Air blowing out at 63 degrees - problem is...

[MPdesign]

Hey guys. Hate to bother you with my own personal problems but those are the ones that hit my pocketbook the most.

I bought the house last October. The coil and condensing unit were added at that time to a furnace and new ductwork both circa 2007.

The house doesn't cool on hot days (3 ton - Atlanta - 1,500 SF, new windows, attic insulated)

The guy just left and he had put gauges on the condensing unit initially and said that it was leaking. Said that because the pressure was just above 220 (I think) and should be over 300. I then showed him my return duct layout.

My returns - I have 1@6" and 1@8". I knew this was not enough so I showed him.

He changed his story and said that was definitely the problem and not a leak.

He did appear to be a knowledgeable guy - more so than average - but he could not fully explain to me - so he does not fully understand himself.

I would think that if not enough air crossed the coils that it would initially get VERY cool - until it froze up. The air coming out is only 63 at the closest register though...

Is the lack of properly sized return duct really the culprit or is it the compressor/coil system?

Note* the coil is enclosed with no access panel (above the furnace in the crawlspace) and cannot be viewed.

Thank you for your valuable insight. [:-magnify

Cheers

[Terence McCann]

Hey guys. Hate to bother you with my own personal problems but those are the ones that hit my pocketbook the most.

I bought the house last October. The coil and condensing unit were added at that time to a furnace and new ductwork both circa 2007.

The house doesn't cool on hot days (3 ton - Atlanta - 1,500 SF, new windows, attic insulated)

The guy just left and he had put gauges on the condensing unit initially and said that it was leaking. Said that because the pressure was just above 220 (I think) and should be over 300. I then showed him my return duct layout.

My returns - I have 1@6" and 1@8". I knew this was not enough so I showed him.

He changed his story and said that was definitely the problem and not a leak.

He did appear to be a knowledgeable guy - more so than average - but he could not fully explain to me - so he does not fully understand himself.

I would think that if not enough air crossed the coils that it would initially get VERY cool - until it froze up. The air coming out is only 63 at the closest register though...

Is the lack of properly sized return duct really the culprit or is it the compressor/coil system?

Note* the coil is enclosed with no access panel (above the furnace in the crawlspace) and cannot be viewed.

Thank you for your valuable insight. [:-magnify

Cheers

You have a lack of return air for sure. A lack of return air would cause a much colder discharge air close to the evap and it would obviously get warmer at the further reaches. Low air flow = low suction = low head but other problems would show up. You sure the evap isn't freezing? Can you see the suction line coming out of the evap? If so is it frozen? Did the tech take superheat - at the coil and at the condensing unit? You may have more than one problem here.

Need head pressure, suction pressure, superheat and sub-cooling.

If the unit isn't in a 140 degree attic try taking the blower door off and check temps.

[Phillip]

Just from the info you have given the return air is too small. The return duct should be at less the size of the supply duct and the filters should be larger than the duct work.

Terry may be able to size it for you, it has been a long time since I sized ductwork

[MPdesign]

Thanks Terry!

I have not seen any freezing on the lines coming out of the coil ever. I am not sure what superheat is? He only connected gauges at the condensing unit access points.

So if I am understanding correctly, it may be cooling but the duct in the crawlspace is heating it up to 63 (in 15' distance) before it exits.

My plan for testing is to cut the hole for another 8" return in the plenum and cover it with a filter. That should give me enough return air. Then if I run it and it is cooler, then I know that is the problem. I would only test this when the temp is cooler because I know it can only cool down 100 degree outside air so much.

Will this work for testing? Any other recommendations.

After testing, I will add another 8" return to that location.

So it sounds like this may be my problem - but not all of my problem.

[Terence McCann]

Thanks Terry!

I have not seen any freezing on the lines coming out of the coil ever. I am not sure what superheat is? He only connected gauges at the condensing unit access points.

If it's your friend tell him to check superheat and sub-cooling. If you hired this guy fire him and get another tech.

So if I am understanding correctly, it may be cooling but the duct in the crawlspace is heating it up to 63 (in 15' distance) before it exits.

What’s the return air temp?

My plan for testing is to cut the hole for another 8" return in the plenum and cover it with a filter. That should give me enough return air. Then if I run it and it is cooler, then I know that is the problem. I would only test this when the temp is cooler because I know it can only cool down 100 degree outside air so much.

Take the blower door off first. You can always cover up part of the blower compartment with the cover to get more r/a.

I’d size the r/a for you but I still can’t find my friggin’ duculator. Going to order a replacement now. If Bain pops in he’ll get it. Use 0.05 static John.

You don’t have enough information on the a/c side to make a proper determination.

BTW, do you know what the term superheat is? If not:

Superheat is defined as the difference between the temperature at which the refrigerant boils at the given pressure in the evaporator, and the temperature of the refrigerant gas as it leaves the evaporator. In essence, it's how much extra temperature the refrigerant picks up after it has boiled.

http://contractingbusiness.com/resident ... imp_17963/

[MPdesign]

Not a friend - he is the original condenser and coil installer (did not install ducts or furnace)

I will measure the return but it should be about 78 - the temp of the house plus a few degrees.

The guy mentioned something about super heat but he thought that the lack of return air could be throwing off all of the gauge readings somehow.

Thanks for the blower door recommendation.

[Terence McCann]

The guy mentioned something about super heat but he thought that the lack of return air could be throwing off all of the gauge readings somehow.

Yes, a lack of return air can throw off reading but if he had checked superheat, found that is was 30 degrees with a lack of return air then you are short on gas too. You're typically looking for 12 degrees superheat (or there abouts).

[Marc]

The house doesn't cool on hot days (3 ton - Atlanta - 1,500 SF, new windows, attic insulated)

The guy just left and he had put gauges on the condensing unit initially and said that it was leaking. Said that because the pressure was just above 220 (I think) and should be over 300. I then showed him my return duct layout.

My returns - I have 1@6" and 1@8". I knew this was not enough so I showed him.

He changed his story and said that was definitely the problem and not a leak.

I would think that if not enough air crossed the coils that it would initially get VERY cool - until it froze up. The air coming out is only 63 at the closest register though...

Thank you for your valuable insight. [:-magnify

The weather has been unusually hot this summer and Atlanta is no exception. At some point, the outdoor temperatures exceed what a particular air conditioner was designed for and performance drops off quickly. In addition to that, the sizing of some systems might be borderline and are inadequate under worse case conditions. In those cases, I often suggest the installation of a window unit to augment the centrally ducted system. I have one myself. Just move the fan switch on the central thermostat from 'auto' to 'on' to better distribute the cooling provided by the window unit until the hot weather passes. If your condition is chronic, you need a good tech (not the one you have now).

As some have said, your returns don't add up to adequate cross sectional area. For 3 tons of cooling, I recommend a cross sectional area of 3 square feet minimum and no more than 15' in length.

For a 100 degree day and an R-22 refrigerant system, 225 psi of head pressure is about normal. For an R-410A system in good condition, expect about 350 psi.

Try to gain a visual and confirm that no ice is present on the evaporator (cooling) coil. Ice will throw off all pressure measurements to the point where they are useless for diagnostic purposes.

Marc

[Terence McCann]

For a 100 degree day and an R-22 refrigerant system, 225 psi of head pressure is about normal.

Marc

Nope-at 30 over on a 100 degree day you're looking at 297# with 22. 225# is what you might typically see on a 85 degree day.

[MPdesign]

Thank you for the return air calculation.

Thanks everyone!

[Marc]

I've always used about 10 degrees or just over that for a minimum when working on resid/light commercial splits. Everyone's different.

Marc

[Bain]

Terry, I use my ductilator about twice a year, and can never locate it on the days I need it. I'll try to look for the bloody thing later.

All the advice given is good. Like Terry said, checking the super heat and sub-cooling are the only ways to determine whether a system is operating correctly or not. A friend and I recently helped another friend whose system wasn't performing up to par. We--but mainly, he--checked pressures and super heat and figured out that the TXV--or thermostatic expansion valve--inside the coil was faulty, and later learned that Trane had a defective bunch a few years ago.

[MPdesign]

Update

I turned the system off completely for 30 minutes. I then removed the blower door to get plenty of return airflow. No freeze was seen on any lines.

It has been on for over an hour continuously.

Air out of vents: 63 degrees. Return air temp 74 degrees. It is after dark now and cooler (84 degrees outside).

[MPdesign]

Can you check the superheat and sub-cooling with the blower door off for max return air

OR should I wait until another duct is added?

[Marc]

Update

I turned the system off completely for 30 minutes. I then removed the blower door to get plenty of return airflow. No freeze was seen on any lines.

It has been on for over an hour continuously.

Air out of vents: 63 degrees. Return air temp 74 degrees. It is after dark now and cooler (84 degrees outside).

A differential of 11 degrees always gets mentioned in my reports. Bust some wall or something out to effect a larger return, then see what happens.

I doubt that only a single issue is present though the undersized return might be the largest of them.

Marc

[Terence McCann]

Can you check the superheat and sub-cooling with the blower door off for max return air

OR should I wait until another duct is added?

You can only check superheat & sub-cooling while looking at pressures with a set of gauges. Taking the blower door off is only a quick test to see what the supply air does.

Sounds like you're short on gas and have a lack of r/a. I ordered a new ductulator but it won't be here for a week or so. Size/fix the r/a first and then tackle the a/c side of things. Don't go williy nilly with the return air i.e. bust out a wall - crap in crap out. It needs to be designed for a .05-.06 static drop. Use a duct velocity of 800 fpm for the main return and 600 fpm for the r/a branch ducts (from Carrier air distribution manual).

If they hosed the r/a is the s/a OK?

[Marc]

There are no negative consequences to having too little pressure drop across the return air duct.

Marc

[Terence McCann]

New Trane Duculator arrived today [:-party].

Based on 500 CFM per ton (your 3 ton system) and at .05 SP drop you would need a 25x12 duct or 20" round.

8" round is good for 160 CFM and a 6" round is good for 75 CFM based on .05 SP.

With an 8" and 6" you are really undersized Mike.

[Terence McCann]

There are no negative consequences to having too little pressure drop across the return air duct.

Marc

Just getting around to addressing this. Couldn't be farther from the truth. Why not stick a 100x100 duct in the master bedroom? How about 30" round r/a ducts in all living areas. One thing for sure, you'd have very little Delta P. There is a method to the madness.

I have to admit though - residential is much more "fly by the seat of the pants" than commercial.

[Marc]

Just getting around to addressing this. Couldn't be farther from the truth. Why not stick a 100x100 rectangle duct in the master bedroom? How about 30" round r/a ducts in all living areas. One thing for sure, you'd have very little Delta P. There is a method to the madness.

I have to admit though - residential is much more "fly by the seat of the pants" than commercial.

Terry, I think this might be just a misunderstanding. I was referring to an AR duct design that consists of only a single air return grille/filter in the hallway of the house. The entire conditioned space of the dwelling is the air return path.

If you were referring to a distributed AR duct system, then I agree with you on all points.

Marc

[Terence McCann]

Even with a single point return Marc you still have to stay within good engineering practice (although, as I type this, I have to laugh at all the wrong systems I see working for many a year - return air open to the entire basement for example - the second floor isn't to comfy however.).

[Bain]

Yeah, sadly there's a whole lot more to this stuff than sticking thermometers in a couple of registers. Earlier this year, I had a new 2 1/2 ton dual-fuel heat pump installed on my main level. It worked fine on heat mode, but it wouldn't cool the living space when the temperature climbed into the 90s.

The system was working fine, but my house, at 90 years old, has all of the registers in the floor since they were installed only for heating. The dreaded stratification t'ing was working in full force. The cold, heavy air was shooting out of the registers, and then being pulled back into the ground-level returns. The ambient temperature at floor level was 68, but 4 1/2 feet higher, where the thermostat was, the ambient was 74.

One solution would have been to elevate the returns, but with plaster walls, that would have been a major pain in the ass. I've since replaced the 2 1/2 ton system with a 3 ton system, altered the ductwork a little to provide additional air-flow, and the house is comfortable again.

Point being, if an inspector dude had checked the delta on the 2 1/2 ton system, he or she would have said everything was groovy, but then the customer would have wondered why the house wouldn't cool below 76 degrees on a hot day.

I'm still not happy with the ductwork, and will eventually get around to partitioning off part of a closet, panning the ceiling joists and getting a higher return, but again, it's never as simple as one would hope it would be.

[Marc]

Yeah, sadly there's a whole lot more to this stuff than sticking thermometers in a couple of registers. Earlier this year, I had a new 2 1/2 ton dual-fuel heat pump installed on my main level. It worked fine on heat mode, but it wouldn't cool the living space when the temperature climbed into the 90s.

The system was working fine, but my house, at 90 years old, has all of the registers in the floor since they were installed only for heating. The dreaded stratification t'ing was working in full force. The cold, heavy air was shooting out of the registers, and then being pulled back into the ground-level returns. The ambient temperature at floor level was 68, but 4 1/2 feet higher, where the thermostat was, the ambient was 74.

One solution would have been to elevate the returns, but with plaster walls, that would have been a major pain in the ass. I've since replaced the 2 1/2 ton system with a 3 ton system, altered the ductwork a little to provide additional air-flow, and the house is comfortable again.

Point being, if an inspector dude had checked the delta on the 2 1/2 ton system, he or she would have said everything was groovy, but then the customer would have wondered why the house wouldn't cool below 76 degrees on a hot day.

I'm still not happy with the ductwork, and will eventually get around to partitioning off part of a closet, panning the ceiling joists and getting a higher return, but again, it's never as simple as one would hope it would be.

Curious. Would you say that it was the additional 1/2 ton of cooling or a higher air velocity at the floor registers that made the house comfortable again?

Marc

[Bain]

Yeah, sadly there's a whole lot more to this stuff than sticking thermometers in a couple of registers. Earlier this year, I had a new 2 1/2 ton dual-fuel heat pump installed on my main level. It worked fine on heat mode, but it wouldn't cool the living space when the temperature climbed into the 90s.

The system was working fine, but my house, at 90 years old, has all of the registers in the floor since they were installed only for heating. The dreaded stratification t'ing was working in full force. The cold, heavy air was shooting out of the registers, and then being pulled back into the ground-level returns. The ambient temperature at floor level was 68, but 4 1/2 feet higher, where the thermostat was, the ambient was 74.

One solution would have been to elevate the returns, but with plaster walls, that would have been a major pain in the ass. I've since replaced the 2 1/2 ton system with a 3 ton system, altered the ductwork a little to provide additional air-flow, and the house is comfortable again.

Point being, if an inspector dude had checked the delta on the 2 1/2 ton system, he or she would have said everything was groovy, but then the customer would have wondered why the house wouldn't cool below 76 degrees on a hot day.

I'm still not happy with the ductwork, and will eventually get around to partitioning off part of a closet, panning the ceiling joists and getting a higher return, but again, it's never as simple as one would hope it would be.

Curious. Would you say that it was the additional 1/2 ton of cooling or a higher air velocity at the floor registers that made the house comfortable again?

Marc

I (think) it was the additional 1/2 ton, as the coil surface is larger, but . . . researching the issue on HVAC forums, I learned that manufacturers can say a condenser and coil are 2 1/2 tons AS LONG as they're larger than 2 tons. Meaning, that if the equipment is 2.1 tons, it's allowable to label it 2 1/2. The 3 ton coil may only be 2.6 tons, but even so, it's now doing it's job. The 2 1/2 ton, original equipment, may have simply been undersized.

I honestly don't know.

[cajun]

to calculate r/a good rule of thumb is 144 square inches per ton of cooling. You cannot determing operation pressure until you have a balanced air distrubtion