Cold Weather Heat Pump Charging

Charging an air source heat pump during cold weather has always been a bit of a problem. The problem is that the amount of refrigerant circulated decreases as the outdoor temperature drops. Why is this? Well, as the outdoor temperature drops, the evaporator temperature has to drop in order to be able to absorb heat from the outdoor air. The lower evaporator temperature produces a lower evaporator pressure. The lower evaporator pressure increases the compression ratio because there is now a greater difference between the suction pressure and the discharge pressure. The higher compression ratio means that the compressor does not circulate as much refrigerant.

At a 45°F outdoor temperature,  a typical air source heat pump produces a heating capacity roughly equal to its nominal cooling capacity. At 17°F outdoor ambient, it produces about half as much heat as it does at 45°F. This difference in capacity is directly related to the amount of refrigerant being circulated. The rest of the refrigerant is just sitting somewhere – normally in either the accumulator or the charge compensator. So a system operating at 17°F outside could have perfect pressures even if it only had half of its factory charge. That is why you can be way off checking a heat pump by pressures in the heating mode.

Some manufacturers provide heating performance pressure charts, but refer to them as “check” charts. They are intended to check the system operation at specific conditions, but are NOT intended as guides for adding refrigerant. The problem is that you don’t have a good way to judge how much refrigerant is stored out somewhere in the system. I can hear a bunch of you saying that measuring superheat and/or subcooling solves that problem. While I AM a fan of checking both, they still just measure the refrigerant that is circulating.

There have been some interesting methods used, such as measuring discharge superheat. For discharge superheat, you measure the temperature and pressure of the discharge line right as it leaves the compressor. It should be somewhere around 60°F warmer than the discharge saturation temperature. So if you have a 410A system running at a discharge pressure of 318 psig (saturation temperature 100°F), the discharge line should measure 160°F. A lower temperature reading indicates an overcharge and a higher temperature reading indicates an undercharge. The surest way to charge a heat pump in the winter is to recover the refrigerant, evacuate the system, and weigh in the correct charge. If you have performed a repair on the refrigerant system, then this will save you time and insure a correct charge.

Checking Airflow is Not Just a Summer Thing

Checking for proper airflow is not just a summer thing. Most techs know that poor airflow is the first thing you should check in the case of freezing up air conditioning coils or lower than normal suction pressures in cooling. We need to be concerned with airflow in the heating season as well. In fact, in the case of heat pumps, airflow is arguably more important in the heating season because the indoor coil is now the condenser. Sure signs of an airflow problem in a heat pump are low suction pressures and icing in the summer and frequent high pressure switch trips in the winter. I have heard several stories about systems which techs “fixed” by adding charge in the summer and taking it out in the winter. In effect, they are overcharging the system in the summer and undercharging it in the winter. Of course this kills both system efficiency and the compressor. In the summer, the overcharge causes liquid dilution of the compressor oil, and in the winter the undercharge makes the compressor run hot. Other signs of poor airflow include open fuse links on strip heaters, open strip heaters, or open thermal limits which eventually fail from opening and closing repeatedly. In gas furnaces, poor airflow will cause a higher than normal temperature rise. In the case of gas furnaces, it is possible to have too much airflow. Too much airflow will reduce the temperature rise below the minimum, which can cause condensation in heat exchangers which are not designed for condensation. Typical temperature rise for mist furnaces is between 40°F and 70°F. However, check the data plate on the furnace for the exact specification.

Perfect Weather for Defrost Problems

WE are experiencing  lot of cold, rainy days here in Georgia. The temperatures hover just above freezing into the mid 40's. This type of weather is perfect weather for heat pump defrost problems. Heat pumps have to defrost more in weather just above freezing than in much colder weather. If the temperature is so cold that water freezes out of the air, there won’t be much water in the air. If there is not a lot of water in the air, heat pump will not develop very much frost. On the other hand, if the temperature is 40 degrees and raining, there is a lot of water available. The coil in the heat pump will be below freezing even when it is 40 degrees outside, so the water will form frost on the coil. The system is more likely to need frequent defrosting at temperatures just above freezing than at much colder temperatures.  This is not to say heat pumps won’t got into a defrost cycle at very cold temperatures, but there is much less ice to defrost. The first thing to check on an iced over heat pump would be to make sure the outdoor fan motor is operating. If the outdoor fan motor is not moving air across the coil, it will frost very quickly in cold, wet conditions. The normal defrost periods would not be able to keep the coil clear. An undercharge can also make a system freeze up faster and keep it from clearing the ice when it does go into defrost. If water created when the coil is defrosted cannot drain away from the unit, a large ice floe can build up underneath the unit. This can create an ice chunk around the bottom of the unit. Heat pump condensers should be elevated enough to allow the water to drain. Sometimes the defrost thermostat just clips onto the coil. Occasionally they become loose and cannot sense the coil temperature, so they will not initiate a defrost cycle in weather that is above freezing. To check a defrost thermostat, just ohm it out. Generally, the defrost thermostat should close when it senses a temperature cold enough to initiate a defrost cycle. This can be checked by disconnecting the defrost thermostat from the circuit and ohming it out. If it is in the correct location, making good contact, and the coil is iced over, the defrost thermostat should be closed (0 ohms). If it is open (OL) it is bad. A thermistor defrost sensor is also checked by resistance, but these typically change resistance with temperature rather than opening and closing like a switch. You have to check the measured resistance against the manufacturer’s specs. Usually, these fail open, so if you have a measurable resistance, the defrost sensor is probably OK. Most defrost boards today have s test function. Jumping across the test pins accelerates time. If the defrost thermostat is closed, jump the test pins. If the board is good, it should initiate a defrost cycle. Do't automatically assume that the problem is a bad defrost board.Changing the board won't help if the problem is caused by one of the other causes.

Airflow in Heating

Technicians who know to check airflow in cooling sometimes neglect to check airflow in heating. But poor airflow can cause heating issues too. If you have a heat pump that is tripping out on high pressure – check for problems that can affect airflow. Dirty air filters, dirty indoor coils, or duct problems can all cause airflow problems. Since the indoor coil is the condenser in the heating cycle, poor indoor airflow will cause high discharge pressures and hot compressors. These can cause tripped high pressure switches or open compressor internal overloads. You would not want to condemn a compressor because the air filter was dirty!

Electric strips may cycle on the their thermal overloads, or the fusible links may open. Anytime you replace a fusible link in a strip, check the airflow and all problems that can cause poor airflow. A stopped up evaporator coil can cause a problem in the heating cycle, even if you do not have a heat pump because it is creating an airflow restriction. Another problem that can be easy to miss is an overly restrictive CLEAN air filer. Most 1" pleated air filters have a very high static pressure drop even when brand new. Homeowners often replace the lightweight filters with these and inadvertently cause airflow problems.

Gas and oil furnaces can cycle on the high limit due to poor airflow. This can be more difficult to catch because most high limits automatically reset. When a furnace cycles on the high limit, the blower keeps running and the burners cycle on and off. Furnaces often cycle on the high limit when they have an airflow restriction. This will shorten the life of the furnace, and is a dangerous situation because you are depending on the safety control to prevent overheating. If the limit would fail to open, the furnace could dangerously overheat. Remember, good airflow in the heating season is really just as important as in the cooling season.

Cold Weather Heat Pump Charging

Checking the refrigerant charge of a heat pump in cold weather can be confusing for many technicians, at least partly due to the many different methods recommended by different manufacturers. Although there are many methods, most are described as heating performance charts or heating cycle check charts. A check chart is just that, a chart for CHECKING the charge during the heating cycle. It is not intended to be used for actually ADDING refrigerant. Why? Lower outdoor temperatures cause a lower evaporator temperature and pressure. Lowering the evaporator pressure increases the compression ratio, so the compressor capacity is reduced. Less refrigerant is being circulated in the refrigerant system because of the diminished compressor capacity at low outside ambient temperatures. The refrigerant that is not being circulated must sit somewhere, usually in the accumulator. For this reason, a charge which is adequate to maintain correct pressures and temperatures at 25°F may not be adequate at 35°F. How? Suppose that the unit was undercharged according to the check chart. If you add just enough refrigerant to bring it up to the required pressures, you have only added enough refrigerant for correct operation at that one condition. When the outside temperature rises, the unit will be capable of pumping more refrigerant. However, since you didn't add any extra refrigerant, there won't be any more to pump. The system will be undercharged. Just adding a little extra is not a great ida either because you really have no way to determine how much extra to add. This is why manufacturers say that the check chart cannot be used to CHARGE the system, only to check its operation. If the system is undercharged, the recommendation from many manufacturers is to recover the refrigerant in the unit and weigh in a total system charge according to the manufacturer's instructions.