First, just to clarify terms, if the box with the burner is heating up hot water to be pumped through tubing for heat, it's a boiler, not a furnace... Heating systems do not normally add moisture to the conditioned space air, nor do they remove it. The RELATIVE humidity is a function of both the temperature and the absolute humidity, but raising the temperature would lower, not raise the relative humidity. Does the indoor humidity only rise when the heating system has been running? If yes, question is where is that moisture coming from. A primary combustion product of burning propane is water, so if there is an exahust vent leak dumping propane exhaust into the house that is one possibility (and a dangerous one, given the OTHER combustion products of propane!) If the radiant heat is from a radiant slab that is in contact with damp soil (no ground vapor barrier) it could be that the the concrete is storing a lot of moisture, and heating it up is releasing it. A third possibility is that the heating system is leaking water somewhere, but the system stays pressurized due to an auto-fill valve. I assume that if it's 75F outside (at any relative humidity) the heating system isn't running, and that if you open up all the windows & doors the indoor humidity will fall. If the humidity only rises when the windows & doors are closed whether the heat is running or not, the sources are internal. Newer houses are sometimes pretty tight and require active ventilation to keep moisture (and other indoor pollutant) levels from rising. With multliple humans bathing/cooking/breathing indoors those levels can rise pretty quickly if you aren't religious about using exhaust ventilation during humidity-spreading activites like showering & cooking. If you have 1001 house plants or a dozen fish tanks bubbling away, those too can be powerful moisture sources. Leaking foundations or saturated slabs are another, that may be difficult to spot if you don't see clear signs of dampness. These issues often don't show up if you normally have high ventilation rates, such as keeping the windows open all summer, and would begin to exhibit themselves when you start keeping the windows closed, which is roughly at the beginning of the heating season. |