Glendale Heating & Air Conditioning | Serving Greater Seattle Since 1938

What is the difference between oil and gas when using a furnace for back up auxiliary heat? The simple fact is that oil and gas furnaces have a very different comfort level.

First of all, when a heating technician uses the terms “auxiliary heat” or backup “emergency heat,” this means that your heat pump system will need help heating your home when the outdoor temperature reaches its minimum balance point temperature for the outdoor condenser to operate efficiently. This is at roughly 40°F.

You see, a heat pump condenser can only exchange the differential of heat between the outdoor ambient temperature and its refrigerant balance point. (Example: R410A refrigerant boils at sea level around 48.5° F.) This balance point can be manipulated slightly with the increase of atmospheric pressures (sea level to mountain top) within the system, but as the outdoor temperatures reach freezing (32° F), the condenser should be locked out by the thermostat to prevent ice formation on the unit fan blades and the possibilities freezing up of the coil.

This is also the point at which the energy consumed by the unit to create heat and discharge it to the home is not economically proficient.

In other words, if your total heat rise from the indoor heat pump coil is less than 5° ΔT total (return air temperature minus warm air discharge temperature), your home will not gain heat quickly enough to quantify the power consumption used to heat it efficiently.

Supplemental Heat Sources

At this point, you need another source of heat to supplement the lack of heat output at the warm air register or indoor head.

A Lennox® forced warm air ducted gas furnace is designed to deliver between 35-55° ΔT degrees of heat rise, depending on the BTU needed for your home (45,000-110,000 BTU). This will boost the heat output on the days in which the heat pump is locked out.

A Thermo Pride® Oil furnace is designed to deliver 60-70° ΔT of heat rise, depending on the BTU needed (60,000-132,000) and indoor blower configurations.

The difference in comfort between the two furnaces is quite substantial, due to the higher recovery rates using an oil furnace for auxiliary back up.

Most people would agree that a 40% relative humidity achieves a feel of a much warmer and more comfortable home. The oil furnace can be set up with fewer furnace CPH’s (cycles per hour) and still deliver a timely, comfortable heat output with more relative humidity sustained within the air. This is because the fewer times per hour air is circulated across a furnaces heat exchanger (internal metal fins that transfer the heat to air) the less moisture is pulled from the air as it is heated. Thus a drying of the air occurs when configuring the thermostat for gas or propane furnaces, due to the often increased amount of CPH needed to deliver heat to the air.

Additional Differences

Another difference between gas and oil is the flame temperature at the point of combustion.

Oil furnaces burn hotter. Natural gas burns at 950-1,150°F. Oil burns between 1,900-3,000°F, depending on firing rate.

The byproducts of oil combustion register much smaller amounts of CO (carbon monoxide) within the flue gases. Obviously, aside from the explosive nature of natural gas and propane leaks, this makes oil heat a much safer furnace in the event of a heat exchanger leak, crack or plugging.

In fact, carbon monoxide alarms are only required when installing a gas or propane furnace.

All fossil fuel furnaces run the risk of sooting and plugging with ash (not just oil furnaces), so it is especially important to have your gas or oil furnace checked and serviced regularly

A power vacuum by a professional, truck-mounted suction device may be required if you are experiencing a blockage, odors or service problems.

These benefits of oil backup heat do come at a premium price and a little more monitoring of the fuel level used during the winter. You don’t want to run out!

The overall price between the fuels is substantially different between per gallon price versus per inch of water column of natural gas or propane.

Factoring in news about recent drops in crude oil prices and new advances in oil furnace design, fuel consumption (when used with a heat pump system) can be extremely low for a 12-month average.

The days of an average oil heated home using 500 gallons per year are changing quite rapidly. To say oil heat is the more affordable option for backup auxiliary heat would simply be untrue in this day and age.

However, I feel obligated to note that superior comfort often comes with a premium price tag.

Think of air travel these days; people still purchase first-class seating even as expensive as it is already. Any professional sports stadium, a suite or club level lounge is always much more expensive but far more comfortable. Luxury cars and trucks these days — they all require a premium price for their comfort and use.

If you always factor in the furnace’s ability to deliver truly comfortable conditioning when you are choosing your heat pump systems backup, no matter what the cost, it will always be money well spent! 

— Chadwick Fulton

Technical applications

Glendale Heating

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