Note: Descriptions are shown in the official language in which they were submitted.
CA 02783070 2012-07-17
COMPENSATING FOR GAS APPLIANCE DE-RATE AT HIGH ALTITUDES
BACKGROUND OF THE INVENTION
[0001] A known problem with a conventional gas-fired furnace, as well as
with other types of
gas-fired heating appliances, is that the furnace outputs considerably less
heat (when using gas
having the same heating value) when it is used at high altitudes. For example,
above 5000 feet
the heating capacity of a typical gas furnace will be reduced by about 20
percent compared to the
heating capacity of the same furnace, using gas having the same heating value,
at sea level (per
the National Fuel-Gas Code Handbook; Section 8.1.2 High Altitude). Because of
this, a
consumer has heretofore been forced to buy a larger and thus more expensive
furnace to obtain
the same heating output at a high altitude location as a smaller furnace at a
lower altitude. In
view of this it would be desirable to provide a gas-fired furnace, or other
type of gas-fired
heating appliance, with the capability of increasing its heating output enough
to compensate for a
high altitude use of the furnace without having to up size the furnace.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] FIG. 1 schematically depicts a representative gas-fired air heating
furnace
incorporating therein a high altitude heating capacity compensation system
embodying principles
of the present invention;
[0003] FIG. 2 is a schematic diagram of the heating capacity altitude
compensation system;
[0004] FIG. 2A is a schematic diagram of an alternate embodiment of the
heating capacity
altitude compensation system; and
[0005] FIG. 3 is a schematic diagram of an input touchpad and lookup table
portion of the
heating capacity altitude compensation system.
DETAILED DESCRIPTION
[0006] Schematically depicted in FIG. 1 is a representative gas-fired
heating appliance,
representatively in the form of a fuel-fired air heating furnace 10, which
incorporates therein a
specially designed furnace control 12 embodying principles of the present
invention and
operative to uniquely provide the furnace 10 with a heating output adjustment
system enabling
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the furnace to output substantially the same total amount of heat at both sea
level and high
altitude.
[0007] The illustrative furnace 10 is of the draft-induced type, having an
outer housing 14
within which a heat exchanger 16 is operatively disposed above a combustion
chamber 18
having a gas burner 20 therein below the heat exchanger 16. Gas burner 20 is
supplied with gas
via a gas supply line 22 in which a modulating gas valve 24, operative to
maintain a
predetermined gas manifold pressure within the line 22, is connected. In
common with
conventional furnace gas valves, the illustrated gas valve 24 has a normal gas
pressure control
setting level of 3.5" W. C. However, according to an aspect of the present
invention, the gas
valve is operable, in manners subsequently described herein, to enable its gas
pressure control
setting to be selectively increased to magnitudes greater than 3.5" W.C. to
thereby increase the
high altitude heating capacity of the furnace.
[0008] During firing of the furnace 10, which is initiated and terminated
under the control of
a thermostat 26 disposed in a conditioned space served by the furnace 10 and
operatively
coupled to the furnace control 12 as shown, hot combustion gases 28 created by
the burner flame
30 travel through the interior of the heat exchanger 16 into the interior of a
vent structure 32 that
is coupled to the upper side of the heat exchanger 16. Passage of the hot
combustion gases 28
through the vent 32 is assisted by the operation of a variable speed
combustion blower 34
operatively mounted in the vent 32. Also during firing of the furnace 10,
return air 36a from the
conditioned space served by the furnace 10 is forced by a variable speed
indoor blower 38
exteriorly across the heat exchanger 16, for example through a duct structure
40, to receive
combustion gas heat from the heat exchanger 16 and thereby create heated
supply air 36b
suitably conveyed to the conditioned space served by the furnace 10.
[0009] Turning now to FIG. 2, the furnace control 12 is part of an overall
heating capacity
altitude compensation system 13 and has a preprogrammed microprocessor portion
12a. In a
subsequently described manner, the furnace control 12 is operative to uniquely
regulate the gas
valve 24, the combustion blower 34 and the indoor blower 38 in a manner such
that the furnace
(FIG. 1) is provided with the same heating output at high altitude as it has
at sea level, thereby
desirably eliminating the previous necessity of upsizing the furnace 10 to
compensate for a high
altitude placement and use thereof. Basically, when the furnace 10 is
installed and operated at a
high altitude, the control 12 operates, in response to a later described
furnace user input to the
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CA 02783070 2012-07-17
furnace control 12 of altitude and gas heating value, to coordinatingly
increase the regulated gas
pressure of the gas valve 24 and the speeds of the combustion blower 34 and
the indoor blower
38 to provide the furnace 10 with a substantially unchanged maximum heating
output despite its
new higher altitude location.
[0010] Referring now additionally to FIG. 3, a user input touchpad 42 (or,
more simply, a
push-button on the main furnace control) is operatively associated with the
furnace control 12
and permits a user to input both an elevation and a gas heating value for the
elevated location at
which the furnace 10 is disposed. Alternatively, this user input device could
only provide for
elevation input.
[0011] User input of a desired elevation value responsively transmits
identical selected
elevation magnitude signals 44 to first and second lookup tables 46 and 48.
Lookup table 46
charts indoor blower speeds against associated elevation values and is
operative as shown to
output to the furnace control 12 a desired indoor blower RPM signal 50
associated with the user
selected elevation range value.
[0012] Lookup table 48 charts gas valve control pressure offsets (i.e.,
above the normal 3.5"
W.C. regulated gas pressure) against selected combinations of elevation range
and gas heating
values (or against only an elevation value as the case may be). When a user,
via the touchpad 42,
selects a desired elevation value and a desired gas heating value, the
elevation magnitude signal
44 and a selected gas heating value signal 52 are transmitted to the lookup
table 48 which
responsively transmits to the furnace control 12 a desired pressure offset
signal 54.
[0013] During firing of the furnace 10, the furnace control 12 regulates
the operation and
speed of the variable speed indoor blower 38 by outputting to the indoor
blower 38 (1) 115 volt
AC electrical power via lead 56 and (2) a speed control signal via lead 58.
The speed control
signal causes the indoor blower 38 to run at an increased speed corresponding
to the magnitude
of the lookup table signal 50 received by the furnace control 12, thereby
causing the indoor
blower 38 to deliver its designed-for CFM of air to the conditioned space
despite the high
altitude placement of the furnace 10 and the resulting ambient air density
decrease.
Alternatively, the indoor blower 38 could be a single speed blower and its
speed control signal
could be eliminated.
[0014] At the same time, the furnace control 10 regulates the operation and
pressure
regulation level of the modulating gas valve 24 by outputting to the gas valve
24 (1) 24volt ac
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electrical power via electrical power lead 60, (2) a regulating pressure
offset signal via lead 61,
and (3) a gas flow regulating signal via lead 62. The regulating pressure
offset signal elevates
the maximum manifold pressure regulation magnitude of the valve (for example,
beyond its
normal 3.5" WC level) to a level indicated by the magnitude of the signal 54
received by the
furnace control 12. This upward valve pressure regulation level adjustment
increases the heating
capacity of the furnace 10 to compensate for its high altitude placement
without the previous
necessity of upsizing the furnace. The flow regulating signal modulates the
gas flow to the
burner 20 as required by the heating demand.
[0015] It should be noted that the regulating pressure offset signal (lead
61) and the gas flow
regulating signal (lead 62) may be the same signal with compensation
calculated and adjusted at
the furnace control instead of the gas valve. Such a signal would have a
predefined relationship
to the output of the gas valve (e.g., a 50% PWM signal that corresponds to 50%
of the valve's
maximum capacity). In any case, the gas valve 24 would need to be pre-set
during production to
be capable of exceeding 3.5" W.C. which is the current maximum value of gas
valves commonly
utilized on residential gas furnaces. While the gas valve 24 has been
representatively illustrated
as being a modulatable gas valve, it could alternatively be a single stage gas
valve, in which case
the gas flow regulating signal (lead 62) could be replaced with a simple
"on/off' signal.
[0016] Operatively coupled to the furnace control 12 by the indicated
electrical leads 64, 66
and 68 as shown in FIG. 2 are two pressure/electric switches - a low pressure
switch 70 and a
high pressure switch 72 electrically coupled to the low pressure switch 70 by
an electrical lead
74. The two pressure switches 70,72 are pneumatically coupled to the
combustion blower 34 by
a suitable pneumatic linkage structure 76 as indicated. In a conventional
manner the speed of the
variable speed combustion air blower (also commonly referred to as a draft
inducer) is regulated
by the pressure switches 70 and 72 via the pneumatic linkage structure 76.
[0017] The combustion air blower speed is changed, via a speed control
signal output to the
blower from the furnace control 12 via a lead 78, based on feedback from the
pressure switches,
the low pressure switch 70 being set to be just closed at the selected minimum
blower speed
(corresponding to the minimum heating capacity of the furnace), and the high
pressure switch 72
being set to be just closed at the selected maximum blower speed
(corresponding to the
maximum heating capacity of the furnace). A lead 80 from the furnace control
12 transmits 115
AC electrical power to the combustion blower 34. In developing the present
invention it has
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been found that this combustion blower control technique automatically
provides altitude
compensation for combustion blower flow capacity by increasing the speed of
such blower at
higher altitudes. As in the case of the indoor blower 38, the combustion
blower 34 could be a
single speed blower if desired.
[0018] Schematically depicted in FIG. 2A is a portion of an alternate
embodiment 13a of the
previously described heating capacity altitude compensation system 13. System
13a is identical
to the previously described system 13 with the exceptions that (1) a pneumatic
branch valve
control line 76a is added and interconnected as shown between the pneumatic
linkage structure
76 and the gas valve 24, and (2) the gas flow regulating signal lead 62
interconnected between
the furnace control 12 and the gas valve 24 is eliminated. In the system 13a
the combustion
blower pressure generated within the pneumatic linkage structure 76 during
firing of the furnace
is transmitted to the gas valve 24 and is used as a control signal to modulate
the gas flow through
the valve.
[0019] As can be seen, the present invention is operative to increase the
manifold regulation
pressure of a furnace gas valve beyond its normal 3.5"W.C. fixed setting, and
to also
correspondingly optionally increase the combustion and indoor blower flow
rates (if these
devices are variable on the system in question) to compensate for the
placement of a furnace at a
high altitude. In this manner, the same furnace can be used at varying
altitudes without altering
its heating output, thereby eliminating the previous necessity of upsizing the
furnace. While this
desirable and cost effective altitude compensation technique has been
representatively described
in conjunction with a furnace, it will be readily appreciated by those skilled
in this particular art
that principles of the present invention could also be advantageously employed
in conjunction
with other types of gas-fired heating appliances.
[0020] Additionally, while the present invention has been described as
being implemented via
an automatic heating capacity altitude compensation system, it will be
appreciated that principles
of the present invention could also be employed by the use of manual
adjustment of gas valve
and blower components of a gas-fired heating appliance. For example, the gas
valve 24 could be
provided with a manual high altitude adjustment structure permitting its
pressure regulation
setting to be manually increased above 3.5" W.C. Additionally, altitude
compensation for the
furnace 10 could be achieved simply by replacing its standard gas valve
(having a fixed 3.5"
CA 02783070 2014-03-26
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W.C. gas pressure regulation setting) with a high altitude gas valve with a
fixed gas pressure
regulation setting greater than 3.5" W.C.
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