Simultaneous Heating & Cooling: It Happens More Than You Realize.

Posted by Dalkia Solutions on Mar 11, 2013 11:21:22 AM

During building energy assessments we often find obvious, no-cost behavior changes savings:   A compressor running flat-out for an idled production line.   Flood lights illuminating an unused parking lot all year long.  Warehouse dock doors that stay open all day.   Our very technical recommendations come with a smiley face:

Step 1.  Shut off the compressor. 

Step 2. Turn off the lights.

Step 3.  Close the door.

But the best visuals come from situations where we find a heating system and a cooling system operating at the same time.

Some are short duration – like a surgeon needing the operating room to be kept at very low temperatures to slow the patient’s blood flow during the procedure, while the surrounding rooms require heating.  Some are longer duration, like the college dorm with old steam radiators cranking too much heat, spurring students to leave the windows open all winter long.

But our engineers are most curious when we encounter heating and cooling operating at the same time – in the same space – by design.  Sounds wacky, but consider these:

1.  Big Box Retail Stores
Big box retail stores typically have packaged rooftop units (RTUs) providing space conditioning.  Most RTUs are configured to cycle fresh air into the store at a certain rate, based on the estimated number of shoppers.  So while the RTU is working to cool and dehumidify on a summer day, it’s simultaneously drawing in hot, humid “fresh” outside air, removing “stale” air that it just finished cooling.  The more outside air the RTU brings in, the harder it needs to work to cool and dehumidify the space.  (Just reverse this for the winter season example.)

Options here?  We add demand control ventilation (DCV) which measures the amount of CO2 in the store and brings in fresh air only when needed, as opposed to hard-wiring 6 to 10 fresh air changes per day.  We can also add energy-recovery ventilators, which use the cold or heat from air being removed to pre-cool or pre-heat the incoming outside air, thereby reducing the RTU’s work load.

2.  Cold Storage Warehouses
Ammonia chillers keep these buildings at below freezing temperatures all year round, preserving stored frozen product.  Yet the building’s concrete slab floor also contains its own heating system.  Without it the subsoil below the slab would freeze, swell and buckle the floor.  The heating system might be hot liquid or electric within the slab, or forced hot air blown under the building’s foundation.  Either way, the chillers fight a heated floor 24 x 7 x 365.

Options here?   Some warehouses have already optimized by using recaptured waste heat from the chillers for the floor heating.  As a large thermal mass, slab floors take a long time to change temperature, so we consider running the heated floor only when necessary, based on the temperature differential at that time.  If the floor is heated electrically we can time-shift the system to nighttime operation, when kWh can be less expensive – or turn the system off briefly at peak periods in order to reduce utility demand charges.

3.  Grocery Stores
Next time you reach into a cooler cabinet to grab a Ben & Jerry’s, take a look at the glass door.  It’s heated.  To combat condensation buildup, case manufacturers have designed in an electric door heating system which operates in tandem with the cooling system.  Interestingly the heater draws nearly four times more electricity than the cooling.

Options here?   We add moisture sensors (called anti-sweat controllers) just within the door frame connected to the glass.  The controllers then cycle heat to the doors only when moisture is detected by the sensor.

Unlike “close the door” and “turn off the lights” most of these solutions require a retrofit and cost money.   While the investments are typically fast payback, they only reduce consumption a modest amount.  But remember we started with two systems designed to fight each other – which is never a good thing in energy efficiency.


Topics: Blog Topics, Refrigeration, Heating