I had the same issue when outfitting two homes with Insteon dimmers (which support both multi-location control and full home automation with an optional computer link) over the past 6 years. For CFL bulbs, it comes down to the design of the ballast. I used a combination of forum research and experimentation to find no-flicker bulbs, and have the following recommendations (in order of availability & price):
- TCP SpringLamp series
- Overdrive Lighting dimmable spiral
- EarthTronics / EarthBulb
- Dimmable CCFLs from LiteTronics or TCP (no high-lumen models)
All of them work wonderfully for me with a wide range of dimming and no flicker. One caveat: very low dim levels cause an audible whine on some of the bulbs, particularly the TCP models.
I've read about positive results with LED lamps as well, but the price isn't right (for me) yet.
Single switches that control a light are called single pole.
When there are two switches that both can turn a light on or off, they are called three way. (I am not sure about the origin of this label, perhaps because it required three wires; it does not mean that there are three or more switches - those are called four way!) In those switches, flipping the on off control on either switch will change its state: if it was off, it will go on, and vice versa.
When a dimmer control is part of a three way setup, one switch usually controls both the level and on/off, while the other switch only controls on and off. Both switches need to be the three way type.
It sounds like someone has put a single pole switch in a system that needs two three ways. Part of the problem is that there needs to be the right wiring between the switches. There has to be two hot wires*. There also may need to be a neutral wire, depending on the wiring configuration.
To determine the solution, we need to know what the wires are in each box that holds the switches, and what wires are currently attached to the switches.
SUPPLEMENT
It is possible to place dimmers at multiple locations if you use certain digitally controlled dimmers. You still need wiring between the dimmers that are able to have two hot lines, called followers, as well as a ground wire. You also need to provide a neutral wire to the fixture. That sometimes is part of the cable that goes through the dimmer boxes, but not necessarily.
Best Answer
Timers come in two varieties:
Timers with a neutral wire. These are connected between the hot and neutral wires of the circuit, parallel to the load. They're powered like any other device, and work by switching the hot (and possibly neutral) wires of the controlled circuit. These timers can control any device, but require a location with access to both the hot and neutral wires.
Timers without a neutral wire. These are connected in series with the load, and work by restricting the current through the hot wire of the controlled circuit. Because they use the load as part of their own power circuit, they can only control resistive loads (essentially, light bulbs and some electric heaters), but can be installed anywhere (in particular, as a drop-in replacement for a light switch where only the hot wire passes through the switch box).
LED and fluorescent bulbs (and motors, and...) don't work properly with the second type of timer, either because they require a sufficient current to let any electricity flow, or because too low a current will damage them.