Every once in a while I come across an air-conditioning system that does not seem to be working properly. No matter how low the thermostat is set, there doesn’t seem to be any cold air coming out of the registers.
Lack of air from the registers can mean a number of things, ranging from a failed fan in the furnace, to leaking ducts or duct baffles closed. But one of the biggest causes is ice build-up in the system.
Outside the home, there is normally the thing most people call the air conditioner unit. This is in fact only a piece of the puzzle. This unit is a combination of a compressor, a large coil, not unlike a radiator in a car, and a big fan. The proper name for this unit is the condenser unit.
Coming from the condenser unit are two, usually copper, pipes. One will be insulated, and the other, generally not. These pipes go through a hole in the wall, and into the tin box above the furnace. This is known as the plenum. Inside this box are the final parts of the Air conditioning system, the evaporator coil and the evaporator drain lines.
So how does this all work?
Bear with me, as the next bit is a bit technical, so I’ll try to make it as simple as possible.
When you compress a liquid, it heats up, and it wants to boil, and turn into a gas. If you cool the gas down it condenses back into a liquid, but still under pressure. If you release the pressure very quickly, the boiling point drops, and the liquid evaporates, and the temperature drops rapidly also.
This can be demonstrated by firing off a carbon dioxide fire extinguisher. Inside the extinguisher is carbon dioxide liquid at high pressure. The normal boiling point of Carbon Dioxide is minus 78.5c, so at room temperature it’s a gas. By compressing it, and cooling it, it turns into a liquid, which is then kept under pressure in the fire extinguisher.
When the pressure is released, the carbon dioxide drops rapidly in temperature, and turns into dry ice.
To test this theory for yourself, place some warm water on the back of your hand and blow on it. Instead of heating your hand, you will feel your hand cool down as the water evaporates.
It is a combination of these things that helps air conditioners work. Pressurising a gas, and then cooling it to form a liquid. Transporting that liquid to somewhere else, where the pressure is released and the liquid evaporates and cools down, and then returning it to where it can be re-pressurized.
So going back to the diagram, the A/C refrigerant, is pressurised in the compressor, where it heats up as a gas. It is then cooled by blowing cold air over the condensing coils from the big fan until it condenses into a high-pressure liquid. Liquid under pressure will try to find it’s way to a lower pressure, so the liquid flows along the pipe, until it gets to the really low pressure in the evaporator coil. Here the pressure drops very quickly, and the liquid evaporates into a gas again, cooling down as it does. This is transferred to the air by the evaporator coils and is blown around the house by the furnace fan.
Warm, moist air traveling over a cold surface will cause condensation, which inside the plenum of the HVAC system turns into water and is drained through the evaporator drain lines. The warmer gas then returns to the compressor to start its cycle all over again.
So what can go wrong?
The temperature drop at the evaporator coil dos controlled by the pressure of gas in the system. Too much, and the gas will not cool enough, so the A/C will be ineffective. This is unlikely to ever be the case, as there is a pressure relief valve at the compressor on the condenser unit to release gas that is over pressure.
When there is not enough pressure however, this is when things get interesting. The gas is compressed much further, because there is less initial pressure in the system. This then starts to decompress around the system, and when the gas reaches the expansion device and evaporator coils it evaporates so quickly, because there is less pressure in the system, that any moisture on the outside of the coils freezes.
This is much like the frost build-up you see on the coils inside a refrigerator’s freezer box.
The fan inside the furnace, keeps supplying warm moist air to flow over the evaporator coils, which creates condensation as it hits the colds coils, and then because of the extreme low temperatures freezes.
The picture on the right shows an evaporator coil that has completely frozen. This stops all airflow through the system, so even though there’s plenty of cold, there’s no cold air.
The problem is this is hidden from view, and so you are not likely to be able to tell if this has happened inside the plenum of the furnace, but there are other tell-tale signs.
The first is the lack of airflow at the registers when the air conditioning is on. Even with the fan at full flow you will feel little airflow from the registers, and the upstairs registers, normally the furthest away may have no air coming from them at all.
The second tell-tale is and ice formation around the high-pressure valve of the condenser unit outside. This is a good indication of low pressure in the gas and excessive de-pressurisation in the system causing ultra low temperatures in the pipes and coils.
You can see an example of this on the photo to the right.
How do you fix this?
It’s going to require a service call to have an HVAC engineer come out and re-pressurize the system. At the same time they can check for leaks that may have created the low-pressure in the first place. BUT WAIT! Before calling out the HVAC engineer, it is important you shut off the A/C for at least 4-5 hours to let the ice melt. The engineer cannot work on the HVAC system f its frosted up, so this will just cost you a call out fee, the problem won’t get fixed and you’ll have to arrange a second call-out.
During the period that the a/c is off, keep an eye on it. There’s a lot of ice that’s going to melt, and you want to ensure it comes out of the drain lines, and not just pour down over the electronics in the furnace below.
Why does the refrigerant pressure drop?
There are three reasons that refrigerant pressure is lost. The first is from a leak. These systems are designed to be very secure with respect to leaks, because the refrigerant is not environmentally friendly. Older systems can leak, but the cause of a leak is usually a valve or fitting not being tightened properly by the installer.
The second reason for a pressure drop is refrigerant theft. This is less common in Canada than in the U.S. but it does go on. Refrigerant gas is not cheap, and the resale value is high.
The third and most worrying reason for pressure drop is that it may be being consumed as a drug. Unprotected refrigerants are easily accessible with a simple pair of pliers or fingers, and the thieves can sniff the refrigerant directly from the valve or store it in a plastic bag for later use. Unfortunately, the “high” or “rush” an individual gets from “huffing” is from the oxygen being displaced in their body. This has resulted in documented cases of brain injury and even death to individuals.
If the HVAC engineer cannot find a leak when they are re-pressurizing, then you should ask if they can fit a tamper resistant cap to the filling valve. They are readily available and not expensive when compared to the call-out fee and replacement refrigerant needed to get you’re a/c working again.