Please explain how one should use a Broan Relative Humidity gauge, which is installed in our bungalow condominium. We have a Desert Spring drum-style humidifier attached to our furnace, as we had in our previous older home. There is also a cold air intake close to the furnace.
Every morning and evening, we adjust the humidifier according to the outside temperature. Sometimes we turn it off completely, and after a very cold night there is still frost build-up of an inch or two on the lower part of the windows.
In the older home, we were told to maintain a little moisture in the lower corners of the window, but can’t seem to accomplish that in this home. The condo is very air-tight and we have the bedroom window open, just a bit, at night.
We seldom run the humidistat, as the furnace man who installed the furnace four years ago advised us to ignore it. Is this correct? Should the humidifier also be running? When we hear a click on the humidistat, is that the humidity reading? We try to keep the Relative Humidity around 40%. We are very susceptible to dry air and so are the hardwood floors.
Thank you for your advice.
As we approach the time of year where outside temperatures reach their lower extremes, often for weeks at a time, this difficult but very common winter question arises. How do we maintain a comfortable level of Relative Humidity within our homes, without having sweating windows and moisture issues that may damage the components of the building? I will try to offer you some explanations, while answering your questions.
The first question to answer is about ignoring the humidistat. You should never ignore any control of a mechanical system in a building without understanding its function. This is especially true of drum humidifiers, as they can become wonderful mould factories if left unmaintained and full of water. The spores produced from this mould can easily circulate throughout the home, carried in the forced air blowing over the humidifier. If any of the occupants of the home have sensitivities to mould or allergies, this can be a very serious mistake. Whatever decision is made about the humidistat, the humidifier needs regular cleaning during the heating season, or a complete shutting down of the water supply and damper, and removal of the interior components to prevent mould growth.
The humidistat attached to the furnace controls and humidifier measures Relative Humidity (RH) in the air in the home, in the location it is installed. RH is the measurement of the percentage of moisture in the air relative to the amount it can hold at a given temperature. In a home with an average 20 – 22 C interior temperature, this may vary considerably with changes in the exterior temperature. Warm air can carry much more dissolved moisture, which is why our homes are much dryer in the winter, even though the outside RH may be very high. When we bring the very cold, high RH winter air into our home and heat it with the furnace, the amount of moisture that can be contained in this air rises dramatically. Therefore, the RH drops significantly as the air warms, giving us low interior RH readings on the humidistat and dry feeling interior air. That is why the suggested RH settings on your humidistat go down as the outside temperature drops, rather than up, as may seem proper.
This can all be quite confusing, but to put this into a rather simplistic context, the reason your windows are frosting up at night is because the reverse of the previous explanation is happening. The warm air in the home is hitting the windows in your home, which are normally quite a bit cooler than the air inside the building. During the day, this may cause a minimal amount of sweating on the windows, as only a small amount of air is cooled sufficiently to cause the moisture in the air to exceed its capacity, or100 percent RH. This moisture often condenses in the cool corners of the window, causing little concern. At night the window temperature drops further, which causes more air to condense on the surface, which may freeze on the coldest nights. That is the source of the frost. As the window warms the next morning, this frost may melt and leak down, possibly damaging the window and wall below.
In your older home, this would not have been as big a problem because the old windows, walls, attic and exterior doors had many air leaks. This made the air in the home dryer in the winter, as the heated interior air could freely leak to the exterior, along with its moisture and the relatively dry outside air could easily infiltrate the home. Some moisture would still condense on the windows, but this was not a big problem. In your new bungalow condo, there are extensive air-vapour barriers in the walls and attic and well sealed windows and doors. There is now minimal dry outside air leaking into the home, combined with a significant reduction of leakage of the warm moist interior air to the exterior. To make a long story short, you no longer need to add moisture to the house air with the humidifier. You may even have to try to get rid of some of this excess moisture with exhaust fans in the kitchen and bathrooms, or a mechanical ventilation system.
To give minimal credit to your “furnace man”, what he was likely suggesting was shutting down your humidifier completely to prevent excess moisture build-up in the home in the winter. That is what I would suggest, as most new homes do not need additional humidity added in the heating season, as older homes did. Buy an electronic hygrometer to measure your RH, which may drop as low as 30% or lower on the coldest days, and should not exceed 50% in the heating season. This may feel uncomfortably dry on really cold days, but will prevent excess condensation and damage to the components of the home.