ac sizing issue

[scottb022563]

Hi, I am trying to figure out my ac issue. I live in Florida (hot). I had my house remodeled and have two ac systems one for each floor. On the ground floor which is 1500 sq feet I have a 3 ton system (carrier 12 seer) and it works great. Upstairs is 998 sq ft. with 10 foot ceilings and in the master bedroom it vaults to 12 feet. Originally installed was a 1.5 ton system and when summer came it ran all day and would not get below 83 during the day. The owner agreed his manual j calculation was off and installed a new 2 ton system (carrier 13 seer with matching air handler) free of charge. Now the upstairs gets cooler but it still runs all day long, it never shuts off. We never push it below 78 degrees, sometimes we leave it at 79-80. At night it will cycle but from sun up to past sun down it still runs. I believe the company will come out and install one more new system but I am not convinced that going up just a half ton to 2.5 will solve the problem. So here is my question.

I have another company coming out Monday for a second opinion.
Since I am nowhere near cycling too fast what do you think of putting in 3 tons instead of just going up a half ton to 2.5 tons?

I am just afraid that I will get the 2.5 ton system and it still will not cycle and run all day. Thanks in advance.

[andserco]

It may not be a capacity issue, but an installation or airflow issue..

[Steve Wiggins]

Your attic heat is leaking into your home through your bathroom exhaust fans or other ceiling penetrations such as recesed can lights.

Reduce the heat load instead of buying a larger a/c unit.

[scottb022563]

ok. good points.

How can i determine if it is heat is coming through my cans or exhaust vents? How can I reduce the heat load?

How can I determine if it is an airflow issue?

I do not think it is an installation issue. These guys do multi-million dollar homes with 19i systems all the time. But who knows.

[comfortdoc]

Start with an accurate Man J load calc. When the man J is done make sure they account for the heat rising from the 1st floor. If the ductwork is in the attic make sure it is not leaking and is insulated properly.

[kallem316]

make sure your t-stat isn't on a exterior wall. the heat from your wall messes with your temp put it on a wall not holding heat.

[cchenot]

Hers the thing if you up size your unit in Florida you will have humity problems
You can take box put enough air in it to cool the box, but if your box has holes in it
Your just trying to polish a terd. The problems not the unit it’s the insulation in atic.

[scottb022563]

the house is brand new just built or remodeled from scratch. every who has worked on it has agreed there is plenty of insulation in the attic. The tstat is on an interior wall but I will check for heat.

[rickboggs]

I have another company coming out Monday for a second opinion

Smart move

[dan sw fl]

Originally posted by scottb022563
Hi, I am trying to figure out my ac issue. I live in Florida (hot). I had my house remodeled and have two ac systems one for each floor. On the ground floor which is 1500 sq feet I have a 3 ton system (carrier 12 seer) and it works great. Upstairs is 998 sq ft. with 10 foot ceilings and in the master bedroom it vaults to 12 feet.
Since I am nowhere near cycling too fast what do you think of putting in 3 tons instead of just going up a half ton to 2.5 tons?

I am just afraid that I will get the 2.5 ton system and it still will not cycle and run all day. Thanks in advance.

As others have noted in the posting,
air flow seems to be a current concern. Increasing the unit size can only increase the air flow concern.

Original design was for 1.5 ton or 600 CFM.
How is the duct work going to be upgraded
to 2.5 ton or 1,000 CFM?

What is the mfg./ model number of the two-ton air handler?
What is the supply air static pressure?
What is the return air static pressure to the coil inlet?

Static pressure reading ...
http://www.dwyer-inst.com/htdocs/air...troduction.cfm

http://www.dwyer-inst.com/htdocs/pre...es2000Spec.CFM

model 2001
with appropriate pitot tube

In addition, why do you suggest that running "all day"
(noon - ~7 pm) is an abnormal situation in FL? I believe that running continuously running for > six hours might be normal for > 70% of FL or any hot climate (SD included for a few weeks) residences.

A problem ( size and/or airflow) exists when you can not maintain 77'F and the unit runs "all day".

Request full explanation of duct work testing
from your service company.

FL (hot) ...
2006 has been somewhat milder than 2005.

[djk4502]

I disagree with some that said insulation is at fault. If a proper manual j was done then system would be sized properly for the load that exists and would cool the home. Now if he wants a lower utility bill and a smaller system by all means insulate as much as you like. But they should have done a accurate load calc. the first time and they seem to be guessing. Changing all these sizes on the system and not changing ductwork is also another issue. Good i idea on second opionion have him take external static pressure reading of current system to see exactly how many cfm your system is moving.

[scottb022563]

let me correct a couple of items.

It is now reading 80 degrees and it is set on 78 so during hot days it does not maintain temperature.

When they installed the 2ton system they opened up a return from a 7 to a 10 giving 300cfm more to the system I beleive.

the compressor is a bryant legacy 223A 2ton

the air handler is a carrier fx4c /fy4a according to the literature.

my downstairs unit runs maybe 20-30 minute an hour even during peak heat

upstairs there are 2 bedrooms and 2 bath with a landing seperating the entrance to each room.

The one bedroom has a vent and return and its bath and closet have a vent.

The landing has one vent.

The masterbath has one vent and the master bedroom has 2 vents and a return. the closet has one vent.


when they checked the systems there was about 20-21 degree drop. at the vents the air coming out is between 59-64 degrees depending on the time of the day.

Each bedroom room has 4 6inch cans, landing has 4, guest bath 2 and the master bath has 6. I measured the temp in one of the cans and it was 102 degrees. Is this a problem? How do I correct this if it is? thanks again for all the advise.

[Edited by scottb022563 on 08-13-2006 at 02:17 PM]

[scottb022563]

if you work in the pinellas county area and think you can resolve my issue. give me a call.

Scott

727.418.9756

[gtjon]

If you are satisfied with the mfr. rating at 100 deg ambiant over the outdoor unit
and
whatever they write about the capacity, and a manual J or Carrier a/c calc was done.
And the home is not losing conditioned air by an over-pressured 2nd story, as found still today....

then consider how much sun is beaming into a space where you "feel the heat".

We calc-'d a job and still had to add 1 ton to 200 sqft in both an office and a mez-pool table- desk area in a second story under a ceiling sky-light wdw.

BECAUSE OF THE LIGHTING SYSTEMS POINTING BEMS AT THE HEADS OF THE OWNERS.

Probably not your case, right?

So without the registers covered, what's blowin on the back of your hand at 2 to 3 feet away ? A breeze , or not?
Yes I got a's in Calculus in engr. too. with HVAC since 1976.
Just tell me.
a quick guide to maximum airflow would be dividing 16 into the CUBIC VOLUME footage of the space / and divide by 390 for the "net tonnage by air flow.
@ sig fig here:
8000 cuft / 16 /390 = blower "tons" @ 1.3 tons for nice-sounding blown air...
x 1.4 max range (I use) = 1.8 tons !! net
What's that "net" capacity with out tossing in deHumd/ for that 1.5 ton? say on a 105 deg day ?

Here at a 2 Ton, you may want to see at least 700 cfm movin and then 6 supplies blowing very well! [to 8]

We now add to 20" of cellulose in attics (like 26" of fiberglass in comparrison by operation tracking...ventilated attics!
that going beyond 4 air changes per hour is getting too commercial.

What is the air movement from the basement thruogh to the 2nd floor?
What's leaking?
Or get an analysis of the performance of that unit NET wet and dry bulb and see if it is measured within 80% [your not in a lab]. -= At above considerations. IS YOUR RETURN AIR ~ 7 FEET HIGH? READ THE AIR INTO IT AND GO TO THE FAN-COIL COOL SECTION AND RED WHAT IS JUST IN FRONT OF THE COOLING COIL. I HAVE SEEN SYSTEMS DRINKING IN AMBIENT AIR FROM TERRIBLE RETURN DUCTING.

[cchenot]

check your duct gain if your air flow is 59 deg at the suply on the unit it should be with in 1 to 2 deg at your grills. your ducts might need o be sealed if its higher.
just because its a new house dont expect the builder to put in any extra insulation in the atic.

[dash]

Originally posted by scottb022563
if you work in the pinellas county area and think you can resolve my issue. give me a call.

Scott

727.418.9756

We work in North Pinellas ,but my guess here may be all you need to resolve this.

If there are floor trusses as opposed to floor joists ,under the second floor,that could be the problem.

Floor trusses have some open space,between the top and bottom chord of the truss.

With the upstairs being smaller then the downstairs,that means the open space of the trusses,is likely exposed to the attic ,of the single story portion of the home.

This can allow hot attic air to circulate,thru the trusses,beneath the floor of the second story.

Often the insulation is installed at the ceiling of the first floor,so hot air from the attic of the first floor ,only effects the floor of the second floor,not the ceiling of the first floor.

We have found this on many jobs ,in your area.The load calculation likely assumes the floor is over conditioned space ,not hot air from the attic.

Check the temperature of the floor ,in the heat of the day,if it's warm,check in the attic of the first floor to see how it's insulated.

If trusses are open to the attic,have the open area insulated with R-19 batts and it's a good idea to add Hi-R foam board over that for a good "air seal".

Also check to see that the walls of the second floor are insulated,where the back side of them is exposed to the first floor attic,these are called "kneewalls",and sometimes the insulation falls down,if not secured on the attic side.


Good Luck!

[t3]

dan sw fl wrote:
>FL (hot) ...2006 has been somewhat milder than 2005.

you're not kidding as we must have the mildest summer in 20 years so far, no storms and outside temps mostly high 80's south of miami. when it rains, temps drop to the 70's

my FPL bill has dropped by about $50 each month, compared to same time last year despite rate increases and I keep house cold

[rickboggs]

you're not kidding as we must have the mildest summer in 20 years so far

Global Warming