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Category Archives: Charging Heat Pumps

Charging TXV Systems

(The content of this post is intended for consideration by trained service personnel only)

Somewhere in a previous post there is some discussion and pics relative to charging heat pumps in the cool cycle. The method of charging depends on the metering device feeding the evaporator coil. Fixed orifice systems have to be charged by the superheat method, TXV systems by the subcooling method. The video below illustrates charging by the subcooling method…

You can get more explanation of heat pump charging details in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

 

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Superheat…or Subcooling?

(The content of this post is intended for consideration by trained service personnel only)

So, here’s the question: How do you charge fixed orifice and txv systems? Same way? No. Fixed orifice systems can only be charged via superheat. TXV systems have to be charged by the subcooling method. The explanations for the answers are related to how TXV’s do what they do and how fixed orifice devices do what they do. Fixed orifice devices deliver a rate of refrigerant flow dependent on the pressure differential across the orifice. TXV’s are active devices that “look” at the suction line temperature and evaporator temperature and maintain a fairly consistent superheat value…Following are illustrations of both. (You can “click” the pics for a slightly larger view)

Fixed Orifice System…R-22

I started a service call on this system knowing there’s a small leak and that the system needed topping off…

An early reading of suction pressure and superheat…

Then I got carried away…”0″ deg superheat.

Removed some refrigerant, and got a number that was about right for the particular indoor and outdoor conditions.

Charging fixed metering systems by superheat is a delicate process, requiring an indoor wetbulb temperature, outdoor drybulb temperature and some “tool” for determining the required superheat for the given system and conditions.

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TXV System…R-410A

This one is a new system start-up.

Early reading…subcooling top right value.

After adding a few ounces of 410A…

You can see a complete explanation of superheat and subcooling in the “Troubleshooting Heat Pump Refrigerant Systems” rental video:

Troubleshooting Heat Pump Refrigerant Systems

 
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Posted by on April 21, 2011 in Charging Heat Pumps

 

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Overcharged System

(The content of this post is intended for consideration by trained service personnel only)

I was called on this unit, found some electrical problems and noticed the compressor sounded not quite right. On checking the compressor amp draw, it was running right at RLA. I messed with it a while and finally connected the gauges and temp sensors. What you’re seeing are the system pressures, saturated temps and liquid line temp. The net result is 50 degrees subcooling.

You can see an explanation and demonstrations of all the common refrigerant system failures in the “Troubleshooting Heat Pump Refrigerant Systems” rental video.

Troubleshooting Heat Pump Refrigerant Systems

(The content of this post is intended for consideration by trained service personnel only)

 

TROUBLESHOOTING HEAT PUMP SYSTEMS…INDOOR / OUTDOOR TEMPS VS SUPERHEAT W/ FIXED ORIFICE SYSTEMS

(The content of this post is intended for consideration by trained service personnel only)

..If you look at a superheat charging chart for a fixed orifice system, you quickly see the required superheat varies with outdoor and indoor conditions. As the outdoor temperatures vary, so does the required superheat…pretty much the same relationship for indoor temperatures. Why? The net force pushing liquid through the metering device is the difference in the head and suction pressures, more or less.

The point being, if the outdoor temperature is 75F you don’t won’t want a “beer can cold” suction line…because by the time the afternoon temperature hits mid-90′s, the increased head pressure will have increased the “net force” pushing the liquid through the orifice, and the system will be overcharged, resulting in a lower than desired superheat.

Likewise, if the indoor temps are “high”, superheats will be high. Most charging charts use indoor wetbulb as the control variable, since wetbulb temps include the humidity factor. As indoor wetbulb goes down, the superheat will decrease, everything else being equal. The following clip demonstrates variations in superheat with outdoor conditions.

You can see an explanation and demonstrations of all the common refrigerant system failures in the “Troubleshooting Heat Pump Refrigerant Systems” rental video.

Troubleshooting Heat Pump Refrigerant Systems

(The content of this post is intended for consideration by trained service personnel only)

 

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