Tag Archives: REFRIGERANT SYSTEM

Understanding Superheat and Subcooling

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

Superheat and subcooling are the terms used to describe two of a heat pump system’s operating characteristics. We, in the service business, generally rely on these numbers to evaluate system performance as well diagnose system problems. The values essentially provide us with information about what’s going on inside the evaporator and condenser coils. And depending on the metering device used in the system, one or the other number is the value used to determine optimum system charge.

If you wanted to define the words non-mathematically, superheat is the increase in temperature of the refrigerant vapor in the evaporator before it exits the coil, and subcooling is the decrease in temperature of the refrigerant liquid in the condenser before it exits the coil. The diagram below offers a visual illustration.

The two numbers are actually calculated temperature values, using simple arithmetic with saturated temperatures and tubing temperatures.

Normal operation always results in some percentage of the evaporator coil filled 100% with vapor and some percentage of the condenser coil filled 100% with liquid. Since the vapor starts out at the same saturated suction temperature, the vapor will take in heat or warm up, before it exits the evaporator coil. Likewise, the liquid starts out at the same saturated condensing temperature, so it will give up heat or cool down before exiting the condenser coil.

So to calculate the superheat, subtract the saturated suction temperature from the suction line temperature. Which in the diagram is 50 – 40 = 10 F superheat.

To calculate subcooling, subtract the liquid line temperature from the saturated condensing temperature: 110 – 100 = 10 F subcooling.

More discussion: HvacR Professional.Com

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

Troubleshooting Heat Pump Refrigerant Systems

You can post questions and get answers from professionals here:

Hvac/R Professional.Com

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R-410A System Pressures

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

Heat pump system pressures are indirect measurements of saturated temperatures…the pressures are simply the result of the particular refrigerant used in the system. If you know what the saturated temperatures are supposed to be, you can easily determine the pressures.

Lets assume we’re connected to the R-22 heat pump system below running in the cool cycle, measuring pressures, tubing temps and calculating superheat and subcooling.

So, we’re looking at a head pressure of 225 psi, a suction pressure of 75 psi, liquid line temp @ 100F and suction line temp @ 55F. If we convert the head and suction pressures to saturated temperatures, the results are a condensing temp of 110F in the outdoor (condenser) coil and a boiling temp of 45F degrees for the liquid refrigerant in the indoor (evaporator) coil, giving us 10F degrees of subcooling, and 10F degrees superheat.

Now, suppose for the sake of discussion, we recover all the 22 refrigerant from the system, replace the oil, replace the 22 TXV with a 410A TXV, re-charge the system with 410A refrigerant, and start the equipment back up…Well, guess what? The system temperatures would be the same values (or at least near the same values). The only thing different would be the head and suction pressures.

We couldn’t actually convert a 22 system to 410A that simply…but you could have a 22 system and 410A system side by side, operating under the same indoor and outdoor conditions and see about the same operating temperatures. The gist of the post is to illustrate the fact that heat pumps (mechanical refrigeration systems) are designed to produce or generate, temperatures. The subsequent system pressures are simply the result of the saturated pressure-temperature relationships for a particular refrigerant at a particular temperature…

More discussion: Hvacr Professional. Com

You can get a full explanation and illustrations of heat pump operating pressures and temperatures in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

Troubleshooting TXV Systems

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

This short video illustrates the symptoms of some of the problems associated with TXV systems…can you diagnosis the numbers?

More discussion: Hvacr Professional. Com

You can see a more in depth explanation of TXV operation and illustrated failures in the “Troubleshooting Heat Pump Refrigerant Systems” and “Troubleshooting TXV’s” rental videos:

Troubleshooting Heat Pump Refrigerant Systems

Troubleshooting TXV’s

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

You can post questions and get answers from other professionals here:

Hvac/R Professional.Com

TROUBLESHOOTING HEAT PUMP SYSTEMS / RECOGNIZING LOW AIRFLOW

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

One of the more common problems found in service work is low airflow across the indoor coil. This situation can be due to coil restriction, inadequate/damaged ductwork or dirty filters to name a few. Much has been written in regards to airflow and it’s critical role for proper heat pump operation and performance. The design value for indoor airflow with A/C’s and heat pumps has, to the best of my knowledge, always been 350-450 CFM per ton. Values less than 350 generally create operational problems like coil frosting in the cool cycle or high head pressure in the heat cycle. Values greater than 450, though the lesser of two evils, would produce situations better described as performance problems…poor moisture removal in the cool cycle, or low discharge temperatures in the heat cycle. But, with typical residential systems, excessive airflow is rarely a problem, simply because residential duct systems are rarely oversized. So, the usual situation becomes one of recognizing and correcting low system airflow.

All residential equipment, be it heat pump air handlers, gas furnaces or packaged units, is limited in its capacity to move air and the limiting factor is system external static pressure. External static pressure is for all practical purposes, a measurement of the resistance encountered by the air as it moves through the duct/air distribution system. The standard for maximum ESP is about 0.5 inWC, and residential duct systems have to be designed and sized, to meet this capability of the blower. Otherwise, the ESP may exceed 0.5 and the airflow per ton will be less than 350 CFM…

With no practical understanding of system operation, determining airflow volume would require taking some kind of measurements and doing some calculations that eventually provide a CFM value. Or, if a specific value of airflow is required, so will be the measurements and calculations. But for me, most of the time, the issue comes down to either having enough airflow, or not. And by “enough” I simply mean, the system will run 24 hours without frosting the indoor coil in the cool cycle, or producing excessive head pressures in the heat cycle. Now, if you’re attempting to evaluate overall system performance and efficiency, which depends to a great extent on airflow, “enough” probably isn’t adequate. But keep in mind, service calls are usually situations where the homeowner was content with the system performance yesterday, but not today. So, I always start with the assumption the airflow has at some point in time, been satisfactory, adequate or “good enough”…and most of the systems I find fall into this category. All I need to do is return the system airflow volume back to or near, whatever it was on Day 1, regardless what that actual value is. And that usually amounts to changing a filter, or cleaning the coil.

Airflow calculations aren’t necessary to decide there isn’t enough. If, in the cool cycle, the suction pressure is low and the superheat is low for fixed orifice systems, or normal for TXV systems, the airflow is low. If, in the heat cycle, the head pressure is high, the cause is either overcharge or low airflow, or both…sometimes, techs will overcharge a fixed orifice system in the cool cycle to correct low suction pressure resulting from a dirty indoor coil. But you can ask a few questions to determine if that’s the case. Common sense dictates indoor coils will restrict over time, due to a variety of possible reasons. So when you see symptoms of low airflow, that’s the first thing to suspect. Occasionally you get lucky and find a dirty filter, but more often than not, the coil is the culprit.

More discussion: HvacR Professional.Com

You can see a more in depth explanation of refrigerant system operation with low evaporator air and illustrated 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…RESTRICTED LIQUID LINE FILTERS

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

Liquid line filters occasionally restrict for whatever reasons, resulting in a significant pressure drop across the filter. The restriction forces more than the normal amount of liquid refrigerant to remain in the condenser coil, which will increase the subcooling value. Head pressure will usually drop a little or remain near normal, while suction pressure will be low and superheat high, due to the evaporator coil being underfed. Of course, where pressure measurements are taken relative to the filter location will determine “head” pressure readings. If the filter is located upstream of the pressure port, the reading would reflect the pressure drop across the restriction and be low. One of the best methods for recognizing a restricted filter is a temperature drop through the filter. The restriction and pressure drop most often creates “flash gas” which generates some refrigeration effect, and causes the filter to cool. A noticeable difference in temperature between the inlet and outlet of the filter is a definite symptom of some restriction. The following clip from the heat pump refrigerant video provides a good visual.

Troubleshooting Heat Pump Refrigerant Systems

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