AC blows cool air but house stays hot

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Old 08-08-12, 07:01 PM
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AC blows cool air but house stays hot

Just bought a house. The AC blower is located in the attic. Every room has a register on the ceiling. In my first day in the house, woke up, temp was 81 on the thermostat. Turned on the AC. It stayed on all day and barely got the house under 79.

Called the AC guy, he cleaned the coils outside, added some refirgerant and said it should be okay. well it wasnt. The AC is on all day but it cannot cool the house off. I called him back. He tested everything and said the unit is taking in 82 degree air and cooling it down to 66 so "everything is doing what it should be doing"

I feel the air coming out of the vents. They are cool, but not cold. The main issue is the airflow is very little. I am used to other houses where the air comes slamming out of the vents. The AC guy says "that is all the system can put out" and leaves. I went into the attic, although I am not sure what I am looking for. He says the attic is hot so that could explain the problem, but im not sure.. what else could be wrong?

He says there is only one intake register so maybe the "system is not getting enough air".. but i doubt that, the old owners said they never had any problems cooling the house and the system has been in for years.
 
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Old 08-08-12, 08:37 PM
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Sounds like you need a new company. You said he says it could be not enoght return. That's BS. It's our (his) job to find out if that's the case not guess. Find another company. Oh all attics are hot yours is no different.
 
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Old 08-09-12, 07:36 AM
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Just bought a house. The AC blower is located in the attic. Every room has a register on the ceiling. In my first day in the house, woke up, temp was 81 on the thermostat. Turned on the AC. It stayed on all day and barely got the house under 79.

Called the AC guy, he cleaned the coils outside, added some refrigerant and said it should be okay; well it wasn't.
The AC is on all day but it cannot cool the house off. I called him back. He tested everything and said the unit is taking in 82 degree air and cooling it down to 66 so "everything is doing what it should be doing"

I feel the air coming out of the vents. They are cool, but not cold. The main issue is the airflow is very little. I am used to other houses where the air comes slamming out of the vents. The AC guy says "that is all the system can put out" and leaves. I went into the attic, although I am not sure what I am looking for. He says the attic is hot so that could explain the problem, but im not sure.. what else could be wrong?

He says there is only one intake register so maybe the "system is not getting enough air".. but i doubt that, the old owners said they never had any problems cooling the house and the system has been in for years.
He tested everything and said the unit is taking in 82 degree air and cooling it down to 66 so "everything is doing what it should be doing"
Well, at 50% relative humidity with very low airflow the indoor temp-split should be even higher than 22-F; additionally, a system will not cool a home with too low an airflow.

It is also charged to that low airflow level &, would be undercharged at a normal 400-CFM per/ton of cooling.

Every system with a hint of low airflow should have an airflow CFM check through the indoor coil, as well as, to each room; it is a shame that does not happen in the vast majority of cases where airflow is a major problem.
 
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Old 08-13-12, 06:40 PM
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I have a similar situation but I already replaced my system with what is probably over sized for the duct work. It's a 2.5 ton a/c system, with fiberglass duct board/flex duct work. The trunk is 16 x 10 duct board. Can I continue to use this trunk if I branch out enough?
 
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Old 08-13-12, 08:06 PM
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I have a similar situation but I already replaced my system with what is probably over sized for the duct work. It's a 2.5 ton a/c system, with fiberglass duct board/flex duct work. The trunk is 16 x 10 duct board. Can I continue to use this trunk if I branch out enough?
Perhaps, but velocities will be somewhat higher than we like...

How well it will work depends a lot on the sizing of the rest of the duct system including supply diffusers & return air grilles & filter grilles etc.
 
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Old 08-15-12, 07:01 PM
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The air handler may need to be moved slightly. Can the copper lines be added onto or should they be one piece only?

Thank you
 
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Old 08-16-12, 06:55 AM
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The air handler may need to be moved slightly. Can the copper lines be added onto or should they be one piece only? Thank you
Yes, copper lines can be added onto.

The best way is to swage the end of the tubing so the other end slides into it; that way you only have one joint to braze verses two with a fitting.

Swaging 3/4" tubing provides a 3/4" overlap; 3/8" a 3/8" overlap, etc.
 
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Old 08-16-12, 02:22 PM
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The temp split should increase once the house gets dehumidified. High humidity reduces the split.

You shouldn't be shutting it off at night.
 
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Old 08-16-12, 10:29 PM
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If it's ok, I'm going to use this thread since it's what is happening with my house.


Years ago not long after purchase of house, I thought I needed a new central a/c system. I did some research but obviously not enough. I had the existing unit looked at but the only thing done was add refrigerant and that did not solve the problem of not being able to cool the house. I bought a 2.5 ton unit with the thought of possibly adding conditioned space by finishing off the garage. I purchased separately and hired an a/c guy to hook it up from a referral. At that time, he was surprised I was able to buy the equipment at all without getting it from an a/c contractor.


The new system did work better, but at the hottest times it still seemed to struggle getting it under 80. This made and still makes no sense to me considering the size of the house(or shack as I like to call it)


Built in 1971, concrete block ranch with frame gable roof, low pitch, small attic; 2 bed 1.5 bath with about 1200 sq. ft but really only about 1000 are conditioned now. This is in west central Florida, Tampa Bay area. Other houses in the neighborhood seem to experience similar results.


I've recently come to realize, I probably purchased the wrong size system for the existing ductwork. As stated, the trunk is 10 x16 outside dimension of 1 inch duct board. It runs about 35 – 40 ft. with the last 10 ft. or so reduced to about 10 x 10. The entire system was the 1 inch duct board except for flex in the return which I'm not sure was original.(was flex used back then?) There are 6 branches, though three of them are the fabricated boots for the supply grills/dampers that are directly attached to the trunk with a rectangular hole cut through both.



Bedroom 18' x 11' including closets. Supply grill 13.5” x 7.5”. Hole in trunk: 6.5” x 8”; hole expandable.

Bedroom 11.5' x 13' including closets. Grill 11.5 x 7.5. Hole in trunk: 6 x 6.5; hole expandable.



Living rm/dining area 14' x 23'. Two grills: 11.5 x 11.5 hole in trunk now 10.75 x 8.5(was about 7 x 7). Second grill(9.5 x 5.5) comes off of 12 x 7.5 outside dimension branch, hole 7 x 3.5(expandable). Downline of same branch(reduced for shed roof) to back room 9' x13'. Grill 11.25 x 7.5; end of branch hole 2 x 10.


Kitchen 8' x 18'. Currently no grill as I removed branch to reroute return side, but will size to 6 x 10 boot. Replaced with 6 x 10 boot with 6” flex.
Bathroom 5' x 8'. Grill 3.5 x 7.5. Duct board branch hole 3.5 x 4 with stack-like boot. Other half bath not in conditioned space(yet).


Return air filter grill 18 x 24 was in wall, now moved to ceiling. The connection to the duct board at the top of the wall was leaking and no telling about the wall cavity itself; the duct board part of return air was also questionable. I'm running 14” flex from this filter grill and need to figure where to add more and add to an additional room off the back room which is a converted screen room with no conditioning. This also has the shed roof limitations; 2 x 6 rafters.


I was thinking I should add returns to each bedroom but not sure on size. With the limitations of the shed roof on the additional room the only thing I know of is some small flex(6”?) to a supply and return grill, but also not sure on size. The room is 9' x 11'.


Eventually there is the idea of conditioning the @ 400 sq. ft. of garage space. Would another 14” flex be too much after adding bedrooms etc.?



The system is disconnected as I use the return air grill hole for additional access, duct sealing etc. Once I place the main return air, I will not have access to the back room in the attic.


Any help is appreciated. Thank you.
 
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Old 08-17-12, 07:01 AM
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Return air filter grill 18 x 24 was in wall, now moved to ceiling. The connection to the duct board at the top of the wall was leaking and no telling about the wall cavity itself; the duct board part of return air was also questionable. I'm running 14” flex from this filter grill and need to figure where to add more and add to an additional room off the back room which is a converted screen room with no conditioning. This also has the shed roof limitations; 2 x 6 rafters.
The Return Air filter grille is not a large enough area for 2.5-Ton, a 1000-CFM of airflow.

The velocity through a cheap throw-away filter should initially be 300-fpm.
A 24x22 will only flow 762-cfm @300-fpm velocity; so you need another large filter grille in the conditioned area with a deep box so there is no restriction there.

An 18x24 is 434-sq.ins / 144 is 3-sf * .69% free-area (Ak) is 2.07 (1000 / 2.07 is 483-fpm, or nearly 500-fpm which is the limit velocity for that filter before debris blows through it. The filter area is actually about 17x23 or, only 391-sq.ins; which is 533-fpm velocity above the velocity required to change the throw-away 1" deep filter...

I'm not going to deal with the rest of the problems at this time.
 
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Old 08-17-12, 08:58 AM
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Wow. Thank you for your reply. This has me concerned it may never be fixable without either changing duct trunk or moving down to a 1.5 ton system. Will the velocities change as I add more return air? Then, even if it is “fixable”, the possibility of short cycling could or will occur?



I was thinking of running a 10” flex to split between the bedrooms with 14 x 14 filter grills, but now I'm thinking a 12” flex split between two 12 x 24 or 16 x 16 filter grills.(if it all will fit) I still have to design/build the return plenum, so there is the option of the deeper air handler filter also. If conditioning the garage eventually came into play, I was considering using the leftover 14” flex for the return there.

(additional info: ceiling height throughout is only 7' 2”. Why? Who knows.)



If this is too much to ask for help on, I completely understand. You have already helped me considerably. Thank you.
 
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Old 08-17-12, 03:17 PM
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According to Hart & Cooley Engineering Data, it takes a 30x24 (720-physical sq.ins.) to flow 1044-CFM (2.5-Ton system) at 300-fpm initial velocity.

You can never have too much Return Air filter area; along with larger proper sized return ducting they both decrease airflow velocity & help increase the volume of airflow the blower can deliver.

I'd have a "Home Energy Efficiency Audit" performed; do the work to reduce the heat-gain & heat-loss; then have a room by room load calc performed for sizing the duct system & the equipment.
 
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Old 08-19-12, 07:28 PM
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Thanks for the advice. I've done some more research. Your website is great.


So...logic dictates. Simple, but not necessarily easy. Smaller holes, faster air & higher pressure; bigger holes, slower air & lower pressure. Velocities must correlate with the pressure, which is why the recommended velocities need to be adhered to. It would appear the return part of this duct system could have been better even with a 1 ton system. If I understand this right, even with a 1.5 ton system to get the return velocity under 300 fpm I would still need to run a second 14” flex with the appropriate size grill or grills. Obviously, this is oversized at 2.5 ton...supply velocity will be 30% too high. But if I have to choose between the two I think I'd rather have the supply side too high velocity rather than the return side starved and too high velocity. If I can fit two 16” flex with proper grills, I'll have to do that. If conditioning the garage area comes into play later, the extra 14” return should bring the total return velocities down to a decent level. Can too much harm come to the system while the supply side is this high?



Thank you
 
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Old 08-20-12, 05:55 AM
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If I understand this right, even with a 1.5 ton system to get the return velocity under 300 fpm I would still need to run a second 14” flex with the appropriate size grill or grills.
That 300-fpm is only through the filter area; not the return trunk area, which could be around 500 or 600-fpm or even higher.

Velocity through Return Air grilles needs to be lower than duct velocities or around 400-fpm.

Velocity is determined by the (free-open-area) sf area that the air is passing through.
 
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Old 08-20-12, 07:02 AM
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Good Morning

Hmm. Well maybe I don't understand it then. I was using the H&C ductulator and splitting the @600 cfm of a 1.5 ton system between two 14” flex ducts to come up with a duct velocity of 281 fpm for the return duct. Is this incorrect?
 
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Old 08-20-12, 08:46 AM
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Hmm. Well maybe I don't understand it then. I was using the H&C ductulator and splitting the @600 cfm of a 1.5 ton system between two 14” flex ducts to come up with a duct velocity of 281 fpm for the return duct. Is this incorrect?
One 14"-metal or one 16"-flex return air duct is large enough for a 1.5-Ton A/C.

It is only the Return Air filter & grille areas that have to be sized extra large physically to get the required free-air-square feet (Ak) areas.

A 25x20 (500-sq.in.) filter will handle 600-CFM.
500 * .69% = 345-sq.ins. / 144= 2.3958-sf; 600 / 2.3958= 250-fpm velocity.

700-cfm / 2.3958-sf = 292-fpm velocity.
 

Last edited by HVAC RETIRED; 08-20-12 at 09:06 AM.
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Old 09-29-12, 11:26 AM
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A question on filters

Hi. I'm still working on this project; fortunately we have another place to stay while I slowly do the work. I was reviewing the information about filters on your website and listening to your audio file where you said each return trunk should have it's own filter. So this made me wonder about the differences of the velocities between the filter grill and the return plenum. I was going to place one 4" filter at the air handler, but now I'm not sure if it would be better to try to install one 4" filter for each return air branch(2 total) or just use filter grills at each of the return boxes.(5 total, 4 sizes) I'm leaning towards the 2 at the return plenum but will the velocity be too high for the filters there?

Thank you very much for all your help.
 
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Old 09-29-12, 03:08 PM
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If the box area behind the grille is deep enough so there is no restriction there then I would install large filter grilles there that hold 4" deep filters.

What sizes are the Return air grilles?

The figures I listed were for a very low pressure drop cheap 1" fiber glass media type filter; so the area you need depends on what the pressure drop of the filter you're going to use has.

Those pleated filters load the debris on there surface area; media filters load through the depth of the filter.

Using 4" deep pleaded filters helps provide more surface area plus less pressure drop, they also load considerably slower than say a 1" pleaded...
 
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Old 09-29-12, 04:06 PM
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For either the 4" or 1" filter I was going to go with the flanders prepleat 40 lpd merv 8. I can still make changes but the final plan at this point is the original 18 x 24 which is a 1" filter grill with the addition of a 12 x 12 grill in each of the two bedrooms, and a 10 x 10 grill in the far corner of a back room. If or when the garage becomes conditioned space, then another 16 x 16 grill will go in there.

I think I can fit a "T" shape return plenum that would enable me to install either two 16 x 25 or 16 x 20 filters; one filter for each return branch. And I'll try to make it to fit up to a six inch deep filter.

This seems like it would work well, I just wasn't sure if the velocity would be too high with the filters at the return plenum compared to the velocities at the grills.

Thank you.
 
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Old 10-08-12, 06:28 PM
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Thumbs down Pretty Sad Tech...

Sounds like that A/C guy needs to come back out and see what is going on. I can't believe his answer is that it's doing what it's suppose to be doing and nothing else he can do. What kind of service is that? These are the kind of techs in the field that make the good guys look bad.
 
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