Sunday, 21 January 2018
Iced Up?


One of the common problems we encounter is a heat pump that has iced up.  A heat pump in good condition should be able to operate in below freezing temperatures without icing over, so why does this happen?

Things to Eliminate

Here are some circumstances that cause icing which you may be able check, fix or eliminate: 

  1. Check to make sure the outdoor unit is not blocked by leaves or covered in any way.  Some people mistakenly think that covering the outdoor unit helps protect it from bad weather in the winter, but this is dead wrong. It always needs proper airflow around it.
  2. Check to make sure you do not have water dripping into the unit by a leaking or blocked gutter.  As temperatures drop, that water will harden into ice and could cause the fan blades to cease up.
  3. Check to make sure there is room for water to drain away from the unit… that it hasn’t sunken into the ground.  If water pools around the unit, it will freeze as temperatures fall, and the ice will impede the operation of the unit.
  4. Check to make sure that nothing is restricting airflow inside the house, such as dirty filters or blocked vents.

If you’ve eliminated these as possible sources of the problem, then it could be a mechanical or electrical problem inside the unit itself.  First, let’s look at how a heat pump should work.

How Heat Pumps Work

The refrigerant inside the heat pump transfers heat back and forth as needed. Even in the winter, there is heat that can be extracted from the outside air to help heat the inside of your home.  But to do so, the refrigerant needs to get very cold… much colder than the outside temperature.  When coils get this cold, water vapor in the air will start to crystalize into ice around them. To prevent ice build-up, the heat pump will periodically go into a defrost mode.  A valve switches so that the outdoor evaporator becomes the condenser.  This allows the coils to get warm enough to melt any ice that may have formed.  While in defrost cycle, the fan turns off so you won’t get cold air blowing on you, or a second-stage heater comes on to offset this cold air.  After the outdoor unit reaches a certain temperature, or after a certain amount of time goes by, the valve switches back and the system returns to normal heating mode, reversing the evaporator and condenser. This cycling on and off happens transparently to the user while the heat pump is in use.

Reasons Heat Pumps Ice Up

  1. Perhaps the most common reason heat pumps ice up, is that the reversing valve gets stuck.  This prevents the heat pump from going into its defrost cycle and the ice on the outdoor coils continues to accumulate until it impedes the turning of the fan blades. 
  2. If the reversing valve is fine, it could be that something is wrong with the defrost timer, sensor, or control module, so that the defrost cycle is not completing or not happening often enough or not being triggered at all due to a faulty thermostat or sensor. 
  3. If the system is low on refrigerant or the outdoor fan motor dies, this could also cause the system to ice up.

Each of these reasons will likely necessitate a service call.

In the Meantime…

If you can visually see your outdoor unit has iced over, turn the unit off.  If you are able to, turn on the emergency heat mode until help can arrive.  Do not continue trying to run the unit “normally” as this will only cause more damage, and don’t wait too long before seeking help, as this may increase the extent of repairs that are necessary.  Do not try to pick off the ice with a sharp object, as the coil and fins will damage very easily, and you may cause a refrigerant leak.


As with most HVAC systems, problems with a heat pump can often be prevented with proper maintenance.  Getting a fall maintenance check before the worst of the cold weather sets in, and getting a spring maintenance check before the heat of the summer, will help lessen the probability that you’ll find yourself all iced up!

If your HVAC system needs attention and you live in the Nashville area, call on Interstate AC Service at 615-832-8500.  We’re here for you.

Posted on 01/21/2018 3:00 PM by Cheryl Austin
Tuesday, 2 January 2018
HELP! My Heat is Blowing Cold Air!

audio, podcast, Heat blowing cold airWe’ll frequently encounter a caller complaining about their heating system blowing cold air.  We may ask a few other questions but it comes down to this: what temperature is the thermostat set to and what temperature is it reading in the house?  The answer to those questions can often expose the problem without a service call.

Gas Furnace vs Electric Heat Pump

feeling cold, cold air blowing, chilledSuppose the caller tells us that the thermostat is set to 70 degrees and the room temperature is reading around 70 as well, but they still feel cold air blowing from the system.  This is often a clue that this homeowner is experiencing their first winter with a heat pump instead of a furnace - in other words, electric heat rather than gas heat.  Here’s the real issue: Gas heat is much hotter coming out of the vents than electric heat.  A gas furnace puts out 130 to 140 degree air.   In contrast, a heat pump may only put out air at about 85-92 degrees. But regardless of the heat source, both types of heating systems are able to maintain the inside house temperature at 70 degrees.  Our body temperature is normally 98.6 degrees.  Gas heat puts out air that is much hotter than our body temperature, so it feels warm.  Electric heat puts out air lower than our body temperature, so by comparison, it feels cold.  Additionally, because gas heat puts out hotter air, it does not need to blow as hard or as long as electric heat to achieve the same room temperature.  Frequent blowing causes evaporation from the skin which naturally cools the body, and may make us feel chilled.  So, if this is your first winter with a heat pump, and the air temperature is keeping close to what whatever you’ve set the thermostat to, there likely is no problem with your heating unit.  It just might take some getting used to.

Switch Fan to AUTO

Thermostat modes, thermostat settings, fan auto, heat modeHere’s another possibility:  Remember how we said blowing air helps cool you?  In the summertime, it is often helpful to turn the fan setting on your thermostat to “On” – so it runs all the time – rather than “Auto” – where it only blows when the unit kicks on.  But in the winter, you’ll want to set the fan back to “Auto”.  Just like on a windy day, blowing air makes us feel colder than the actual air temperature.  In between the heat cycling on, the air inside the ducts may cool below the “usual” heated air temperature, so if the fan is on all the time, it will be blowing cooler air at you until the heating unit kicks back on.  Avoid a service call by just manually switching the fan setting on your thermostat to “Auto” in the winter.

Service Call? Things You Can Check

But, if the room air temperature is really well below the thermostat setting, then you may indeed have a problem!  The outdoor unit may have iced up, or your system may have a bad reversing value or compressor, or the refrigerant may be low (yes, you still need refrigerant for heat!).  Any one of those things will likely require a service call.  But here are some things you can check and might be able to fix yourself.  Is the thermostat switched to cool mode instead of heat mode?  Turn it off, and then flip it to heat mode.  hole in duct, ductwork leakIf your ductwork goes through an attic, crawl space or basement, are there open windows in those areas?  Does the ductwork have a hole in it or has it become separated from the main trunk?  Those things will need to be fixed before it’s possible to determine whether there is an actual mechanical problem with your heating system.

Not As Warm As You'd Like?

If you live in the Nashville or surrounding area, and your heating unit is not keeping you as warm as you’d like, give us a call at 615-832-8500. We’re here for you.

Posted on 01/02/2018 8:03 AM by cherylaustin