ends. An attic fan will reduce your attic from 120 degrees to 105 degrees without much energy usage. It may not save you money, but it will help maintain the comfort level in your home.

My recommendation is to add attic ventilation. Also, most codes now require attic insulation value to be over R of 30. One layer of insulation between the ceiling joists, and one 6" batten layer above that running perpendicular to the joist will give you an R of 38. If your duct work is in the attic, 1" (minimum) duct insulation is required. But I'd recommend 2"...and make sure there are no leaks (the joints should have pressure sensitive foil duct tape or joint mastic if sheetmetal ducting was used).

I also recommend that you be patient with your A/C unit when more normal summer temps arrive.

cheers, Jeff

Al Willits
08-10-2007, 10:14 PM
"""""""""""
Rule #1: Never size an A/C unit based on square foot. The tech should have done a 'manual J' calculation on your whole house.

However, if your unit maintained 75 degrees on a 104 degree day, it's probably sized correctly and operating correctly.
""""""""""""""

The 600 sq foot thing is what techs will usually use to guestamate a system for service, the sales guys normally use the heat loss program.
It is not used as a measurement to install a system, shouldn't be anyway.

It is/can be used a lot in outbuildings like a garage where their use is sporadic or not so critial sometimes, and especially if the homeowner is off to by a window unit for his shop/garage, as the heat loss program can be a bit hard to find for some.

75 at 104 might be a bit high, least for Minn, most units would be losing by then.
May depend on where you live too.

Al

David G Baker
08-10-2007, 11:23 PM
Byron,
I know nothing about air conditioners or the required sizes but having lived in California, I do know about attic fans and insulation. Get an attic fan and make sure your attic is well ventilated and insulated. A well ventilated attic and a good attic fan can lower the attic temperature between 10-15 degrees. Good insulation is critical as well.
A whole house fan is something to think about as well but may be overkill.

Greg Crawford
08-11-2007, 1:57 AM
Remember, the AC unit is trying to get rid of excess heat in the coils outdoors. If it's over 100 degrees, it's hard to get rid of much heat. Is your unit in the shade? This can make a difference on the temp of the coils. You can water the coils down, but if it's done too much, you get mineral deposits and corrosion.

For an attic fan, if it doesn't get enough air from vents in the attic, it will pull inside air through plumbing stack penetrations, HVAC duct penetraions, etc. That means that the fresh, hot outside air is coming into the house. Ridge vents with good soffit vents are supposed to be the best attic ventilation.

I had a low-e film put on my windows and it made a huge difference. I also installed two extra return air ducts for my downstairs unit. The original was 12x36, but routed through a joist cavity that was about 12x12. The unit was starving for air. That also made a huge difference. At least now the house stays about the same temp throughout.

HVAC is a lot more than just an AC and heating unit. There are many factors, and finding someone that actually knows their stuff is hard to find, but they're out there. You just have to do a lot of interviews. After all, you're hiring them.

Now, if I could just keep my shop below 80....

David G Baker
08-11-2007, 10:14 AM
Another thing that will make a difference is Solar Screens on windows. I haven't found any for sale retail in Michigan but I had a roll that I brought with me from California and put it to good use on the East and West side of my house.

James Rambo
08-11-2007, 11:17 AM
I'm not an ac guy (electrician) but, it is my understanding here in florida (maybe everywhere) most of the ac guy say it is standard thinking to expect a 25 degree difference from outside to inside temps. Any larger difference is extra. So, 80 degrees outside possible 65 inside. 105 outside 80 inside etc... . But just what I hear. Also a few years ago the county was insisting on if the ac was in the attic it had to be in an a/c'ed area in the attic. Most of the time it was mounted to the ceiling in the garage.