Thermostatic Expansion Valves (TXV) (TEV) Applications - set up an Adjustments Sub Cooling Super Heat

                    - with Darrell Udelhoven

Thermostatic Expansion Valves (TXV) (TEV) Applications - set up an Adjustments Subcooling Superheat

Why should you choose air conditioners and heat pumps with thermostatic expansion valve metering devices?

Thermostatic Expansion Valves have advantages in nearly any climate zone where you have a variable temperature in the summertime. The advantages skyrocket when you live in a high humidity climate zone.

First, a thermostatic expansion valve with a low eight (8) Fahrenheit superheat set-point will keep the evaporator fully active at any temperature or humidity level.

Second, it is critically important that the Thermostatic Expansion Valve be properly matched to the condensing unit and the evaporator used in that specific installation.

There is a wide variation in the tonnage applications that a particular thermostatic expansion valve will cover. When the matched equipment uses R-22 refrigerant, Goodman Manufacturing uses a one to 3-tonnage thermostatic expansion valve. When matched with equipment using R410A, they use a specific thermostat expansion valve for each tonnage up to 4-ton four and five-ton, which uses the five-ton TXV to cover both tonnages.

Now let us look at the technical aspects of the TXV versus the fixed piston orifice when operating in different climate zones.

The fixed orifice system increases the head pressure and refrigerant flow rate when there is high outside ambient temperatures; the TXV holes the same superheat setting of 8-degrees Fahrenheit.

When the ambient temperatures are low, 70 to 75°F, the refrigerant flow rate drops off and the superheat skyrockets. At 75°F outdoors and 78 indoor wet bulb, superheat skyrockets to 40° F.

The increase in superheat means that your coil is going to be partially filled with vapor that is being Superheated by the airflow through the coil. This raises the temperature of that portion of the evaporator coil and reduces the gross capacity of the coil.

The thermostatic expansion valve will continue to keep the superheat at 8°F, thus you have optimal condensation of humidity and a full capacity - heat loaded coil. The heat load absorbed by the evaporator coil determines the heat that is exhausted out through the condenser to the outdoors. Always strive to get an optimal heatload through the evaporator coil.

Now, must talk about keeping the coil well below the dew point for optimal condensation of moisture. Since the application temperatures and pressures are higher with the new higher seer air-conditioning systems, where humidity is a problem it would be wise to reduce these cubic-feet-per-minute (CFM) airflow to 350-CFM.

The larger evaporator coil with the low TXV 8°F setpoint and the oversized evaporator coil with an abundance of fin surface coupled with the lower velocity of the 350-CFM airflow across the coil will result in adequate dehumidification.

Checking Pressures - Errantly Thinking it is a Fixed Orifice System 

When checking TXV controlled suction pressures for low charge keep in mind that if the TXV controlled evaporator is receiving flash gas, suction pressures can read normal.

Due to a low charged condenser, the TXV sensor bulb on the suction line will cause the Power Element to be pushed down, - opening the valve and this can raise the suction pressure and temperature readings to normal or above.

The compressor cannot keep up & the evaporator temp & pressure rises with too much vapor filling the evaporator coil, reducing its capacity. The lack of cold vapor could lead possibly to an overheating the compressor & its oil. Check the discharge line temp to see that it is under 225°F. 

For various reasons, many times TXV controlled evaporator's are not listed in the installation instructions.

Every condenser & furnace should be labeled that it has a TXV metering device, or the usual errant assumptions will ensue, that it has a fixed orifice device!

To HVAC techs came out to my brother's home to check his TXV AC system.  They checked the suction line pressure per usual, that read close to 80-psig, and they said I wouldn't put any more Freon in there.

I said, that's a TXV controlled system. They did not have any thermometer to check line temperatures; I said, I the superheat should be around 12°F on a TXV system. They said the superheat should be over 20°F; someone told them that before they came out. well, they left saying they would get Dan to come out, I will let you know, - but doubt that he will ever show up. (More on these critically important TXV application discussion topics soon)

If there are errors in this page it was typed with drag and voice software. My hands and fingers are to miss the to type very much as I make many typos now that I am constantly correcting! I have not checked this page for errors you.

Darrell Udelhoven – udarrell   Posted: 8/30/07
Posted: 8/30/07

When the humidity is higher, the evaporator increases its ratio of latent heat absorption.

Air Temperature Drop through evaporator coil

Air Temperature Drop Through Evaporator Coil (1987 Period)
Indoor temperature and humidity load variations graph.
Refrigeration & Air-Conditioning (ARI) Second Edition, 
Page 624, © 1987

Just a 'rough' brief demonstration of how the latent heat capacity of the DX coil "increases with the increase in room relative humidity.

A modern 2-ton 13-seer system would produce around .70 of a ton or 8,400-Btu/hr, however at 70% Relative Humidity its capacity would increase to around 1.1 ton or 13,200-Btu/hr or over half of the 2-tons would be used for the latent heat-load. "That is around a 36% increase in latent capacity" with some reduction in sensible capacity, --due to a higher humidity!

Conmdenser's Return Air Latenmt Load Graph

Page 618, Refrigeration & Air-Conditioning (ARI) Second Edition, C 1987
Those lower SEER units had higher condenser splits than 12-SEER and higher units.
Sorry, I defiled the graph, 90-db outdoor, 80-db indoors with 67 wet bulb or,
50% RH represents the condenser splits shown above graph.
Graph: 80-DB & 80-WB line-intersect is 100% Relative Humidity.


 I assume no responsibility for how anyone uses the information on my web pages
All HVAC/R work should always be done by a licensed Contractor! This information is only placed on these pages for your understanding & communication with contractors & techs.

This information is for the edification of contractors and techs. I am NOT liable for your screw-ups, you are liable for what you do! - Darrell Udelhoven

Darrell's Refrigeration Heating and Air Conditioning - Federal Refrigerant Licensed - Retired Licensed Contractor
Darrell's Refrigeration Heating and Air Conditioning- Retired
Darrell Udelhoven 

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Posted: 08/30/07