Gas and Oil Home Heating Furnaces - pressures not right on lennox unit
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k.rogan
07-09-02, 06:33 PM
I have a LENNOX HS10-311V-4P unit, not sure how old it is. The A coil was replaced on it last year by my now ex-home warranty company with a "new high effiency one" that is half the size (long story.)
Any way this summer it was doing fine until it got into the mid 90's outside and my unit quit cooling right in the afternoons. I checked the pressures and they were 65/185. I thought I was a little low on R22 so I added a couple of pounds and my pressures were then 90/200. They should be about 70/265. Any ideas? The cold tube on the compressor stays cool after it shuts off, so I do not think the high pressure side is bleeding back.
The A-coil and the outside unit are both clean, the inside blower and the outside cooling fan are running fine.
I would appreciate any suggestions.
Any way this summer it was doing fine until it got into the mid 90's outside and my unit quit cooling right in the afternoons. I checked the pressures and they were 65/185. I thought I was a little low on R22 so I added a couple of pounds and my pressures were then 90/200. They should be about 70/265. Any ideas? The cold tube on the compressor stays cool after it shuts off, so I do not think the high pressure side is bleeding back.
The A-coil and the outside unit are both clean, the inside blower and the outside cooling fan are running fine.
I would appreciate any suggestions.
lynn comstock
07-09-02, 10:14 PM
Real understanding of the gauges comes when you forget pressures and think about the temperatures of the boiling or condensing refrigerant…or you can skip to the information in CAPS.
The gauge reading are always influenced by:
1) Outdoor temperature.
2) Indoor temperature.
3) Refrigerant (usually R-22).
4) Airflow across the outdoor coil (about 1000 cfm per ton).
5) Airflow across the indoor coil (about 400 cfm per ton).
6) The efficiency of the system (a design issue).
7) Running time…it takes 8 to 10 minutes for the system to arrive at steady state conditions. Interpretation of the gauges depends on equilibrium (steady state) in the system.
IF these factors are normal, the coils are clean, the compressor is OK/normal, the metering device is OK/normal, the components are matched and designed to work together (LOTS OF IFS)...and assuming the refrigerant is R-22:
The head (high side) pressure depends on the TEMPERATURE at which the refrigerant (R-22) condenses in the condenser (outdoor coil). Add 15 to 25 degrees F to the outdoor temperature (say 90+15=105 and 90+ 25=115) and look at the TEMPERATURE information for R-22 on the gauge (green ring of numbers).
THE HIGH SIDE NEEDLE SHOULD POINT TO A TEMPERATURE BETWEEN 105 AND 115 (in the green ring of numbers) AND THE PRESSURE IS READ ON THE BLACK RING OF NUMBERS FOR PRESSURE.
The suction pressure depends by the TEMPERATURE of the boiling (evaporating) refrigerant (R-22) in the evaporator (indoor or cooling coil). This temperature should be about 35 to 40 degrees F below the Return Air temperature. IF the return temperature is 75 the evaporating temperature of the R-22 should be between (75-40=35) and (75-35=40).
THE LOW SIDE NEEDLE SHOULD POINT BETWEEN 35 AND 40 DEGREES (in the green ring of numbers) FOR R-22 AND THE PRESSURE IS READ WHERE THE NEEDLE POINTS ON THE BLACK RING OF NUMBERS FOR PRESSURE.
These TEMPERATURE rules apply to any refrigerant used for comfort cooling...even one that you are seeing for the first time in your life.
The gauge reading are always influenced by:
1) Outdoor temperature.
2) Indoor temperature.
3) Refrigerant (usually R-22).
4) Airflow across the outdoor coil (about 1000 cfm per ton).
5) Airflow across the indoor coil (about 400 cfm per ton).
6) The efficiency of the system (a design issue).
7) Running time…it takes 8 to 10 minutes for the system to arrive at steady state conditions. Interpretation of the gauges depends on equilibrium (steady state) in the system.
IF these factors are normal, the coils are clean, the compressor is OK/normal, the metering device is OK/normal, the components are matched and designed to work together (LOTS OF IFS)...and assuming the refrigerant is R-22:
The head (high side) pressure depends on the TEMPERATURE at which the refrigerant (R-22) condenses in the condenser (outdoor coil). Add 15 to 25 degrees F to the outdoor temperature (say 90+15=105 and 90+ 25=115) and look at the TEMPERATURE information for R-22 on the gauge (green ring of numbers).
THE HIGH SIDE NEEDLE SHOULD POINT TO A TEMPERATURE BETWEEN 105 AND 115 (in the green ring of numbers) AND THE PRESSURE IS READ ON THE BLACK RING OF NUMBERS FOR PRESSURE.
The suction pressure depends by the TEMPERATURE of the boiling (evaporating) refrigerant (R-22) in the evaporator (indoor or cooling coil). This temperature should be about 35 to 40 degrees F below the Return Air temperature. IF the return temperature is 75 the evaporating temperature of the R-22 should be between (75-40=35) and (75-35=40).
THE LOW SIDE NEEDLE SHOULD POINT BETWEEN 35 AND 40 DEGREES (in the green ring of numbers) FOR R-22 AND THE PRESSURE IS READ WHERE THE NEEDLE POINTS ON THE BLACK RING OF NUMBERS FOR PRESSURE.
These TEMPERATURE rules apply to any refrigerant used for comfort cooling...even one that you are seeing for the first time in your life.