Air bypassing Evaporator coil?

[talbot3]

I have an AC system that came with my house that I feel is performing sub par. I have had it serviced each summer and the technicians that have inspected the system have not discovered anything wrong with it. Recently, I became motivated to self-diagnosis the problem and have used this site as a resource. Equipped with hygrometers, thermometers and a fluid manometer I began taking some measurements while the unit was operating.

I have discovered that the temperature and relative humidity in the supply duct just past the evaporator coil has a wide variance depending on where the probe is located in the duct. For example on one side directly over the coil I measure a dry bulb temp of 50 degrees and an RH of 98%, but on the otherside of the duct (moving the probe 12") I measure a dry bulb temp of 64 degrees and an RH of 67%. The air right before the blower is 74 degrees with an RH of 50%.

I have also discovered that the static pressure drop across the coil is only 0.06 inches of water. Based on the manufacturers literature this should be 0.2 with a wet coil.

All this leads me to believe, that a large portion of the return air is flowing around the coil and not through the coil like it should. Please tell me your thoughts. My plan now is to find a professional to verify the problem and provide a fix.

[Shophound]

All cooling coils bypass a certain amount of air without cooling or dehumidifying it. To what extent this occurs depends on rate of airflow, and if the system is properly charged with refrigerant or not, and to a certain extent what type of metering device it has.

The low pressure drop is likely due to a coil not pulling out as much moisture as when the OEM measured for a wet coil. At 74 degrees and 50% RH you should be dropping plenty of water out of the air onto that coil.

Your technicians should be able to tell you what your superheat and subcooling values are for the refrigerant are at the time the system is checked for these values. If they don't disclose it upfront, ask. If they don't measure for it at all, you need a different tech.

[danielworkerbee]

Check me if I'm wrong, but the humidity should be lower after the evap. coil than the return air Rh 50%.
Do you have a bypass humidifier ? Is the damper blade open ?
Could you attach some photo's of the system ?
Get a photocopy of the test data from the last tune-up that was done and post the info.
luck dan

[shhvac14]

The relative humidity after the evap will actually be higher than that of the return air because it is cooler. The cooler it is, the closer it is to it's dew point. Once it mixes with the rest of the air in the house and gets warmer the humidity level will be lower.

[talbot3]

Check me if I'm wrong, but the humidity should be lower after the evap. coil than the return air Rh 50%.
Do you have a bypass humidifier ? Is the damper blade open ?
Could you attach some photo's of the system ?
Get a photocopy of the test data from the last tune-up that was done and post the info.
luck dan

Daniel,

The system is as follows (I will try to get some pics). There is an outside compressor and the lineset runs about 8 feet into the garage into the evaporator coil. The coil is located on top of the furnace which sits on a plywood plenum. The return duct attaches to the plenum. There is no humidifier or any duct work bypassing the coil. I looked at what the tech left me last year when the AC was serviced. There was no data on the superheat or subcooling so it looks like I need to find another company for the servicing.

Based on my calculations 74 degree air at 50% has 64 grains of water per pound, 64 degrees at 67% has 60 grains and 50 at 98% has 53 grains. So the coil is removing some moisture.

[talbot3]

All cooling coils bypass a certain amount of air without cooling or dehumidifying it. To what extent this occurs depends on rate of airflow, and if the system is properly charged with refrigerant or not, and to a certain extent what type of metering device it has.

The low pressure drop is likely due to a coil not pulling out as much moisture as when the OEM measured for a wet coil. At 74 degrees and 50% RH you should be dropping plenty of water out of the air onto that coil.

Your technicians should be able to tell you what your superheat and subcooling values are for the refrigerant are at the time the system is checked for these values. If they don't disclose it upfront, ask. If they don't measure for it at all, you need a different tech.

Thanks for the advice shophound. Is it normal in your experience to have drastic differences in the air temp and humidity in the duct work immediately after the coil (say about 12" to 18" past the coil). How much distance or time is typically required for the bypass air and air that goes through the coil to mix completely? Should I try to take the temp and humidity readings further from the coil? I have a tech coming by monday morning and will be sure to ask about superheat and subcooling.

[teddy bear]

I have an AC system that came with my house that I feel is performing sub par. I have had it serviced each summer and the technicians that have inspected the system have not discovered anything wrong with it. Recently, I became motivated to self-diagnosis the problem and have used this site as a resource. Equipped with hygrometers, thermometers and a fluid manometer I began taking some measurements while the unit was operating.

I have discovered that the temperature and relative humidity in the supply duct just past the evaporator coil has a wide variance depending on where the probe is located in the duct. For example on one side directly over the coil I measure a dry bulb temp of 50 degrees and an RH of 98%, but on the otherside of the duct (moving the probe 12") I measure a dry bulb temp of 64 degrees and an RH of 67%. The air right before the blower is 74 degrees with an RH of 50%.

I have also discovered that the static pressure drop across the coil is only 0.06 inches of water. Based on the manufacturers literature this should be 0.2 with a wet coil.

All this leads me to believe, that a large portion of the return air is flowing around the coil and not through the coil like it should. Please tell me your thoughts. My plan now is to find a professional to verify the problem and provide a fix.


This is a good working system. You have 50%RH and no sweating ducts.
What do you want? As long as you have an extended cooling load, excellent.
The fun starts when the a/c cycles on the t-stat and the moisture evaporates off the coil back to the home through the ducts. During wet cool weather, the a/c short cycles before the moisture goes down the drain or even worse. The outside infiltrating fresh air dew point is 65^F and the occupants are adding moisture. The inside %RH is rising and there is no cooling load. Keep measuring and keep us posted.
Regards TB

[udarrell]

I have an AC system that came with my house that I feel is performing sub par. I have had it serviced each summer and the technicians that have inspected the system have not discovered anything wrong with it. Recently, I became motivated to self-diagnosis the problem and have used this site as a resource. Equipped with hygrometers, thermometers and a fluid manometer I began taking some measurements while the unit was operating.

I have discovered that the temperature and relative humidity in the supply duct just past the evaporator coil has a wide variance depending on where the probe is located in the duct. For example on one side directly over the coil I measure a dry bulb temp of 50 degrees and an RH of 98%, but on the other side of the duct (moving the probe 12") I measure a dry bulb temp of 64 degrees and an RH of 67%. The air right before the blower is 74 degrees with an RH of 50%.

I have also discovered that the static pressure drop across the coil is only 0.06 inches of water. Based on the manufacturers literature this should be 0.2 with a wet coil.

All this leads me to believe, that a large portion of the return air is flowing around the coil and not through the coil like it should. Please tell me your thoughts. My plan now is to find a professional to verify the problem and provide a fix.
The pressure drop across the evaporator coil is an indicator of how much CFM of airflow is going through the coil.

For example; a new clean EX*36F 3-Ton wet coil with a pressure drop of .21 is supposed to be delivering 900-CFM; at, an extremely high, .41 of an inch SP drop 1300-CFM. An EX*36J, gets a 1000-CFM at .20" drop; & 1400-CFM at .33" SP drop; more normal.

If your coil is clean, it should have a .20" SP drop across it.

Is the airflow real low?
It would appear that there is a very low CFM of airflow going through that indoor A-Coil.

Also appears there is not, a necessary, even airflow across the evaporator coil's circuits; or plugged distributor or coil circuits.

Is it a TXV or piston flow rater metering device?

Tech should remove evap cover & look for frost; also check temps, etc.

Check the temp rise off the outdoor condenser & list it here; temp off the condenser minus out door Temp.

They should always check & confirm actual delivered airflow when diagnosing a system.

All coils have a by-pass factor, as all the air doesn't get conditioned as it passes through the coil.

That system has problems...

[Shophound]

Thanks for the advice shophound. Is it normal in your experience to have drastic differences in the air temp and humidity in the duct work immediately after the coil (say about 12" to 18" past the coil). How much distance or time is typically required for the bypass air and air that goes through the coil to mix completely? Should I try to take the temp and humidity readings further from the coil? I have a tech coming by monday morning and will be sure to ask about superheat and subcooling.

If the refrigerant circuits within the evaporator are a bit short on refrigerant, it can lead to symptoms similar to what you measured. Same for a metering device that offers no superheat control (which is any metering device outside of a TXV). Only one metering device can keep a properly charged system loaded with the correct amount of refrigerant at all times, and that is a TXV.

That said, I would measure further down the duct, but not too far.

Bottom line: is your a/c system not keeping your house comfortable, or are you focusing on what you saw at the evaporator but otherwise you're comfortable in the house?

[talbot3]

The pressure drop across the evaporator coil is an indicator of how much CFM of airflow is going through the coil.

For example; a new clean EX*36F 3-Ton wet coil with a pressure drop of .21 is supposed to be delivering 900-CFM; at, an extremely high, .41 of an inch SP drop 1300-CFM. An EX*36J, gets a 1000-CFM at .20" drop; & 1400-CFM at .33" SP drop; more normal.

If your coil is clean, it should have a .20" SP drop across it.

Is the airflow real low?
It would appear that there is a very low CFM of airflow going through that indoor A-Coil.

Also appears there is not, a necessary, even airflow across the evaporator coil's circuits; or plugged distributor or coil circuits.

Is it a TXV or piston flow rater metering device?

Tech should remove evap cover & look for frost; also check temps, etc.

Check the temp rise off the outdoor condenser & list it here; temp off the condenser minus out door Temp.

They should always check & confirm actual delivered airflow when diagnosing a system.

All coils have a by-pass factor, as all the air doesn't get conditioned as it passes through the coil.

That system has problems...

udarrell thanks for the insight. The system is suppose to deliver 3 tons. The static pressure just before the coil and right before the blower is within the maximum external static pressure listed on the side of the furnance. This and my "impression" of the air flow at each register leads me to believe I do not have an air flow issue.

The coil has a piston metering device.

I will try to check the temp rise at the compressor. Where should I measure the temperature? Is this the temp of the air leaving the compressor fan? Can I get an accurate reading with an IR temp gun?

[talbot3]

This is a good working system. You have 50%RH and no sweating ducts.
What do you want? As long as you have an extended cooling load, excellent.
The fun starts when the a/c cycles on the t-stat and the moisture evaporates off the coil back to the home through the ducts. During wet cool weather, the a/c short cycles before the moisture goes down the drain or even worse. The outside infiltrating fresh air dew point is 65^F and the occupants are adding moisture. The inside %RH is rising and there is no cooling load. Keep measuring and keep us posted.
Regards TB

Teddy bear,

You may be right. The reason I am looking into this is that temerature in the house will not get below 80 degrees on the hottest days of the year (Savannah, GA). Based on a load calc I did using HVAC-calc, I should only need 18000 Btu sensible and 23000 total. My system in theory should be capable of 36000 total. So my 3 ton system is performing like a 1.5 ton. Ideally, I would like to have a properly sized and installed 2 ton system.

[talbot3]

If the refrigerant circuits within the evaporator are a bit short on refrigerant, it can lead to symptoms similar to what you measured. Same for a metering device that offers no superheat control (which is any metering device outside of a TXV). Only one metering device can keep a properly charged system loaded with the correct amount of refrigerant at all times, and that is a TXV.

That said, I would measure further down the duct, but not too far.

Bottom line: is your a/c system not keeping your house comfortable, or are you focusing on what you saw at the evaporator but otherwise you're comfortable in the house?

Shophound,

The metering device is a piston type. The house is very comfortable now, but my concern is the dead of summer where the current system has performed poorly for at least the last 4 years.

[Shophound]

Shophound,

The metering device is a piston type. The house is very comfortable now, but my concern is the dead of summer where the current system has performed poorly for at least the last 4 years.

Piston metering devices require the charge in the system to be very close in order to get maximum performance from these systems. A few ounces off either way can affect performance. I've proved this to myself repeatedly when checking charge on these systems. It's an eye opener the first time you realize it.

If the system is overcharged, it may cool but not dehumidify well. If it's undercharged it won't cool or dehumidify very well. When the charge is correct it will dehumidify and cool adequately.

[udarrell]

udarrell thanks for the insight. The system is suppose to deliver 3 tons. The static pressure just before the coil and right before the blower is within the maximum external static pressure listed on the side of the furnace. This, and my "impression" of the air flow at each register leads me to believe I do not have an air flow issue.

The coil has a piston metering device.

I will try to check the temp rise at the condenser/compressor. Where should I measure the temperature? Is this the temp of the air leaving the condenser/compressor fan? Can I get an accurate reading with an IR temp gun?

Then you are not taking the readings correctly, or air is by-passing the coil.
What are the supply & return temps, or did I miss that?

To take the A-Coil's Pres.-drop; you have to get on the discharge-side of the blower & the discharge-side of the A-Coil; NOT on the other intake-side of the blower.

Use a regular thermometer for air temps.
You take a reading of the outside temp, depending on the sun situation, & (then when it's warm enough both indoors & outdoors) measure the discharge air temperature coming of the condenser.

Post what both are; at 50% relative humidity indoors the temp rise on a 10 to 12-SEER condenser should be, depending on conditions, around 17 to 21-F.

If way too low or way too high that tells you where to look for the problems.

If it is way below that, then the indoor coil is not absorbing enough heat.

[talbot3]

Then you are not taking the readings correctly, or air is by-passing the coil.
What are the supply & return temps, or did I miss that?

To take the A-Coil's Pres.-drop; you have to get on the discharge-side of the blower & the discharge-side of the A-Coil; NOT on the other intake-side of the blower.

Use a regular thermometer for air temps.
You take a reading of the outside temp, depending on the sun situation, & (then when it's warm enough both indoors & outdoors) measure the discharge air temperature coming of the condenser.

Post what both are; at 50% relative humidity indoors the temp rise on a 10 to 12-SEER condenser should be, depending on conditions, around 17 to 21-F.

If way too low or way too high that tells you where to look for the problems.

If it is way below that, then the indoor coil is not absorbing enough heat.

Udarrell,

I was able to turn the system on for 20 minutes over lunch and take some readings. First let me clarify my pressure readings. I am taking the coil pressure reading exactly as you describe and my dywer fluid manometer is reading 0.06 inches. I also have taken the total external static pressure for the system and it is within the specs listed on the furnace. I have located tubing for the manometer just above the coil, just below the coil, and inside the blower compartment. To take different reading I just hook up the correct tubes to the high and low side of my manometer.

Here are the readings I have took using a number of Extech 445713 hyrg-thermometers:

Outside: 72 degrees at 55% RH
Compressor FAN Air temp: 98-101 degrees
Return just before blower: 73 degrees at 53%RH
Supply duct 24" above coil: 62 degrees at 71%RH

Assuming I have 1200 CFM of airflow (I will ask the tech to verify this monday, but I will probably have to purchase an anemometer to do so), I calculate the capacity to be 14,250 Sensible, 5,820 Latent, and a total of 20,070.

Does this make sense for a 3-ton system operating under the above conditions?

editted to add temp and humidity measuring device.

[udarrell]

Udarrell,

I was able to turn the system on for 20 minutes over lunch and take some readings. First let me clarify my pressure readings. I am taking the coil pressure reading exactly as you describe and my dywer fluid manometer is reading 0.06 inches. I also have taken the total external static pressure for the system and it is within the specs listed on the furnace. I have located tubing for the manometer just above the coil, just below the coil, and inside the blower compartment. To take different reading I just hook up the correct tubes to the high and low side of my manometer.

Here are the readings I have took using a number of Extech 445713 hyrg-thermometers:

Outside: 72 degrees at 55% RH
Compressor FAN Air temp: 98-101 degrees
(The split is high which does not comport with that low BTUH finding...)
Return just before blower: 73 degrees at 53%RH
Supply duct 24" above coil: 62 degrees at 71%RH

(I needed digital wet bulb temps in tenth increments; not easy for me to accurately convert the %RH to wet bulb)

Assuming I have 1200 CFM of airflow (?) (I will ask the tech to verify this Monday, but I will probably have to purchase an vane anemometer to do so), I calculate the capacity to be 14,250 Sensible, 5,820 Latent, and a total of 20,070.

Does this make sense for a 3-ton system operating under the above conditions? (NO)

Edited to add temp and humidity measuring device.

I was gone all afternoon, just got home.

I have not figured the BTUH yet based on the data you gave me; but looked up a 3-Ton Goodman GSX14-SEER; At 75F-outdoors; 70F indoors & a 63F wet bulb, it shows 34,600-BTUH - 20,070 is 14,530 less than mfg'ers Rating.

I got some things I have to do after I eat, I'm hurrying to figuring this; may not have got everything shaved just right. I got 19,926-BTUH, your figure is probably closer than mine! It those conditions; that BTUH is way low for a 3-Ton system...

However, the above Goodman Published Ratings illustrate where the ballpark figures ought to be it close to those conditions.

Over a ton less at quite similar conditions, - is too often, a finding...

Congratulations for taking real initiative to learn what is actually going on with the system. :cheers:

[talbot3]

I was gone all afternoon, just got home.

I have not figured the BTUH yet based on the data you gave me; but looked up a 3-Ton Goodman GSX14-SEER; At 75F-outdoors; 70F indoors & a 63F wet bulb, it shows 34,600-BTUH - 20,070 is 14,530 less than mfg'ers Rating.

I got some things I have to do after I eat, I'm hurrying to figuring this; may not have got everything shaved just right. I got 19,926-BTUH, your figure is probably closer than mine! It those conditions; that BTUH is way low for a 3-Ton system...

However, the above Goodman Published Ratings illustrate where the ballpark figures ought to be it close to those conditions.

Over a ton less at quite similar conditions, - is too often, a finding...

Congratulations for taking real initiative to learn what is actually going on with the system. :cheers:

Thanks for taking the time to look a this. It definitely sounds like my system has an issue if my measurements are correct. I am fairly confident in the return and supply temps. I measured the compressor temp by taping the temp probe to the louvre over the fan and leaving it for about 5 minutes. The compressor and probe were exposed to direct sunlight during this time. I guess I won't know what's really going on until the superheat is measured, the coil inspected, and the airflow is determined.

[talbot3]

I have been using a handy app for my iPod called psychrometric lt to calculate enthalpy, wet bulb, absolute humidity, etc. It tells me 73 F at 53%RH is a wet bulb temp of 62 F and enthalpy of 27.53 btu/lb. 62 F at 71% is 57 wet bulb and 24.0 btu/lb.

[udarrell]

Thanks for taking the time to look a this. It definitely sounds like my system has an issue if my measurements are correct. I am fairly confident in the return and supply temps. I measured the compressor temp by taping the temp probe to the louver over the fan and leaving it for about 5 minutes. The compressor and probe were exposed to direct sunlight during this time. I guess I won't know what's really going on until the superheat is measured, the coil inspected, and the airflow is determined.

Well, it's counter-intuitive to have a low indoor BTUH & a high condenser split. I'd need to be there to trouble shoot it.

I only deal with accurate digital wet bulb temps to be at all accurate on the BTUH output. Therefore, I know my BTUH figure is well off the real target figure.

I need the wet bulb in tenths to properly use the enthalpy chart; so with %RH there was too much room for me too error when trying to convert to a usable wet bulb temp. Guess you've been to my Website...
Time for me to hit the sack.

Good luck Monday... Darrell

[udarrell]

I have been using a handy app for my iPod called psychrometric lt to calculate enthalpy, wet bulb, absolute humidity, etc. It tells me 73 F at 53%RH is a wet bulb temp of 62 F (I had that one right) and enthalpy of 27.53 btu/lb. 62 F at 71% is 57 wet bulb (I had that one wrong!) and 24.0 btu/lb.
It is great to have those modern-day miracle working tools, using %RH, I have to work to get anywhere near accurate going from %RH to Wet bulb in tenths, until I find my misplaced chart, even then I need wet bulb in tenths, which my old charts don't show.

[talbot3]

So I had a tech come out and measure the superheat and subcooling. The superheat was way high at 38 and the subcooling was low at 5. The tech calculated that the system was lacking about 1.5 lbs of r-22. He added the refrigerant and checked the superheat again which had lowered to about 15. He said it would continue to lower till it was close the target. He said the subcooling readings would not be accurate for some amount of time because he had just cleaned the fins with water. My indoor and outdoor unit are about 10 yrs old and the furnace is over 30 yrs. He asked if I had thought about system replacement due to the leak and the condition of the furnace, and I said that I was interested. He is going to put together a bid on a new system and told me if I decided that I did not want to replace the system he could come back out to fine tune the superheat. I was very impressed with his overall knowledge and his willingness to explain things.

conditions on arrival:
Return: 73^F 53%RH
Supply: 62^F 66%RH

conditions after refrigerant added:
Return: 72^F 51%RH
Supply: 52^F 95%RH

So it looks like my issue was the charge and the low static pressure drop at the coil was due to an unsaturated coil.

Thanks for the help. I might ask for some more opinions if I decide to replace the system.

[udarrell]

I take it, as usual, airflow CFM was not checked!

Another method we can use:
E-mail from Jim Bergmann to me, is relevant here:


Also you might consider including that any A/C unit that is operating at 400 cfm/ton of airflow will have a change in enthalpy of about 6.66 btu/lb of dry air regardless of the tonnage. If a home owner verifies this they can be assured that the charge and airflow are in the ball park. If the change in enthalpy is above 6.66 they likely have low airflow. Below 6.66 it could be an airflow or a charge problem.

Comes from BTUH= 4.5 x CFM x change in enthalpy

12,000 / (400 x 4.5) = 6.66
60,000 / (2000 x 4.5) = 6.66


On Wed, Apr 27, 2011 at 9:37 PM, Jim Bergmann

[talbot3]

I take it, as usual, airflow CFM was not checked!

Another method we can use:
E-mail from Jim Bergmann to me, is relevant here:

You are right that airflow was not checked beyond a static pressure measurement.

That's a very useful tip from Jim. The change in enthalpy before the added charge was 4.2 and after increased to 5.6, but the sensible change went from 14000 to about 26000. My hygrometers are probably not accurate enough to get a good enthalpy calculation.

[Shophound]

So it looks like my issue was the charge and the low static pressure drop at the coil was due to an unsaturated coil.



Which is what I suspected as stated in Post #2 of this thread.

Darrell...interesting tidbit from your friend! I'm writing that one down.

[lynn comstock]

I like the tip Udarrell provided in post 22. I will add that the 12,000 BTU/ton output assumes ARI conditions.

In the field we need to also consult the manufacturer's extended performance data. At 110°F OD, most equipment will do something less than 9000 BTU\ton. At 85°F OD the capacity will be considerably higher than 12,000 BTU/ton.

Split systems lose something also for long (don't forget the elbow line losses) or undersized line sets, so we need to pay attention to that as well into our thinking.

[Hheating]

talbot3,

Would you like a job as my AC service tech? Cause you are already doing better than most of the guys out there (unfortunately)

[talbot3]

talbot3,

Would you like a job as my AC service tech? Cause you are already doing better than most of the guys out there (unfortunately)

Trust me you don't want that. I cannot even maintain my own system! The best I can do is try to learn what questions to ask. I have attached the spread sheet I wrote to help calculate my system capacity. Hopefully, someone else may find it useful.

http://hvac-talk.com/vbb/attachment.php?attachmentid=181952&stc=1&d=1304446808

just change the .doc to .xls

Udarrell:

I know I should have pressed for an accurate measurement of airflow, and I still have plans for this to be verified.

Shophound:

You nailed the problem. Very impressive that you accurately diagnosed the problem over the internet!

[lynn comstock]

talbot3,Would you like a job as my AC service tech? Cause you are already doing better than most of the guys out there (unfortunately) SAD, but True.

[Shophound]

Shophound:

You nailed the problem. Very impressive that you accurately diagnosed the problem over the internet!


Where I work, you can walk or step inside most of our air handlers on the discharge side of the cooling coils, close the door, and move around easily. It didn't take me long to realize I could take my digital psychrometer in there with me. Sure was an eye opener!

I also know that air moving in a duct can thermally stratify. That's a mind blower first time you come across it!

[udarrell]

Which is what I suspected as stated in Post #2 of this thread.

Darrell...interesting tidbit from your friend! I'm writing that one down.
I was a bit surprised to get an e-mail from Jim; as nearly all E-mailer's want me to help them to learn something or to resolve a problem.

Jim Bergmann does those excellent Videos for TruTechTools, - that I now have links to from several of my HVAC Web pages.

Bergmann's Videos are the best quality with smooth delivery, that I've viewed.

Of, course the enthalpy difference would be different when using 350-cfm or 450-cfm per/ton of cooling, at ARI conditions.

Lynn Comstock's on-target post ought to be followed & adhered to in the field. The problem is some... well I don't have to tell anyone here... it is really sad.

Talbot3 cares enough to get it right; he works to learn what the problem is & how to correct it... Three Cheers for Talbot3 :cheers: