Checking numbers

[daviddsims]

Got a 4 ton unit that is acting up on me. Below are my numbers:

Outdoor DB=92
Suction Pressure=82
Discharge pressure 235
Liquid line temp=110
Suction line temp=68
Indoor DB=73.5
ID WB=64
Compressor amps 10.5 out of a fla of 14
Cap tube system

A superheat of 20 is telling me that I am undercharged when my target should be about 7 degrees. Does this sound like I am on the right track?

[Techlite]

Whats the delta T across the evap

[daviddsims]

10 degrees

[Techlite]

little low on charge just need to recheck the superheat after space temp gets down to setpoint

[Techlite]

go for 12 sh

[hewitt]

What type of metering device and what is your supply and return temp.I ASSUME this is a R22 system based on pressures?

[daviddsims]

Cap tube system with 73.5 return and 64 supply. It is an r-22 system.

[Damien]

My TSH shows somewhere around 7, with the conditions you stated. I'd say you're on the right track

[newoldtech]

Is the ductwork tight?

[hewitt]

What is your cfm at return

[hvacrmedic]

You have a bad (under-pumping) compressor. High suction, low head, low amp draw, low deltaT. Adding refrigerant will only run the suction pressure up higher, and it's already way to high for those conditions on an old cap tube system.

[hewitt]

X2

[garyed]

The only way it would be undercharged is if you were sucking air from the attic across the evaporator. If the temps you measured are what are actually going across the evaporator then I agree with Hvacrmedic, you have a weak compressor.

[FlyersFan]

How clean is the condenser coil? High head because of a dirty condenser could be throwing off the rest of your numbers.

[joemach]

I would see if the compressor can pump the system down to at least 0 psi. If not, I would say bad compressor.

[daviddsims]

I measured the indoor temps at the return grill. I was hoping I was not going to hear weak compressor

[hvacrmedic]

I measured the indoor temps at the return grill.

Unfortunately that doesn't change things.

I was hoping I was not going to hear weak compressor

That doesn't either.

[Damien]

You have a bad (under-pumping) compressor. High suction, low head, low amp draw, low deltaT. Adding refrigerant will only run the suction pressure up higher, and it's already way to high for those conditions on an old cap tube system.

So obviously experience comes into play in recognizing the symptoms. As a greenhorn how can I learn to recognize when pressures are too high. Manf. charging charts?

I would guess a subcool number would be good to know in this situation? Add a bit of refrigerant and watch numbers?

[hvacrmedic]

So obviously experience comes into play in recognizing the symptoms. As a greenhorn how can I learn to recognize when pressures are too high. Manf. charging charts?

I would guess a subcool number would be good to know in this situation? Add a bit of refrigerant and watch numbers?

Here's the condensed version.

Part 1, Evaporator TD

Evaporator temperature (SST) determines the cooling capacity of the system. If you have 400 cfm per ton moving through the coil, and you are trying to get the nominal capacity out of the system, then the coil temperature must be a certain temperature in order to do that. At 80° indoor dry bulb, and 67° indoor wet bulb (rating conditions), the SST is going to have to run at 45° in order to achieve the desired capacity. Let's look at the performance data for a typical evaporator coil.



As you can see by the highlighted numbers the SST must be running at 45° plus or minus a few tenths in order for the evaporator capacity to match the system's rated capacity. The evaporator capacity changes dramatically when going from 45° to 40° and to 50° SST. At 50° SST the system is running at only 78% or so of nominal capacity. Obviously a 50° SST would correspond to a higher than normal suction pressure on this system. These physical characteristics of the coil define what is called the "Design Evaporator TD". Since physics isn't concerned with brand names, we should expect to get the same results looking at any other evaporator coil's data sheet. It is under these universal test conditions that the coils are sized and get their capacity ratings. IOW, it doesn't matter what brand of unit you're dealing with, if the manufacturer followed protocol, you must have a 45° evaporator at 400 cfm when the return air is at 80° dry bulb/ 67° wet bulb. The "Design TD" is therefore

80 - 45 = 35°

Part 2, High Efficiency Systems and Over-sized Evaporator Coils

In this section I'd like to go over the reason that high efficiency systems tend to utilize much larger evaporator coils. The primary reason is that the same capacity can be obtained at a higher SST. Looking back at the performance table in Part 1, you should be able to see that the 3 ton coil produces 2 tons of capacity at a SST of 50° with 300 cfm per ton. So if we stick that 3 ton coil on a 2 ton system, we could achieve 2 tons of capacity at 600 cfm and with a SST of 50°. Now if we increase the condenser coil size as well, we can get the head pressure to run lower, and as a result the compression ratio is greatly reduced. Efficiency is greatly improved because we're still generating a full 2 tons of capacity even though the compressor is doing less work. So in a nutshell this is why the coils are over-sized on higher efficiency systems. However, note that the Design Evaporator TD on this new high efficiency system is only 30°. Protocol has been broken. Well, so be it, it's impossible to make progress without breaking old rules.

Part 3, How to use this Information as a Diagnostic Tool.

The most valuable information in this lesson is this: The Evaporator TD will not change as the indoor temperature changes. If the evaporator entering air temperature drops 5°, then the evaporator coil will also drop 5°. The rate of sensible heat transfer depends directly upon the TD and is proportional to it. If TD changes, then capacity will also change. So with a constant capacity system, the TD must remain constant. As such, evaporator TD becomes a valuable diagnostic tool. You can use it to determine whether your suction pressure is running too high or too low, you can use it to evaluate system capacity when air flow is known, or you can use it evaluate airflow when the capacity is known, etc. etc.

Well, that's all good news, but unfortunately it isn't exactly true. In reality system capacity is not a constant, but will vary up and down with both indoor and outdoor temperature changes, and also with indoor RH levels. It does give us a good ballpark figure though, and if the actual TD is different than design TD, well you should be able to determine whether this is due to any of these changes in conditions, or maybe due to something else. For example, if the TD is higher than 35° and ambient is 100° while indoor dry bulb is 70°, then there's a problem. The system capacity is lower than the rated capacity under those indoor/outdoor conditions, whereas the higher than normal TD is saying that the capacity is higher than the rated capacity. This presents a contradiction. This should tell you that the indoor airflow is too low, causing a higher than normal TD. This is just one example of how you can use TD as a diagnostic tool in the field. A TEET chart will provide you with the same information in some cases, but it does so from a different angle. You can use both of these tools together to rule out possibilities in some cases, that is, when the data from either is giving you more than one possibility. The possibility that both point toward will likely be the actual issue.

Part 4, How to determine what the design TD should be for a system when the product data isn't available.

If the nominal cfm is 325-350 per ton (high SEER), then the design TD should be about 30. If the nominal cfm is 400 per ton, then the design TD should be about 35.

In practice you'll find that this information isn't 100% accurate all the time, but it has served me well over the past several years. FWIW I use a slightly different version of TD. It goes as follows: Since RH affects the TD, I thought it would be an improvement to use wet bulb temp rather than dry bulb temp to calculate TD. With this change in the formula the temperature difference will remain more constant with changes in latent load. The evap load is after all a direct function of return wb temp, that being a relationship that the SH charging charts all rely upon. That being said, the wet-bulb-SST TD should be 22° for low efficiency (400 cfm per ton nominal) and 17° for high efficiency (325-350 cfm per ton nominal). These numbers were obtained by substituting the design wb temp for the design dry bulb temp in the earlier equations. To wit:

67(wb) - 45 = 22°
67(wb) - 50 = 17°

Cheers.

[beenthere]

Or he has a piston system with the piston not seating. And thinks the distributor tubes are cap tubes.