Residential A/C - Indoor air temp. off the chart?

[Bentus]

Hi everyone, excited to be a new member.

I am a serice technician for an apartment complex and this week has brought high temperatures and high humidity and I have a problem.

I was checking the refrigerant charge on one of our air conditioners today, which is a 1.0 ton R-22 split system. I was using the charging chart supplied with the condensing unit and the indoor air temperature entering the evaporator was 85* F, but the chart only goes up to 76* F.

What would you do in this sort of situation?

Thanks!

[Gary]

I would first check to see if it was a TXV or cap tube system.

[Bentus]

Thanks for replying Gary, and I'm sorry it has capillary tubes.

Upon further inspection it appears the error is on my part, I was taking a "dry bulb" temperature as opposed to a "wet bulb" temperature.

I guess I'll have to look into buying something to take a wet bulb temperature. Can you recommend such a tool? I'd like something that looks a little more professional than a thermometer with a wet rag around it : )

No offense to anyone : )

[Gary]

You want to start with airflow. Airflow through both coils has an enormous effect on temperatures and pressures, so make sure you have good airflow through both coils before you do anything else.

I use drybulb temps for everything. I measure the temps entering and leaving the coils (measured close to the coils). I'm looking for about 20F dT (air in/out difference) across the evap coil. Much more than this is an airflow problem. Much less than this is either high air in temp and/or not enough refrigerant in the low side.

When the air at the thermostat is down to temp setting, I want the air leaving the coil to be about 20F lower than the temp at the thermostat and I fine tune the airflow (blower speed) to achieve this. Why? Because that coil leaving air when it warms to room temp will be about 50%RH.

When all of this is done, then use the chart to trim the charge.

[Bentus]

Thank you for the detailed response, it is very much appreciated.

To clarify a few things:

I use drybulb temps for everything. I measure the temps entering and leaving the coils (measured close to the coils).

So I could measure the temp of the air entering the return air vent and measure the temp of the air leaving the supply vent closest to the coil?

Or should I open up the return vent in the ceiling and actually access the coil?

I'm looking for about 20F dT (air in/out difference) across the evap coil. Much more than this is an airflow problem. Much less than this is either high air in temp and/or not enough refrigerant in the low side.

How is it that a greater differential between temperatures can be caused by poor airflow, I thought poor airflow would make the evaporator less effecient at pulling heat and humidity out of the air if it was dirty or blower fan speed is not correct? I must be misunderstanding something.

Also, what would you consider a high air in temp? Lately I have been dealing with apartments that have an indoor temp of 80-90F.

An all too common practice around here is to hook up a set of gauges and a refrigeant tank to the condenser, and likely "top it off". Or diagnose by whether or not the suction line is sweating. I'm really trying to stray away from this type of maintenance : ) Thank you.

[Gary]

So I could measure the temp of the air entering the return air vent and measure the temp of the air leaving the supply vent closest to the coil?

Or should I open up the return vent in the ceiling and actually access the coil?

The closer to the coil the better. There are usually flexible duct connectors at the air handler. That's where I prefer to insert the temp sensor.

How is it that a greater differential between temperatures can be caused by poor airflow, I thought poor airflow would make the evaporator less effecient at pulling heat and humidity out of the air if it was dirty or blower fan speed is not correct? I must be misunderstanding something.

The system keeps cooling, but there is less air to cool... so a smaller volume of air gets very cold. Reduce the airflow enough and the coil freezes.

Also, what would you consider a high air in temp? Lately I have been dealing with apartments that have an indoor temp of 80-90F.

Because the A/C isn't working?

[Magoo]

All the above that Gary has posted.

Are the indoor air filters clean, is the out door condenser cleaned regularly. High ambient temps can highlight problems with coils and filters.

magoo

[Bentus]

The system keeps cooling, but there is less air to cool... so a smaller volume of air gets very cold. Reduce the airflow enough and the coil freezes.

Alright now I understand! Good explaination.

Because the A/C isn't working?

Let me elaborate a bit. We experience mainly 2 a/c problems.

1.) Resident comes home from work to a hot apartment because they weren't running their a/c for whatever reason, forgot to turn it on before they left in the morning or want to save energy. They run their a/c for an hour or so after work and call in a complaint that it's not cooling effectively. Now for starters I'd like to determine if it really is working properly before I jump into troubleshooting. Would a simple check of air in and out temps be sufficient to determine if the system is working properly? I'd like to give people benefit of the doubt but I get a lot of false calls and can't just beleive what everyone says.

2.) Resident's a/c has been running non-stop. Air temperature in the apartment can't seem to drop below 78F. The concensus around here is either "it's just too hot outside" or " that apartment gets a lot of sun", makes sense I guess but I'd like a little more technical evidence. The cha

Both of these examples assume it's very hot outside, 90F+, with high humidity. I try to use my gauges to help diagnose these problems but my data never seems consistent. I have some troubleshooting literature but they use terminology like "higher than normal suction pressure" and I am not sure what is normal.

These a/c's have been worked on by many different people with all levels of experience and inexperience over the years, so I'm left to deduce if there are problems with the system or if someone overcharged it at some point in time

[Bentus]

Are the indoor air filters clean, is the out door condenser cleaned regularly. High ambient temps can highlight problems with coils and filters.

magoo

Thanks for commenting Magoo.

I have made it a habit to replace dirty filters and inspect/clean the condenser coils. The evaporator coils are another story.

This property is 10 years old and I doubt many of the evaporator coils have been cleaned at all over the years. The more I read the more it seems that dirty evaporator coils may be causing a lot of our problems.

The trick is our evaporator coils are paired with a hot water coil for our hydro heating system, which is all contained in the ceiling behind the return vent. Getting into the coil to do a visual inspection is time consuming, which doesn't bother me, but my boss doesn't like the idea.

So through proper analysis of all of the other elements of this system, could one determine the evaporator coil is in fact dirty without the need for a visual inspection?

[Bentus]

I guess what I'm getting at is that I'm getting conflicting information from my company's training programs and my co-workers.

My issues seem to be telling if a system is operating at it specified efficiency, or if the problem is related to higher than normal outdoor temperatures or flaws in building construction/design.

[Bentus]

Oh and my apologies for the multi-posting and all the quotes. A/C is my favorite area of my work. : )

[Gary]

1.) Resident comes home from work to a hot apartment because they weren't running their a/c for whatever reason, forgot to turn it on before they left in the morning or want to save energy. They run their a/c for an hour or so after work and call in a complaint that it's not cooling effectively. Now for starters I'd like to determine if it really is working properly before I jump into troubleshooting. Would a simple check of air in and out temps be sufficient to determine if the system is working properly? I'd like to give people benefit of the doubt but I get a lot of false calls and can't just beleive what everyone says.

The indoor unit absorbs heat, while the outdoor unit rejects that heat. If the outdoor unit is rejecting a heavy load of heat, then we can assume that it is absorbing a heavy load indoors.

There are 4 temperatures to check out on the outdoor unit:

Condenser air in temp.
Condenser air out temp.
Saturated condensing temp (SCT).
Liquid line temp.

Air out minus air in = dT. If the cond dT is more than 30F, there is an airflow problem.

SCT minus liquid line temp = subcooling. If the subcooling is more than 15F, the system is overcharged (that's your buddies who keep topping it off) or there may be non-condensables.

SCT minus air in = TD. This varies with system designs, but should be 20-35F... with 20F indicating a light load and 35F indicating a heavy load. Higher efficiency systems tend to have lower TD's.

[Gary]

Are all of the units identical?

[Bentus]

We have multiple floorplans with varying square footages each. One bedroom units have 1.5 ton Payne Condensing units and the 2 bedrooms have 2.0 ton Payne condensing units.

And upon further inspection the evaporator coils are boxed in sheet metel with no way to access the coils without cutting the pvc condensate drain line, evacuating the system, and completely removing the coil from the ceiling.

[Gary]

The trick is our evaporator coils are paired with a hot water coil for our hydro heating system, which is all contained in the ceiling behind the return vent. Getting into the coil to do a visual inspection is time consuming, which doesn't bother me, but my boss doesn't like the idea.

So through proper analysis of all of the other elements of this system, could one determine the evaporator coil is in fact dirty without the need for a visual inspection?

Seems we have come full circle back to wetbulb temperature.

Condensate forming on the coil means the air at that point is at 100%RH... so the drybulb temp and the wetbulb temp of the air leaving the coil are the same... well... pretty close.

The drybulb temp then rises as it moves through the duct, but the moisture content remains the same, therefore the wetbulb temp remains the same.

The wetbulb temp at the closest supply vent gives you a close approximation of the drybulb temp at the coil outlet.

Does this make sense?

[Bentus]

I believe so.

If we were to put wet and dry bulb thermometers at the coil outlet and at the nearest supply vent, the coil outlet wb and db temps would be the same. As the around the coil has been cooled to bring the air down to saturation temp., 100%RH, so the rag on the wet bulb can't evaporate any moisture.

The db air temp at the supply has risen, lowering the RH, so the wb gains a cooling effect similar from the wet rag, maintining 100%RH on our wet bulb thermometer.

Do I have a grasp on this hypothetical situation?

[Bentus]

Err.. my assumption that RH and saturation point were one in the same appears to be incorrect.

[Gary]

That sounds about right.

Now let's take it a step further:

Let's suppose that our temp at the thermostat is 75F and our temp at the ceiling return vent is 76F (warmer at the ceiling).

To achieve our humidity target we would want the wetbulb temp at the nearest supply vent to be 21F lower than the temp at the thermostat... 54F.

Our ideal dT then is 76 - 54 = 22F dT.

You might be wondering why I changed from 20F below stat temp to 21F below stat temp. Wetbulb is a little bit lower than drybulb at the coil outlet because of the bypass factor of the coil.

[Gary]

At what dT do we say the airflow is insufficient? How high is too high? How dirty is too dirty?

I would draw the line at anything above 25F dT.

[lowcool]

gday bentus you could hire one of those hand held inspection cameras with a light on the end,drill a hole in return air and check the coil that way,and look for breakdown of insulation in ductwok also

[Bentus]

I just got out of work, another scorcher today but no a/c issues so far. I need to re-read through this post a couple more times later today to get a better grasp of these concepts, I'll return later.

Thanks for all of your help and insight (and patience) Gary, it is very much appreciated.

Hi lowcool, that is a good idea. We have been trying to get one of those types of cameras here on site but at the earliest it would be worked into our budget next year, if it's approved at all. Highly unlikely but I'll keep my fingers crossed.

Thanks guys, I'll be back later.

[Bentus]

To take a wet bulb temp is to measure the temperature of a volume of air with the maximum amount of evaporative cooling effect in relation to the relative humidity, am I understanding this fully?

As I understand it, a wet bulb temperature at 50%RH would be lower than a wet bulb temperature at 75%RH, correct?

I am really starting to get a hang of the whole concept it's just taking some time, the way you explan A/C and ventilation is way different than I had been taught.

A few things I'm still hung up on:

Earlier you had said "To achieve our humidity target we would want the wetbulb temp at the nearest supply vent to be 21F lower than the temp at the thermostat... 54F."

How do you know that the dT should be around 21F? 20F because thats what most manufactures recommend or do you use some temperature/humidity charts to tell you at what temperature the air has to hit the coil to condense the moisture?

You had also said earlier the supply air should be warming to room temperature and ending up around 50%RH. Is that because you know that the 50%RH air hitting the coil is going to be 100%RH, and condensing at the point when it drops by 20F?
I may be confusing myself but making progress.

dT, I understand what it is, but what does dT stand for?

And lastly, bypass factor. Is that just terminolgy for explaining the fact that the air leaving the evaporator won't be exactly 100%RH, therefore creating a difference between dry and wet bulb temps?

I'll check back and continue my reading, have a good weekend.

Bentus

[Gary]

dT = delta-T = delta Temperature

A delta is a measure of the change in a single substance or flow of substance. For example, the dT is the change in temperature of the air as it flows through the coil.

It's counterpart would be TD:

TD = Temperature Difference

A TD is a comparison of the temperatures of two different substances or flows of substances. For example, the evap TD is a comparison of the temp of the air entering the evap coil to the (saturation) temp of the refrigerant flowing through the coil.

[Gary]

Bypass factor:

As the air flows through the coil a small percentage of it manages to slip through without actually touching anything. This is the bypassed air. This mixes with the conditioned air and causes a small difference between the drybulb and wetbulb temperatures. The bypass factor of each coil varies, but roughly speaking the wetbulb is about 1F lower than the drybulb as the air leaves the coil.

[Gary]

Relative Humidity:

RH = The amount of moisture the air is currently holding as a percentage of the amount of air it is able to hold. When we say the air is at 50% RH, we are saying that it is holding 50% of what it is capable of holding.

There are two ways to change the RH: We can change the air's moisture content or we can change the air's ability to hold moisture. Warmer air can hold more moisture and vice versa.

Air at 75F is able to hold twice as much moisture as air at 54F, therefore if we have a volume of air at 54F @ 100%RH and we warm that air, we end up with air at 75F @ 50%RH. The moisture content of the air has not changed but it's ability to hold moisture has.

[Gary]

I am really starting to get a hang of the whole concept it's just taking some time, the way you explan A/C and ventilation is way different than I had been taught.

For better or worse, I do things my way... and I do my own research. Where the textbooks disagree with me, I assume they are wrong until proven otherwise.

Call me a radical, but you can't find the better way to do anything without challenging the status quo.

[Yuri B.]

Hello.
My common sense : the higher is air temp (at the same pressure), the more free space is there between atoms and molecules such as O, H, N, CO2 etc for the molecules of H2O to take place.

[Yuri B.]

I meant to account for the "mystery" of
The moisture content of the air has not changed but it's ability to hold moisture hasHeated air expands, the distances between its particles increase leaving more space for H2O (or whatsoever substances' vapour).

[Yuri B.]

Again failed to quote (forgot to mark).

[Gary]

I meant to account for the "mystery" of
The moisture content of the air has not changed but it's ability to hold moisture has

Heated air expands, the distances between its particles increase leaving more space for H2O (or whatsoever substances' vapour).

That sounds entirely plausible, Yuri.

(I'm thinking Abby Normal will probably jump in here soon to give us the straight scoop.)

[Gary]

Here is the data needed for a full evaluation of the system:

Evap air in temp
Evap air out temp (or wetbulb temp at the closest supply vent)
Suction line temp at the evap outlet (and/or the compressor inlet)
Low side pressure or saturation temp

Cond air in temp
Cond air out temp
Liquid line temp at the condensing unit
High side pressure or saturation temp

Temp at thermostat

[Bentus]

Hello.
My common sense : the higher is air temp (at the same pressure), the more free space is there between atoms and molecules such as O, H, N, CO2 etc for the molecules of H2O to take place.

Ah yes! Had to think back to high school science class: Warm air particles are excited, spaced out, and moving independently. Allowing for the transport of more water vapor.

If we relate this to, say, water itself: warm the water to evaporation temp., and the water vaporizes, particles are excited and spaced out. Cool the same water to freezing point and the particles become slow and sluggish, freezing into ice.

[Bentus]

For better or worse, I do things my way... and I do my own research. Where the textbooks disagree with me, I assume they are wrong until proven otherwise.

Call me a radical, but you can't find the better way to do anything without challenging the status quo.

That is the same reason why I've come here, I have come across many different sources that conflict with the information I have been given by my employers.

Normally in situations like this I'd just start poking around until I came up with my own conclusions but there are just to many variables to factor in here. Ownership has changed hand at least twice and I don't know how many techs have been in and out of here. No telling what they had or had not been doing.

I'm trying to put the maintain back into maintenance...uh wait...

[Bentus]

Here is the data needed for a full evaluation of the system:

Evap air in temp
Evap air out temp (or wetbulb temp at the closest supply vent)
Suction line temp at the evap outlet (and/or the compressor inlet)
Low side pressure or saturation temp

Cond air in temp
Cond air out temp
Liquid line temp at the condensing unit
High side pressure or saturation temp

Temp at thermostat

I'm going to use this guide to try to do a full evaluation of my a/c this weekend, it should improve my understanding of these concepts we've been talking about.

Got to work on an a/c today, but unfortunately it was an electrical issue which isn't as challenging for me : )

[Gary]

Normally in situations like this I'd just start poking around until I came up with my own conclusions but there are just to many variables to factor in here.

It's all about eliminating the variables. Once we know that the airflow through both coils is good, then it's down to refrigerant charge (high side charge and low side charge) and refrigerant flow (metering device and compressor).

[Gary]

I'm going to use this guide to try to do a full evaluation of my a/c this weekend, it should improve my understanding of these concepts we've been talking about.

Did you get a chance to gather the list of temperatures over the weekend?

[Bentus]

Haven't had a chance yet, hopefully these storm clouds will pass and I'll be able to get out there today or tomorrow.

Bentus

[Abby Normal]

I meant to account for the "mystery" of
The moisture content of the air has not changed but it's ability to hold moisture has

Heated air expands, the distances between its particles increase leaving more space for H2O (or whatsoever substances' vapour).

Actually we use the term how "warm air can hold more moisture than cold air"

This is technically incorrect but I myself have no issues with someone visualizing it like this.

Water has the PT relationship. When there is 100% RH, the vapour pressure of the water in the air is the same as the vapour pressure of water saturated at that temperature.

Dalton's Law of Partial Pressures sort of explains why water will slowly evaporate until it is heated up until its vapour pressure equals atmospheric pressure.