You didn't mention anything about your current 'heating system': don't know how long it will be before you replace that. What I'm getting at, is that whenever you do replace it, it will likely be a high efficiency unit(> 90% efficiency), which will require a condensate drain. If it is located in a basement or utility room where you do your laundry, the condensate can be drained via a wash tub or standpipe. Worst case, you may have to buy a condensate pump (probably less than $100 at a home center). If you go this route, make sure that the pump has an overflow safety switch, which should be wired through the furnace's 24v supply. This way, if the pump fails (or stops for some reason), you will not get a flood. Another thing to consider: Units without a fan other than a drum type (not too many of those around), utilize a bypass duct to draw hot air through the wet medium, then back into the return air stream. This amount of air is 'stolen' from the supply air. With a 1, 2, or 3 stage furnace, it's not a problem. If you are considering a modulating furnace, which runs mostly at relatively low fan speeds, it could be a problem. You might want to hook it up to the hot water supply. Good luck. V

I had some similar issues when I installed mine (I'm not a pro either). I installed mine on the return air trunk because that is where I had room. Seems to work fine there. I also wanted to have it run whenever the fan was blowing so I probed the contacts on the furnace for anything that was producing 24V whenever the fan was running. I couldn't find any so I had to go to RadioShack and get a relay that I could run off of the power going to the fan itself (may have been 240V). Then you need to find the contacts that are producing 24V (or in your case if you are using a 24V transformer, you can use that for the power to the solenoid valve). Wire it so when the fan is energized, the relay closes the connection from the transformer to the solenoid and you have what you need. If you can find any contacts that provide the 24V whenever the fan is running, no need for the additional relay. The standard installation is for the humidifier to run only when the thermostat is calling for heat. If I just wanted this, I could have just run the solenoid off of the appropriate furnace contacts. I bought a separate humidistat that I mounted next to my house thermostat because it is in a return air stud bay. You could do something similar with your sensor. If you have a transformer, I would guess that it converts 110VAC to 24VAC for to power the solenoid. As I mentioned, I did not need a separate transformer because there must be one internal to my furnace that is producing 24V (many thermostats run off of 24V that they get from the furnace wiring).

You can install the one I mention above outside of the return, but you have to run the sensing tube into the air return. So it would have to be mounted on the wall of the air return with a hole that penetrates the return for the sensing tube to be mounted. This may not be feasible, so this is why Honeywell also instructs you to mount it in return duct. Typically once you set it up, it's not a setting that will get changed because if the humidity is too high (if set up right) the humidifier will not come on. But to each their own. I typically only set up wireless controls when there is no feasible way to run wires... as wire runs are longer term solutions with many more options to chose from in the form of different controls. Remember, you're talking to someone who is called to trouble shoot problems with these kinds of systems. Nothing is better than a hard wired solution.

With or without a humidifier, you might have comfort problems Nobody installs a whole-house humidifier unless they're having a problem of some kind. Many people absolutely can't stand having dry skin during winter. It can actually be painful, and a humidifier probably will help your skin from getting too dry. https://www.cooling-heating.net/ In other homes, the air gets so dry that wood furniture cracks and gaps form between floorboards. Pianos go out of tune. Everything just feels off-kilter (or off key). Super-low humidity just isn't fun. Any time your indoor relative humidity dips into the middle-20% range, it's a good idea to increase it. The thing is, humidifiers have problems of their own: They might cause mold to grow in your home. This happens more often than you might think. Since humidity sticks to cold surfaces and your home contains many cold surfaces during winter, the environment is often perfect for fuzzy, black stuff to show up. You might not see the mold: While humidifier-induced mold sometimes clings to ceilings and walls, the mold that results from your humidifier isn't always visible. It might be inside your walls or - and this is pretty common - lining the inside of your ducts. The "moldy ducts" scenario is more likely if you have uninsulated ducts or a lot of your ductwork resides in an unconditioned area (like your attic). It's cold in your attic, so moisture accumulates on the ducts when your humidifier is running. Then mold starts to grow there. Mold, as you well know, can really do a number on indoor air quality. It's bad to breathe it in. And if you've got asthma, a moldy environment can actually be dangerous. The bottom line is that you don't want to grow mold in your home, but you don't want your air to be too dry either. What can you do to increase indoor humidity without increasing the risk of mold growth? Air sealing keeps dry air out How did the air in your home get so dry in the first place? The answer is simple: Cold air from the outdoors infiltrated your home through gaps and cracks in the building envelope. Seal off the points of entry for that dry air, and you'll increase your relative humidity. If you do a good job, you won't need a humidifier at all. Think about it like this: Warm air migrates to colder environments. When you turn on the heat, the hot air generated by your furnace eventually seeps out of your home to the outdoors. To maintain a pressure balance inside a structure, the warm air that leaves your home is continuously replaced by cool air that gets in. This is air infiltration. It's the draft you feel under your front door. It's the cold air that seeps in around your windows. Most of all, it's the cold air that sneaks in through the plumbing, electrical, and ductwork penetrations between your first floor and your crawl space. When you seal off these sources of air leakage, you achieve two things: You close off the "escape hatch" by which warm air leaves your home. You prevent air infiltration from the outdoors, which keeps your indoor humidity from getting too low. Stated more simply, you should strongly consider air sealing your home before purchasing a whole-house humidifier. By doing so, you'll be in a great position to mitigate uncomfortable dryness and avoid all of the problems associated with humidifiers. The best way to discover your biggest sources of air leakage is via a blower door test. That way, you can determine both the air infiltration rate and identify the biggest sources of leakage. If you do opt for a humidifier… Air sealing should always be your first course of action to alleviate problems caused by dry indoor air. That said, there are circumstances when you might want a humidifier anyway. For example, maybe your household activities don't generate enough humidity to make you comfortable, even after you've air sealed your home. You need a humidifier to even things out, so to speak. Whatever your reasons for installing a whole-house humidifier, these best practices will help you avoid potential mold and indoor air quality issues: Get a steam humidifier:Steam humidifiers boil water on their own. Unlike bypass humidifiers, which "steal" a little hot air from your supply duct to generate water vapor, steam models are more efficient. Basically, you get more humidity during a shorter run time. Bypass models also dump hot air right back into your return plenum, which can increase the temperature of incoming air and cause your system to overheat. A proper installation can mitigate this, but it's still a risk. Only run the humidifier when the furnace is on: If your ducts are in an unconditioned space (most people's ducts are), running the humidifier while the ducts are cold increases your risk of mold growth.