Charging the heat pump
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Refrigerants are under high pressure, may displace oxygen in the room and when exposed to high temperatures may break down into poisonous gasses. If you are not confidant, do not continue.
How are heat pumps different to charge than air conditioners
For years heat pumps were considered just like air conditioners for charging purposes. This resulted in many failures of major components.
When charging a heat pump in the summer in the cooling mode, there are many similarities. However, when cooler temperatures prevail the differences become apparent.
In the heat mode, the indoor coil is the condenser. In the cooling mode with the outdoor coil as a condenser, the air flow across the coil was pretty much always the same unless the coil was plugged with leaves, etc.
With the indoor coil as a condenser, the air flow is dependent on the sizing and condition of the ductwork, how clean the blower wheel is, how clean the coil is (indoor coils become dirtier faster than outdoor coils because indoor air is much dirtier), and how dirty the air filter is.
So where we a had a fairly constant airflow outside and could depend to a certain extent this part of the system, we could concentrate on other factors.
Also, with the indoor coil as a condenser, there is more opportunity for compressor damaging high head pressures.
Also, if the indoor air is restricted and the unit is controlled by a fixed orifice expansion device, charge balance may be impossible.
The early years of heat pumps were a mass graveyard of compressors.
So, how do we charge them?
Well, it depends on the outside temperature. Usually below 35F we charge them by weight, although that is also not always accurate. The lower the temperature the harder they are to charge. I will be charging a heat pump at low temperatures to demonstrate how it can be done.
The video below gives the method to recharge a water source heat pump.
When charging a heat pump in the summer in the cooling mode, there are many similarities. However, when cooler temperatures prevail the differences become apparent.
In the heat mode, the indoor coil is the condenser. In the cooling mode with the outdoor coil as a condenser, the air flow across the coil was pretty much always the same unless the coil was plugged with leaves, etc.
With the indoor coil as a condenser, the air flow is dependent on the sizing and condition of the ductwork, how clean the blower wheel is, how clean the coil is (indoor coils become dirtier faster than outdoor coils because indoor air is much dirtier), and how dirty the air filter is.
So where we a had a fairly constant airflow outside and could depend to a certain extent this part of the system, we could concentrate on other factors.
Also, with the indoor coil as a condenser, there is more opportunity for compressor damaging high head pressures.
Also, if the indoor air is restricted and the unit is controlled by a fixed orifice expansion device, charge balance may be impossible.
The early years of heat pumps were a mass graveyard of compressors.
So, how do we charge them?
Well, it depends on the outside temperature. Usually below 35F we charge them by weight, although that is also not always accurate. The lower the temperature the harder they are to charge. I will be charging a heat pump at low temperatures to demonstrate how it can be done.
The video below gives the method to recharge a water source heat pump.
Charging the heat pump in winter.
One of the hardest procedures involving heat pumps is charging in the winter. Most manufacturers recommend charging split systems by weight.
Charging by weight is not always as simple as it may seem. First the unit comes with a factory charge. Depending on manufacturer and expansion device type, the factory charge may be enough for up to 25' of lineset. Longer linesets will need additional charge. So, how much charge does the unit need?
If you have the factory install manual and/or service manual, the info is generally there. The problem comes when there is no manual or the system is made up of unmatched components. When systems are manufactured, the indoor coil and even the air handler are matched to work with the outdoor unit. When systems are mixed, especially when the indoor coil is not matched, there may be unusual pressures and temperatures and charging by weight may not be accurate.
Also, when servicing equipment, a liquid line drier is usually installed. It may be a replacement drier for one that was originally installed when the system was manufactured. Or it may be installed when the service is done. In either case, the replacement drier will be larger than any original drier.
Because it contains liquid, refrigerant must be added to compensate for the drier. Drier manufacturers have specs for how much their driers hold.
So, drier size can bring in some questions about how much charge to add.
It is not so critical if the unit uses TXVs to control flow, but if it uses fixed orifices, the charge is critical.
The video below begins the explanation of methods to charge the heat pump in winter.
Charging by weight is not always as simple as it may seem. First the unit comes with a factory charge. Depending on manufacturer and expansion device type, the factory charge may be enough for up to 25' of lineset. Longer linesets will need additional charge. So, how much charge does the unit need?
If you have the factory install manual and/or service manual, the info is generally there. The problem comes when there is no manual or the system is made up of unmatched components. When systems are manufactured, the indoor coil and even the air handler are matched to work with the outdoor unit. When systems are mixed, especially when the indoor coil is not matched, there may be unusual pressures and temperatures and charging by weight may not be accurate.
Also, when servicing equipment, a liquid line drier is usually installed. It may be a replacement drier for one that was originally installed when the system was manufactured. Or it may be installed when the service is done. In either case, the replacement drier will be larger than any original drier.
Because it contains liquid, refrigerant must be added to compensate for the drier. Drier manufacturers have specs for how much their driers hold.
So, drier size can bring in some questions about how much charge to add.
It is not so critical if the unit uses TXVs to control flow, but if it uses fixed orifices, the charge is critical.
The video below begins the explanation of methods to charge the heat pump in winter.
One of the ways to charge the heat pump in winter is the discharge temperature method.
This method uses the temperature of the compressor discharge pipe to determine charge.
This method is sometimes used when the unit uses a fixed orifice for the expansion device.
The technology behind this that there is a relationship between the temperature and liquid content of the suction gas coming in to the compressor and the discharge temp.
For example, if the returning gas was all gas and at a temperature of 40F degrees and the heat of compression adds 100F, the discharge temp would be 140F.
If some liquid is in the return gas, the latent heat will reduce the discharge temperature.
If system airflow is at proper levels, as the outside temperature reduces, the system head pressure will reduce. The reduced head pressure will reduce the refrigerant flow thru the orifice. The balance of refrigerant flow can be read fairly accurately by the discharge temperature. You may think superheat would be the best way to check charge in low temps, but it does not work well at low temps. The video below goes thru charging with this method.
This method uses the temperature of the compressor discharge pipe to determine charge.
This method is sometimes used when the unit uses a fixed orifice for the expansion device.
The technology behind this that there is a relationship between the temperature and liquid content of the suction gas coming in to the compressor and the discharge temp.
For example, if the returning gas was all gas and at a temperature of 40F degrees and the heat of compression adds 100F, the discharge temp would be 140F.
If some liquid is in the return gas, the latent heat will reduce the discharge temperature.
If system airflow is at proper levels, as the outside temperature reduces, the system head pressure will reduce. The reduced head pressure will reduce the refrigerant flow thru the orifice. The balance of refrigerant flow can be read fairly accurately by the discharge temperature. You may think superheat would be the best way to check charge in low temps, but it does not work well at low temps. The video below goes thru charging with this method.
Charging by superheat in winter is not an effective charging method. The video below covers an attempt to charge this way.
The video below cover the operation of charging the heat pump by weight.
more coming