Some devices can reduce your X10 signal strength by "absorbing" the signal right off the AC line. Sometimes called "signal suck" or "signal black holes." The signals get pulled in and can't get out to the modules they are meant to control. The device can be anything that's wired to or plugged into your AC.

An uninterruptible power supply (UPS) is good for doing this. Most UPS's not only supply power during a power outage or some sort of power event, but they are also designed to filter out anything in the AC going to your computer or other sensitive electronic equipment. This is actually a good thing to have a UPS do. It's not really so good for X10 though. Technically an X10 signal is line noise in that it's not considered to be part of a clean AC waveform. A UPS will not only block (or severely reduce) X10 signals via chokes and filters from getting to the devices they are protecting, but some can actively seek out and remove anything it considers line noise without the AC even having to pass thru the UPS itself. In other words, your signal strength will go down just from the UPS being plugged in.

I have an APC UPS that does this very same thing. When this happens, you need to plug the UPS into an X10 filter to block the X10 signals from ever making it to the UPS so they can't be absorbed. It actually lowers the X10 signal strength so much that the entire circuit that the UPS is plugged into will have a significant lowering of X10 signal strength. In fact, overall signal strength throughtout the entire home is reduced by just the UPS. Plugging the UPS into an X10 filter will correct this.

You can also use an XPF wired in filter to block X10 from ever going thru the circuit that a UPS is plugged into. This will put the UPS on a filter without it actually being plugged into a separate XPPF filter module. Plus the XPF can handle 4 times as much current as what the XPPF can.

Not only does a UPS affect signal strength, but most anything can. A lot of newer high tech devices have power supplies in them that filter and remove anything on the AC supply to provide a clean AC power supply. So here, you have the same situation as a UPS. You need to put the device that's absorbing the X10 signals on a filter. The filter will block X10 signals from ever reaching the device so they can't be absorbed.

The quickest, easiest, and cheapest way to determine if a device is causing your X10 signal strength to go down is to unplug it. It's so simple it's almost scary. If you have modules that do not work correctly when a device is plugged in, but then they start to work when the device is unplugged, it's a pretty good bet that is the device that's causing the problem. Most devices today are still connected to the AC line when you have them off. They're actually in a low power state or standby. Even though they appear to be OFF, they are still ON in that their power supply is connected to your AC. That's why it's best just to unplug the device when trying to determine if it's causing the problem or not.

If a suspect device is hardwired in like a furnace for example, shutting it off at the breaker will be the same as if it were unplugged.

Oddly enough signal interference can look and act like signal absorption. The signal is interrupted or "drowned out" by electrical noise from a variety of devices. Much like you can't hear one person in a stadium full of people talking because the background noise drowns out the person you are trying to listen to, AC noise makes it hard for X10 modules to "hear" their orders of ON/OFF/DIM, etc.

Sometimes noise can be solved by using the XPNR noise reducer that "grabs" the noise and sends it to neutral (or ground) removing it from the AC line. Putting the offending device on a filter can also remove the noise by blocking it from ever getting a chance to travel out onto the powerline. A noise reducer can't block signal absorption like a filter, but a filter can do both. In the case of a noise reducer, the load of the device does not pass thru it so it can be smaller than a filter. On the other hand if you use a filter, you need to take into account the current that the device uses and use a filter that is rated for at least that amount of current.

I had a customer once who had to put a scroll compressor from his heating/cooling system on 2 XPF filters to block the noise being generated. Evidently they can be very noisy electrically speaking. Also, fluorescent lights (compact or standard) can generate a lot of noise causing X10 signal reception problems. By using an XPTR signal strength indicator, you can determine if it's line noise or signal absorption. The XPTR will tell you the signal strength, which if it goes down when a device is ON, this would indicater signal absorption. The XPTR also has an error indicator which would tell you that there is a noise problem.