New Homeowner - Basic AC questions

J.M.

Thanks Mike. No good deed goes unpunished, so two quick follow-ups:
If I understand you correctly, since our house is at 83, we should not expect any drop in the temperature until the whole structure (walls, furniture, etc.) is cooled and the humidity is sucked out.
Second, do I need a professional to install an attic ventilation fan, or can I purchase something at a hardware store and put it in the window myself?
Thanks,
JM

J.M.

new homeowner's basic AC questions

Sorry for two more dumb homeowner questions:
(1) The first time we use our central AC system for the system, how quickly should it reduce the temperature? Then after that, how quickly should it reduce the temperature?

We just turned on our AC for the first time this season. In fact, we installed it last fall and today are using it for the first time. It's 83 degrees inside and quite humid, 94 degrees outside and 33% humidity according to websites. We've set the thermostat to 74-degrees. About how many hours should it take for the temperature to hit 74? Will efficiency improve after the first use? Or how will we know that it's time to contact the contractor to make sure it's working right?

(2) When we installed our new heating and cooling system (the AC is in the attic), we asked about an attic fan, and we were told it's not necessary. Is that so? Or should we put in a fan in one of our two attic windows to help reduce the load on the AC?

Thanks,
J M

Empire_2

Not Dumb, but very smart Q:........

> Sorry for two more dumb homeowner questions: (1)

> The first time we use our central AC system for

> the system, how quickly should it reduce the

> temperature? Then after that, how quickly should

> it reduce the temperature?

>

> We just turned on

> our AC for the first time this season. In fact,

> we installed it last fall and today are using it

> for the first time. It's 83 degrees inside and

> quite humid, 94 degrees outside and 33% humidity

> according to websites. We've set the thermostat

> to 74-degrees. About how many hours should it

> take for the temperature to hit 74? Will

> efficiency improve after the first use? Or how

> will we know that it's time to contact the

> contractor to make sure it's working

> right?

>

> (2) When we installed our new heating

> and cooling system (the AC is in the attic), we

> asked about an attic fan, and we were told it's

> not necessary. Is that so? Or should we put in

> a fan in one of our two attic windows to help

> reduce the load on the AC?

>

> Thanks, J M

Empire_2

Not Dumb, but very smart Q:........

Since you Q:, is very complex in it's nature I will try to make it simple. Let's start with Q: #2) If your out door ambient is 93*, you can bet that the attic is approaching 135 to 150*. Venting in ALWAYS a good idea even with the best duct insulation.

The basic Idea behind a/c is maintaining a set temp through out the day to control the R/H%, and as a result, the temp will drop as a result. Reading you post and seeing that you turned it on at 83*,...Well you have already let the load in the structure rise way above the designed parameters. Your walls, floors, furniture etc... are at this higher temp and will need to be cooled. Not to mention the 33%R/H which is probably higher because you can really feel it. Provided the A/C system is Of the proper Size, and in this case well insulated, you can expect a 1* drop 20 to 40 min AFTER the structure has been CONTROLLED to it desired temp.

Size is everything in all cases. Too large and the R/H% will not be taken out, but temp can drop. You would feel calmmy, ( No pun intended Clammy):-) To small and it cannot lower the sensible load in the structure. If you are concerned call the contractor in to do a follow up on the system. They should love to see You again and set your mind at ease!!!.;-) Plus maybe get a recommendation or 2.

Mike T.

J.M.

Thanks Mike. No good deed goes unpunished, so two quick follow-ups:
If I understand you correctly, since our house is at 83, we should not expect any drop in the temperature until the whole structure (walls, furniture, etc.) is cooled and the humidity is sucked out.
Second, do I need a professional to install an attic ventilation fan, or can I purchase something at a hardware store and put it in the window myself?
Thanks,
JM

Eugene Silberstein 3

Attic Fans

As Mike mentioned, your attic is going to be very warm. To simplify things, just remember that a warmer temperature substance will give up heat to a cooler substance. In addition, the rate at which this heat is transferred is related to the difference between these two temperatures.

For these two very important reasons, keeping your attic cooler will reduce the rate and the amount of heat transfer from your attic to the occupied space and the air inside the duct system.

So, as far as an attic fan goes... GO FOR IT!

And, yes, you should definitely call in a professional to install the attic fan. Make certain that it is thermostatically controlled to prevent constant operation when the outside anmbient temperature is low.

Best of luck.

The Professor

Techman

JM

Evening All! Another way of looking at it is this ,once the structure is HOT and HUMID you now have a "pulldown load " as compared to a "maintaining load" when the structure is at the desired temp. The "pulldown " is more expensive to acheive. Also around 75%of the work being done is removing humidity leaving around 25% to drop the temp ,HOURS later ,when you do reach the desired temp the ratios are reversed . Most AC systems are sized for the "maintaining load ". Good Luck!Attic fan is a must, was 82* outdoor WAS 110* in the attic.!!!!!!!!!!!!!!!!

J.M.

Thank you

Thank you all for the straightforward and comprehensive guidance.
JM

Empire_2

JM,..Follow to your follow up...

The temp will drop, but since the residual heat/R/H is still in the inside you will find that the A/C will cycle back on pretty quickly as the fore mentioned gives up its' heat load. Once it has been stabilized there shouldn't be so often of cycles.

Its like preheating the oven...They tell you that for a reason, to get your core temp where it needs to be so when you introduce that load whether it be hot or cold,... The core temp is or has been pre-?temped if you will,.Get it?

Not so scientific, but trying to make an analogy.;-)

Mike T.

Hey Geno, and Techman....

J.M.

Hi all.
Well, it appears we've got a problem with our new central air system. It's been since 2pm Mon (now 7am Tues). The outside temp has dropped to 75*, and I opened attic windows, so the attic is cooler (than the rest of the house). But our house is 81* downstairs and 85* upstairs. At 2pm when we started, it was 78* downstairs and 83* upstairs. Kind of disappointing for a new systems's first time out. Any thoughts? I'll definitely be calling the contractor. . .
JM

Empire_2

Give em a call

Lile I said before it's a follow up visit within the 1 year. Could be something easy.

Mike T.

Brad White_9

What are the discharge temperatures

at the unit AND at the grilles?

To be an effective dehumidifier and air conditioner, the discharge air temperature should be about 20 degrees or more cooler than the room target temperature; it may start out wider than that (warmer than you would like) until it catches up). What's more, the dewpoint of that supplied air wants to be lower than the space being served. If not the humidity will climb.

In an ideal world, the discharge air would be 55 degrees with a 54.5 or lower wet bulb temperature, practically saturated is the idea, but less total or specific humidity than the space target condition. You may or may not have the means to measure the dewpoint or wet bulb temperatures (A psychrometer is the tool), but an RH meter might be the best short-hand means.

Back to the measured temperatures: If you are getting say 58 degrees off of your air handler and 60 or more out of the outlets, that indicates heat gain through the ductwork to me. Ventilating the attic is one strategy but insulating the ductwork and ensuring the integrity of that insulation, is paramount. Other thoughts: If the air is barely cool it may mean that your system is not fully charged (God Forbid a leak).

A thought on whole house fans: Great to run at night to purge the space and draw in air via opened windows. Does not help the humidity issues as I am sure you know.

Best use is for a "sensible heat (temperature-based) pulldown". Once the temperature is as low as can be achieved, there is less load on the AC system so it can perform better.) Once in operation the system can complete the sensible cooling mission but also get on with the "latent heat (moisture-based) pulldown".

Good Luck

Brad

D107

attic fan versus whole house fan

Just to clarify for another homeowner:

I assumed JM was talking about an attic fan that is installed through the attic eaves in order to vent the attic of hot air. The only negative on that as I understand it, is that the negative pressure generated (if there are no soffit vents) will suck hot air into the attic from the floor below, defeating the purpose. (I know this gets into ridge vents and vented/unvented attic choices a la buildingscience.com.)

I assumed the 'whole house' fan Brad mentioned was (what we used to call an attic fan when I was a kid)is to cool the house only and not the attic. It would be installed in the top floor ceiling and draw in reasonably cool air through open windows in the evening. The attic would have a louver
to allow the positive pressure air from the fan to escape.

David

Brad White_9

That is how

I would define them, David- especially the last one I referenced in my post.

Thanks for clarifying!

D107

now that I re-read, JM spoke of a fan in the attic window

which is an alternative to the attic fan through the eaves, meant to cool the attic. I've heard of that--if you have another window opposite that can let in air from the fan pull.Some even suggest a fan in each window--one pushes, one pulls--but of course in daytime you're probably bringing in alot of hot air, but I guess you could lower the temp a few degrees. Maybe that plus a ridge and soffit vent.

Thanks,

David