Honeywell H1008 User Manual
Honeywell H1008
Automatic Humidity Control
with
HumidiCalc+ Software
Copyright © 1998 Honeywell Inc. • All Rights Reserved
Rev 1.0
May 27, 1998
Automatic Humidity Control
The Honeywell H1008 Automatic Humidity Control with HumidiCalc+ Software
is the world’s most advanced stand-alone humidity control. Simple to install, easy to
troubleshoot, it is designed to save the installing contractor time, while maximizing
benefits to the homeowner. It’s much more than a humidistat - it’s a sophisticated
electronic control with the ability to maintain a comfortable humidity level in a home,
while preventing condensation on windows over a wide range of weather conditions.
Back to Basics - Expressing Humidity
When most people think of humidity they probably are thinking of “relative humidity.”
But there is a significant drawback in expressing humidity in terms of relative humidity.
In order for relative humidity to be meaningful, the temperature of the air needs to be also
known. Simply stating that the “humidity is X %” does not really indicate how much
moisture is present in the air. The best measure of the humidity level in the air is
dewpoint, not relative humidity. Dewpoint is a measure of the absolute amount of
moisture in the air. If the temperature of the air is raised or lowered the dewpoint will
remain constant, unless moisture is added or taken away. Figure 1 shows the effect that
changing temperature has on relative humidity. With conditions of constant moisture
content in the air (dewpoint of 50 F) the relative humidity varies between 35% and 70%
as the temperature varies between 60 F and 80 F.
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
Relative Humidity - % RH
0.0
55 60 65 70 75 80 85
Figure 1 – Variation of Relative Humidity with Changes in Temperature at a
Dry Bulb Temperature - deg F
Constant Moisture Level
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Defining a Comfortable Humidity Level
Much research has been conducted on what combination of temperature and humidity
creates the most comfortable indoor environment. The 1997 ASHRAE Fundamentals
Handbook1 defines how comfort is perceived as a function of temperature and humidity.
Figure 2 shows a summary of the ASHRAE conclusions.
65
70 % RH80 % RH90 % RH
Dewpoint (F)
60
55
50
45
40
30
20
10
60
Figure 2 – ASHRAE Winter Comfort Zone
65
70
75
Dry Bulb Temperature (F)
In designing a humidifier control, what is of particular interest is the lower region of the
60 % RH
50 % RH
Comfort Zone
40 % RH
30 % RH
20 % RH
10 % RH
80
comfort zone. Figure 2 shows that a comfortable humidity level is defined in terms of
dewpoint, rather than relative humidity. The lower end of the winter comfort zone is a
dewpoint of 36 F, for dry bulb temperatures ranging from about 69 F to 76 F. This
strongly supports the notion that a comfortable humidity level is best described by
dewpoint, not relative humidity. In fact within the temperature range of 69 F to 76 F,
the relative humidity can vary from 23 % to 81% and still be considered to be in the
comfort zone, as ASHRAE has defined it! You can begin to see the problem in trying to
use relative humidity to control humidity level. Relative humidity is not a direct
measurement of the amount of moisture in the air. As shown in Figure 1, it is very
dependent on the temperature of the air being measured.
1
"Conditions for Thermal Comfort", 1997 ASHRAE Fundamentals Handbook, page 8.12,
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The message to take away from all this is:
If you are in the business of trying to provide a comfortable indoor
environment, controlling a humidifier by sensing relative humidity may
not result in optimum control.
Especially poor performance may result when the temperature in the living space
experiences large swings, like in instances where setback thermostats are used.
In these applications, a humidity control that controls based on dewpoint will result in
optimum control over all conditions. Honeywell H1008 Automatic Humidity Control
with HumidiCalc+ Software does just that. While most humidity controls simply have
a humidity sensor, the H1008 Automatic Humidity Control has both humidity and a
temperature sensor. This allows dewpoint to be calculated, and the humidification
equipment controlled to achieve the desired dewpoint.
The Quest for Dry Windows – All Winter Long
As we said above, “If the dewpoint could always be maintained at 50 F, there would be
no complaints about humidity.” However, as much as we would like to live year-round in
an environment with a 50 F dewpoint, unfortunately, that can’t always be done. Here’s
why:
The need to humidify for a comfortable indoor environment is driven by the fact that all
homes breathe, exchanging outside air for inside air. When the amount of moisture
contained in the outside air is less than that in the inside air, the humidity level will go
down. If the outside air is very dry, at some point moisture will need to be added to
maintain a comfortable humidity level. The drier the outside air, the more humidification
that will be needed. When temperatures get very cold outside, the air can hold a very
limited amount of moisture. Thus, the very driest air occurs when the temperatures get
very cold.
When comfortable indoor humidity levels are maintained during extremely cold weather,
problems with condensation can occur. Usually this condensation will occur first on
windows, because windows generally have less insulation value than walls. If the
temperature of the inside surface of the window is lower than the dewpoint of the inside
air, condensation will occur. Over time, this condensation can be very damaging to the
finish and the structure of the window.
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Figure 3 illustrates why condensation occurs:
Window Panes
Inside = 70 F
Window = 40
Outside = 0 F
Figure 3 – Temperature Profile of Double Pane Window
Figure 3 shows the temperature profile through a typical double pane window. If it is 70
F inside and 0 F outside, depending on the insulating value of the window, the
temperature at the window surface may be around 40 F. Therefore in this example,
under these conditions the maximum dewpoint that this house could withstand without
condensation forming on the window would be 40 F. If the outside temperature were to
increase, the window temperature would also increase, meaning that the house could
stand more humidity. Conversely, if the outside temperature were to decrease, the
window temperature would decrease, meaning the humidity level in the house would
have to decrease further to prevent condensation.
A second factor in determining window temperature that is not always considered is
indoor temperature. Raising and lowering indoor temperature will similarly raise and
lower the window temperature. This can be a significant factor in applications where
programmable setback thermostats are used. If a thermostat sets back at night, the
allowable humidity level in the home will be less than during periods when the
temperature is at its normal setting.
This sets the stage for the quest to achieve both a comfortable humidity level and dry
windows.
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Outdoor Temperature Compensating Humidity Controls
Honeywell, with the Perfect Climate Comfort Center, was the first to introduce an
electronic humidity control system for the residential market designed to automatically
adjust humidity levels based on outdoor temperature. This system also considers the
effect of indoor temperature in its algorithm, providing optimum control for all
combinations of indoor and outdoor temperature.
The Honeywell H1008 Automatic Humidity Control, when installed with the C7089H
Outdoor Temperature Sensor, functions as a true dewpoint control, just like the Perfect
Climate Comfort Center.
In 1997, Research Products introduced its AutoTrac
2
control. The AutoTrac is a
stand-alone, electronic humidistat that, when installed with an outdoor sensor, will adjust
the relative humidity setpoint per a programmed reset schedule. We have tested the
AutoTrac and found that it does perform as advertised. It attempts to control to 45 %
RH, until the outdoor sensor value reaches the point at which a ½ % RH reduction in
setpoint is made per F change in outdoor sensor value. Figure 4 graphically shows the
linear reset curves.
50.0
45.0
40.0
35.0
30.0
25.0
20.0
Relative Humidity - %
15.0
Knob Setting = 1
Knob Setting = 2
Knob Setting = 3
Knob Setting = 4
Knob Setting = 5
Knob Setting = 6
Knob Setting = 7
10.0
5.0
0.0
-30-25-20-15-10-5 0 5 101520253035404550
Outside Temp - deg F
Figure 4 – Research Products’ AutoTrac Outdoor Temperature Compensation
2
AutoTrac is a trademark of Research Products Corporation.
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The AutoTrac control does not have the ability to provide true dewpoint control. This
may result in overhumidification at times when the temperature is greater than normal
conditions and underhumidification if the temperature is less than normal conditions. For
example, if during night setback the temperature was 60 F, if the AutoTrac were able
to achieve its 45 %RH, the dewpoint would be 39 F. If the temperature were raised to
72 F during recovery, the relative humidity would only be 30 %. All opportunities to
humidify during setback would be lost.
Some additional items noted during our testing of this product (conducted in the fall
of 1997):
If the outdoor sensor fails open, or is not connected, the humidistat will not
function, with no feedback to the user.
If the sensed outdoor temperature exceeds 50 F, the humidistat will not function,
losing opportunities to humidify under those conditions.
The concept of adjusting the indoor humidity level depending on what is happening
outdoors in one that we embrace – in fact, Honeywell pioneered the concept with the
Perfect Climate Comfort Center. What we heard that installing contractors do not
embrace is the time and effort required wiring an outdoor sensor – up to an hour on some
jobs. So, armed with an unequalled knowledge of comfort systems, we set out to design a
humidity control with exceptional performance, that would also make life easier for the
installing contractor.
H1008 Automatic Humidity Control with HumidiCalc+ - The Basic Idea
The idea for the H1008 Automatic Humidity Control was born by considering some very
obvious facts:
Wiring between the humidity control and an outdoor sensor can be very
difficult
Humidifiers and duct mounted humidity controls are usually installed near the
HVAC equipment
Wiring between the humidity control and the HVAC equipment would usually
be pretty easy
If you could replace the wiring from the humidity control to the outdoor
sensor with wiring to the HVAC equipment, it would save a lot of time
Here’s a few more:
By monitoring the W terminal on your furnace, you could tell how much it
was running
The colder it gets outside the more your furnace runs
You’re probably starting to see where this is headed. Here’s the basic idea - if we could
estimate the outdoor temperature by monitoring the calls for heat, we could conceivably
replace the outdoor sensor with some simple wiring between the HVAC equipment and
the humidity control.
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H1008 Automatic Humidity Control with HumidiCalc+ - The Technical
Approach
We found that there is more than a casual relationship between furnace run time and
outdoor temperature. In fact, in a single stage heating system, in steady state, the
percentage of burner on-time (% on) is linearly related to the difference between the
balance point (in this context, the temperature at which it is warm enough that the furnace
never runs) and the outdoor temperature. To carry this one step further, an equation can
be derived which expresses % on as a function of four variables:
1. Outdoor temperature
2. Balance point
3. ASHRAE heating design temperature
4. Oversizing of furnace, relative to ASHRAE design temperature
Figure 5 shows two examples of this relationship.
80.0
60.0
40.0
20.0
0.0
-20.0
0 10203040506070809010
0
0% Fu r nace Ov er siz e
100% Fur nace Oversize
-40.0
-60.0
Outdoor Temperature - deg F
-80.0
-100.0
Percent On
Figure 5 – Relationship Between Outdoor Temperature and Percent On
The upper line shows an application with a balance point of 55 F, where the furnace is
exactly matched to the heat loss (0% furnace oversize) at the ASHRAE 99.6% design
temperature for Minneapolis (-16 F). In this case the curve is a straight line connecting
the balance point at 0 % on, with the design temperature at 100 % on. The lower line
shows the same application with a furnace with twice the btu output (100% furnace
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