Does anyone know of a sensor that would work with two double hung windows? My home has two side by side in each room ( I think they are called Mulled windows)? This is going to get expensive to buy sensors for all of these individually.
sorry I dont know of one but I’m interested in what others have to share.
In the meantime Lowes iris sensors are on sale, with coupons you can get contact sensors for around $18/pc
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https://www.lowes.com/pd/Iris-1-Sensor-Indoor-Door-And-Window-Sensor/999925302
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Double hung windows and mulled windows are two different things. In a double hung window, each of the two sashes can move either up or down. So you can have the window open from the top as well as from the bottom
“Mulled” just means two windows (or two doors) set side-by-side with some kind of finishing trim around them. It doesn’t say anything about how they open. You could even have mulled double hung windows.
In terms of sensors, there are two different questions to start with.
First, how many ways can the windows open?
Second, what information do you want when they do open?
A number of people use one sensor with two magnets, but they put the second magnet about 4" in. This allows the window to report as “closed” when in fact it’s open just enough for air but not enough to let someone climb through.
But other people find they need multiple sensors or they need much more complex sensors that recognize each different endpoint.
A lot of people will switch and use hardwired sensors if they have a lot of different windows or different window positions just because those will be much less expensive.
So if you can post a photo or drawing of your windows showing how they open and which exact conditions you want reported, then we might be able to make some suggestions. 
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thank you for the response. My windows are all Double hung Mulled. So they are like the bottom picture that you posted with double hung panes. I would really like to just use the sensors for security type purposes alerting me when they are opened.
Thanks
You would need one sensor per window, so side by side is at least two sensors. I doubt an intruder is going to break the bottom window just to move the upper sash down and climb over that to get into your home. I just did only my lower sashes.
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That is a good point! No real reason to put then on the top panes I guess. I just wish someone made one with one sensor and two magnets or something similar. may just have to bite the bullet and buy two for each window.
Wait…couldn’t you put the sensor in the middle, where the two windows meet? Then if either opens it reports opened?
Edit: I’m so dumb…how would the windows open without popping the magnet part off. lol.
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The magnet by itself doesn’t do anything. All the magnet piece does when it gets close enough to the sensor piece is to pull a tiny metal needle, called the “Reed” into a position where it closes a contact inside the sensor piece. Then the radio inside the sensor piece reports that the sensor is “closed.”
If you are going to use wireless sensors, that’s about the only way to do it. Which is why it matters exactly what information you want to get reported.
In the example I gave, where there is one sensor and two magnets, the sensor is being moved from one magnet position To the second magnet position, so it can report “closed” at either spot.
But if you want to monitor multiple places on one window, you either need a sensor/magnet combination at each spot or you have to go to a different kind of sensor.
Most commonly when there are a lot of windows people will go to wired system where all of the windows have to be closed or the electricity can’t go through the full loop. So any window open is reported as an intrusion. But in the inexpensive systems Of this type you don’t know which window is open. Just that the full loop is not closed. In security systems this is usually called a “zone”: all the windows have to be closed in order for the system to report that the zone is closed. And any window being open means the zone will be reported as being open.
There are much more complicated and more sophisticated sensor Systems, but they all cost a lot more money.
You should be able to set up a quite inexpensive wired zone system if you are OK with just knowing that any one of the windows is open. And if you are OK with the wires. @heythisisnate might have some more suggestions for that.
But if you want to get specific information for each window, or you don’t want to use wires, then you are back to needing a sensor for each position that you are monitoring.
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You might be able to do this with a Sensative strip, which is a very thin sensor designed to fit inside a door or window frame. But I’m a little less optimistic about it then I would be about a door frame because windows usually have a tighter fit to keep out the rain. 
Also, the Sensative strips are expensive. Two regular contact sensors might actually cost less. But the strips are an option that some people have used for fancy windows, particularly the French door kind.
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I would really like to know which window(s) is/are open, at least which room anyway. Also being that we just remodeled the entire house, I doubt that the wired system would work for my setup at this point in the ballgame. I may end up just buying a few at a time and go a room at a time until i get them all done. If I am counting from memory correctly it would take about 15 sensors (there is one single window) to do my home.
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So…maybe I’m not dumb. I’m now wondering if you could embed a magnet in the bottom frame of the top window and put the sensor on top of the frame of the bottom window so they line up. That should work, right?
What you are saying should work. if you mounted the magnet on the top window pane at the bottom and then mounted the sensor on the bottom window pane in the same location at the top of it then the sensor would detect when either widow pane is moved. That still leaves me buying a sensor for each window that is side by side though.
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Actually, this is exactly how I do it. I use the Iris contact sensors which have slightly smaller magnets than the ST Multipurpose sensor. The trim on the top sash is just fat enough to accommodate the magnet without it getting knocked off when the window is opened. I can open either the top or bottom sash using one sensor and one magnet this way. I can provide a pic of one when I get home.
Yep! No solution for the side-by-side windows, but then I’m a bit of a stickler for details and prefer to know exactly which of my windows are open so that limitation doesn’t bother me.
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How about this for low cost… Get a Z-Wave window sensor that has a pair of external contacts as well. Then buy some inexpensive magnetic window sensors and wire them all in series and attach to the external contacts on the z-wave sensor. If any of the Windows is opened, you’ll get a notification. You’ll just need to conceal the wires from the additional magnetic window sensors. You won’t necessarily know exactly which sash opened, but it is good enough for home security.
Here a discussion about one such device…
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