Heat Pumps: What's the Difference between Evaporator & Condenser Coils?

How does your heat pump work?

You just flick the thermostat, don't you? That's how it works, isn't it?

Well, yes and no.

If you understand the basics of how your heat pump system works, it will encourage you to look after it and ensure it never lets you down. After all, it will always fail when it's most inconvenient, won't it?

That's called Murphy's Law.

This simple diagram lays it out for you (even I understand it!).

Evaporator and condenser coils

Main components of your system

The main components of your heat pump are two coils – the evaporator coil, which is inside your home and the condenser coil, which is located with the compressor and expansion valve, and the other main components in your system, all housed in the unit which sits outside your home.

What are the coils?

The coils are essentially heat exchangers. The evaporator coil provides indoor cooling by pulling the heat from your home and blowing the warm air across the coil. The copper or aluminum tubes act as a heatsink, which means heat from the air is transferred to the refrigerant passing through the tubes, which evaporates, trapping humidity, before traveling on through the copper tubes back through the compressor. The cooled air is pushed through your home, and the process continues.

As the refrigerant passes through the compressor to the condenser, the pressure on the refrigerant raises its temperature to 100 F, causing condensation in the condenser coil, finally, the unwanted heat is released into the atmosphere with the help of a fan. The system repeats the cycle, keeping your home cool in summer.

In winter a reversing valve reverses the flow of the refrigerant through the coils; the indoor coil becomes the condenser, and the outdoor coil becomes the evaporator, so it harvests heat from outside, even on cold days, and transfers it through the unit inside to provide comfortable heating for your home. Air always holds some heat, and a heat pump will provide heating for your home even when the outside temperature is low. Air at 64 F still has around 85% of the heat it had at 70 F.

Ingenious!

What is the refrigerant?

The refrigerant works efficiently because it has a very low boiling point. For example, the boiling point of water is 100 F, whereas some heat pump refrigerants have a boiling point as low as -48.5 C (-55.3 F).  The refrigerant in the system is a cold vapor at around 40 F. It is sealed inside both coils and circulates under pressure through the system continuously when it's running.

The copper or aluminum tubes used in both condensers are encased in thin aluminum fins, which give the tubes a larger surface area to harvest more heat. The switch from copper in a steel frame to aluminum tubing in an aluminum frame in the past few years is because steel rusts quickly, and can shorten the life of your coils. Also, copper is more susceptible to attack by the acids and chemicals used in your home, leading to tiny holes forming in the coil. These leak and lose refrigerant, which will lower the pressure in the unit, making it less efficient over time.

The condensers are both sealed units, they should last as long as your system, on average between 15 and 20 years. However, occasionally, leaks can occur in either coil, leading to a loss of refrigerant and your system not running as efficiently as it should.

You may notice it takes longer to cool or heat your home. That's when you will realize there could be an issue. Leakages may not always be visible, as it only takes a pinhole to lower the system's pressure.

Fixing leaky coils

Leaks in the copper or aluminum tubing in an evaporator coil can happen for a variety of reasons, including accidental damage, so it pays to be careful when you are cleaning the unit. If the leak is in part of the tubing away from the coil, it may be possible to repair it using solder or epoxy. This is a job which should be done by a professional HVAC technician as it's easy to cause further issues in other joints when using solder.

If the leak is inside the coil, then it's likely the only solution is to replace the unit, which is expensive. You should take advice from a qualified professional HVAC technician as it is vital to ensure the new coil is matched to your system. A mismatch will cause a loss of efficiency and push up your energy bills, which is not what you want.

The job of replacing a coil unit is rarely a DIY task. You may be able to switch the unit over, however, there's the issue of harvesting the refrigerant and re-gassing the unit, which needs professional input. Best left to the HVAC experts for safety and a guarantee the job will be done correctly with the right expertise and tools.

Formic Acid

Formaldehyde in your home from furnishings and carpets, can build up and combine with moisture on your evaporator coils to make formic acid. This eats into the tubing and creates tiny holes, which then leak under the system's high pressure. It's important to ventilate your home properly for your health and the long life of your evaporator coil.

Condenser coil leaks are also uncommon, but when they happen, it's usually because of lack of maintenance and a buildup of dirt and debris that all encourage corrosion. Located outdoors, they are subject to weather and changing temperatures, so it's essential they're kept clean and free of debris. A leaking condenser coil is usually serious, and it may not be possible to repair, leading to a very expensive bill.

Cleaning evaporator coils

Regular cleaning of the evaporator coil and the filter is essential and should be done at least twice a year. It's a simple job, almost as easy as flicking the thermostat!

Use a soft brush and gently clean the fins. Don't rub too hard; the fins are very fragile. Remove all debris using a vacuum cleaner and use a coil cleaner spray over the coils, which doesn't require rinsing. Once a year, have the unit professionally cleaned and checked by an HVAC technician. They will deep clean the evaporator unit. It's much cheaper than replacing it.

Clean any debris from the drain tray and ensure the drain is clear. A few drops of household bleach drizzled into the drain with some clean water will ensure there's no buildup of mold. The same applies to the condenser unit outside, as it's more liable to get dirtier; it does need regular attention, but no power wash! Soak the fins with clean water and a soft brush. The fins are easily damaged if you rub too hard, and it will reduce the efficiency of your system.

Keeping the system clean is an easy task if you do it regularly. It costs nothing but could save you a ton of cash in the longer term. As they say – just do it!

Freezing up

It's a common scenario in winter for your heat pump to freeze up. If the coils become coated with thick ice, it will prevent the unit from running correctly. One of the reasons this happens is poor airflow through the unit, which allows moisture in the air to freeze on the coil.

Freezing up happens most often when it's cold and raining. Keep the units clear of all obstructions, leaves, snow, or any debris that could block the airflow. Keep the coils clean. Your system should automatically switch to defrost mode if it freezes up, however, if it doesn't, it could indicate a fault with the defrost cycle, or the electronics, in which case an HVAC technician is needed to prevent damage to your unit.

Conclusion

A little attention to basic maintenance tasks can save you significant issues and cash later on.

Remember Murphy's Law?

Regularly clean the coils and filters and always make sure your exterior unit is clear of obstructions, debris, plants, and snow. If you do that, it will reward you with excellent service year in year out. An annual check-up by a qualified HVAC technician is recommended, and perhaps you could consider a maintenance contract as your unit ages. It's an excellent insurance policy. But if you do your bit, it will be as easy as flicking the thermostat to get warm.