Air Source Heat Pump Basics

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Air Source Heat Pump Basics

Air Source Heat Pump Basics

When it comes to heat pumps and air conditioners, they’re actually very similar.  Today I wanted to share my experience with heat pumps and focus on how they operate to give you cooling in the summer and heating in the winter.

Many new techs get intimidated when they work in an area with predominately gas/electric systems and have to work on a heat pump.  The same goes for those who work on heat pumps a lot and then have to work on a gas furnace. It’s foreign to those new techs, and they feel less confident working on those systems.

Today we’re going to try and clarify those mysterious thoughts so you can better understand them.

So, if you didn’t already know, in cooling mode, the heat pump works just like an air conditioner, in that a 24-volt signal comes to the outdoor contactor to turn that unit on.  The contactor switch gets pulled in, which allows the 240 volts from one side of the contactor onto the other side so that voltage can make its way to a capacitor that supports the compressor and the condenser fan motor.  

The refrigerant cycles through the system and basically makes the indoor evaporator coil, a cold coil, and the outdoor unit’s coil the hot coil.  At the evaporator coil, a fan blows across the cold coil and sends that cold air into the duct system and onto the rooms of the house.  We remove the heat from inside the house at the outdoor unit and pull it out to the outdoor coil to be released into the atmosphere.

A heat pump just has a reversing valve that reverses the flow of refrigerant to make the indoor coil the hot coil and the outdoor coil the cold coil.  So we’re trying to extract heat from the outside and bring it inside, which can be done down to a certain outdoor temperature.  After that, there is very little heat in the air to extract, so heat strips will kick in to supplement that effort.  

Heat strips are a wire coil that is strung out across a rack and sits in the blower motor’s airstream.  They get so hot they glow orange. They’re great, but they’re expensive to run.

A regular occurrence with a heat pump in the heating season is for the outdoor unit to go through a defrost cycle.  You can imagine the cold outdoor coil interacting with cold outdoor temperatures can cause some freezing.  Anytime that outdoor coil gets below 32°, the outdoor being the cold coil develops frost on it.  It can’t keep operating this way, or frost will develop into a straight-up ice block!

A temperature sensor is mounted onto the outdoor coil to monitor this situation.  When enough frost develops on the coil, a defrost board in the outdoor heat pump control panel will energize, sending it into a defrost cycle.  Another sensor is included here. It’s an ambient temperature sensor.

The difference between the ambient and the outdoor coil temperature is the difference or delta-T measurement. This delta-T measurement is what ultimately determines the need to go into defrost. 


Defrost Cycle Only Works in Heating Mode

Now that the sensor on the outdoor coil has signaled to the defrost board it’s time to go into defrost, a few things will happen.  The defrost board is the quarterback for this whole play too.  Because the outdoor coil was the cold coil and freezing up, we need something to melt that outdoor coil so it can function properly again.  So the reversing valve lets out a big whooshing sound and reverses the flow of refrigerant so we can essentially go into air conditioning mode again and make the outdoor coil the hot coil.  Also, at the outdoor unit, you’ll notice the heat pump’s fan motor stop running.  This is to help warm the coils up faster.  Because if we were drawing cold air across the outdoor coils when we were trying to warm them up, it would be counterproductive.

Inside at the air handler, the fan still blows, which means there is cold air coming out of the ducts.  But the air handler’s heat strips come on to neutralize the cold air.  All this happens until the outdoor coils are warm enough to go back into heat mode and become the cold coil again—maybe 45 to 90 seconds.

One last time the reversing valve makes a big whooshing sound and switches the flow of refrigerant back to heating mode, the outdoor fan turns around on, the heat strips inside turn off, and the indoor coil becomes the hot coil again.


Defrosting Heat Pump

Real quick, I wanted to mention that this method of defrost control is called “defrost on demand.” Some older systems defrost cycle is called “electronic time-temperature defrost.”  Basically, defrost is started every 50, 70, or 90 minutes and has a sensor that tells it to go back to heating mode.


Let’s get into some more details about the reversing valve and the defrost board in other blogs.  For now, I just wanted to go over the sequence that happens for heating and cooling to work on the heat pump system.  There are ways to test the reversing valve for diagnostic reasons.  There are also ways you can force the unit into defrost when performing maintenance or a diagnostic call on a heat pump.  Another test you can do is test the sensors to see if they are dialed in enough to make the right call to go into and come out of defrost.

Thanks for checking in on our blog.  See you next week!

Fox Family Heating & Air serves Sacramento, Rancho Cordova, and all of Northern California. If you’re looking to schedule HVAC service in these areas, give us a call!


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