How do you know this is a EMI/RFI problem?
While interference from microwave ovens, refrigerators, and other high-wattage appliances and other high speed digital devices is not uncommon, it is still not supposed to happen in a properly wired facility and properly operating appliances and electronics. Interference in crowded wireless environments (large apartment complexes, for example) are not uncommon either.
But, safety first, and therefore the first place I would look is facility wiring and grounding. Every household should have a
AC Outlet Tester. I recommend one with a GFCI (ground fault circuit interrupt) indicator as it can be used to test bathroom and kitchen outlets too. These testers can be found for your type and voltage outlet, foreign or domestic, at most home improvement stores, or even the electrical department at Walmart.
With one of these, you can ensure your outlets are correctly wired and provide a decent ground to Earth. If the tester reports an open ground or reversed hot and neutral, you need to have a certified electrician in there to sort that out.
The next thing I would do is try to relocate wires and distant ends (the wirelessly connected devices) so their paths (including the "line-of-sight" wireless path) don't run near or through those high wattage appliances. Wireless network performance is greatly affected by the number and content of walls, ceilings, and floors in between the endpoints. By content, I mean the construction (brick, wallboard, wood or metal studs) and if the barrier contains wires and/or metal pipes.
Have you tried a
different wireless channel?
A wireless extender is typically used to extend the range, not boost power - so you can connect beyond 300 feet or so. I don't see an extender increasing security concerns any more than using wireless already does.
What happens if you physically move the printers closer to the WAP (wireless access point)?
