Environmental Cents
Heat pumps use a lot of energy, but they're also one of the most-efficient heating cooling systems available for your home. Knowing a few terms and questions can reduce energy use and save money.
by Bruce Saller
WRITER
Heating and cooling our homes is the source of our greatest energy use according to the U.S. Energy Information Administration.
Here in central Virginia, the most efficient way to heat and cool your home is with a heat pump. But which one is best for your home? To decide that, you’ll need to understand some basic terminology.
Understanding Ratings
There are two efficiency ratings for a heat pump:
SEER - Seasonal Energy Efficiency Ratio - is the number that represents the average cooling output in British Thermal Units per Watt-Hour (BTUs/WH).
Because 1 WH = 3.4 BTU, a unit with a 17 SEER removes 5 WH of heat for every 1 WH of power (500% efficiency).
A 16 SEER unit will use 25% more energy than a 20 SEER unit.
HSPF – Heating Seasonal Performance Factor - is the average heating output in BTUs per WH. A unit with an 8.5 HSPF delivers 2.5 WH of heat for every 1 WH of power (250% efficiency).
An 8 HSPF unit will use 25% more energy than a 10 HSPF unit.
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, for example, has an HSPF of 9.5 (average efficiency of 280%), but its efficiency is 450% at 62 degrees, 320% at 42, 280% at 32 and 265% at 22. It still outputs 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.
Saving some money
Here are some ways to save energy using a heat pump:
Find the heating performance tables for your heat pump and determine what causes the backup heat to turn on. Information may be in the owner’s manual. Alternately, you can contact your service provider.
Consider adjusting the outside temperature setting to a lower temperature based on the performance table. High humidity and wind can reduce performance, so don’t be overly aggressive in reducing the temperature.
Consider increasing the backup heat time delay or temperature delta settings.
Do not turn the thermostat down more than a couple of degrees at night once the morning temperature is below 40 (reduces time at lower performance).
Only increase the thermostat temperature by one or two degrees at a time (reduces backup heating)
Also, be aware that a new heat pump can qualify for a federal tax credit of 30%, or up to $2,000.
Bruce Saller is a retired engineer and active participant in the Fredericksburg Clean and Green Commission. This series provides practical steps we can all take to reduce our carbon footprint - and often save money.
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Thank you for this. Did not realize about central PA for rating. Source?
FYI though - Recently the rating systems for both HSPF and SEER changed to more accurately reflect field conditions for ducted systems. Good explanation here:
https://www.ekotrope.com/blog/the-impacts-of-seer2-and-hspf2#:~:text=The%20main%20difference%20between%20the,warrant%20a%20new%20rating%20system.
You are correct though about how the heat pump's usage is under-utilized in cold conditions.
Another thing not mentioned however, is the importance of right sizing the heat pump for the home.
That involves two critical steps, which many HVAC installers are incapable or unwilling to do correctly.
For a home's heating and cooling system to be right sized, the installer for a new home is required by code to complete a calculation (or its equivalent) called Manual J. This is a program that evaluates the home's windows, walls, insulation, solar orientation, etc. to determine what it's true load is during both the heating and cooling system. Then the equipment is sized according to that calculation.
Why does it matter? Cost and cooling season performance. Heat pumps, air conditioners, furnaces are typically priced according to size. And as they grow bigger, they use bigger fans, which means bigger and more ducts are required to distribute the air. And you're paying someone to install that system. Which adds to the cost (it will cost twice as much to install 4 ducts as two, in both labor and material).
Even more importantly, is during cooling seasons, when bigger is not necessarily better. In fact, it may be harmful. Air conditioners and heat pumps work essentially the same way, in that they condition the air two ways in the cooling season.
One is by cooling the air down, or removing the sensible heat load. They do that by blowing air across a cooled refrigerant coil - cooling the air coming back into the home. Bigger is better works for that part. A bigger system will cool your air down faster than a smaller one.
But the problem is, as part of the same process, the system removes HUMIDITY from the air passing over the coil. Moist, hot summer air condensates into droplets, is collected into a condensate line, and drained from the home. If your system only runs for two minutes instead of 10-15, it never gets the opportunity to remove that moisture. So the home becomes cooler, with the same (and increasing) moisture, to where it feels "clammy" (like a cool fog).
Often times, home owners will need to install a dehumidifier to address this problem, which eats into their energy savings, as these are typically energy intensive operations.
Another counterintuitive advantage of the longer run times, is that it is typically more energy efficient for the fan motor than continually stopping and starting (short cycling). Think (if you're of an age to remember these things) how much easier it was to keep a merry go round going, even with a bunch of kids on it, as compared to starting the merry go round up.
The good news is that many manufacturers with any sense of quality are using fans with ECM motors, you should insist yours have that if nothing else. Slow ramp up, slow ramp down. If they have a variable speed motor (more pricy) then even with a poorly designed/sized system - the damage is minimized by the quality of the equipment.
Or - if all of that sounds way too complicated for you, just look to buy a home that has been certified by a reputable program such as Energy Star, Earthcraft, LEED for Homes, PHIUS.
Many of your large scale builders have been doing this for years and from what I've seen, are doing a good job. My daughter bought one new about 3 years ago, and I bought one that was 10 years old 2 years ago from the same builder - they were Energy Star rated. I tested and inspected both of them, and have been very pleased with the results.
Best wishes.