re: Question for air conditioner guys.

Thanks for your input.

The new whirley is in an open space of the attic and has free access to about half of the house as the other two whirleys occupy the other half.

As I've had everything else troubleshot, I'm feel like it has to be the attic. The guy that checked the duct work is the one that brought up attic venting and he had no monetary interest in pointing it out as he does not do that kind of work. He said our attic was 30 degrees hotter than any attic he can remember being in.

The two, original whirleys were spinning like crazy at 9 AM the other day before the heat even started setting in so I know they needed help. Just hoping that getting some airflow to the new one will take care of it.

The only other answer will be that my foundation is built on a portal to hell that suddenly appeared 2 years ago.

A 3 1/2 ton is good for 1800 to 2100 sf. Our 2000 sf is right in the sweet spot. It may not be overpowered like I said, but it should do better than 75 in the heat of the day.

I'm not an AC but here are a couple of very basic items that I didn't see asked or mentioned.

What is the differential temp between your thermostat reading and the air coming out of the vents.

Assuming the differential is less than 20 degrees, then while the unit is running see if the air coming off the fans is warmer than normal. If so you need to have the coils cleaned on your outside unit.

You can clean the coils by turning the unit off, remove the outer housing and spray the coils down with a garden hose.

This
Unless the house is extremely well insulated 3.5 tons isnt enough.

With average insulation, average windows, and nothing out the ordinary like a raised house, etc. I'd have a five ton unit if I were you.

Your unit is undersized.

You will need to add 2 more attic fans to get your house down to 70 in this heat.

What seer is the unit?

How old is the house?

Can you add more attic insulation?

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Do you have a electric meat thermometer or regular electric thermometer? If so, put it in the register and see what the temp is of the air coming out of the register. You should be 14-16 degrees cooler than the temp outside.

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That's the way to do it, but the comparison is incorrect. Think about it. If it's 100F outside, you'd be blowing 85F air out of your register. That would suck! Instead of the outside air, you should be comparing to the temperature of the air entering the return, and that difference, called "the split", is nominally 20 degrees. It it's lower, the system could be malfunctioning, or the duct might have inadequate insulation, which is common when hanging flex from rafters in non-foamed attics, a stupid but common practice. The longer the duct run, the greater the heat gain. To eliminate duct variability, you could measure just before and after the evaporator coil, being careful not to drill into it.

I saw earlier that an AC should maintain 20F cooler than outside. Think about that. It means on a 100F day, you're at 80F inside. That would suck! Instead, what you get out of the split depends on a lot of factors such as the insulation in the house, tightness of the house, duct integrity, air changes per hour, shade trees, etc.

1) If you're getting a 20 degree split between the return and the coldest vent the unit is doing what it is supposed to do.

2) In very hot attics many techs foil tape the return door on the air handler which is a typically very poor seal from the factory, no big deal when the air handler is put in a basement in Ohio, big deal when the air handler is in a LA attic.. ANY amount of 120f-140f air that is getting sucked into the return will kill efficiency.

3) Consider foaming the underside of the attic if you can't get it figured out. Dropping the attic temp will help with every aspect of the systems operation.

4) Look carefully for return leaks in the ductwork, especially if flex. This may explain why the problem popped up 2 years ago and has persisted between two different systems. A failed piece of large flex duct on a return may be pulling in a lot of very hot attic air.

*ETA* I saw a post above that said air blew out when you cracked the door. This would be consistent with my #4 of a large return leak. The only way to get positive pressure in a residential system (assuming no design outside air makeup like commercial) is for the return to be drawing air from some other place than the conditioned space. If your home is older (built like swiss cheese) it would be very hard to generate positive pressure without a lot of outside air being brought in.

Hope that helps.

Congrats on the oversize. Humidity is probably 60% in the shoulder seasons.

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That's the way to do it, but the comparison is incorrect. Think about it. If it's 100F outside, you'd be blowing 85F air out of your register. That would suck! Instead of the outside air, you should be comparing to the temperature of the air entering the return, and that difference, called "the split", is nominally 20 degrees. It it's lower, the system could be malfunctioning, or the duct might have inadequate insulation, which is common when hanging flex from rafters in non-foamed attics, a stupid but common practice

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This is correct.