Many people believe that hydro-air systems do not dry the air like furnaces do. Since dry air can cause a number of health ailments, this would be a significant advantage that would justify a hydro-air system's increased cost. However, the science behind this claim is dubious at best.
As anyone living in a cold climate well knows, winter is synonymous with dry air. Conventional heating systems tend to remove humidity from the air, which can cause sore throats, nose bleeds, and headaches, as well as aggravating asthma and other conditions. Because of the way most air heating systems work, dry air is unavoidable. An alternative system known has hydro air is gaining popularity, in part due to the popular conception that hydro air systems do not dry the air like furnaces.
Is this true? Do hydro air systems really spare the moisture in a home's air, or is the whole idea a load of hot air (pardon the pun)? First, let's discuss how hydro air systems work.
How Hydro Air Systems Work
In hydro air systems, the air does not come in direct contact with the combustion process. Hydro air systems combine water heating and air heating mechanisms into a single system; additionally, the system can be modified to provide air conditioning. The heat source in a hydro air system is a boiler, generally gas or oil-fired. After the water is heated, it passes into a small-diameter tubing which takes it into an air handler. The air handler contains a water coil, an air fan, and (usually) an air conditioning unit.
As the system pumps hot water through the water coil, the fan blows air over the coils. Due to the large surface area of the coils, heat is quickly transferred to the air. Next, the air is pumped into heating ducts and into the rest of the house. There are several advantages of this type of system, including ease of zoning, but for the purposes of this article the alleged advantage of a hydro air system is that the hot air does not come in direct contact with combusting gases, which supposedly prevents water vapor from being sucked out of the air.
Image: Flickr - Oskay - Free Coil
Do Hydro Air Systems Spare Humidity?
The important thing to note about hydro air systems is that they are fundamentally not very different from hot air furnaces. Both heat air by means of a heat exchanger. Furthermore, the humidity of air is primarily a function of outside temperature - when you bring very cold air up to a hot temperature, the relative humidity goes down regardless of the heating process used.
The idea that hydro-air brings up humidity levels seems to be a myth based on a misconception of how air humidity levels work. It may be the case that homes with hydro-air systems have higher relative humidity levels, but this is most likely due to the fact that these homes are newer and have better insulation. A "leaky" home with poor insulation and sealing may have low relative humidity due to constant cycling of air.
How to Prevent Air Dryness
If dry air is a problem in your home, the best way to increase relative humidity is to install a humidifier directly onto the furnace. This option may be expensive, but it is effective. Alternatively, homeowners could install room humidifiers into bedrooms and other frequently-occupied areas.
On a related note, check out this article - What is the Best Indoor Humidity Range - to learn more about the ideal indoor humidity range.