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Residential HVAC

Having issues with refrigerant readings…

Discussion · Sep 11, 2026

Having issues with refrigerant readings… Installed a Guardian RC318 (1.5 ton condenser, R454B) with a new Guardian 40,000BTU furnace and new evaporator coil. Evap came with factory installed TXV (made sure TXV was positioned like the manual says). Also new lineset about 30’ long with new filter drier. Brazed all joints while flowing nitrogen. Vacuumed system to 109 microns and it passed the decay test. Existing ductwork is for a downflow system and the evaporator sits right on top of the concrete (horrible design, I know). Set cooling fan speed and got about 540 CFM at about .52” tesp. Started everything up and suction pressure was 125psi with 16 superheat. Head pressure was 250psi with 1 degree subcooling. I thought it needed charged, but after weighing in refrigerant for about an hour, the suction pressure went up a little and the superheat dropped slightly, but the head pressure and subcooling never changed. Suction temp was about 63 and liquid temp was about 89. Am I missing something? The system is cooling and the thermostat even satisfied before we left to about 74 in the house (outside temperature about 80).

Outcome shared

Ok so after installing a 4-ton unit this week, having the same issue, I couldn't take it anymore and had to call my supply house rep. He knew exactly what I was missing, which is great, but also a little embarrassing. He explained to me that since the indoor coils they sell and we buy are not the new microchannels York is putting out, there is a separate paper they put in the condenser manuals that have ADDITIONAL refrigerant charge depending on what type of indoor coil you are installing. So, with my additional lineset length plus the additional coil charge, I was undercharged. So, we will be going back to the last couple of jobs we've done to make sure everything is set correctly. We had no complaints of these new units not cooling so we obviously put enough refrigerant in the systems to have them running, but not enough for them to run where they need to be. I'm glad I found what was going on, but I'm upset that I didn't find that paper. I guess since I've been installing units for years, I didn't even think to look through the manual other than for the startup sheet. Just another learning curve. Read that FANTASTIC manual, even if you think you know what you're doing.

Replies

Reply · Sep 11, 2026

Also I should add, the table on the condenser said with the outside DB and indoor WB I should have a head pressure of 272psi.

Reply · Sep 11, 2026

The numbers indicate the system is undercharged with a flashing liquid line, but the real mystery is why an hour of charging didn't change the high side. Guardian condensers use microchannel coils, which have a very small internal volume. If any significant amount of refrigerant actually made it into that coil, the head pressure and subcooling would have skyrocketed almost immediately. Here is the breakdown of what those pressures are doing and the two most likely culprits for why the system wouldn't take the charge. ### 1. R454B Pressure-Temperature Breakdown Since R454B is a zeotropic blend with temperature glide, you have to look at Dew Point for suction and Bubble Point for the liquid line: * **Suction (125 psi):** The Dew Point is ~45°F. With a 63°F suction line, your superheat is around 18°F (very close to your 16°F reading). The coil temperature is perfectly normal for a 74°F indoor load. * **Head (250 psi):** The Bubble Point is ~87°F. With an 89°F liquid line, your subcooling is actually slightly negative. The refrigerant isn't fully condensing, and the liquid is flashing before it hits the TXV. ### 2. The Charging Paradox If the system was truly taking refrigerant for an hour, a 1.5-ton microchannel would be severely overcharged. Since the head pressure and subcooling flatlined, the issue falls into one of two camps: **The Refrigerant Never Actually Entered the System** * **Vapor vs. Liquid Charging:** R454B must be charged as a liquid. If the cylinder didn't have a dip tube and wasn't inverted, charging vapor into the suction line is incredibly slow and causes fractionation. You could stand there for an hour and barely move ounces. * **Schrader Core Depressor:** The hose depressor might barely be cracking the suction Schrader. It fills the hose but severely restricts flow into the system, trickling it in at a snail's pace. * **Tank vs. System Pressure:** If you were trying to throttle liquid into the liquid line while the system was running, a tank of R454B at 80°F ambient sits at about 225 psi. The system pressure (250 psi) would push back, physically preventing any refrigerant from entering. **The Compressor is Bypassing** * 125 psi suction and 250 psi head is exactly a 2:1 compression ratio—which is extremely weak. At an 80°F ambient, an R454B system should run a head pressure closer to 280–300 psi to maintain a normal condensing temperature difference. * If the compressor has a damaged seal, a stuck internal relief valve (if scroll), or leaky discharge valves (if rotary), it will just churn the refrigerant. It will pump enough to cool the house, but it will flat-out refuse to build a column of liquid or drive head pressure up, no matter how much charge you dump into it. ### The Concrete Pad Airflow As for the downflow coil sitting directly on the concrete—you are right that it is a horrible design. The cold air slamming into the slab causes massive turbulence that can block airflow across the lower apex of the A-coil. However, severe airflow restrictions normally drive suction pressure *down* (starving the coil). Your 125 psi suction means there is plenty of heat load on the coil, so the immediate issue is refrigerant flow or compression capacity, not the duct design. What did your refrigerant scale say you actually put in?

Reply · Sep 11, 2026

One other observation. If the outdoor ambient was 80 and the LL temp was 89, that seems way too much. Years ago Lennox used what was called the approach method to charge TXV's. That was based on charging until the liquid line temperature "approached" the outdoor ambient temperature. Usually that was some TD of 2 to 4 degrees. Using that method, you are way undercharged. The more refrigerant in the condenser, the more heat of compression is removed, and it will eventually reach the temperature of the outdoor air passing through the condenser. Never will it go below it unless a restriction is present, but that is not the situation here. Just good to know.

Reply · Sep 11, 2026

Thanks for the descriptive responses. I appreciate it. I only put in about 10oz of refrigerant. The tank we have has a dip tube so that could possibly be the culprit if it is damaged somehow. The condenser is supposed to have refrigerant for about 15' of lineset, so an additional 10oz should have been plenty for an extra 15'. I thought the TXV might not have been operating properly and be wide open, but I would assume it would not cool the space, and the superheat would be very low because it started out at about 35 and with adding the 10oz it dropped to 16. Also, the air flows into a small plenum under the concrete and then has three 8" round ducts supplying the house. Absolutely horrible design but the old equipment lasted for 20 years that way and worked "fine".

Reply · Sep 11, 2026

Actually, after thinking about it, whenever I charge systems, I crack the hose slightly to let out air and liquid refrigerant did come out of the hose. So not sure the cylinder is the issue.