Special Feature Article

Geoff McDonell 
P.Eng.  LEED


Here's a 1929 classic from our friends in Ottawa, Ontario, Canada

Click the picture to download this document


This is a picture of Robert Bean's steam humidifier mounted on his make up air units...its all part of the IAQ equation.

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From the same mechanical room, this is a dual control set up. One is for constant air temperature discharge and the other for supplemental space heating.

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Recommended Reading


Climate Zones

Copyright (c) 2005, Building Science Corporation


Have Fun With This Comfort Tool.
The Comfort Calculator developed by Dr. A. Marsh and the guys at Square One Research PTY LTD...is for you - go ahead, click the picture above...play with the humidity and see what happens. 



Residential Humidity Control Strategies for Radiant Conditioned Homes By Geoff McDonell  P.Eng.  LEED AP
Copyright 2005, All Rights Reserved

A radiant temperature control system will satisfy the biggest portion of the human comfort equation – mean radiant temperature.  However, an energy efficient home still requires the other two legs of the stool:  ventilation, and humidity control.  Once the temperature control function is taken care of by the radiant system, a tightly constructed, energy efficient home needs a dedicated outdoor air ventilation system, plus, in most climate zones of North America, some kind of humidity control system to add or take away moisture in that outdoor air used for ventilation, as well as to control internally generated humidity inside the home.

The human comfort range for ambient relative humidity is between 30% and 60% at an ambient temperature range of 65F to 75F.  The warmer the air temperature, and the more humid it is, the warmer it will feel.  Reducing the relative humidity (RH), while keeping the air temperature the same will make you feel cooler.  It’s not only about comfort- at ambient humidity’s below 20%RH, your mucous membranes dry out, and itchy, watery eyes result as well.  At RH levels over 70%, mold and other organic growth issues become very significant.  Controlling humidity in the home is just as important as ventilation and temperature control.

The first step is to make sure the house has an excellent envelope:  infiltration of outdoor air is minimized, opening windows are operated properly, and all doors and windows are weather-stripped and sealed at the frames.  The next step is to treat the incoming outdoor air being supplied to the house for ventilation air.  In wintertime, this will be cold, dry air, and will need added humidification to maintain the interior of the house in the comfort range.  In summertime, in many climate zones of North America, humidity will have to be removed from that outdoor air to keep the house comfortable.  If you control the rate of outdoor (fresh) air going into the house, you can control the indoor humidity levels.  Uncontrolled outdoor air going into the house from leakages and infiltration = uncontrolled indoor humidity. 

Simple things like having a bathroom fan on while taking a bath or shower will keep the humidity levels in the house down.  But the key is where is that fresh air coming from to make up for the exhausted air?  If it is untreated outdoor air, then uncontrolled indoor humidity conditions will occur in summer and winter climates.

So, how do we make sure the humidity levels in the house will be controlled?  In a typical warm air furnace system, common to cold climates like Canada, it’s a matter of adding a humidifier to the main supply air duct and control it with a wall mounted humidistat.  In a warm humid climate, the cooling coil of the air conditioner will usually remove enough moisture from the air when it runs to supply cooling on a call from a thermostat.  The drawback of these “all-air” systems is that the humidity is controlled only intermittently as the central furnace or air conditioner runs to supply heating or cooling from the thermostat. 

How can this be done better?  The key is to separate the temperature control function from the ventilation function for the home heating/cooling plant.  Then the ventilation air can be controlled to provide humidified or de-humidified air on call from a space humidistat, and “on call” to provide make-up air when exhaust fans in the house are operating (bathroom exhaust, stove range hoods, etc.).  Some other system can then react to the heating or cooling requirements of the house – ideally radiant heating and cooling systems that will provide total human comfort. 

There are also stand-alone options for humidity control:

Re-circulating humidifier units – adds moisture to the ambient air in a room.

Re-circulating de-humidifier units – reduces the moisture in the ambient air in a room 

These types of units simply plug into an electrical outlet and using an internal fan and control system can add or eliminate moisture to the ambient air in a room.  The catch is that these units can only do one thing or the other, and are an energy user, and can increase the operating cost of the house, as well as requiring maintenance and replacement after a relatively short service life (typically 7 to 10 years maximum).

The ideal system would use an air to air heat exchanger ventilator system that acts as the main exhaust unit and fresh air make-up unit, where the humidity control can be centralized to treat the whole house.  These units can be obtained with humidifiers, de-humidifiers and filtration systems to provide “whole house” indoor air quality treatment.  These are normally controlled from both a humidistat as well as interlocks to point source exhaust fans.  The energy recovery aspect of this type of system provides a very energy efficient solution, with low operating costs and a reasonably long service life (15-20 years). 

There are many climate zones in North America that will require both humidification in wintertime, and de-humidification in summertime.  Centralizing the humidity control at one main ventilator can provide a relatively simple, energy efficient, stable indoor comfort control. 

Once the indoor air quality is being maintained by the high performance building envelope and the energy recovery ventilator, the temperature control in the house can be easily handled with a radiant system.  In peak conditions, the ventilation system can also be used to provide “touch-up” heating and cooling as necessary. 



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