Attic air handler


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Old 07-05-13, 04:25 PM
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Attic air handler

Today was 92. Humidity about 53%. T-stat set for 78. House was at 81. This was during the afternoon. Now that the sun has gone down, the temp is dropping inside the house. This is a 3.5 ton unit. One zone, 1500 sq ft house.

I think I may have a dirty coil in the handler. Merely basing this on the fact that I have lived in this house 10 years and never cleaned it. I do, however, change the return filter religiously. I also don't know if the prior owner ever bothered cleaning the coils. My question is, how do I get to the coil. I believe it is on the return side, correct ?. (right side of pic).

Do I have to take the covers off the unit to access the coil ?. I assume the far side of the unit is identical to the near side ?. It was well over 100 up in that attic so I didn't stay very long LOL.

One more question - what temperature should the air be on the supply side ?. I am sure it is a range, but I have no idea what that should be.

aaaand......one last tid-bit.....humidity inside the house was basically the same as outside, but I could see water dripping outside from the attic unit (goes into gutter and down the spout)....didn't seem to be as much as other times, but still dripping.....


Thanks
 
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Last edited by fxcarden; 07-05-13 at 05:19 PM.
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Old 07-05-13, 04:41 PM
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That cover that faces us in the picture should get you access to the coil.
Have you changed the filter(s) recently. The filters are what protect the coil from dirt and dust.

You can determine the system's split by posting the return air and the supply air temperatures. Try to measure them as close as possible to the air handler.... post the numbers here.
 
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Old 07-05-13, 05:18 PM
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Yeah - only problem is, I don't have a temp probe.

I'll try to take the cover off one of these nights when it's bearable up there. Maybe next rainy day/night would be a good time to at least have a look.

Filters are changed regularly due to allergies, so yes.
 
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Old 07-06-13, 05:59 AM
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Feel that copper line, it should be warm to the touch.
If its hot or you can feel refrigerent "spitting" through it, you might be a little low on refrigerent.
Also check and clean the cols on the condenser outside. Make sure there is adequate room around it for proper air flow.
 
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Old 07-06-13, 09:13 AM
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OK so I went up there. The copper line feels barely warmer than the palm of my hand. The plywood on the floor feels warmer than the line. Again, the attic is a furnace right now. As for "spitting", if there is any, I can't tell. When I touch the line I feel vibration, but I think that is from the unit running. I do not hear any noises that would indicate "spitting".

Currently I have 89 outside with 50% humidity. Inside, it is 78 at 53%. T-stat is set to, and has been holding 78 for 3 hours. The water continues to drip outside.
 
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Old 07-06-13, 09:54 AM
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Since water is dripping the unit is at least working.

You suspected the coil in the attic may be dirty..... how is your airflow from your ducts ?
 
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Old 07-06-13, 10:21 AM
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Airflow seems about the same as always.
 
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Old 07-06-13, 02:52 PM
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Carefully feel the two copper lines at the condensing (outside) unit. The smaller line should be warm to hot, at least noticeably warmer than the ambient air temperature while the larger line should definitely be cold. The larger line MAY be sweating or have a very light frosting but it should not be a block of ice. You may need to carefully cut and peel back some insulation to see . feel these copper lines.

Post back with the results of these tests.
 
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Old 07-06-13, 03:27 PM
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Smaller line is warmer than ambient as you described. I cannot see frost on the larger line, but the copper pipe appears to be wet right before it disappears inside the insulation. The insulation is slightly wet for about 2 feet after that, then as it curves upward to go up the side of the house, everything is dry as a bone.
 

Last edited by fxcarden; 07-06-13 at 04:48 PM.
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Old 07-06-13, 04:26 PM
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Those simple tests lead me to believe that you are a bit low on refrigerant. Not seriously low but low enough to adversely affect performance. Unfortunately, adding refrigerant is not a DIY job as you have to be licensed by the US Environmental Protection Agency to work with most refrigerants. You will need to call a technician.
 
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Old 07-06-13, 08:01 PM
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Having the fan speed set too high can cause a warmer than normal suction line, as can a very high return air temp. (say, from duct leakage)

At the end of the day, it's not possible to tell what's happening without measuring refrigerant pressures and temperatures.

But, a 3.5 ton should have no trouble what so ever cooling 1500 sq ft; odds are that it's delivering 2 tons or less of cooling.

Many things can reduce capacity:

1. Leaky or poorly insulated ducts in the attic
2. Insufficient airflow due to-> dirty coil, undersized or poorly installed ducts, restrictive filter.
3. Improper charge
4. Indoor coil mismatched to outdoor unit

Has it always cooled poorly? Can you post some pictures of the ductwork?
 
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Old 07-06-13, 08:16 PM
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"
One more question - what temperature should the air be on the supply side ?. I am sure it is a range, but I have no idea what that should be.
"

15-20F difference between return and supply depending on the indoor humidity if it's moving enough air. The difference alone can't tell you if the system is working correctly.
 
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Old 07-07-13, 06:42 AM
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On "normal" summer days (in the 80's) , the unit works fine . Once it goes above 90, it starts to struggle......if it hits 100, it is toast. Here is the duct work. Return on the right, obviously. The single return takes a 24x24 filter. I usually put those allergy filters in for spring, but now I have one of those regular blue mesh types, so I don't believe filter restriction is an issue.

[ATTACH=CONFIG]14781[/ATTACH]

I am wondering if the fact that all this is in the attic which is the hottest part of the house is impacting the efficiency. Maybe I should put one of those automatic exhaust fans in the attic to keep it cooler.
 
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Old 07-07-13, 01:03 PM
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What size is that return? It looks really small for a system of your size. I also see that the duct straps are cutting into some of the supply ducts.


Really, the first thing to do is verify that the system is moving enough air and if not, figure out why.

For that, you'll need to call a technician who has to the tools to measure airflow and static pressure; I bet the vast majority don't. (there's a diy way if you've got electric heat strips)

If it's moving enough air, the refrigerant charge should be checked, and there's more to it than just measuring pressures. Problems other than too much or too little refrigerant can cause issues (mismatched coil, wrong orifice, restriction, compressor with bad valves, etc.)


It's not a "inflate to x psi" type thing.

On the other hand, if the airflow is way off, there's no point of even putting gauges on the system.

--------------------
I can guaranty you that it's not working properly regardless of how hot the attic is - at less than 500 sq ft per ton, you should be able to hang meat in there.
 
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Old 07-07-13, 01:38 PM
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the return is 16 inches wide
 
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Old 07-07-13, 02:02 PM
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16" is too small for 3.5 tons ; it might be worth adding a second return.

If practical, you can try disconnecting the return; if airflow noticeable increases, you can use some more return air.

The supply ducts might be undersized as well; there could also be other problems - ie incorrect charge. As previously stated, it's not possible to get the charge right if the blower isn't moving enough air.

Here's a duct calculator: Residential Air Duct Calculator - EfficientComfort.net

Required airflow 400 cfm per ton

Return air pressure drop should be fairly low - 0.05"-0.08" per 100 ft depending on the length. (duct calc has to be set for flex to prevent undersizing. Flex ducts of a given size always have a higher pressure drop than metal)

1400 cfm needs a 20" return give or take a little.

Can you post some info re: the supply side. Duct sizes, number of branch ducts, etc.
 

Last edited by user 10; 07-07-13 at 02:21 PM.
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Old 07-07-13, 03:22 PM
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Maybe I should put one of those automatic exhaust fans in the attic to keep it cooler.
Absolutely !! I have a gable mounted 36" whole house fan and an automatic rooftop ventor. They both serve important functions. My rooftop vent runs pretty much all day. The attic is tolerable with it running. I've added soffit vents so that there is a natural air flow path. You don't want the vent fan drawing cool air up to the attic from the house. When I run the whole house fan I open a hallway closet with a screened hatch access to draw air out of the house.
 
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Old 07-07-13, 05:29 PM
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More info: the model number of the compressor is 38TR042300, in case that means something. From what I can find, it checks out at 3.5 tons.
 
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Old 07-07-13, 05:42 PM
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@Muggle: The duct calculator is giving me a result of 21 inches. Can you confirm ?.

At 16, I'm not that far off so it suggests adding a small (say 8-10) return line would insure enough return flow ?. I only have the one return for the entire house in the hall upstairs. A few years back someone suggested adding a 2nd return for the 1st floor, but he wanted too much $ to do it, and his choice of wall was not acceptable to me.

I just find amazing that as soon as the sun stops hitting the roof, the temp drops 2-3 degrees in a little over 1 hour. The more I think about it, the more I thing that attic exhaust fan can only help, even if it is a small one.
 

Last edited by fxcarden; 07-07-13 at 06:18 PM.
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Old 07-07-13, 08:04 PM
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One thing to remember is that area/pressure drop is not proportional to diameter, so the difference between 16" and 20" is very significant.

You can never have too much return on a system like yours.

However, prior to spending money, it's a good idea to do some testing. I recommend...

1. Measuring the temperature difference between air coming into and leaving the condenser unit. This can give us an idea of how much heat the a/c is removing

2. Measure the temperature difference between the supply and return at the air handler. (post indoor humidity level if you have a meter) A high difference may indicate that it's not moving enough air.

A low difference may indicate that it's moving too much air or the refrigerant charge is off.

3. Temporarily disconnect the return air duct to rule out undersized return. If you get much better airflow with the return disconnected, you've just found a significant bottleneck. (If the supplies are also undersized, airflow may not increase even if the return is too small)

4. If you have electric heat and know the kw rating, run the system in heat mode and measure the temp difference. (this can be used to calculate airflow)

5. Get the charge checked, especially if it used to cool better.
 
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Old 07-07-13, 08:08 PM
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I just find amazing that as soon as the sun stops hitting the roof, the temp drops 2-3 degrees in a little over 1 hour. The more I think about it, the more I thing that attic exhaust fan can only help, even if it is a small one.

Read more: http://www.doityourself.com/forum/ai...#ixzz2YQ5ChChe
Suggestion:

Verify that the a/c is working as it should prior to spending money on attic ventilation.
 
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Old 07-08-13, 12:36 AM
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If your system was functioning properly, I don't think you would be having the problem you're experiencing. Your temperatures over there in New Jersey are low compared to out here in Phoenix, Arizona. A week, or so ago, it was 115 to 117 out here. Daily it is 104+ out here, EVERY damn day! Overnight lows of 92 etc. It is like living in a blast furnace here in the summer..

Anyway, I have a split system like you. Air handler in the attic. Been in my house 10 years also, and have never cleaned the coil in the air handler. Change my intake filter regularly, like you.

My AC unit, 4 ton, YORK, R22, 21 years old, last serviced 8 years ago, with apparently a non-original universal fan motor with improperly routed reversing wires. House is 1575 sqft. I set the t-stat at 81, it NEVER gets hotter in the house than 82 (even when it was 117 outside). Output vents in house supply air that feels like the cool breeze of a YORK peppermint patty..

My attic has got to be hotter than yours, and I have no special ventilation fan installed in it.. Matter of fact, you could probably kill someone, if you shut them up inside my attic with the air handler for 12 hours.. There are soffit vents up there though..

This is extreme heat out here and the old YORK keeps running like the Energizer Bunny.. Even with such performance, I kinda just feel lucky, and want to check out my capacitors and try to fix my reverse wiring arrangement.. Stock up on some spare caps, and maybe a contactor..

My friend had a capacitor failure in his, with temperatures rising to 91 inside the house. Unfortunately, I was over at his house when this happened, during a 4th of July party.. $250 service fee (after a 10% discount), to fix it and save the party and allow us to sleep there that night. His AC unit is only 8 years old and the cap that died was only 2 years old.. That has shocked me into doing a maintenance check on mine because it is ancient in comparison, even though it seems to be running good..
 
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Old 07-08-13, 06:26 AM
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I'm gonna call a tech and have him.....

a) check the refrigerant charge
b) check the airflow
c) check the coil and clean if needed

thanks for all the responses, but this stuff is above my skill level and physical ability.
 
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Old 07-08-13, 01:04 PM
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In this order...



c) check the coil and clean if needed
b) check the airflow
a) check the refrigerant charge

I would screen several companies over the phone; pretty much anyone who doesn't know what static pressure is or lacks the tools to measure it isn't worth your time or money.

Airflow combined with measured (wet and dry bulb) temps can tell the tech how much heat the unit is extracting.

When checking the charge, request that the tech checks the high and low side pressures and temperatures. It's not possible to tell what's actually happening based on pressures alone. (don't be afraid to be a pain in the neck)
 
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Old 07-08-13, 05:16 PM
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One thing I've never been afraid of is being a pain in the neck.

Got THAT covered, sir.
 
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Old 07-14-13, 10:49 AM
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Just to follow up on the "theory" side of things......

So we determined up above in this thread that my return (16 inch) is not big enough.. It should be 20 inches. With that in mind, does the following make sense?

Air handler wants to push x CFM per unit of time.....return duct being too narrow means air flows at higher speed, causing it to pass over the coils too fast and therefore not cool down enough ?.

Am I on the right track here ?.
 
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Old 07-16-13, 02:36 PM
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Not quite.

Your house is at atmospheric pressure and as air flows through the duct to the air handler, the pressure drops. "suction" doesn't exist - it's caused by a small area of low pressure in the presence of normal pressure. The air at "normal" pressure around the return vent literally rushes into the lower pressure area.

Having an undersized return duct causes the air to move too fast through the duct, which results in a greater pressure difference between the room and the inlet of the air handler due to friction/turbulence in the duct.

The lower the pressure is at the air handler intake, the less air the blower can move across the coil.

Reducing airflow reduces cooling capacity.
 

Last edited by user 10; 07-16-13 at 04:31 PM.
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Old 07-17-13, 09:27 AM
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Right - I think we are both saying the same thing.

I tried an experiment, I opened the grille, and removed the filter while the unit was on. The pressure sucked it right back into place. I don't think that should happen, right ?.
 
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Old 07-17-13, 10:43 AM
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Is the return very noisy?


----------
If there's a refrigerant charge problem, adding a second return may not improve performance that much. I would call in a pro if you haven't already.
 
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Old 07-17-13, 11:07 AM
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No. Not noisy at all. I mean, you have to be right next to it to hear any air moving. Even then, barely noticeable.
 
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Old 11-23-13, 05:35 PM
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So - reviving this thread - I had 2 different companies come and look at the unit for a possible replacement. One guy said the return should be 18 inches, and he also says I should have a 4 ton unit. The other guy said it was fine at 3.5 tons, because 4 tons would short cycle, and the 16 inch return was ok too. He said 16 inch return can handle 1500 CFM, and my supply ducts can only put out 900 CFM ?. I have (10) 6-inch flex lines for supply. Not sure about this math ?.
 
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Old 11-25-13, 08:05 AM
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Also, one guy proposed a Bryant unit. Never heard of them. Are they any good ?.
 
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Old 11-25-13, 10:22 AM
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Return should be 20 inches if run in flex 18 still to small
 
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Old 11-25-13, 10:26 AM
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4 ton would be 22inch, 3.5 ton 20 inch if both done in flex. 20 and 18 could work but its very close if they are replacing it do it right. They need to do a heat load for the equipment size, with out that they are guessing. both those company's shorting you on return size what else are they doing wrong.
 
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Old 11-25-13, 06:22 PM
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^Did any of these guys actually take (performance) measurements or do they just want to sell you a new system?

One thing that I'm sure of is that you shouldn't even need 3.5 tons of capacity for 1500 sq ft.
 
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Old 11-25-13, 06:29 PM
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I have (10) 6-inch flex lines for supply.


Here's a duct calculator: Residential Air Duct Calculator - EfficientComfort.net

The friction rate on the supply shouldn't exceed 0.1" per 100 ft.

The rate on the return should be far lower than that - maybe 0.5" +/-

6" flex is only good for about 80 cfm at most.

The rule is 400 cfm/ton.

Considering that those ducts are flex, you don't even have enough for 2.5 tons which needs 1000cfm. (with a 3.5 ton air handler, you'll get a bit more through that system -> maybe 1100-1200 cfm. Static pressure measurements can confirm this)

Low airflow combined with other problems could be causing your lack of cooling. You could theoretically spend thousands in a new 4 ton system and get similar results. SEER and tonnage ratings go out the window when you have crappy ductwork.
 
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Old 11-26-13, 03:40 PM
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Alrighty then. Thanks for the info.


No performance measurements, since on the day they came, the outside temp was about 20 degrees.

When you say "crappy duct work"......do you see something grossly wrong with mine other than the smallish return ?.
 

Last edited by fxcarden; 11-26-13 at 04:03 PM.
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Old 11-26-13, 03:44 PM
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I can't make any sense of those calculators, but thanks.
 
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Old 11-26-13, 04:35 PM
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^Grossly undersized supply and return for a 3.5 ton system; you're trying to push 1400 cfm of air through 800-900 cfm of supply duct.

With low airflow, you're not likely to be getting full capacity to begin with; the installers may even have overcharged the system to keep the coil above freezing.

But, it's really important to have measurements taken before making changes...


1. Static pressure on the supply and return -> can be used to tell how much air the air handler is moving and identify problems (your supply is probably worse than your return. problems on the return side may not show up until you fix the supply)

2. Wetbulb supply/return air temperatures with the system operating in cooling mode - can be used to calculate actual capacity

Without doing any testing, the best thing you can do is guess. If the system is cooling fine, you don't need more capacity; something is wrong with your house - be it lack of attic insulation, high leakage, poor attic ventilation, leaky ducts, etc.
 
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Old 11-26-13, 05:25 PM
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The attic is well insulated. I haven't been able to see any leakage, but won't discount some. I do think the attic could use an exhaust fan. I think at this point, I will wait until spring when the system can be run, and I'll have someone come and see it operating. At that point, I'll have them clean coils, check the charge, etc, perhaps change the return if not too costly. I will also look into an exhaust fan for the attic. There is no way in hell I am putting a 4 ton unit in here. I agree that 3.5 is plenty, and there is a flow issue.

Too bad you don't live in NJ. I'd have you do it.
 
 

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