A vacuum pull is the step where the engineer removes air and moisture from the pipework before refrigerant is released. You may hear a vacuum pump and see a gauge. Expect them to check that the result holds, rather than run a pump for a moment and immediately open the service valves. Moisture and air left in the system damage it. This page does not give a vacuum method, a target, or anything you should do with a pump yourself. Ask for the pull to be noted on the commissioning record.
Air and moisture in new pipework
New pipework contains air and, often, a little moisture. The vacuum pump is there to remove them before the refrigerant charge meets that pipe. Leaving them in is a known way to spoil a new compressor over time. You do not need to operate the pump to benefit from the step. The company should be F-gas certified. The vacuum is part of commissioning the refrigerant circuit, not an optional extra for keen customers. Energy Saving Trust's description of air-to-air heat pumps assumes that commissioning has been done.
A pump, a gauge, and a sheet that names the company
You may hear a pump and see a gauge. Expect them to check that the result holds, rather than run a pump for a moment and immediately open the service valves. Warranty conversations go better with that page in the folder.
Frequently asked questions
Will I notice the vacuum being done?
Often, yes. A pump runs and the engineer watches a gauge, then checks the result holds. If the visit goes straight to opening the refrigerant valves, ask whether the vacuum has been done.
Can I pull the vacuum myself to save time?
No. It is part of commissioning a refrigerant system. This page does not describe how to do it. There is no householder procedure on this page. Refuse any plan that leaves the pull to you.
Does a vacuum mean the pipes do not leak?
A vacuum and a pressure test answer different questions. The pressure test is whether the joints hold. A vacuum does not prove a leaking joint was acceptable, and a pressure test does not dry the pipe. You want both. Neither one is a DIY check with a household tool.
What does a rushed finish look like?
An install that jumps from flaring the pipes to opening the outdoor unit valves, with no pump and no wait, has skipped the step. A token few seconds is worth a polite question. You can ask them to stop and explain. You should not be invited to finish it off after they leave.
Does a vacuum replace the rules for old pipe?
No. If old pipe is being reused, the vacuum does not replace the rules for reuse: right diameter, clean, pressure tested. A vacuum on contaminated pipe is not a flush.
Should the commissioning sheet mention the vacuum?
Yes. Ask for the pull to be noted, held to the engineer's satisfaction against the manual, with the serial numbers and the company name on the same sheet.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.