Output Air Temperature

I had a contractor install an A/C unit about two summers ago. The output air temperature is around 68 degrees F from the opening of the air duct. The weather is hot and the indoor temperature stays around 80 degrees with the thermostat set at 65 degrees. This has been the case since the installation.

My question concerns what output air temperature I should expect from this model.

ANSWER:
From Gene Silberstein
Consulting Engineer
Whitestone, N.Y.

Generally speaking, a split-type air conditioning system will yield a temperature split across the evaporator of 17 degrees to 20 degrees F. This means that, if the temperature of the air returning to the evaporator coil is 80 degrees, the temperature of the air coming off the coil should be in the range of 60 degrees to 63 degrees. You indicated that the temperature of the supply air was 68 degrees with an 80 degree room temperature, resulting in a temperature split of 12 degrees.

In order to determine if the system is providing adequate cooling, you should take temperature readings of the air in the return duct very close to the evaporator coil and of the air as it leaves the evaporator coil. Taking the difference between the two temperature readings will give you a more accurate temperature split. If the temperature split you calculated is in the 17 degree to 20 degree range, the problem is likely in the duct system, not the refrigerant circuit. If the temperature of the return air at the air handler is much higher than the temperature of the air in the occupied space, there may very well be air leaks in the return duct.

The hot air in the attic is being pulled into the air handler, decreasing the operating efficiency of the system. In addition, if the return duct is not insulated, the heat from the attic will be transmitted through the duct into the air handler. This is assuming that the return duct is passing through the attic and the air handler is in the attic as well. If the temperature of the supply air at the air handler is lower than the temperature of the air coming from the supply grilles in the occupied space, the supply duct may not be insulated properly. Improperly insulated supply ducts will permit attic heat to transfer through the duct to the supply air, thereby increasing the temperature of the supply air. Your problem may be one of these or a combination of both.

If the temperature split you calculated is in the 12 degree range as you originally reported, your duct system is likely in good shape and you should turn to the refrigerant circuit. A low temperature split across the evaporator can result from an underfed evaporator (when there is less refrigerant flow through the evaporator than desired), a refrigerant undercharge, or noncondensables in the system.

An underfed evaporator is characterized by low suction pressure and high head pressure. One possible cause for this is a restriction in the liquid line, quite possibly the liquid line drier. By taking temperature readings at the inlet and outlet of the liquid line drier and figuring the difference between the two, a blockage or partial block can be detected. There should be a very low temperature difference. A large temperature difference indicates that the drier is blocked and should be replaced. Another possible cause for this could be the thermostatic expansion valve. If the inlet strainer is clogged, it will yield the same result.

A refrigerant undercharge is characterized by low suction pressure and low head pressure. Low refrigerant charge results from a system leak or improper equipment installation. If the system never worked properly, chances are that the system was installed and started up on a very hot day, when operating pressures are higher than on cooler days.

Noncondensables in the system are characterized by high suction pressure and high head pressure. In order to determine if the system contains noncondensables, the system should be shut down and allowed to rest for at least 30 minutes. After this, gauges should be installed on the system. The pressure in the system should correspond to the ambient temperature on a pressure-temperature chart.

If it is determined that the system contains noncondensables, the refrigerant needs to be properly recovered. Once the refrigerant has been removed from the system, the liquid line drier should be replaced. If the system has one, the suction line drier should be replaced as well. If the system does not have a suction line drier, one should be installed. After this, the system needs to be properly evacuated before new refrigerant is introduced.

Milk Storage Tank

I am working on an old milk storage tank used on a farm in upstate Pennsylvania. The manufacturer is Sunset from St. Paul, Minn. The farmer - like all farmers - has little money. He wants me to install this used tank. I thought of converting the refrigerant from R-12 to -134a. But some people I have talked to said not to do so.

The valve is an Alco T0304F0. The tubing is 5 SH7, MOPD 45. It is a 3-ton, two-bolt valve. The unit uses a Tecumseh compressor.

I would appreciate any help. Is the valve still available? I have no problem staying with R-12, or would you recommend something else?

ANSWER:
By Emerson Flow Controls
St. Louis

You should do the following: 1. Take off the old power assembly and cage (from the old valve). 2. Cap off the old "to" body flange with the A3436 flange cap. 3. Install a completely new TCLE 3FC (including a new body flange) upstream of where the old body was capped off. The new valve will work with either R-12 or -134a.

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Publication date: 03/01/2004