CA2777901A1 - Method and apparatus to improve the operating efficiency of dual heating - Google Patents

Abstract

Method and apparatus for improving the operating efficiency of forced air, dual heating systems with fossil fuel or electric furnace and heat pump, to heat and cool an indoor living space. The invention includes a programmable control module with separate electric element heater or equivalent when used with fossil fuel furnace. Resulting in significant duty cycle and operating efficiency improvements for both heat pump and furnace. The control module is programmable and acts as a central hub connecting control wiring from house thermostat, furnace, heat pump, electric element heater and indoor, outdoor temperature sensors. In this configuration the heat pump always participates in the heating process, down to the low temperature cutoff point.

Description

Background of the Invention Forced air, dual heating systems consisting of a fossil fuel or electric furnace and a split air to air heat pump are widely used in colder regions of the continent especially in locations where electricity rates are reasonable compared to the cost of fossil fuel.
= A typical dual heating system for residential use, can consist of a 120.000 BTU mid efficiency oil or gas furnace and a 36.000 BTU air to air heat pump. When an electrical furnace is used the installed capacity is typically an equivalent of 90.000 BTU because of higher efficiency. Actual heating capacities vary depending on building seize, heat loss characteristics and geographical location.
A heat pump is the most efficient heating appliance for heating and cooling an indoor , living space when using forced air. Its capacity requirements for cooling the living space = are less when compared to winter heating needs. The installed heat pump output capacity in a dual heating configuration is a compromise between summer cooling and winter heating needs. Over seizing the heat pump to gain additional heating output capacity, results in increased cycling of the heat pump during summer cooling and milder weather conditions. This results in degradation of its operating efficiency, as well as increased wear and tear on moving parts. A properly seized heat pump typically has sufficient heat output capacity to maintain house thermostat daytime set point at outdoor temperatures just below freezing point, without supplementary furnace make up heat. The point when a heat pump as part of a dual heating system can no longer supply sufficient heat to maintain the house thermostat at daytime set point is called "balance point".
Below balance point the indoor temperature slowly decreases. The house thermostat detects the temperature drop, disables the heat pump and enables the furnace.
The more powerful furnace now rapidly raises the house temperature back to daytime set point, disables the furnace and again enables the heat pump. As the outdoor temperature drops = further below balance point, the heat pump operates for shorter intervals and the furnace increasingly longer. When the outdoor temperature reaches a certain low temperature point, the heat pump is disabled under control of an electric utility provided external dry contact switch or by mean of an outdoor thermostat contact as typically used for dual heat With electric furnace. The furnace now takes over the heating process for as long as the outdoor temperature remains below the low temperature threshold typically set between -10 to -15 C.

A typical dual heat system consumes most of its annual heating budget when outdoor temperatures vary between "balance point" and low temperature heat pump cutoff point.
The installed furnace heating capacity is derived by calculating the total worst case heat loss of the heated space based on regional winter temperature data.
In a dual heating configuration the furnace spends most of its service life delivering "make-up"
heat for the heat pump when below "balance point". While the electric furnace supplies 100% efficient "make-up" heat, the fossil fuel furnace peak output efficiency is reached only if heat demand exceeds furnace capacity or during extreme cold days of winter.
Practically while supplying "make-up" heat, the average operating efficiency of a fossil fuel furnace falls far below its rated capacity, making it an inefficient heating source.
An oil furnace typically has a heat chamber made from cast iron. Each time the furnace is enabled it takes several minutes to heat up the heat chamber. During this time, only limited heat transfer takes place between the furnace combustion housing and air flowing through the furnace. This result in a delayed supply of heat to the indoor space, resulting in a slow rise in house temperature at first and accelerating after the furnace has warmed up. This sudden intense surge of heat is uncomfortable for its occupants, especially when nearby the warm air heat duct registers.
After the furnace has raised the house temperature back to thermostat stage one, call for heat the furnace is disabled and the heat pump enabled again. When near "balance point"
the furnace latent heat buildup of the heat chamber, raises the indoor temperature further upwards, quickly satisfying stage one call for heat and disabling the heat pump only after a short run time. This unwanted temperature overshoot again contributes to unwanted temperature fluctuations causing additional discomfort for its occupants.
After the indoor space has cooled down, thermostat stage one, call for heat again enables the heat pump.
When further below "balance point", this latent heat buildup from the furnace is absorbed by the heat loss component of the building without raising the indoor temperature significantly. Intense, excessive heat produced by a furnace when operating below its rated capacity also causes temperature stratification in the heated space.
This occurs when the excessive warm air pumped through the ductwork and through warm air registers, rises to the ceiling where the temperature can be 4 to 7 C higher compared to floor temperature where heat is needed. This leads to temperature buildup near the ceiling resulting in increased heat loss. Excessive heat also contributes to a reduction in relative humidity in the heated space further increasing the need for humidification.
The natural gas furnace is more efficient to operate and a preferred choice in dual heating systems, mainly because of lower natural gas fuel cost.
The oil furnace remains a preferred option in many urban and rural areas where no natural gas distribution network exists and propane gas as a commodity generally is more expensive compared to fuel oil.
Most thermostats utilized to control dual heating systems with heat pump, are, brand name, manual or automatic house thermostats with two stages for heat and one stage for cooling and separate E heat output. This type of thermostat is actually designed for controlling heating systems that consisting of a heat pump with a heating coil for backup or aux. heat as utilized in regions that enjoy temperate winter weather conditions where a heat pump with heating coil are sufficient to meet winter heating needs. Utilizing this type of thermostat in a dual heating system with oil or gas furnace without additional circuitry would result in enabling both heat pump and furnace at the same during stage two call for heat. This is not desirable as it causes damage to the heat pump among other undesirable side effects. Therefore a separate fossil fuel kit or equivalent circuitry is required to enable the heat pump for stage one and oil or gas furnace for stage two heat.
This kit also accommodates a low temperature heat pump cutoff and furnace enable command either from an external utility load control signal or from the E heat command.
This adaptor circuit is normally installed near the furnace and acts like a hub between house thermostat, heat pump and furnace control wiring. This circuit can be as basic as one or two inter wired relays or as elaborate as a small printed circuit board that includes additional features such as humidistat control and heat pump alarm options.
Another popular configuration on the market with similar functionality combines a house thermostat and fossil fuel kit and consists of a non programmable electronic thermostat combined with a user console. It is interconnected with a 3 wire communication and power supply wire link to a matching remote interface module sometimes called "heat pump controller". From the remote module the furnace and heat pump control wires, the utility low temperature cutoff as well as temperature sensors and other features such as humidifier control and heat pump alarm are connected.
Most automatic thermostat models on the market are not adjustable for hysteresis or temperature differential adjustments for stage one or two on/off operation.
Factory hysteresis setting is typically 1 C for each heating stage. When used in dual heating with a fossil fuel furnace the hysteresis range is too tight, resulting in frequent cycling between heating appliances, thereby adding to operating inefficiencies, effecting heating cost, discomfort as well as increased wear and tear on the heating appliances.
When nighttime thermostat setback is used, some automatic thermostats on the market have a feature called Smart Response Technology. It uses past response data to estimate the time the heat should be raised to arrive at the desired day time set point. This allows the heating system additional time to supply make-up heat to compensate for higher heat loss at lower temperatures. Another manufacturer uses ERM. This delay function allows the heat pump to run about 6 minutes longer for each 1 F in temperature differential between set back and daytime set point temperature before stage two, heat is enabled.
When used in geographical areas with more extreme cold weather conditions and where temperatures can vary dramatically on a daily basis, both techniques are inadequate because it does not provide adequate time for the heat pump to raise the indoor space temperature to any significant level before the furnace is enabled.
Heat pumps are most efficient at outdoor temperatures well above freezing. As at outdoor temperature drops the net heating output capacity of the heat pump is reduced.
This is called COP or the Coefficient of Performance of a heat pump. It can vary between manufactures and models as well as the age of the unit and compares to the efficiency of a standard electric heating element. Around the freezing point the COP can be 3 or 300%, or for every 1KW of electrical power supplied to the heat pump the equivalent of 3kW of heat is generated. At -10 C the COP typically has dropped below 2 or 200%. At balance point in a dual heating configuration the heat pump can still supply very efficient heat as compared to any other heat source. However because it cannot supply sufficient enough heat it is disabled.
As the outdoor temperature drops frost buildup on the outdoor coil can occur.
In heating mode, the outdoor heat exchanger or evaporator coil extracts heat from the outdoor air, cooling it off when flowing through the evaporator coil, resulting in a temperature drop between intake and exhausted air. Frost crystals build-up between the outdoor heat exchanger coil fins starts to occur when the outdoor temperature drops below "frost point" or a temperature below 0 C (32 F), at which moisture in the air condenses as a layer of frost crystals on any exposed surface.
Leaving this unchecked it causes ice crystals to grow between the heat exchanger fin plates, thereby effectively choking off airflow through the outdoor heat exchanger and reducing heat transfer between outdoor and indoor heat exchanger, resulting in potential compressor damage. The main factors that determine the frost point in the heat pump outdoor heat exchanger are relative humidity and the air temperature when passing through the evaporator coil. The frost point increases as outdoor air relative humidity increases and/or outdoor temperature decreases. A typical heat pump starts to develop frost ice crystal buildup at an outdoor temperature of +3 C at 70% relative humidity.
Depending on the heat pump manufacturer or model the relative spacing between heat exchanger fm plates varies. The wider the spacing the longer it takes for ice crystals to grow, effecting heat transfer and airflow. As the outdoor temperature drops and/or the relative humidity increases, frost buildup accelerates.
To combat this unwanted scenario, a defrost control mechanism is installed in the heat pump outdoor unit. In older heat pumps this mechanism can be a set of mechanical relays. Heat pumps today are equipped with a printed circuit board using electronic sensors and timers for defrost control and can be either Time based or Demand based.
Time-based defrost control uses a mechanical thermostat clamped onto the outdoor evaporator coil. The thermostat contact closes when the evaporator coil temperature drops to several degrees above freezing point, enabling the time based defrost counter.
The contact opens up again after the outdoor coil temperature reaches approx.
+24 C.
Time based defrost control has a field adjustable timer to initiate a defrost cycle typically following 30, 60 or 90 minutes accumulated heat pump run time, provided the outdoor thermostat is remains in closed position during the entire time. When accumulated heat pump runtime reaches the preset time delay the heat pump, defrost control circuit enables the reverse valve thereby reverting the heat pump into cooling mode. Now heat from the indoor space is transferred to the outdoor coil, melting away any frost buildup.
The unwanted side effect of the defrost cycle is that warm indoor air is converted to cold air resulting in lowering the indoor temperature causing discomfort to its occupants as cold air is now flowing through the supply air registers into the living space. To combat this cooling effect the heat pump defrost module is equipped with an output lead to enable the furnace during each defrost cycle to temper the cold airflow that can last between 3 to 8 minutes depending on the outdoor temperature and make / model heat pump. In many installations this lead to enable the furnace is not connected in order to reduce fossil fuel heating costs. After the outdoor coil temperature at the outdoor thermostat has risen to the upper limit of the thermostat the contract opens resetting the defrost timer. Outdoor coil thermostat characteristics vary between manufactures and models. An unwanted side effect of time based defrost is that defrost cycles are initiated with or without actual frost build-up on the outdoor coil. Depending on the geographical location usually the majority of all time based defrost cycles initiated are not required because the conditions to create ice buildup are not present, making this method of defrosting the outdoor coil wasteful, inefficient and unpleasant to the occupants.
During milder winter weather with the furnace connected to operate during heat pump defrost cycles the powerful furnace can cause the indoor temperature to rise above thermostat set point, resulting in premature shutdown of the heat pump before completing the defrost cycle. After the thermostat calls for stage one heat again, the heat pump needs to resume the defrost cycle by raising the evaporator coil temperature again to open the coil thermostat. Taking additional time and energy to complete the defrost cycle.
Demand based, defrost control is found in more expensive heat pump models.
Most demand defrost controls monitor temperature differential between outdoor air and evaporator coil as a factor to determine if frost build-up exists on the outdoor coil, making this method is more energy efficient and preferable.
Furnace air handler airflow and PSC motor noise are a common annoyance factor in forced air heating systems. Most PSC or split capacitor induction motors, have up to three speed settings offering a small range of speed control. In addition PSC
motors consume lots of electricity during normal use. Some furnaces are equipped with a control switch allowing the user to select low fan speed when the house thermostat is not calling for heat or cooling in order to maintain airflow throughout the indoor space, reverting to high speed when the thermostat calls heat or cooling. Because of limited PSC
fan motor speed range the low fan speed still produces considerable fan and airflow noise.
According to most building codes, combustion furnaces require a separate fresh air intake from the outside of the building to facilitate furnace combustion.
Typically a 4" pipe is connected from the outside of the building onto the return air plenum near the furnace. Not installing or intentionally closing off this source of outdoor air forces the furnace to draw outside air through cracks and openings in the building for combustion.
For older, poorly insulated buildings this is generally not an issue. However with rising heating costs buildings are made more airtight to reduce heating cost. Not providing for a separate outdoor air intake for furnace use can now become a serious health issue for its occupants.
During heating season this fresh air intake pipe also causes excessive influx of outdoor air when the furnace is not used. This adds to unwanted indoor space heat loss, placing more demand on the heating system. Outside the main heating season when the indoor space requires little or no heating and air changes per hour (ASH) are low, the fresh air intake also brings replacement air into the house aided by the furnace air handler fan.

Field of the invention The invention generally relates to control systems for heating ventilation and air conditioning (HVAC). More specifically it pertains to implementation of fundamental changes in heating equipment configuration, features operation, and methods for dual heating systems consisting of a fossil fuel furnace and air to air heat pump to condition indoor living space for residential and small commercial buildings, resulting in significant improvements in operating efficiency and duty cycle for both heating appliances. As added benefit temperature fluctuations normally experienced by standard dual heating systems are no longer an issue, resulting in improved home comfort, lower heating cost and reduced appliance maintenance.
With the invention significant improvements are achieved following the introduction of a programmable tri-heat control module and separate dynamic controlled electric plenum heater. The control module is equipped with at least two temperature sensors.
One to measure outdoor temperature to determine building heat loss and dynamic heat output for the electric plenum heater. Another sensor measures the return air temperature to determine when the house thermostat is in nighttime setback. The hardware can be readily added in existing dual heating systems or as part of a new installation.
Technology used to design and build this apparatus can be solid state, logic, microprocessor, mechanical or solid state relays or any combination thereof.
The electric plenum heater can be switched in stages using standard mechanical or mercury relay contactors or continuously modulated for added precision in heat output control. Tests have shown that both methods result in satisfactory results.
However from equipment cost and long term reliability point of view the electric plenum heater with multiple switched heating elements is preferable.
The tri-heat control module incorporates a number of features and operating modes for user specific programming to improve the heating process. Main benefits to the user are reduced heating cost and less need for routine furnace maintenance as well as improved home comfort by reducing indoor space temperature fluctuations caused by switching between heat sources and heat pump defrost cycles. Reduced use of the oil furnace also lowers oil burner noise and fumes released when chimney draft is not optimal.
The tri-heat control module is also applicable in dual heating systems with two or more stages electric furnace and heat pump. In a typical all electric, dual heating system the heat pump indoor coil is located in the bottom compartment, followed by a blower fan compartment, and multistage electric furnace in an upper compartment.
Configurations may vary between manufacturers and applications. The electric furnace is used to supply measured heat for aiding the heat pump to maintain house thermostat daytime set point as well as furnace only heat when outdoor temperatures are below heat pump low temperature cutoff point. Although typical achievable heating cost savings for this configuration do generally not measure up with potential heating cost savings achieved using a fossil fuel furnace, the use of the tri-heat control system results in improvements in heat pump efficiency by allowing it like in the previous application to operate whenever stage one or two heat is called, down to low temperature cutoff point.
A standard programmable automatic house thermostat with two stage heat, and one stage cool, is required as user interface for most dual heat applications.
For dual heat systems with fossil fuel furnace a "fossil fuel kit" or similar device is required. The house thermostat enables the heat pump with stage one call for heat. Stage two, call for heat enables both heat pump and electric plenum heater. When the temperature drops below the low temperature cutoff point the furnace is enabled either at stage one or two call for heat (user selectable at the tri-heat control module). When in E-heat, the furnace enables and both heat pump and plenum heater are disabled. The invention also includes the use of an application specific manual or automatic thermostat with 2 wire communication link for the remote tri-heat control and interface unit.
While the heat pump operates intermittently in a standard dual heating system, a much improved duty cycle, and output efficiency are achieved with tri-heat control by always allowing the heat pump to contribute to the heating process when the house thermostat calls for stage one or two heat down to the low temperature cutoff point.
Whenever the house thermostat calls for stage two heat, measured heat is supplied by an electric plenum heater installed upstream from the heat pump indoor coil. To achieve a good balance between building heat loss and heat supply to maintain thermostat daytime set point and prevent temperature stratification, the electric plenum heat output is regulated by the tri-heat control unit to be slightly higher compared to the net heat loss of indoor space which includes the heat gain produced by the heat pump. Calibration of the electric plenum heat output is achieved by offsetting the outdoor temperature sensor output with the actual building heat demand. This process can be achieved manually or automatically.
Below "balance point" the heat pump generates continuous, efficient heat only interrupted by defrost cycles or when the house thermostat is turned down.
For dual heating systems with fossil fuel furnace, heat recovery following nighttime thermostat setback often requires the fossil fuel furnace to raise the indoor temperature back to thermostat daytime set point. Often even at outdoor temperatures well above the freezing point. The tri-heat control unit is equipped with a feature called "first-heat"
enable. Following a house thermostat setback it detects a temperature drop in the return air duct. Following an outdoor temperature dependent delay that decreases as the outdoor temperature drops, "first heat" mode is enabled to confirm the house thermostat is in nighttime setback. When raising the house temperature back to daytime set point "first heat" allows the heat pump an additional programmable or outdoor dependent controlled, dynamic15 to 120 minutes delay to raise the house temperature to daytime set point. If daytime set point is not reached after the delay expires, a measured amount of dynamic electric plenum heat is enabled to assist the heat pump in raising the indoor temperature to daytime set point. This method makes first-heat heat recovery more cost efficient and timely in reaching daytime set point following a pre determined delay. Several tri-heat control programming modes can be used for heat recovery following house thermostat nighttime setback.
For manual night time setback and fast daytime heat recovery without furnace use the tri-heat control can be programmed to enable the heat pump only for a set time delay, followed by adding electric plenum heat in "boost mode" to enable all available electric element heat for maximum heat output, independent of actual outdoor temperature. When daytime set point is reached, tri heat control detects the higher return duct air temperature and disables first heat mode. This reverts, plenum heat to outdoor temperature dependent dynamic mode. In house thermostat daytime, heat-maintain mode stage one heat enables the heat pump. Following a stage two call for heat the electric plenum heater is enabled in outdoor temperature dependent dynamic heat mode supplying measured heat to raise the house thermostat back to stage one call for heat, thereby disabling the electric plenum heater. In this fashion the daytime thermostat set point is regulated within a very narrow temperature margin.
For scheduled nighttime set back and daytime heat recovery using the house thermostat in automatic programmable mode the tri-heat control module can be programmed for dynamic heat pump /dynamic plenum heat mode.
Following a call for heat from the house thermostat the heat pump starts heating the indoor space. Following an outdoor temperature dependent delay that decreases as the outdoor temperature drops further, the dynamic output controlled electric plenum heater is enabled. As the outdoor temperature drops more available heating elements are switched on. In this fashion the heat recovery process is completed within a narrow margin of error in arriving at daytime temperature set point within a fixed time delay independent of the outside temperature or heat loss of the indoor space. Like in the previous heating mode sequence the electric plenum heater reverts back to dynamic mode once it has reached thermostat day time set point. A number of different heating modes can be programmed for different heating system configurations and preferences.

At outdoor temperatures above 0 C stage two, electric plenum heat is can be disabled to allow the heat pump to complete the heat recovery process without plenum heat.
Most electronic automatic thermostats on the market only have a narrow temperature differential or hysteresis between stage one or two call for heat enable/disable. When used in dual heating systems it results in frequent cycling of the furnace when operating below the low temperature cutoff point and the heat pump when operating above balance point. This contributes to inefficiency and increased heating costs especially during fossil fuel furnace use. The tri-heat control module is equipped with a feature allowing the user to program individually or for both furnace and heat pump increase the on/off hysteresis following a call for heat from the house thermostat.
This reduces the number of heating cycle intervals. When enabled the heating appliance turns on only when the indoor temperature has dropped to house thermostat stage two call for heat. The heating appliance is disabled again after thermostat stage one, call for heat is satisfied. This feature automatically reverts back to normal 2 stage heat operation whenever the outdoor temperature drops below +3 C and the heat pump low temperature cutoff point.
Heat pump defrost cycles can be a major cause of unwanted temperature fluctuations in the indoor space. When the fuel furnace is connected to turn on during defrost cycles it produces a blast of hot air from the intake vents often interrupting the defrost cycle as the house thermostat call for stage one heat is now satisfied. When not connected to enable the fuel furnace, it can produce a blast of cold air from the vents, cooling down the indoor space, shutting down the defrost cycle prematurely to enable the furnace. During heat pump defrost cycles the tri-heat control unit enables the electric plenum heat in dynamic mode. By adding dynamic heat to temper the cold air generated by the heat pump during defrost the air temperature at the intake vents throughout the house is neutral, not hot not cold. In this fashion the indoor temperature remains stable allowing the heat pump defrost cycle to complete avoiding extended defrost cycles.

Because the heat pump duty cycle and total operating time increases dramatically with the tri-heat control module An optional demand defrost module can be installed for a heat pump with time based defrost. This module is installed at the outdoor heat pump unit. It controls the defrost cycle by controlling the outdoor coil thermostat contact output to the existing heat pump defrost board by adding an additional switch in series with the outdoor coil thermostat. This switch is controlled by the demand defrost module.
Making the installation of this demand defrost module easy, without modifications needed to the existing time based defrost board. With this module in place the heat pump defrost control circuit can initiate a defrost cycle only when actual when sufficient frost build up exists on the outdoor coil to warrant a defrost cycle.
Dirty air handler filters or failure of the blower fan can result in incomplete heat exchange at the indoor coil, raising the head pressure. This can lead to permanent compressor damage when left unattended. To prevent this from occurring, the tri-heat control module can be programmed to interlock with an external air flow switch located in the plenum heater to detect minimal airflow in the supply air duct. When insufficient airflow is detected it disables both plenum heater and heat, until sufficient airflow is restored. During this time the fossil fuel furnace is enabled for heating the indoor space.
The electric plenum heater used is generally custom OEM supply CSA/ UL
approved and build to individual requirements especially for mostly non standard seize and heating capacity add-on installations. Special control wiring between electric plenum heater is required to accommodate binary electric element switching and supervisory functions are tri heat control application specific. The typical application with multi stage heating elements deploys a two stage electric plenum heater allowing for three independent heating stages. Stage one heating element is typically 4kW, stage two 8kW.
The tri-heat control module increases the electric plenum heater output as the outdoor temperature drops. Stage one heat, 4kW stage two, 8kW, stage three, (4 +8kW) resulting symmetrical staged heat with less control hardware resulting in lower plenum heater unit cost.
Most forced air dual heating systems are equipped with a PSC blower fan motor.

Noise and vibrations generated by fan motor and air flow through duct work are unwelcome side effects of a forced air heating system, especially for older buildings where ductwork construction no longer meet today's standards. The PSC motor is also inefficient and costly to operate because of its high electricity consumption when compared to the ECM or electronically commutated, motor which typically consumes up to 65% less electricity to operate. The ECM blower fan motor is increasingly more used for HVAC applications such as high efficiency gas furnaces. The tri-heat control module is equipped with optional ECM fan motor speed control output, to accommodate variable blower fan speed control for idle speed, heating and cooling needs. Idle and maximum fan speeds are adjustable according to the heating /cooling requirements.
Without heating or cooling requirement and the house thermostat in fan on mode the tri-heat control is adjusted to idle speed which can be set between 100 RPM to maximum speed to provide air circulation throughout the indoor space without producing noticeable fan motor or airflow noise. When stage one heat pump in heat or cooling mode is enabled by the house thermostat the fan speed increases slowly to allow head pressure to build up before increasing to heat pump defined maximum speed. When in heat mode with one or more electric heating elements enabled the fan speed increases proportionally.
When the more powerful furnace is enabled the fan speed increases typically to maximum speed.
Dual heating system is equipped with a low or medium capacity fossil fuel combustion furnace require a separate fresh air intake to supply outside air to the heated indoor space as essential part of the furnace combustion process. An unwelcome side effect is cold air infiltration into the heated space. As the outdoor temperature drops an increasingly larger air pressure differential develops between cold outdoor, and warm indoor air. Cold outside air is forced into the house making the fresh air intake pipe also a major source of unwanted heat loss especially during the colder days of winter.
The tri-heat control module accommodates an optional fresh air damper equipped with end-switch for installation into an outside air intake pipe. The fresh air damper now prevents cold outside air from entering the heated space when the furnace is not enabled.
Following a thermostat E heat command or low temperature cutoff command the damper =
opens and the end switch closes enabling the furnace. For buildings without separate fresh air intake heat exchange mechanism the tri-heat control unit can be programmed to open the fresh air damper when the outdoor temperature exceeds +3 C to allow fresh air into the building with aid of the forced air blower fan.
Description Of Prior Art.
To be completed Summary Of The Invention To be completed Brief Description Of The Drawings.
Fig.1 Shows a basic single wire diagram of a dual heating system with fossil fuel furnace tri-heat control and electric plenum heater.
Fig.2 Shows a basic single wire diagram of a dual heating system with electric furnace and tri-heat control.
Fig.3 Shows first heat recovery sequence curve with fixed plenum heater enable delay and above balance point heat maintain.
Fig.4 Shows first heat recovery sequence curve with dynamic plenum heater enable Delay and below balance point heat maintain Fig.5 Shows the advantage using hysteresis doubling for furnace operation.
Fig.6 Tri-heat control functional block diagram (not included) Fig.7 Not defined Fig.8 Not defined Fig.9 Not defined Fig.10 Not defined Description Of The Preferred Embodiment To be completed The Embodiment Of The Invention In Which an Exclusive Property Or Privilege Is Claimed are Defined As Follows:
To be completed