I went though a similar debate last year and ended up getting a whole house humidifier. I never remember reading that you should be able to get the humidity level to normal levels though normal means (shower, cooking, etc). In a big house, that is most likely not possible. Do you have a humidistat now, or does your stand alone unit give you a reading? I would recommend you getting one of those first to see how bad it truly is. If I remember correctly, if your humidity is below 30%, you could actually be causing harm to yourself and your home.
Our house approached 25%, which is dangerously low. Some of the driest deserts in the world don't get that low. For example, Las Vegas ranges from 21%-39%. You may be living in conditions that are way too dry.
All that being said, I knew we needed to do something. You mentioned worrying about mold, sure this could happen if you raise your humidity very high, but think about it, in the summer your home probably approaches 80%-90%. With humidity that hight, and the warm temperatures, that natural condition is more likely to produce mold than your 70 degree house in the winter. Plus, you will find just increasing your humidity to like 35%-40% with your whole house humidifier makes it very comfortable. AT that level, mold is unlikely an issue. The added bonus us that the more humid the house is, the warmer it feels. Again, just like in the summer, a humid 90 degrees is much warmer than a dry 90 degrees. We found we were able to decrease the temp of the house simply by adding a little humidity. Used to be 72 degrees at about 25-30% humidity, we now keep it at 70 and 35-40%. So we save heating costs there.
Ok, so what kind to buy? I researched the various types of whole house humidifiers, and it comes down to three types.
- Drum - These are the least expensive and easy to install. They install on your cold air return line, and use a motor and belt to essentially lift cold water from a drum into the the air and let evaporation and the passing air movement to pick up the moisture. These require considerable maintenance it has standing water and risk of mold due to the standing water. You have to replace the belt frequently. I opted to not get this one due to the standing water and maintenance issues.
- Flow Through - These can be installed on return or supply line. These are the middle of the road in cost and performance. Again, they also use evaporation, but you do not have standing water. There is a filter that water flows through (drip) and the air blows through the filter. The water however, needs to be drained, and new water is always trickling though. Since it always uses new water, the risk of mold is almost none. Uses no electricity, but does requires a water line and is always using new water.
- Mist / Steam - The most expensive, but most effective. They actually produce steam, rather than relying on evaporation. Because of this, you can more easily control the desired humidity. Can come in cold and hot water forms, injecting hot or cold steam into your supply line. Requires both electricity and a water supply line. Risk of mold is low due to no standing water, but the hot steam could cause issue in your ducts if you run it too often and too high I imagine.
I opted for the steam model, mainly because of the low maintenance and ability to control the desired humidity more easily. I installed it in an afternoon, it was easy to do. Most of them come with high end humidistat / thermostat, that would replace your existing house thermostat. Then the system knows how to control the furnace to optimize the output. I went with the Honeywell TrueSTEAM series and it has worked great. Good Luck!
Here are a few other descriptions:
http://www.just-humidifiers.com/works.php
http://www.guide4home.com/care-hf/whole-house.htm
That's plenty of space to install a humidifier, provided it's air intake scoop is designed to operate or can be installed horizontally. Freezing of the supply line or its drain is a concern; steam humidifiers are recommended for unconditioned spaces. Drain it inside, install a catch pan and a shut-off switch, like you have for your AC coil. Sharing that installed pan's drain and safety switch will not help; it only shuts down the AC condenser, not the entire furnace.
Best Answer
In Washington, DC you want to remove as much water as possible from the dank soup that passes for summer air. Your AC does that. You definitely do NOT want to add any more humidity in the summer, there.
In a desert climate you might, but then you'd probably also be looking at a swamp cooler, which does both cooling and humidifying - in a DRY climate.