A very wet system keeps forming ice while it is being evacuated. Why might the technician break the vacuum with dry nitrogen?
To raise the pressure so that ice can melt and the water can be removed. Water that freezes under deep vacuum leaves very slowly and can stall dehydration, so the technician adds dry nitrogen, then evacuates again.
- To raise the pressure so the ice melts and can boil offCorrect answer
- To reach an evacuation level required by Table 1
- To let nitrogen react with the water and remove it
- To cool the system so the ice stays in one place
Why “To raise the pressure so the ice melts and can boil off” is correct
Water that freezes under deep vacuum leaves the system very slowly and can stop the dehydration. Raising the pressure with dry nitrogen lets the ice melt, and the technician can then evacuate again to remove the water.
Source: EPA: Test Topics
Why the other answers are wrong
- To reach an evacuation level required by Table 1 — Table 1 sets recovery levels before an appliance is opened, not steps in dehydration.
- To let nitrogen react with the water and remove it — Nitrogen is inert and does not react with water; it only changes the pressure and sweeps moisture out.
- To cool the system so the ice stays in one place — The goal is to get the water out, and leaving it frozen does the opposite.
The source, word for word
Need to evacuate system to eliminate air and moisture at the end of service
Nitrogen in any application;
This one rests partly on trade knowledge. EPA's outline says systems must be evacuated to remove air and moisture, and the rule exempts nitrogen from the venting ban. Breaking the vacuum with nitrogen to deal with ice isn't in either one. It is common field practice, so follow the equipment manufacturer's procedure.
The sequence
This is a dehydration step, done after the refrigerant has been recovered and the repair is finished.
- Evacuate until progress stalls or ice is suspected.
- Valve off the pump and add dry nitrogen through a regulator to raise the pressure.
- Allow time for the water to melt and spread, then release the nitrogen.
- Evacuate again, and confirm the result with a micron gauge.
Why moisture has to go
Water left in a system can form acids and sludge, and ice at the metering device. Use only dry nitrogen, because oxygen and compressed air are not acceptable.
Keep practicing
- Which gas to use for leak testing
- Why pressure rises after reaching recovery vacuum
- EPA 608 study guide
- EPA 608 Core practice test
Unofficial, not affiliated with the EPA or any EPA-approved certifying organization. Questions are original practice written from public EPA material, not real exam questions.