|
Geoff McDonell
P.Eng. LEED AP
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.
Click Picture to Enlarge
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.
Click Picture to Enlarge
Recommended Reading
Climate Zones
Copyright (c) 2005,
Building Science Corporation
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.
REFERENCES/RESOURCES:
http://www.hhinst.com/archives.html
http://www.cmhc-schl.gc.ca
http://oee.nrcan.gc.ca
http://irc.nrc-cnrc.gc.ca/pubs/ctus/14_e.html
http://healthandenergy.com/dehumidification.htm |
