ENVIRONMENTAL CENTS: Understanding Heat Pumps
Understanding heat pump ratings and efficiencies can have a significant impact on your energy costs.
By Bruce Saller
COLUMNIST
Heating and cooling our homes is our highest use of energy according to the U.S. Energy Information Administration. In our climate, heat pumps are an efficient heating and cooling solution. There are two efficiency ratings for a heat pump:
SEER - Seasonal Energy Efficiency Ratio. Average cooling output in British Thermal Units per Watt-Hour (BTUs/WH). Since 1 WH = 3.4 BTU, a unit with a 17 SEER removes 5 WH of heat for every 1 WH of power (Coefficient of Performance (COP) of 5). A 16 SEER unit will use 25% more energy than a 20 SEER unit.
HSPF – Heating Seasonal Performance Factor. Average heating output in BTUs per WH. A unit with an 8.5 HSPF provides 2.5 WH of heat for every 1 WH of power (2.5 COP). An 8 HSPF unit will use 25% more energy than a 10 HSPF unit. On the east coast, HSPF is calculated using weather around central Pennsylvania, so we will see better performance than the rated HSPF.
While cooling performance is fairly stable, heating performance changes significantly with temperature. My heat pump has a rated HSPF of 9.5 (COP of 2.8), but its COP is 4.5 at 62 degrees, 3.2 at 42, 2.8 at 32 and 2.65 at 22. It produces full heat at 42 degrees, but only 90% at 32, and 80% at 22.
Heat pumps have a backup resistance heater (about 100% efficient) that normally comes on for one of these reasons:
Emergency heat has been manually turned on.
The outside temperature is below a certain temperature.
The house has failed to achieve the thermostat set temperature within a certain amount of time or is colder than the set temperature by a certain number of degrees.
These are normally controlled by the thermostat. As installed, my thermostat was set to turn on the backup heat under 40 degrees, or if the thermostat did not reach room temperature within 15 minutes. I had the installer change them to 25 degrees/30 minutes and have not had any issues.
If you have a gas or oil furnace, consider installing a dual fuel heat pump which uses a gas/oil furnace instead of electric resistive heaters for auxiliary/emergency heat.
A new heat pump may be eligible for a federal tax credit of 30% up to $2,000 if installed by 12/31/2025. A rebate up to $400 is also available from Columbia Gas if you install a natural gas dual fuel heat pump.
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Umm - yeah, but no.
Left out a pretty important change that was recommended around 2016 and finally implemented in January 2023.
The rating calculation. Heat pumps bought today are rated in SEER2 and HSPF2, not SEER/HSPF respectively.
The reason was that they found the SEER/HSPF ratings were being obtained in near perfect conditions, rather than real world expected conditions (which again, may be much better than real world ACTUAL conditions).
They only changed one metric, but it had a pretty significant impact. SEER/HSPF were being measured in factory conditions, in systems subjected to .1 inches of water column pressure. The is an extremely low pressure (static pressure increases due to things like dirty filters, coils with condensate, leaky ducts, blower fans set to flow rates higher than needed, or that can be accommodated by design - efficiency falls with increased resistance).
A more realistic metric, and one that designers now use in their calculations unless using very well designed systems (variable speed ECM motors installed in systems calculated and installed to Manual D), is .5 IWC.
Poorly designed/installed systems commonly have measured static pressures well over 1.0 IWC, 10 times the factory rated performance.
The same system that is rated at 14.0 SEER performs closer to 13.4 SEER in a system with .5 IWC, which again - is what is usually found in real world systems when properly installed.
The impact for SEER is around a 4% reduction in performance.
Of bigger note, is the performance under heating conditions. The difference between HSPF and HSPF2 is around 15%. A system rated around 8.8 HSPF really gives around 7.5 performance when installed in a system with .5 IWC rather than the near perfect .1 IWC found in factories rather than the real world. So today, that same system would be rated at 7.5 HSPF2 rather than 8.8HSPF.
Another interesting point that is alluded to above and now being more recognized by the industry, is the importance of commissioning the heating and air conditioning (as well as ventilation, but that's another story).
If I do an energy rating on a home system that is properly installed and is then commissioned, it often results in a 3-5 point reduction in the home's overall energy rating (lower is better, like in golf).
Doesn't sound like much, but since most homes energy ratings are in the 55-60 range if it is new construction built to current code, it can be about 5-10% better performance merely by confirming that the HVAC system is properly installed, like it should have been anyway. Not too much to ask for something you may pay - $10 - 20k for.
5 simple steps.
Making sure the system is designed according to industry standards (a proper Manual J - rather than just rule of thumb (which has been code for about 20 years but is often not done, or is done incorrectly)).
Making sure the air isn't leaking from the duct system out of the house or behind the walls rather than being delivered to where it is needed.
Confirming the amount of air blown by the motor is correct based upon load and design.
Confirming the blower motor watt draw is correct and efficient.
And finally (and this is a big one), confirming the amount of refrigerant in the system is correct. This one step alone has been attributed regularly to a 20% drop in system performance.
If all of that seems overwhelming (and it can be, I know), I would suggest the following, especially for new construction:
Look to use a contractor who is ACCA (Air Conditioning Contractors of America) certified.
Insist on seeing the Manual J or similar calculation used to determine the system. Again, doing this calculation has long been a code requirement. It should be a detailed calculation. Match that against industry standards and the actual home. They should be close.
Likewise, Manual S calculations that show the equipment to be installed aligns in size with the calculated load.
Or have the home certified by a program like Earthcraft, Energy Star, or Passive House, where someone does those things for you.
But again, realize that investments in sound, methodical, informed planning at the beginning of a project of this size is a wise investment.
There's a reason why carpenters say to measure twice and cut once.
The planning matters.
.