How to charge or add

Freon or Refrigerant to your

Air conditioner or Heat pump. 

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Spring 2006:

   The price of energy has almost doubled since this page was originally written. There are many things that you can do to your system that will help its performance, some more than others. One of the most important things you can do with your air conditioner or heat pump that will reduce energy consumption is to make sure that you have the right amount of refrigerant in the system and to make sure the refrigerant is metered where in needs to go at the correct rate. If you have a system that has fixed metering i.e. capillary tubes or a fixed orifice then it will never work as well as it will if it has a thermostatic expansion valve. Most new systems will have thermostatic expansion valves on them and they can be added to many older systems with modifications and in many cases giving the system performance that it never had.

    The following material is somewhat controversial as in many contractors would like me to go away. There are many who would like to make sure that the average homeowner will never work on their own equipment and the industry has gone to great lengths to get the government to regulate the use of refrigerants by the average person. In short blame the last person that you voted for who got elected to office. If you want to learn more about law here is a good fun place to start.

    I will not discuss the legality of handling refrigerants please take that up with the appropriate authorities if you can figure out who they are. I will simply be telling you how to know if your system needs refrigerant and how to know how much to put in. But surf-ice it to say that there is a correct amount of refrigerant that should be in your system and adding any more will not make it work any better but will cause damage.

    On a showing of "Dateline NBC" several years ago they set up 8 companies in the Houston Texas area. After checking out this Trane system thoroughly they let each company look at it and every one either added 2 pounds of R22 or said it needed 2 pounds. by the time they were done this system had over 4 pounds (that is what they reported it could be more) of excess refrigerant in addition to what was already in there. I can understand the first charge but after that the head pressure on that system must have been over 300 psi and by the end of the show the system must have been over 400 (the compressor should have gone off on internal relief). I am guessing that is why they chose to use a Trane or American Standard system. That just goes to show how crooked this trade has become.



    Just about every one is familiar with a gauge manifold, it is those two gauges that have hoses coming out that everyone who has ever done any refrigeration work has. There are two gauges on that manifold one for low pressure (also known as suction) one for high pressure (also known as head). By using these gauges you can tell a lot about how your system is working but the gauges will not tell all you need to interpret them as well as monitor other things like the temperature of the refrigerant lines.

    The Suction pressure is a reflection of how well the evaporator portion is picking up heat (higher) and how well the compressor is removing it (lower). The Head pressure is a reflection of how well the condensing coil is getting rid of the heat (lower) and how much load the system is putting on it (higher). It may sound like a contradiction but high or low pressure is a good thing depending on what is causing it. In general you want the suction pressure to be as high as possible without it being caused by an over charged condition and the head pressure to be as low as possible and still maintain a full liquid line. High efficiency units will have higher suction pressure and lower head pressure than an older unit depending on the size of the coil and the airflow. If you get a steady ready of 90-100 psi suction then you may have a compressor that is not pumping properly. It will act like a system with an open TEV or a missing metering device.



Simply put what makes one unit more efficient than another is bigger coils in relation to the compressor capacity. This is the dirty little secret that no one wants to talk about. Because if they did talk about it then it wouldn't be a secret.

Want a really efficient system add a second compressor that is half the system rating (picture added soon), American Standard/Trane has one that is a 4 ton system with 2 2 ton compressors, or a Lennox (Copeland) Two Speed or the Bristol "Twin Single" (T/S) if you can find one. Update I have found one and it is installed in a 5 ton Rheem system in Fort Washington Maryland (should ring some bells).
 It is currently working in high mode only while I order the start components. When this system is finished I will write a full report on it. If you want to buy a System using this compressor you can order it in the "York Stealth" heat pump or one made by Carrier and a Ground source system made by Water Furnace. If you want one of these systems new, you will pay about 3-5 times what I would charge on a bad day to engineer you a 2-stage system or with some fancy electronics: a true variable speed system.

Enough of my rant now back to the subject at hand. 



Definitions:
    To be added...


    From all my experience I can tell you that an air conditioning system is correctly charged when the liquid line leaving the condenser is full of liquid refrigerant. Adding more refrigerant will simply waste space in the condenser and reduce efficiency and raise the head pressure. The other issue is how well the refrigerant is being fed into the indoor coil. Most older residential systems use capillary tubes, newer equipment uses a drilled piece of metal called an accurator, piston or orifice, real systems use a thermostatic expansion valve (TEV) to regulate refrigerant flow. As far as I am concerned your system is not working correctly unless it has a thermostatic expansion valve. The difference is that mechanical restriction works only for certain conditions but a TEV. will deliver the right amount of refrigerant no matter what the head pressure is and allow a perfect charge (full liquid line without a drop more). Without this device you will have situations where you have to add excess refrigerant (full liquid line and a filling condenser) to raise the head pressure to push the refrigerant through the coil or the restriction (or a lack there of) lets too much refrigerant through and you have a liquid line that has hot gas mixed in with it and does what is called "flooding" or "slugging" the compressor (a bad situation). In either case the system is not working correctly and there is no "right" amount of charge that will make it work right. If you have restricted capillaries like we have seen on too many Rheem/Ruud units the coil will turn to a block of ice (cooling mode) and the compressor will run very hot no matter how much refrigerant you add to run up the head pressure. The solution to this is to replace the indoor coil with one that has a TEV. and a larger capacity and watch the system work better than when it was new.

    I have recently added a TEV/TXV to two system that made them work better than the day that they were installed. One was a GE heat pump located in Annapolis Maryland that is over 20 years old. The owners claims that it is colder now (with the TEV/TXV) than it was when it was installed 20 years ago. The other one was a York heat pump in FT. Washington Maryland (not the one with the Bristol Twin Single) and the owner claims that it is colder now than it was 15 years ago when it was new.

    In short adding a thermostatic expansion valve to a system that uses and orifice/piston system can make it work for the first time. I have since had many experiences where simply adding a TEV/TXV makes all the difference in the world.

    If you are having problems with your system performing and you do not have a TEV/TXV then add one (if you have an orifice system) before wasting any more time trying to make it work.


Rheem/Ruud indoor coil with thermostatic expansion valve.
    Image of Rheem/Ruud heat pump indoor coil with thermostatic expansion valve (cooling mode). A system that doesn't have one of these inside (cooling) and out (heating) cannot be the best that it can possibly be.

    Note: This 3 ton coil had a leak and was replaced in the late 90s. It has since been repaired and donated to a needy family along with a Carrier 5 ton outdoor coil (no compressor at the time) and a 2.5 ton Rotorex rotary compressor in a house that had a 4 ton gas air conditioner and all the surrounding houses have 4 or 5 ton systems, this one cools just fine. No wonder the other contractors hate me!

 Changing the indoor coil to one like this can bring new life to an older malfunctioning system and it can work better than a new one that doesn't have this feature. We have found that most cooling problems start with a dirty or undersized indoor coil that you can't see than the outdoor unit that you can see.

    My partner Gerald had a flattened stack of indoor coils piled to the ceiling of the garage but no replaced outdoor units. In the last 3 years we have replaced no outdoor units but have a pile of leaking or very dirty indoor coils. What does that tell you?



Image of Heat pump TEV/TXV scheme
    This image of a heat pump TEV/TXV scheme was donated by 4Z5AY. The only technical change would be the connection on the left should look like the connection on the right and there would usually be a Bi-flow (heat pump) Filter Dryer to the left of the sight glass not individual dryers.


    The suction pressures shown are for cooling mode. Heating mode will be lower. Head or high pressures will be similar from heating to cooling. One of the methods of charging that I have read about that seems to make sense when explained properly is to add charge as long as the suction line keeps getting colder as soon as it starts to show low super heat (I will explain that later) then there is too much refrigerant; you don't want to flood the compressor or run the head pressure to high. What I usually do on a system that is not a TEV/TXV. is to charge until the suction pressure gets above 55-60 psi then add more until the suction line starts to get cold. remember more efficient systems will have higher suction pressures (80 psi is not uncommon). I also watch my head pressure to make sure there isn't other problems (175-225 is good depending on the condition of the outdoor coil, anything above 250 is cause for concern). If the pressure is above 300 then there is definitely something wrong like a very dirty coil or the system is simply overcharged. The compressor (hermetics {piston} not scrolls or rotary) will have a ball of sweat around where the suction line enters, what Mr. Forest Grauel calls a "happy compressor". If the compressor becomes a ball of sweat then the system is feeding too much refrigerant into the indoor coil. If it is dry then there is a restriction (suction pressure will usually be low; below 50 psi cooling mode and ice will start to grow).
Image of Sporlan liquid line sight glass
    Image of a sporlan sight glass connected to a filter dryer after a liquid line solenoid valve. When the sight glass is clear like this the system is fully charged. Adding any additional refrigerant will just back up into the condensing coil (outdoor in cooling mode, indoor in heating mode) If the suction pressure is still low and the refrigerant is not feeding properly then you have a problem with the metering devices. Adding this device is the easiest way to diagnose a problem system.

    If your system has the refrigerant out for any reason; add one of these before charging. You will thank yourself 100 times over.



Please also See the page on icing. Which is closely related!!!

What is "Super heat"?

    You will hear and see this term all the time in reference to refrigeration. Simply put it is the difference between the temperature of a vapor line in relation to the temperature scale on a pressure gauge for a particular refrigerant or how much liquid is feeding the evaporator in relation to how fast it is being boiled off. for example (R22) if the suction gauge reads 70 psig  then the evaporating temperature is 41 degrees but if the tubing is 51 degrees then you have 10 degrees of Superheat. A typical range for residential air conditioning is 8-18 degrees with some error based on extreme conditions. Once you understand Superheat you can diagnose obvious problems. For example a system that is under charged or has a stuck (closed) metering device will have high super heat (over 20 degrees) at the compressor and a system that is grossly overcharged or has a dirty indoor coil will have very low Superheat about 3-7 degrees with low suction pressure and the suction line will be very cold. It is ok and quite normal for the Superheat to change dynamically while the system is running, you will have to interpret what you are seeing.
 

What is "Sub cooling"?

    Sub-cooling is similar to Superheat but happens in the condensing portion. Refrigerant when condensing will happen at a particular temperature which is very close to the temperature scale corresponding to head pressure for a given refrigerant. After the refrigerant is condensed it will try to assume ambient temperature but will never reach it. The difference between liquid line temperature and condensing saturation temperature is Sub-cooling and is a very good indication of "refrigerant level", but only when proper Superheat is indicated or you could have a misleading indication. Typically 20 degrees of Sub-cooling is desirable and the closer the liquid line temperature is to ambient the better (indicating an efficient system). Checking Subcooling in the heat mode of a heat pump has to be done carefully because you have influence of the space between the indoor coil and the point of measurement. For best heating you will want most of the refrigerant to be condensing in indoor coil without backing it up with refrigerant.

As a general rule Subcooling = Refrigerant charge quantity, Superheat = Refrigerant cycle performance. Check both!!!



 

    The following conditions are for R22 in the cooling mode and relate to indoor coil problems which contrary to human nature we find to be 80% of the reason for poor cooling.

    Human nature says that the Thermostat is "where is all comes from" and if you can just get a new enough outdoor unit with the right brand name everything will be ok, B.S!

I will have to make a chart for outdoor coil problems. The goal for outside is to get a full liquid line and the head pressure as low as possible. On a newer unit 190-225 PSI of head is normal, on some older units (25+ years) 300 PSI of head on a 90 degree day is not out of the question and be normal. Anything higher wash the coil and consider the unit may be overcharged.

    *Warning: There are situations where due to the conditions of you system you may have lower than normal suction pressure and/or higher than normal head pressure. Usually lower than normal suction pressure due to low airflow (either intentional or due to restrictions) and any attempts to raise the suction pressure by overcharging will cause damage by flooding the compressor. The trick is to determine if this pressure is normal for this system, which is usually caused by airflow problems. For example some systems 50-55 PSI (30-35 degree evaporating temperature) is normal. The secret to being a good mechanic it to determine if this is a normal pressure and leave it alone. See Superheat above.
 
 
Normal Superheat 
Normal suction pressure
Suction pressure~65-90 psi
5-15 degrees superheat. depending on size of indoor coil, air flow and condition.* Pressure will track indoor conditions.
Head pressure~190-250 on newer units, 300 not out of the question on an older unit on a 90+ degree day. * Pressure will be a function of outdoor temperature. Full liquid line, sight glass will be clear, compressor will draw near rated current (NEVER charge by current draw).

 
Low super heat low suction pressure Dirty indoor coil, no or poor air flow.  Coil will ice up quickly. One of the most common problems! 0-5 degrees superheat suction pressure 35-40 psi Suction line will freeze out to to the compressor. Compressor will be ruined because of oil dilution.
Low super heat higher than normal suction pressure Over feeding refrigerant or overcharged If overfeeding head pressure will be slightly lower than normal if overcharged head pressure can be sky high. 65-80 psi 5-10 degrees superheat.
High superheat lower than normal suction pressure. Refrigerant not feeding properly. Compressor is being destroyed by overheating. Could be slightly undercharged or a problem with the metering device. If overcharged head pressure can be sky high. 40-60 psi suction. 20-30 degrees superheat.
High superheat very low suction pressure. Refrigerant not feeding at all  Could be almost out of refrigerant. compressor will overheat and be destroyed. 20-40 psi
Compressor will get very hot and shut off on internal protection.

 
Low super heat, high suction pressure. Lower than normal head pressure. Compressor is not pumping properly. Or possibly metering device is missing or stuck open. A heat pump check valve (indoor coil) is blown. Replace compressor. Check for open metering device or blown check valve. Compressor will be flooded.
80-100 psi. Pressures will equalize almost instantly

 



 I Work very long days and sometimes I end up working on this stuff at 3 or 4 am after having been up since 10 am. So some of the information is incomplete and there are some errors here which I will try to correct when I get a moment to sit down and go through this. So I will continue this page when I have more time but for now please follow the links below. If there are any volunteers out there who would like to help me write this please knock your self out.  In a nut shell don't over charge your system.

Final words: As a general rule Subcooling = Refrigerant charge quantity, Superheat = Refrigerant cycle performance. Check both!!!



 This page will be continued later:
Scott Meenen can be reached at 301-591-1646
 

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