Cost Effective Use of Auxiliary Electric Resistance Heating Requires a Paradigm Shift from Fossil Fuel Heating


If you’re not careful, the way in which you adjust thermostat settings for your centralized forced-air geothermal heating system can result in unnecessary use of auxiliary electric resistance heating (AERH). A good understanding of how these systems respond to thermostat setting changes can go a long way to avoiding unnecessary energy use and unpleasant surprises in your monthly electricity bill.

AERH is a general requirement for all 2-stage and many variable-speed water-to-air (wta) heat pump systems. (Single-stage wta heat pumps are effectively obsolete in New York State as they don’t qualify for the GSHP Rebate Program.) AERH is often a necessary component of these systems in order to avoid system short-cycling (which is inefficient and results in excessive compressor wear) during mild weather conditions (e.g. Spring and Fall) and minimize your annual electric energy consumption. Wta heat pumps are typically slightly undersized relative to the building’s design heat load – the amount of heat required to maintain comfortable inside temperatures when outside temperature is 2 to 6F (in western New York). This reduces short-cycling but requires another source of heat to assist the heat pump during the occasional bitterly cold outside temperatures. That’s the job of AERH which is commonly built into the wta packaged unit – heat pump and integrated blower assembly in one enclosure – and controlled by the thermostat.

Here’s an illustrative example of how it’s intended to work:

  1. Let’s say the thermostat differentials – typically adjustable only by a service technician – for a 2-stage wta heat pump and blower assembly are set at 0.5F, 1.0F, and 3.0F for first stage, second stage, and auxiliary heat, respectively. So when the indoor temperature is at least 0.5F below the thermostat setting – let’s assume 70F for this example, first stage heating is available to address the difference; when it’s at least 1.0F, second stage heating is also available. It’s not until room temperature falls to more than 3.0F below the thermostat setting that AERH becomes available.

a. For this example, the outside temperature is initially in the 30s and first stage heating — ~80% of maximum heat pump capacity – kicks in when room temperature drops below 69.5F until it recovers to 70F.

b. As the outside temperature begins to drop… eventually to single digits, first stage heating is unable to overcome the heat loss and room temperature dips below 69.0F, activating second stage – maximum heat pump capacity – heating.

c. Second stage heating may be adequate to return room temperature to 70F, but as outside temperatures continue to fall, eventually it too can’t keep up. When room temperature drops below 67.0F, AERH is activated to assist the heat pump’s second stage until room temperature recovers to the thermostat setting.

2. AERH also serves as a back-up if the heat pump is temporarily out of service, e.g. a loop pump fails and the compressor shuts down to protect itself from impending damage. While you’re waiting for a service technician to repair or replace the loop pump, you still have heat.

These are the two scenarios that AERH is designed to address, but if sudden and significant increases to the thermostat setting occurs (e.g. recovery from overnight setback), AERH can be activated even though you’d rather it wasn’t. Why should you care? Heat from electric resistance is 4 to 5 times more costly than that from the heat pump – enough said. Here’s an unnecessarily common example:

You’re accustomed to adjusting the thermostat setting down several degrees overnight – say 70 to 64F – to save energy and, more specifically, money – more on why you should break that habit in a minute. By morning and when the thermostat setting has been returned to 70F, room temperature has dropped to 64F. If your thermostat is a Geostar / WaterFurnace and the thermostat setting increase was programmed (as opposed to manually adjusted), the heat pump will run at maximum output without aid from AERH for 30 minutes, then if room temperature is not yet above the differential for AERH – 70-3=67F for the previous example, AERH will be activated until room temperature exceeds second stage differential – 69F for previous example.  If your heat pump has been sized appropriately, you can be pretty certain room temperature will still be well below the thermostat setting. Why? Your fossil-fuel furnace paradigm is that recovery from setback only takes 20 to 30 minutes – not so for heat pumps. It’s very common for furnaces to be significantly oversized for the duty; after all, there’s very little downside to over-sizing furnaces – a very modest increase in equipment cost but short-cycling has virtually no effect on efficiency or wear.  Not so for heat pumps: because they’re not oversized, recovery from setback will be slower… much slower.

So what are you to do to avoid inadvertently activating AERH… and unpleasant surprises when your electric bill arrives?:

  • The simplest answer is to stop adjusting the thermostat. You’re not saving much by routinely lowering the setting, because these systems are so energy efficient that it doesn’t cost much to operate them.
  • If you still need to setback at night (because you sleep better in cooler temperatures), then minimize the setback amount and/or ask your installer to increase the AERH differential and/or recover from setback in two programmed thermostat increases separated by a few hours, e.g. to 67F at 3am then 70F at 6am.
  • Do not manually adjust your thermostat more than a few degrees at a time, and never more than the AERH differential.

To get the most from your geothermal heat pump system, understand how it responds to thermostat inputs… and shift from your fossil-fuel paradigm.