Nitrogen pressure testing and purge brazing

Where the pressure test fits

The order on a new split-system install, from the manuals cited here:

1. Braze the line set with nitrogen flowing.
2. Pressurize the line set and indoor coil with dry nitrogen and hold.
3. If the pressure drops, find the leak with soap bubbles, repair, and test again.
4. Release the nitrogen and evacuate to the manual’s micron target.
5. Open the service valves, charge, and leak test again with a detector suited to the refrigerant.

The unbranded R-454B manual states the rule directly: after completing field piping on a split system, the field pipework is pressure tested with an inert gas and then vacuum tested before refrigerant charging. Indoor coils ship with only a holding charge of nitrogen, Rheem notes, so keep tube ends sealed until you connect them.

Rheem’s sequence is the same: pressure test first, and proceed to evacuation only if the line set and coil hold pressure. A vacuum hold does not replace the nitrogen test in either manual.

Test pressures by manufacturer

The rule that covers all of them: use the pressure in the manual for the equipment you are installing, and never exceed the manual’s maximum or the nameplate design pressures. The test pressure also has to be at least what the manual calls for; the unbranded OEM ties its minimum to the nameplate low-side design pressure.

Read the test on a gauge rated for it. Rheem specifies R-454B manifold sets reading up to 800 psig on the high side and 250 psig on the low side with a 550 psig low-side retard, and hoses with an 800 psig service pressure rating.

Regulator, relief and hold

Always feed nitrogen through a pressure regulator. Oxbox says to “always use a pressure regulator and safety valve” so that only low-pressure dry nitrogen enters the tubing during brazing. For the test itself, set the regulator to the test pressure, fill, then close the tank valve as Rheem instructs, so the system is isolated from the cylinder during the hold. A system still connected to an open regulator will keep topping itself up and hide a leak.

Rheem’s minimum hold is 15 minutes. The other two manuals do not give one. Mark the gauge reading and the line temperature at the start, then compare at the end.

Oxbox also warns never to repair a brazed connection while the system is under pressure. Release the nitrogen first.

Temperature changes the gauge, not just leaks

Nitrogen in a sealed line set behaves close to an ideal gas: at a fixed volume, absolute pressure is proportional to absolute temperature (Gay-Lussac’s law). If the line set warms or cools during the hold, the gauge moves even with no leak.

Work it in absolute units: psia = psig + 14.696 (sea level) and °R = °F + 459.67.

500 psig test at 90°F, line set cools to 70°F:

• P1 = 500 + 14.696 = 514.7 psia, T1 = 549.67 °R, T2 = 529.67 °R
• P2 = 514.7 × 529.67 ÷ 549.67 = 496.0 psia = 481.3 psig
• The gauge drops about 19 psi with no leak.

250 psig test at 90°F, cooling to 70°F: 264.7 × 529.67 ÷ 549.67 = 255.1 psia = 240.4 psig, a drop of about 10 psi.

It works the other way too. A line set filled to 500 psig at 70°F that then warms to 90°F in the sun rises to 534.1 psia, or 519.4 psig, which is above Rheem’s 500 psig maximum. Fill to the test pressure when the line set temperature will stay steady, or at a pressure low enough that warming will not push it past the manual’s limit.

Compare start and end readings at the same line temperature where you can. A drop with no temperature change is a leak.

Flow nitrogen while brazing

All three manuals require nitrogen in the tubing during brazing:

• Oxbox: dry nitrogen should always flow through tubing being brazed, because brazing temperature is high enough to oxidize the copper unless an inert atmosphere is provided. Keep the flow until the joint has cooled. Only a small flow is needed to displace air. Its procedure removes the Schrader cores from both service ports, feeds low-pressure nitrogen into the liquid line port, and lets it exit at the suction service port.
• Rheem: “When using a torch during brazing, nitrogen must be flowed while brazing.” Flow nitrogen into a pressure port and through the tubing, but do not let pressure build inside the tubing, which can cause leaks. Once the system is full of nitrogen, turn the regulator off.
• Unbranded OEM: purge the refrigerant lines and indoor coil with dry nitrogen, wrap the valve body with a wet rag, and continue the dry nitrogen purge until all brazing is complete.

Rheem also requires an existing R-410A or R-22 line set reused for an R-454B unit to be purged and flushed with nitrogen first, and recommends replacing the line set if possible. Joints must be brazed or welded, or mechanical fittings listed to UL 207.

Fieldpiece’s vacuum pump manual adds that purging with a few psi of dry nitrogen before evacuation helps pre-dry the system, and its triple evacuation uses nitrogen breaks between pull-downs.

Never oxygen, never compressed air

Rheem: “Do not use oxygen to purge lines or pressurize system for leak test. Oxygen reacts violently with oil and possibly with R-454B, which can cause an explosion resulting in severe personal injury or death.”

Both R-454B manuals say compressed air or oxygen shall not be used for purging refrigerant systems. Rheem adds that R-454B and air should never be mixed in tanks or supply lines, and that leak checking should be done with nitrogen. The unbranded manual says not to break a vacuum with air. Air also brings in moisture that POE oil absorbs; see refrigerant oil types.

A2L service sequence

For opening an A2L circuit for repair, Rheem’s section 10.8 (the unit complies with the partial-unit requirements of UL 60335-2-40) and the unbranded OEM manual give the same procedure:

1. Remove the refrigerant, following local and national regulations, into the correct recovery cylinders.
2. Evacuate.
3. Purge the circuit with inert gas (optional for A2L).
4. Evacuate (optional for A2L).
5. Continuously flush or purge with inert gas when using a flame to open the circuit.
6. Open the circuit.

Purging means breaking the vacuum with oxygen-free nitrogen, filling until working pressure is reached, venting to atmosphere, and pulling down to a vacuum again; repeat until no refrigerant remains. After the final nitrogen charge, vent the system down to atmospheric pressure so work can take place. Keep the vacuum pump outlet away from any potential ignition source and make sure there is ventilation.

If a leak needs brazing, Rheem says to recover all the refrigerant or isolate it with shut-off valves in a part of the system remote from the leak. If a leak is suspected, remove or extinguish all naked flames.

Before recharging, the system is pressure-tested with the appropriate purging gas. It is leak-tested after charging but before commissioning, and again before leaving the site. Rheem specifies an A2L-compatible leak detector for the final leak test. See the A2L refrigerants guide for the rest of the A2L handling rules.