Mini Split R410A Refrigerant Working and Test Pressure

Mini splits require the refrigerant or freon to operate and they operate at a certain range of pressure. Some people charge the refrigerant of their mini splits by themselves and thus, they are wondering what should R410A mini split pressure be?

The low side pressure of a typical R410A mini split is between 110-140 psi depending on the operating conditions. However, it should not fall below 100 psi or raise beyond 160 psi.

The working pressure of mini splits is depending on factors such as the weather and cooling load. Hence, it is important that we understand the fundamentals behind the working pressure of a mini split.

What Should R410A Mini Split Pressure Be?

Most mini splits today are using the R410A refrigerant. So, they should be operating in between 110-140 psi of pressure on the low side. Most of the time, we don’t measure the high side pressure.

The low side pressure is the pressure of the refrigerant running inside the suction line. It is the pressure of the refrigerant before the compressor.

On the other hand, the high side pressure is the pressure of the refrigerant running inside the discharge line. it is the pressure of the refrigerant after the compressor.

However, the low side pressure of R410A mini splits should never fall below 100 psi or raise beyond 160 psi.

At 100 psi, the temperature of the R410A refrigerant is at 0°C or freezing point of water. Hence, the surface temperature of the cooling coil will also be near the freezing point thereby causing the air passing through the cooling coil to freeze up.

When the cooling coil is frozen, its heat exchange capability drops and thus, further decreases the low side pressure and eventually causes the mini split to trip due to low pressure.

On the other hand, the temperature of the R410A refrigerant is at 14°C when the low side pressure is at 160 psi. Hence, the surface temperature of the cooling coil will also be about 14°C which is very close to the dew point of air at dry bulb 25°C and RH 55% which is 16°C.

While approaching the dew point temperature, the dehumidification capability of the mini split drops, potentially causing high humidity problems.

Furthermore, as the surface temperature of the cooling coil is around 14°C, the supply air temperature will also be around 14°C which is considered high and possibly causing insufficient cooling.

Factors that Affect Mini Split Working Pressure

Mini splits adopt the refrigerant cycle. Naturally, the working pressure varies based on the operating condition. Below are the factors that affect mini split working pressure:

  • Outside air temperature – If the outside temperature is higher, the operating pressure is higher.
  • Cooling load – If the cooling demand is higher, the operating pressure is higher.
  • Fan performance – If the fan of mini splits deteriorated, less heat is dissipated thereby causing the operating pressure to raise.
  • Dirty condenser coil – If the condenser coil is dirty, heat exchange is ineffective thereby causing the operating pressure to raise.

How to Pressure Test a Mini Split?

Most of the time, you only need to pressure test the refrigerant lines or copper pipes that you’ve installed for the mini split instead of connecting the mini split and pressure test the entire system.

This is because most mini splits are factory-tested. Manufacturers should replace a new unit or make necessary rectification for free if you found that the internal pipes of the mini split is leaking.

In construction projects, we don’t pressure test any of the mini split itself. Instead, we bypass the mini splits and only pressure test the copper pipes so that when there are internal leakages, we have the evidence to claim for a full warranty.

But, if you’re going to pressure test the entire system, make sure to check the maximum or design pressure of the low and high sides. It is usually stated on the outdoor unit nameplate or written in the installation manual.

As a general rule, the pressure test should have a pressure of about 20% more than the expected working pressure. So, if an R410A mini split is operating at less than 160 psi of pressure, the pressure test should be around 190 psi. However, we need to account for the high side pressure.

Usually, the high side pressure of an R410A mini split is between 230-300 psi. However, the maximum high side pressure could go up to about 320 psi, especially in hot and humid countries.

Even in cold and dry countries, inefficient heat transfer due to dirty mini splits can push the high side pressure to around 300 psi.

Hence, the pressure test should be around 380 psi, subject to the safe working pressure of the copper pipes.

Following are three tables stating the working pressure of different types of copper pipes:

Pipe DiameterWorking Pressure (Annealed)Working Pressure (Drawn)
3/8″465 psi855 psi
1/2″409 psi766 psi
3/4″334 psi625 psi
1″284 psi532 psi
1-1/4″279 psi522 psi
1-1/2″275 psi516 psi
2″249 psi467 psi
Type M Copper Pipe to ASTM B88
Pipe DiameterWorking Pressure (Annealed)Working Pressure (Drawn)
1/4″730 psi1368 psi
3/8″638 psi1197 psi
1/2″584 psi1094 psi
5/8″511 psi958 psi
3/4″469 psi879 psi
1″405 psi760 psi
1-1/4″365 psi684 psi
1-1/2″337 psi631 psi
2″300 psi573 psi
Type L Copper Pipe to ASTM B88
Pipe DiameterWorking Pressure (Annealed)Working Pressure (Drawn)
1/4″851 psi1596 psi
3/8″894 psi1676 psi
1/2″715 psi1341 psi
5/8″596 psi1117 psi
3/4″677 psi1270 psi
1″527 psi988 psi
1-1/4″431 psi808 psi
1-1/2″404 psi758 psi
2″356 psi668 psi
Type K Copper Pipe to ASTM B88

A typical 9000 BTU mini split needs 1/4″ and 3/8″ copper pipes for its liquid line and gas line respectively. If you choose to use the more common type L copper pipes, you’ll notice that it is much easier to find the annealed type (comes in roll form) than drawn type (straight pipe) at 1/4″ and 3/8″ pipe size.

The limit of the pressure test is depending on the type and size of copper pipes you use for the mini split. For instance, type L annealed copper pipes at 1/4″ and 3/8″ have a working pressure of 730 psi and 638 psi. In this case, you are safe to do the pressure test at 380 psi.

Performing a higher pressure test can better minimize the chances of having a refrigerant leak.

Sometimes, improper brazing work may not cause a leak during normal days when the mini split working pressure is low. However, during hot days plus a few months later where the mini split got a bit dirty, the poor brazing point may give off due to the higher working pressure.

Nevertheless, in construction projects, many people pressure test the copper pipes at 250-300 psi only due to the low working pressure of the low specification but cheaper copper pipes.

If you want to know more about where to put a mini split, feel free to check out my post Best Place to Put the Mini Split Air Handler and Condenser.

If you need me to help with your HVAC system, purchase the Email Consultation service.

This article was originally published on Actions will be taken for unauthorised republication of this article.

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