Are you considering the heat pump option for your all-electric HVAC?

What you need to know

By Steve D. Johnson

You can safely say that the heat pump conversation is “heating up” in the North America.  There is high interest and the queries come up in any conversation regarding climate control for battery buses. Why?  Because it makes sense for the zero-emission battery bus when heat requires almost twice the electric energy as air conditioning.  This was a very early learning.  The theory is that the heat pump would reduce the amount of power taken from the battery if it replaced electric resistance heaters, thereby increasing the Kilowatt hours of the battery and extending the range of the bus. 

Heat pumps use the HVAC unit’s electrically driven compressor and heat exchangers in the existing refrigeration system to extract heat energy from the outdoor air and use that heat to warm the designated space.

How does the heat pump work and when is it most practical?

Heat pumps use the HVAC unit’s electrically driven compressor and heat exchangers in the existing refrigeration system to extract heat energy from the outdoor air and use that heat to warm the designated space. The size of the system and the ambient temperature determine how much heat is available.  Most residents in warm and moderate climates are familiar with heat pumps.  In the very warm climate areas, it can be the sole source of heat.  As you move to the moderate and cold climate areas, a back-up heat source may be needed for unseasonable and record-breaking cold snaps.   While the theory of operation is sound, and when in use the heat pump is a more efficient source of heat, the current heat pump designs for buses have a major limitation when compared to the heat pump used in residential HVAC. 

The limitation is the threshold temperature at which the system is no longer capable of extracting heat from the atmosphere.  Typical bus HVAC heat pump systems will operate down to 19 degrees F (7C) or 5 degrees F (-15C) while a residential heat can still extract heat at minus zero F temperatures.  What is the reason for this difference is a frequently asked question.  The simple answer is the sizing of the heat exchangers.  A bus HVAC system can only weigh so much and will be allowed only so much space on the bus.  Everything, including the heat pump, must be designed within these limitations.  New technologies like the use of CO2 will dramatically improve the efficiency and output of the heat pump. 

As for the practical use of the heat pump, consider that a bus with the full capability of using engine heat to heat the bus through the HVAC system, still needs auxiliary or back-up heat in the colder climates.  To believe that the heat pump will resolve the issue of heat for the battery bus is a huge misconception.  Some form of electric heat or glycol heat should be available for back up even when the heat pump is the primary source.  Yes, I said glycol heat even though we are speaking of buses that don’t have engines.  Glycol is being used to keep the battery and other electronics from overheating.  Why not use this available heat source?  More supplier to OEM integration will have to take place to realize this opportunity.

What other considerations should I be made aware of?

DESIGN: To add the heat pump option to an existing system requires additional components and tubing which also add complexity to service and diagnostics.

COST: The unit will be more expensive and, depending on the climate, resistance heat or some other form of back-up heat will still be required.

SERVICE: The condenser becomes the evaporator in heat pump mode. The condenser fans and compressors operate in winter, meaning additional run time with the compressor subjected to severe operating conditions.  The expected life for these components changes.  There will be more components to service and diagnose when failure occur. The condenser coil must be tube and fin and will also be subjected to severe operating conditions with the freezing and thawing.

There is one other thing you might be interested in knowing.  The use of heat pump technology on buses in U.S. Transit lags far behind Europe and China.  All of the global HVAC suppliers have a heat pump option and most all have a presence in the US.  The high level of interest from the end user base and the US Transit OEMS says that things will be happening soon.  Stay tuned.

Steve D. Johnson, Sr. serves as product marketing manager, Bus HVAC, at Thermo King, Minneapolis, MN. Thermo King is a world leader in transport temperature control systems for buses. Thermo King also manufactures auxiliary power units, which dramatically reduce engine idling. All Thermo King products are backed by a nationwide dealer network. Visit