House cool down question

[stryker221982]

I recently had a new central air unit and furnace installed today is the first day when used the new AC system. it got to be 88*F outside and I had the thermostat set to 72 with a 2* swing so at 74 it kicks on.

Question is should it take 55 minutes to cool it down to 72 every time the system kicks on. We have an R50 in the attic and we recently had the whole exterior walls filled with retrofoam insulation. all new energy efficient from champion.

55 minutes to cool 2* just sounds like a long time.

[vstech]

if you turned it on for the first time, and it took 55 minutes, then the house has humidity that needs to be removed. humidity is heat that is in the home, that cannot be measured with a simple thermometer.
if you leave the system set to the temp you want it, or say 4 degrees warmer to save energy when you are not there, the humidity load will be lower, and it'll achieve setpoint faster.

[btuhack]

Hmmm,

Equipment capacity is rated with a 80* indoor temp, lower it to 72 and figure a conservative 10% infiltration/duct leakage and a 5 ton system is now producing 4(+\-). Double the duct loss and 5 tons might only handle 2 tons of work.

Talk to your contractor, or get an independent 2nd opinion. Thermal image of the walls will tell you what kind of job the insulators did.

[Rcb2875]

To add to what vstech said, you have to remember that you are also cooling the ducts, walls, and every surface in the home from a given temp. Your home should maintain about a 50% relative humidity with the air conditioner maintaining a set temp. (if sized properly)
With high humidity cooling capacity is lost while dehumidifying the home so it will naturally take longer from high temps and humidity. It also makes a difference in the return placement/'s and how efficiently the system can cycle the air through the home.

[motoguy128]

Where do you live. 88F is above design conditiosn in some parts of the country. In San Francisco or Seattle, a properly sized system should run almost constantly to maintain 72F at 88F outdoor temperatuers.

[stryker221982]

I live in Michigan, going back to what vstech said this wasnt the first time it was turned on this was in between cycles, it would cool down to 72 wait 20 to 25 minutes and it would kick back on and run for 55 min get back down to 72 wait another 20 to 25 min and start all over. will the blower fan speed in the furnace have any factor on cooling?

[vstech]

ok, lets expound on the issues. how large is your house?
long run times are designed into some a/c systems to aid in comfort from humidity, and 72F is quite a bit below the standard designed systems normal set temp. your heat gain calculator may have only designed it to spec, which is 78F... 72 is going to be a larger temp difference to outside temp, so heat will be entering the house faster, and this may not have been calculated into the design of the system...

[motoguy128]

I live in Michigan, going back to what vstech said this wasnt the first time it was turned on this was in between cycles, it would cool down to 72 wait 20 to 25 minutes and it would kick back on and run for 55 min get back down to 72 wait another 20 to 25 min and start all over. will the blower fan speed in the furnace have any factor on cooling?

Even in SE Michigan (warmest part) the design temp is only Ithink 90F. SO you're system sounds abotu normal if you have a huge 2F swing in the thermostat. I'm not sure why it's such a large swing. Actually if anything it's still oversized just a little, but the next size is probably too small.

When your near design conditions, the unit shold take a very long time to drop the temperature.

Other things to consider. IF you're short on airflow (too small of ductowrk or too restrictive or dirty air filter) you'll lose capacity.
If the outdoor condenser is dirty or has lots of bent fins, you'll lose capacity.


I"m having a new system put in my home soon. It's sized correctly and I expect it at 95F on a sunny afternoon to run continously to mainting 75F with no ability to reduce the temperature further. At higher temepratures, it will fall behind.

IF I has a larger unit installed, it would cycle on and off and in more mild temepratures which represents 90% of the actualy cooling seaso, especially for a system that serves bedrooms where you want ot be comfortable at night, it would cycle even more. I have ot run my current system 2-4F cooler to be comfortable because it's limited in it's ability to drop the humidity in low load conditions.

[skippedover]

The missing link in the explanations here is that the unit is rated on Btu's PER HOUR. If the home has a total heat content that is less than the AC Btu/hr number, then cooling to a lower set-point is achievable. The difference between the heat content of the home and the desired set-point will determine how long the system needs to operate to reach set-point. The more moisture (humidity) in the home, which is part of the total heat content, also plays in the actual temperature factor. When the heat content is exactly equal to the system capacity, it runs non-stop to maintain temperature set-point but has no excess capacity to lower the temperature. And when the heat content exceeds system capacity, the indoor temperature goes up, not down and set-point becomes irrelevant. Trying to determine correct or incorrect operation with a watch is like trying to measure the temperature of water with a string. There's just no real correlation.

[chucko615]

55 minutes to cool 2* just sounds like a long time.


With a name like stryker it sounds like you might be in the Kalamazoo area. 2 degrees in 55 minutes isn't too bad, the system needs to run to dehumidify your home. I don't understand why you have such a big temperature swing? I always size my systems for a 95 degree day.

[motoguy128]

With a name like stryker it sounds like you might be in the Kalamazoo area. 2 degrees in 55 minutes isn't too bad, the system needs to run to dehumidify your home. I don't understand why you have such a big temperature swing? I always size my systems for a 95 degree day.


Even that is a bit conservative. 2-1/2% deisgn is 88F for Grand Rapids. Where I am in Iowa it's only 91F.

[chucko615]

I'm not disputing design temps and yes I know it's 88F for here. But I like to add a little cushion on the systems I install.

[jpsmith1cm]

LooseOnTheLead

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

You can find the rules for posting and qualifications here (http://hvac-talk.com/vbb/showthread.php?t=957002).

Additional infractions may result in loss of posting privileges.

Your post has been deleted.

[beenthere]

I recently had a new central air unit and furnace installed today is the first day when used the new AC system. it got to be 88*F outside and I had the thermostat set to 72 with a 2* swing so at 74 it kicks on.

Question is should it take 55 minutes to cool it down to 72 every time the system kicks on. We have an R50 in the attic and we recently had the whole exterior walls filled with retrofoam insulation. all new energy efficient from champion.

55 minutes to cool 2* just sounds like a long time.

Lets see. 55 minutes to cool 2 degrees. So it would take 22.5 minutes to cool 1 degree. And 11-1/4 minutes to cool 1/2 degree. Which would put your cycle rate at about 3 cycles per hour if you had a Honeywell thermostat. So run time sounds about right for your thermostat having a 2 degree temp swing.