A/C running all the time

Eugene Silberstein 3

Of course, in my opinion, all houses should have an attic fan. Setting that aside, I have a couple of questions for you that may help clear things up a bit at this end.

1 - What was the outside ambient temperature when the reeadings were taken?

2 - What was the space temperature when the readings were taken?

3 - Did you take a return air temperature reading at the air handler itself?

4 - Where was the suction line temperature that you used to make your superheat calculation taken from?

5 - How far apart are the air handler and the condensing unit?

The low side pressure reading will be pretty much right on point if the space temperature is about 79 to 80 degrees.

The high side pressure will be pretty much right on point if the outside ambient temperature is about 90 degrees.

If the temperature readings in questions 2 and 3 are the same, you do not have a return air dict leak. If the temperature in question 3 is considerably higher than that in question 2, Brad is 100% correct and you have a leak in your return duct.

The one thing I can suggest right off the back is to inspect the attic isulation and make certain that it is in tact and in place. You may also want to suggest that the homeowner increase the R-value of the insulation. This will help in both the summer and the winter.

Keep us posted

Mikey B

I have one homeowner that always is complaining that his A/C runs all the time. Unit is 1 year old 2ton 13 seer Heil in a app 900SQFT home. He leave his Tstat at 68 and when not home 70. The attic has a ridge vent but no attaic fan or side vent. i would say about 120 in attaic on a 90-95 day in direct sun. he only complains when its over 90 and very humid. Call every week says unit cycles great but when hot outside runs all the time, IE worried about electrical bills and wants fixed. Pressure are 236-75. super heat 10 TD, in house spread 15-16. I need help please. i not the one to drop the ball on the customer and say go try somebody else. I don't want to also say put a attic fan in solve your problems then it don't work.

Charles G.

A.C. unit

2 ton unit @ 900 sq ft. Ceiling heights? Geographic location? Install location? Heat load Calc? 22* differential may be a little optimistic, esp. w/high RH. Answer the above questions and we'll see what's what.

Brad White_9

Let's see....

2-Ton System, 900 SF... Seems amply sized, not knowing glass areas, orientation, insulation and other things. But for an average house, 450 SF per ton seems rather generous...

(As for pressures and temperatures superheat, I will leave that to others for now; I am not a refrigerant tech but will learn soon enough that end. Let me assume the numbers seem reasonable but I really do not know. Dr. Silberstein and I should be having a conversation sometime..

The airside delta-T's could be higher, perhaps too much airflow on the evaporator but more likely I would say air leakage elevating the return side temperature as a possible source of capacity loss or artificial load.

So what I am left with is, the hotter it is outside the longer it runs. So what I would gather is, it sounds normal but his heat gain is larger than he thinks. AC is supposed to run longer in hot weather same as a boiler will run longer in cold weather.

But I would check the ducts for leakage first.

My $0.02

Brad

Mikey B

Hopeful i can answer all ? for your help. The house is a raise ranch just cooling the upstairs with 8 FT ceiling. The house is an electric heat home and insulation in attic was doubled layered, brand new windows and roof r vavle unknown.3 bed room with 6" vents,kitchen with 6" den area 2-7" vents. 30x24 return with 14" flex linset about 30-35' Air handler installed is hydro air i guess for future heat (optional) i took temp when 92 outside to try to find issues of concern. all the facts were on the cover of the unit that i went by and all i think resonable. return air temp 74, supply average 58-60 super heat i look on panel and wanted 9 super heat. the only thing i didn't do is humification test(no instrunments) TX valve on hydro 13 seer Heil. i took retun and supply temp at register not in attic. superheat was taken at condenser. MA. location and we have been having a major heat wave. I not the greatest at A/C am i missing something, should i take temp at air handler and what should the ideal coil temp be. could the heat coil be lacking air flow blower on high. what should be volice at vent i can measure with meter. Thanks for all your help.

Mikey B

Also what is space temp, never eard that?

Ron Schroeder

space temp

I think he means in the living space probably meaning more temp of air just before entering the returns.

Brad White_101

Ideal Temperatures...

Supply air at 58-60 degrees into the space is not going to dehumidify the space too well and will not cool it to the 68 degree setpoint unless you have a LOT more air at that temperature. The only consolation is if the air coming off the coil is lower than 55 degrees saturated or at least below the dewpoint of the space it serves. My point here is that you may be getting that temperature off the coil but the air is being heated on the way to the diffusers.

The fact that the return air (space temperature, same thing for our purposes) is 74 tells me that the system is working. If the T-Stat is at 68, no wonder it runs all the time...

Where did you measure the 58-60 F temperatures? At the diffusers? What is the coil leaving air temperature by comparison?

If you have 30 to 35 feet of 14" diameter flex.... ouch.

Your system should be moving about 800 CFM, 400 CFM per ton, more or less. Anywhere between 700 and 850 CFM. A 14" diameter duct, if solid smooth sheetmetal, would be well sized. But flexible duct with all of those corrugations, any shear offsets, etc. and you could lose airflow by pressure drop. Now, normally that would mean that you would have colder air, not warmer... So I suspect still, a leak of return air.

Can you measure the return airflow at the grille and compare that to a traverse measurement at the unit? Delta = Leakage. Get a friendly balancing contractor with a nice Shortridge or Alnor Balancing hood. I am partial to Shortridge but let's not look a gift horse in the mouth here, shall we?

Measuring velocity has no meaning unless you correspond that to an area in order to read flow. Use a flow hood if you can borrow one, assuming you do not have one.


Here is my take on air volume: Let's assume you have a 2-ton system, 24,000 BTUH and 75% of that is sensible heat (18,000 cfm going to temperature reduction). You enter the coil at 74 degrees and leave at say 59 degrees (average of 58-60), thus a 15 degree airside delta-T.

18,000 /(15 degrees x 1.085) = 1105 CFM versus the 800 or so you should be running.

You have a leak there assuming the refrigerant side is checking out.

If you are running 800 CFM airflow, your air entering at 74 would leave at a nice, cool 53.3 degrees.

Check for duct leaks.

Mikey B

i don't have tool that you speak of. so if i measure return air at grill and then at unit return and it incresed (by how much?) i have a leaking return causing hot air to be sucked into unit and therfor causing A/C not to cool proper. Thanks

Brad White_9

You may have to rely on temperatures

absent the ability to measure air flow.

I would make a temperature profile. Just record the temperatures at each point from the space near the return air (where the thermostat should be, ideally), then into the RA duct, again at one or two more points toward the unit, then at the unit itself. Then upstream of the coil (to pick up any casing leakage) then downstream of the coil (your coldest air). Then downstream of the fan to pick up the fan heat as little as it might be. Then down the supply duct to the diffuser.

Make a simple line diagram to visualize the progression.

Any discrepancy would jump out at you if you have a significant leak. For example if the RA just upstream of the unit is 76.5 degrees then is entering the coil at 86 degrees from a 110 degree attic, you have found something...

I wonder if there is a way to use the pressure-enthalpy charts to determine what percentage of the capacity is going to sensible heat (I do not know how but perhaps Professor Silberstein does). Each coil and condensing unit arrangement as a system and at different conditions will yield varying percentages of sensible (temperature) cooling and latent (moisture removal) cooling all within the same total capacity. The ratios fluctuate especially with humidity. More humidity, more is removed. A system in Sedona, AZ will have more sensible capacity than the same system in Eunice, LA, which will have much more latent capacity (and needs it). Does that make sense?


Once you have an idea of relative sensible heat capacity (SHR or sensible heat ratio), you can make a determination of airflow this back-door way. But the temperatures will give you the qualitative information you seek if it exists.


Take the coil delta-T in degrees F., multiply x 1.085 (a constant for the specific heat of air over an hour at sea level density, close enough for temperature adjustment) and divide that into your presumed sensible capacity of the unit. That will give you airflow in CFM.

Rodney Summers

man what would this wall be without brad white :-) very interesting brad. you amaze me every time a read your post's your a smart guy.

thanks
Jason

Jim Bergmann_3

Airflow

Brad,

The latent sensible split varies with airflow, and entering wb conditions. The back door you are trying to use will lead you to trouble. You cannot estimate the latent sensible split unless you know entering wet bulb, or RH and the airflow is set to the manufactures design criteria. For the evaporator to operate with the designed capacity and sensible latent split the airflow has to be set to the manufacturer’s design criteria at the evaporator coil. Temperature drop across a coil will vary with the latent load (humidity) the more humidity, the more cooling energy goes to converting water vapor to water. The temperature drop across the evaporator can fall within 16° to 24° degrees with ease. Therefore it is imperative to set the airflow to the proper range and not to rely on the temperature drop across the coil to verify system performance. It is important to understand that the conditions of the air entering the coil will not normally affect the designed temperature difference of the coil. (The difference in temperature between the return air and the coil surface) It will however affect the temperature of the air leaving the coil.


Also important to remember is the sensible latent split varies with airflow and therefore cannot be used to calculate CFM. Airflow must be measured with a pitot tube, vane anemometer (preferable) or as a last resort the temperature rise method operating the gas or electric heat strips

When air is conditioned (Heat is added or removed.), the gross total heat removed or added is the sum of the sensible and latent heat commonly referred to as load. Because the operating characteristics of the cooling and heating equipment are affected by sensible and latent loads it is important that they be considered when verifying the operating conditions of the heating or cooling equipment

Gross Total Heat Formula (GTH): This is the total heat added (heated) or removed (cooled) from the air that is being conditioned.

The formula: GTH = 4.5 x CFM x delta-h

Where: GTH = Gross total heat,

CFM = Airflow in cubic feet per minute,

delta-h = change in enthalpy in Btu per pound of dry air,

Total Sensible Heat Formula (TSH): This is the heat added or removed that causes a change in temperature of the air without adding or removing humidity to the air.

The formula: TSH=1.08 x CFM x delta-t,

Where: TSH = Total sensible heat,

CFM = Airflow in cubic feet per minute,

delta-t = Change in temperature across the cooling coil or heat exchanger,

Total Latent Heat Formula (TLH): This is heat added or removed that causes a change in state of the air (Humidity added or removed) without changing the temperature of the air.

The formula TLH = 0.68 x CFM x delta -grains,

Where: TLH = Total latent heat,

CFM = Airflow in cubic feet per minute,

delta -grains = Change in grains of moisture in the air,


Derivation of the Air Constants

When using any of the air formulas it is important to understand how to correct for changes in the air density if the air being measured is not standard air. The air constants apply to standard air at 70° F and 14.7 Pisa, (29.92”hg.) and 0%rh. If air being measured is outside of these parameters, it may require that the constant be recalculated. For most situations the standard air formulas can be used, but if precise measurements are desired, adjustments to the constants should be made. Remember, fans are doing work; they are moving in reality pounds of air. The amount of air they will move in CFM remains constant with a variable mass flow rate, so the cubic feet of air they will move over any given time period will remain the same. The difference is in the density of the air or the number or the pounds per cubic foot. This is important because coil selection software calculates required coil capacities based upon pounds per hour (lb/hr) of air passing through the coil, not CFM.
The constant 4.5 is used to convert CFM to lbs/hr

4.5 = 60 min/hr or (60min x 0.075 lbs/cu ft)
13.33

Where 13.33 is the specific volume of standard air (cu.ft/lb) and 0.075 is the density (lbs/cu.ft)

If the air being measured is not standard air, the air density will vary with the barometric pressure and the absolute temperature. To recalculate the air density, measure the temperature and obtain the barometric pressure use the following formula:

Air Density (lb/cf) = 1.325 x Bp/Tabs

Where: 1.325 (Constant to keep consistent units)
Bp = Barometric Pressure
Tabs = Temperature (Absolute)

Example 1.325 x 29.92/(70°F + 460°F) = 0.0748 ~ 0.075 lb/cu ft
This is how standard air density is calculated

If you were measuring air coming out of a furnace, and the air was 154° F the air density would change as follows

1.325 x 29.92/(154°F + 460°F) = 0.0645

If heated air were used in this formula, the constant would be:
(60min x .0645 lbs/cu ft) = 3.87 instead of 4.5 used for standard air.


The constant used in the sensible heat formula 1.08 is used to convert CFM to lbs/hr and factor in .24 the specific heat of standard air (BTU/lb/°F)

Where: 1.08 = (0.24 x 60)/13.33 or 0.24 x 4.5

4.5 = 60 min/hr or (60min x 0.075 lbs/cu ft)
13.33

0.24 BTU = specific heat of standard air (BTU/lb/°F)

The constant 0.68 used in the latent heat formula is used to factor out the amount of heat contained in water vapor in BTU/LB

Where 0.68 = (60/13.33) x (1060/7000) or 4.5 x (1060/7000)

Where: 13.33 is the specific volume of standard air (cu.ft/lb)

1060 = average latent heat of water vapor. (Btu/LB)

7000 = grains per lb or water.

4.5 = 60 min/hr or (60min x 0.075 lbs/cu ft)
13.33

The temperature profile is an excellent idea, but I am afraid the airflow will have to be set correctly using another method.

If you look at the attachment from an ICP product lit, you will see the split varies with airflow as well as entering air conditions.

Thanks

Jim Bergmann

Eugene Silberstein 3

Thanks JIm

Thanks for taking the time to post that.

Whew!

Brad White_101

We agree, Jim!

I was trying to get to a qualitative result but you are correct for quantitative results. (Ever try slinging a psychrometer in a small duct? Don't. )

Seriously, your method will work better, but I would stand by mine as a shortcut to get a rough idea. Without manufacturers' matched coil and condenser data, all is a guess anyway. Using my method will at least get you a range of sensible heat absent airflow data. I have used it with balancing data to corroborate it, and it comes out to within 10-15%. Close enough to gauge where the gross issues lie.

I was alluding to the humidity issue noting how the capacity changes within that range. Generally I have found home systems to have an SHR between 60 and 85 percent depending on, as you say, the psychrometrics.

As the Professor said, thanks for posting that. It is an excellent primer.

udarrell

The house is an electric heat

> home and insulation in attic was doubled layered,

> brand new windows and roof R value unknown. 3 bed

> room with 6" vents, kitchen with 6" den area 2-7"

> vents. 30x24 return with 14" "flex" linset about

> 30-35' Air handler installed is hydro air i guess

> for future heat (optional) i took temp when 92

> outside to try to find issues of concern. all the

> facts were on the cover of the unit that i went

> by and all i think resonable. return air temp 74,

> supply average 58-60 super heat i look on panel

> and wanted 9 super heat. the only thing i didn't

> do is humification test(no instrunments) "TXV valve"

> on hydro 13 seer Heil. i took retun and supply

> temp at register not in attic. superheat was

> taken at condenser. MA. location and we have been

> having a major heat wave. I not the greatest at

> A/C am i missing something, should i take temp at

> air handler and what should the ideal coil temp

> be. could the heat coil be lacking air flow

> blower on high. what should be volice at vent i

> can measure with meter. Thanks for all your help.




You indicate that it has a TXV refrigerant metering device.
A TXV normally has a Superheat setting of around 12-F and you need to check the subcooling for proper charge, and then also verify that it is getting close to 12-F SH.

My question to all of you is: How does a TXV affect any of the formulas you are using to determine CFM, etc., as compared to a fixed orifice metering device?




- udarrell - Darrell

RadPro

Sizing

Hi all,

I remember an old timer giving me different examples of how to quickly size a system. One of the ways involved the temp difference you wanted to achieve. He explained to me that sometimes a unit was sized for a 15 degree delta T. If you used 95 degree outside and subtracted 15 you get 80 degrees. If you wanted 20 delta T you would achieve 75 degrees and that is comfortable for most people. If normal temps in your area are 85 to 90 and now you are getting abnormal ambients your system is too small!

I would think a load calculation would be the first thing to do to see want size is needed for the load at varying conditions.

Correct me if wrong, but most homes do not need 68 degrees to be comfortable! I would say if he is concerned about it, to install a 2 speed system to handle the abnormal loads, it will run on low most of the time and handle his humidity.

The problem with systems that "run all the time" are customers that don't want to live in the real world. If it doesn't run it can't dehumidify and certainly can't cool.

Reminds me of a call about a month back. In the afternoon the area got too warm. Went to check-found stat set at 64 degrees and the unit was running all the time! I asked how "hot" does it get? Mid 70 was the answer in the afternoon. I would hope so!

If the customer wants a walk in cooler, you have to design a different system.

Don in Mo

jim lockard

Diff. temp.

74 degree return air temp. with 90 to 95 degree OST. sound good to me. May be roll the stat to 74 and let it be. J.Lockard

udarrell

I agree Jim, 74 with a 90 to 95 OST ought put the conditioned space in a very comfortable zone, providing the coil is cold enough to get the humidity down where it belongs.

Air movement in the rooms is also very important to achieve the "Human Comfort Zone."

- udarrell

Brad White_102

Essentially correct, Don

in that the delta-T (supply air temperature to room temperature) ratio is only part of the equation.

Normally in design we use 55 degrees as a default supply air temperature to maintain a 75 degree space temperature at 50% coincidental RH. But:

The quantity of air supplied at whatever temperature has to be sufficient to absorb the heat gains to the space (people, lights, appliances, solar and transmission, etc.)

If the delta-T is fixed, you will need to adjust the air volume.

If the air volume is fixed, you need to adjust the temperature of that air supplied to the space.

As for air volume, air 15 degrees cooler than the room will require 33% more air volume than air at 20 degrees cooler than the room, all for the same cooling load.

Conversely, Hi V systems such as Unico use roughly half the air and cool it to the low 40's. Same sensible load, better dehumidification by far.

The "55 degree default" has a basis in humidity control. Air at that temperature, even if saturated (100% RH) has the same specific humidity of air at 75 degrees and 50% RH. Point being, the air you supply wants at some point to be cooled to below the dewpoint of the room served. The degree below that dewpoint is dependent on the humidity load to the space.

In other words, you can control temperature with any air cooler than the room itself, given enough quantity. But unless depressed adequately below the served space's dewpoint, forget about humidity control.

We entirely agree that with the thermostat set below the space temperature, of course the system will keep on running!

Best,

Brad

S Ebels

MB Carlin

Where's the duct located? In the attic or does the place have a basement?

jim lockard

Making water

Udarrell just the fact that you have 74 degree RETURN AIR with a 90/95 OST tells me the system is working. Trying to hit 68 degrees with any amount of humidity in the air may be beyond the limits of your A/C system. Wonder if they could post a indoor wet bulb/dry bulb temp. J.Lockard