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  • Dual Fuel Systems

Dual Fuel Systems


What are they and how do they work?


Dual fuel systems are two systems of heating using a heat pump and a fossil fuel. The fossil fuels used are natural gas, oil or propane. Combining these types of heating systems maximizes the energy efficiency of any heating system. In fact there is no other system that provides a better payback to investment than a dual fuel system.

When comparing energy efficiencies of equipment the payback or return on investment must also be considered to give a true aspect of the equipment's overall performance. For example a natural gas furnace with an air conditioning system may cost $2,400 for our example. That same system using an air to air heat pump instead of a standard air conditioning system will cost $256 more. Depending on your location the cost to provide heat from the standard system versus the dual fuel system will cost $200 to $300 more per year to operate. The choice to a dual fuel system becomes very obvious with the increased savings in heating costs. The costs for cooling are the same with either system since the Seer or air conditioning efficiency of both systems are equal. 

During the outside temperatures of 35 to 70 degrees a heat pump is a more economical method of heating in comparison to a fossil fuel system such as a natural gas furnace, oil or propane furnace. With resistance electric heat a standard watt of electricity produces 3.4 btus. However a standard watt of electricity in a heat pump will produce 10.2 btus. That's 3 times more heat than with resistance heat. Immediately you can see the performance efficiency of a heat pump. In HVAC terms this is referred to as the Coefficient of Performance. Heat Pumps today have a coefficient of performance of 3.0 or higher. The heat pump only produces this high level of performance at 35 degrees and above. Below 35 degrees outside temperature the coefficient of performance drops rapidly.

In a standard heat pump there is electric heat which will operate to maintain the temperature and produce the required btus when the outside temperature is too low and the heat pump efficiency and capacity are too low. Note that as the coefficient of performance decreases so does the amount of heat produced by the compressor in the heat pump. That is why in a standard heat pump electric heat is added. This electric heat is often referred to as back up heat, emergency heat, auxiliary heat or heat strips. When a heat pump compressor operates below 35 degrees the efficiency and energy costs escalate in comparison to the heat produced. At this point the electric heat strips are in operation to provide the required amount of heat. And as the outdoor temperature decreases the amount of heat required to keep the house comfortable increases. As the outdoor temperature decreases even further the compressor efficiency is so low that it is better to shut off the compressor and only use the electric heat. For example when the outside temperature is below 20 degrees there is no benefit to operating the outdoor compressor. The amount of heat produced and the coefficient of performance has dropped to a point that is almost equivalent to the performance of the electric heat strips. In addition to operate the compressor at this level of performance causes unnecessary wear and tear.

In a dual fuel system instead of using costly electric heat the dual fuel system uses a fossil fired furnace. For example if you live in an area where natural gas is available the fossil fuel system would be a gas furnace. When the efficiency of the heat pump decreases to a point where it is no longer efficient and the coefficient of performance is 2 or less the gas furnace will operate as the secondary source of heat.

If you are using oil or propane the propane or oil fired furnace become the secondary source of heat.

To compare the costs of fuels Click Here or go to this link:  Here you can see the difference in costs between many different types of fuels and electricity. To compare the operating costs of a heat pump and another type of fuel, follow this example. If you have natural gas take a recent gas bill form the winter months. Determine the amount of cubic feet of gas consumed during that given month. Then take the grand total of the gas bill including all taxes and divide that by the cubic feet of gas that was consumed. That will be your actual real cost per cubic foot of gas. Now take your electric bill and do the same only this time you will determine the actual kw or kilowatts of electricity that were consumed. Now again take the grand total of the electric bill including all applicable taxes and divide this amount by the actual kw of electricity and you will have your cost per kw.   Figure the amount of natural gas

Another added bonus to a dual fuel system is the heat pump also produces air conditioning for the hot summer months.

As for initial costs a dual fuel system is generally only approximately $200 more than a standard high efficiency furnace with an air conditioning system. But the energy savings are so much more that the extra investment will generally pay for itself in one or two years. After that time the energy savings go directly to your budget and you continue to save more every year as energy costs escalate. With the heating and air conditioning energy costs the largest part of your household budget these savings are very significant. With natural gas and all other energy costs skyrocketing, the question of whether you should install a dual fuel system becomes more of a question as to why you wouldn't want a dual fuel system.

If you could like to learn more about heat pumps Click Here or go to this link:

If you would like to learn more about gas furnaces Click Here or go to this link:


Typical questions about Dual Fuel Systems

I live in the northern part of the country and the winter temperatures drop to 10 degrees or less in the winter. How would a dual fuel system be of benefit to me with such cold winters? Actually in your area is where a dual fuel system will save the most. If you look at a meteorology weather chart for your region you will find on an average winter there are many hours of every month where the winter temperatures will be 35 degrees or warmer. Especially in the months of October, November, March and April and even into May there are many days where the temperature is 35 degrees or more. In fact for most regions of the north there are also more times in the coldest of winter months where the temperature is 35 degrees or above. If you were to add up all those days or hours you would find there are significant amounts of time where the heat pump would be operating efficiently and providing heat at a cost below that of natural gas, oil and especially propane heat. Please make note that this information applies to almost all areas of the country with exception only to California where extreme electrical costs of .23 to .42 per kw make a dual fuel system inefficient. However for the rest of the country where electrical costs are .06 to .14 per kw the dual fuel system is definitely the best energy efficient system.

How much will I save by installing a dual fuel system over a furnace with air conditioning? Energy costs as well as energy savings are very complex to calculate. Energy costs are dependant on (a.) the cost of the fossil fuel (b.) the cost of electricity in your particular region of the country (c.) the amount of heating and cooling degree days (d.) type of construction and insulation in your home and (d.) the efficiency of the heating and air conditioning equipment and (e.) most importantly where you set the thermostat. All these factors play an important role in determining what your energy costs and energy savings will be. However a dual fuel system will almost always have a payback of two years or less for most parts of the country.

Where won't a dual fuel system make sense? In California due to the high electrical energy costs, a dual fuel system is cost prohibitive. In more temperate climates such as Florida, Alabama, Southern Texas, etc. are areas where a heat pump with electric back up would be more cost effective. In these areas the amount of time the electric heat strips would be required would be very minimal.

How efficient should the equipment be for a dual fuel system? For a dual fuel system using natural gas with a heat pump the furnace should be a two stage 92+ AFUE gas furnace with variable speed. The heat pump should have a minimum of 13 Seer for air conditioning and a heating efficiency or HSPF of 7.8 or more. There is no benefit to installing a dual fuel system with lower efficiency equipment. Also remember that next year the standard seer permitted to be manufactured by the Department of Energy will be 13. This change in standards will introduce variable speed as a standard item for almost all applications.

Would I need a lot of fancy controls to operate this type of system? Today the new Ultra High Tech Thermostats such as the White Rodgers 1F95-377 or the Robertshaw 9870i thermostats can do all the features the dual fuel kits used to perform. We do recommend the addition of an outdoor thermostat in conjunction with the thermostats to determine the outdoor temperature where the dual fuel system switches over form the heat pump operation to the gas furnace or fossil fuel.

There is also the older traditional method of controlling the system with a conventional heat pump thermostat and an all fuel board. However with the new technology thermostats this method is more labor and more complicated for installation and troubleshooting.

How difficult is it to install and set up a dual fuel system in comparison to a standard system? If you're capable of installing a standard system then a dual fuel system will require only three more control wires. With today's new high tech thermostats the dual fuel systems are easier to install and less complex than ever for an installer.

What are the basic component parts required for a dual fuel system? To fully understand a dual fuel system there are basic elements of the system you will need to understand to make a decision whether dual fuel is for you.

The most basic component, the heat pump, is an air conditioning system with a reversing valve.

To better understand how a heat pump provides heat think of this example. Anytime you walk past a window air conditioner on the outside of a window in the summer time remember what you feel? What you feel is a strong gust of hot air being discharged to the outside. And on the inside you feel cool air discharged to remove heat form the house. So think about this process. A window air conditioner is removing heat form the inside and rejecting it to the outside. How does it do that? Inside the air conditioner is a pump called a compressor. The compressor pumps refrigerant inside copper tubing. The refrigerant generally referred to as Freon circulates through the copper tubes. Newer refrigerants are called R410 or most times referred to as Puron which is simply the name Carrier has given to this refrigerant. As it circulates through the tubing the Freon absorbs heat form the inside air. As this refrigerant absorbs heat it changes from a liquid to a gas. Always remember that the compressor is a gas pump, not a liquid pump. When the refrigerant passes through the outside coil and air passes over it from the fan, the refrigerant rejects the heat to the outside air.  As this hot refrigerant gas rejects heat to the outside air it changes from a gas to a liquid. So in simplistic view the compressor pumps the heat form the inside air to the outside air by compressing the heat out of the refrigerant. This describes how a basic air conditioning system operates. The difference between an air conditioner and a heat pump is a valve that is placed in the copper tubing. This valve is called a reversing valve. The purpose of the reversing valve is to change the flow of the refrigerant. So in order to also heat a heat pump changes the flow of the refrigerant and now takes heat form the outside air and rejects the heat inside the house.

But it's cold outside when I need heat in my house. How does the heat pump produce heat from the outside air when the air is cold? Incredible to believe but that air outside still has a lot of heat in it. Cold is a relative term and what we may sense to our bodies as cold still has heat. However to get the heat out of that air we need a compressor. The purpose of the compressor is to extract the heat out of the air and put it in the house. The compressor can do that efficiently down to outside temperatures as low as 25 degrees. Below that temperature the compressor is no longer efficient at extracting the heat. So another source of heat is required for colder environments. Typically that source of heat is electric. In a dual fuel system that source then becomes either natural gas, oil or propane gas.

I've seen heat pumps in operation and ice forms on the outside of the outdoor coil. Is that normal? Yes in fact you would expect to see ice form on the outdoor coil in lower temperatures. Remember the outdoor coil now becomes like the indoor coil for an air conditioner. So as the outside temperature changes and becomes colder the coil temperature becomes colder also. As that outdoor coil temperature drops below 32 degrees the coil will begin to freeze. As the outside air passes over the coil the moisture in the air freezes on the coil and begins to build up.

Won't that build up of ice block the air flow and decrease efficiency? Yes! So every 30, 60 or 90 minutes of operation the heat pump will go into a defrost operation.  This defrost cycle reverses the flow of the refrigerant into the normal air conditioning mode and heats up the ice and melts it off the coil.

A typical defrost cycle occurs as stated above every 30,60 or 90 minutes depending upon the area and setting of the electronic control board during installation. Naturally as you travel further south into more moderate climates the heat pump will need less time between defrost cycles. For example in Florida a typical heat pump would be set up to provide defrost cycles every 90 minutes and in the north such as Pennsylvania the defrost board would be set to provide defrost cycles for every 30 minutes.

At those timed intervals the defrost control board senses the outside coil temperature. If the temperature of the coil is below 28 degrees the heat pump will begin defrosting. This will set the system to operate as air conditioning to remove any ice form the outdoor coil. When the outdoor coil warms to 65 degrees or ten minutes the defrost cycle is completed.

When the heat pump goes into the defrost cycle it is air conditioning, correct? Yes

Won't that make the house inside cold when this happens? Yes but what also happens is the back up heat source will be energized at the same time to prevent any chilling effects caused during the defrost cycle. In the case of a dual fuel system with a gas furnace, the gas furnace will be in operation during the defrost cycle to prevent any chilling of the air.

It would seem that as the air gets colder outside that ice will form quicker and so there will be more need for defrosting of the coil. Doesn't that make the heat pump inefficient? Yes! As the outside air temperature drops below 35 degrees naturally there will be more ice forming on the outdoor coil very quickly. For that reason it is not energy efficient to operate the heat pump below 35 degrees or lower.

How do I know when it is inefficient to operate the heat pump and at what temperature should the heat pump shut off? The temperature at which the heat pump can no longer provide sufficient heat to heat the house is called the balance point. Each house has a different balance point. The balance point differs based on the construction and efficiency of the house and the size equipment selected. For example a house with a 2.5 ton heat pump that has 2000 square feet and is energy efficient will have a lower balance point than another 2,000 square foot house that is not energy efficient.

Is a heat pump sized for the heating requirements of the house or the air conditioning requirements? In most applications the heating requirements for any house are greater than the cooling requirements. Only in moderate climates such as Florida is the reverse true. So in more northern climates there is a greater difference between the heating btu requirements and the air conditioning requirements. If a heat pump was sized for the heating requirements, the heat pump would be extremely efficient for heating. However for air conditioning the system would be grossly oversized resulting in very poor air conditioning.

To give a better example of this let's take a 2,000 square foot energy efficient house as described above. The correct size heat pump for this house located in Pennsylvania would be 2.5 tons because the air conditioning requirements were 26,880 btus or 2.24 tons. So we selected a 2.5 or 2-1/2 ton heat pump. Our heating requirements for this house however are 41,240 btus or 3.44 tons. If we selected our heat pump based on that requirement we would select a 3.5 ton heat pump. That selection would be excellent for heating and our balance point or the temperature where the heat pump could no longer provide sufficient heat for the house would be 24 degrees. Whereas the 2.5 ton heat pump would have a balance point of 30 degrees. The 3.5 ton heat pump would be energy efficient and perform beyond our wildest expectations for energy efficiency until we went to turn it on for air conditioning. Then that 3.5 ton heat pump would only operate for 3 to 5 minutes per cycle and never operate long enough to dehumidify the air. The net result would be an air conditioning system that would definitely cool the house but never remove any moisture from the air. And we would end up with a house that would produce a summer indoor climate that would be so clammy and humid that it would be unbearable. The second problem would be the short life span of the equipment. Short cycling equipment doesn't last long. Each time you start up any piece of equipment whether it is your car air conditioner furnace, etc. you put the most stress on the equipment. A car that has mostly highway miles on it versus a car with all city miles is going to generally have fewer repairs and longer life span. The same is true of heating and air conditioning equipment. So that is why we size heat pumps for air conditioning instead of heating.

How do the new Ultra Hi Tech thermostats operate and what makes them different from other types of thermostats? There are many thermostats available today and quite frankly they can be very confusing. For dual fuel systems the two thermostats we recommend are the White Rodgers 1F94 or the Robertshaw 9870i.

The White Rodgers 1F94 thermostat is a great dual fuel thermostat providing intelligent energy operation. Each of these thermostats provide individual programming on a daily basis for each day including automatic daylight savings time changeover. One important feature of the Robertshaw thermostat is true humidification and dehumidification control. These two features are very important because adding a humidifier to a heat pump system with variable speed requires extra controls and wiring. With the Robertshaw thermostat there is a true connection in the thermostat with humidity sensing. In addition the Robertshaw thermostat also provides true dehumidification control of the variable speed drive system. No other thermostat provides direct control of the variable speed drive system on the market today. Yes there are other thermostats that claim to have dehumidification control including White Rodgers and Honeywell. But all those other thermostats simply lower the air conditioning temperature setting 1.5 to 2 degrees. This is not true dehumidification but only a change or lowering of the set point of the thermostat.

Since all variable speed drive systems are made by one manufacturer, General Electric, they are all basically the same. On all variable speed drive systems there is a dehumidification cycle. This feature actually decreases the fan speed by 20% to slow the air passing over the indoor coil thus allowing more moisture to be removed from the air. True dehumidification occurs without falsely lowering the temperature of the house which can cause discomfort. Most contractors are unaware of this feature and as a result the dehumidification cycle was almost never utilized. If it was utilized a separate dehumidistat had to be installed. And if the house also a had a humidifier then a separate humidistat was required also. So before a typical application utilizing temperature, humidity and dehumidification required three controls mounted on the wall. Aside from the fact it looked obnoxious it was also difficult to understand and set up. The Robertshaw 9870i thermostat replaces all those controls and is relatively easy to set up and more user friendly. Although more costly than the White Rodgers 1F94 thermostat which would also require a humidistat and dehumidistat, the Robertshaw thermostat is the only true thermostat for providing total temperature and humidity control of a variable speed drive system. For complete specifications for the Robertshaw 9870i thermostat Click Here or go to this link:

For a dual fuel system with a humidifier we highly recommend the Robertshaw 9870i thermostat in conjunction with a outdoor changeover thermostat.

When you refer to variable speed drive on an indoor air handler blower or on a furnace, does the fan change speed according to the amount of air conditioning or heating required?


Absolutely no! Probably one of the most misunderstood features of today's high tech heating and air conditioning equipment is variable speed drive. All variable speed drive motors and their controllers are made by one manufacturer and that is General Electric. General Electric has the entire market for the variable speed drive units and recently introduced the latest 6th series with new patents. So as long as General Electric continues to dominate this market prices will remain somewhat high. But in addition we have all come to know General Electric as producing very high quality products that last.

The variable speed drive systems provide 3 basic but important advantages over standard multi-speed motors.

1. VDS or variable drive systems are quiet. A typical multi-speed motor is anything but quiet. You don't hear the VDS motors start or stop. VDS systems are Direct Current or DC voltage motors. Because of this the motor does not start immediately and go to full speed. The motor also do not have a high inrush of current during start up that is common with a multi-speed motor.

2. VDS systems use far less power than a multi-speed motor. Typical multi-speed blower motors for furnaces or air handlers will consume anywhere from 500 to 800 watts of power. A VDS motor only consumes about 75 watts of power.

3. Because VDS motors are direct current the start up and shut down of the motor is soft and slow. This slow start allows the air conditioning coil to become colder a lot more quickly providing better dehumidification. On heating applications such as gas furnaces this feature also allows the air to flow form the system more gently while gradually increasing air speed to eliminate those typical hot blasts of air that were always associated with older style multi-speed motors. On shut down the same is also true. Instead of just shutting down form 100% air flow to 0 air flow the variable speed drive system slows gradually until it stops. This feature of the VDS systems allows for more even temperatures as well as improved comfort and quietness.

Other features are added to the variable speed drive systems to improve comfort as well. For example on the new Goodman GMV9 furnaces there is a feature called auto comfort mode. This auto comfort mode improves the dehumidification performance of the air conditioning cycle. In a typical air conditioning system on a multi-speed blower or a standard VDS blower system it can take anywhere form 12 to 15 minutes until the indoor coil is cold enough to begin proper dehumidification. Since most cycle run times are anywhere from 10 to 15 minutes during the average summer months there is usually not enough run time in the cycle to provide proper dehumidification. As a result on days where the outside temperatures are less than 90 degrees, which is usually the case during most of the summer, the result is a cool clammy house. With the auto comfort mode the blower will take more time to achieve full speed. There are four settings in the auto comfort mode. The "A" setting provides minimum dehumidification and is similar to the standard VDS start up operation. However if set to the maximum "D" setting, the fan will take up to 5 minutes to achieve full operation. Final result is the indoor coil will become cold enough within two minutes to begin dehumidifying instead of the typical 12 to 15 minutes. For most areas of the country where high humidity levels in the summer months are unbearable, this change in time to achieve full bower speed operation creates the same level of comfort on a mild temperature high humidity day as it does on a hot humid day. What an incredible change of performance and comfort!

I'm reading more and more about two stage systems. What does two stage mean? Are there two compressors? Or on a furnace are there two gas valves and two heat exchangers? Whether you refer to a gas furnace or a heat pump or an air conditioner, two stage means there are two levels of capacity in the same unit.

Today everybody wants to have a 2 stage variable speed gas furnace. Why? Because of the increased energy efficiency and the high level of comfort achieved. A gas furnace with 2 stages has a tremendous advantage over the same single stage furnace. If you were to compare two gas furnaces with equal AFUE efficiencies with one being single stage and the other being two stage with variable speed you will see the two stage furnace saves more energy and provides a higher level of comfort. In fact the single stage furnace in a house with a temperature recorder will provide a typical 3 degree or more deviation form the thermostat setting. That same house with a 2 stage furnace will provide a 1 degree variation room the thermostat setting. The two stage furnace will use less gas and electricity. The two stage furnace will also run longer.

How can a furnace that operates for longer periods of time use less energy? Think of the furnace like a car. A single stage furnace would be the equivalent of operating your car at full throttle every time you turn it on. Could you imagine trying to drive on an expressway and each time you needed to maintain speed  







For a natural gas dual fuel system the components are as follows:



The gas furnace with blower:  

The indoor cased heat pump coil:  


The outdoor condensing unit:


The outdoor electrical disconnect for the condensing unit:


The outdoor electrical whip:



Risers for the outdoor condensing unit:



Condenser pad base to place below the condensing unit:


Refrigeration lineset to connect from the outdoor unit to the indoor coil:  Available in 30 and 50 foot lengths.

House thimble for decorative outlet for the above lineset:


Thermostat to control the system:


Thermostat wire to connect to the thermostat to the air handler and to the outdoor condensing unit:  



Humidifier for the air handler:


High Media filter for the air handler:

Auxiliary drain pan for the air handler in the horizontal position:


Condensate pump:


Vibration Isolator pads for the indoor air handler to reduce noise transmission:


Duct materials to provide the air distribution system:

DESCO will select and assist you in the selection of all components to put together a complete heat pump system including the duct system. For more information visit