Build Your Own HVAC - The Basics

Booming Voice: So Petey, are you ready to build your first HVAC System?

Petey: Oh boy, I sure am!

V: Okay, let’s get started! By the end of our chat you will have your very own Heating, Ventilation and Air Conditioning System.

Key HVAC Components

There are a few key parts in a vapor-compression HVAC system:

1. Refrigerant
2. Compressor
3. Expansion Valve
4. Condensing Heat Exchanger (Condenser)
5. Evaporating Heat Exchanger (Evaporator)

Before Starting

Refrigeration is fundamentally a system that uses a special medium (refrigerant) and mechanical components to transfer energy from one location to another. Before we move onto the actual component selection let’s do a quick run-down of how these elements work to bring you cold air.

Refrigerant

The most critical component of compression based HVAC is the refrigerant. Refrigerant is the medium used to transfer energy…if you think of a train as an analogy, the copper pipes are your train tracks, the train is the refrigerant, and the energy is the people riding the train. People get on at one stop, ride the train to the destination, then the train continues back to the pick up more people. In the case of air-conditioning, refrigerant picks up energy from inside your home, then delivers it outside.

A Matter of Phases

Matter, in this case refrigerant, can exist in different states, HVAC operates by transitioning refrigerant between a vapor and a liquid. (note to self: said like a true stoner “dude…matter exists in multiple states brah.“) So, what’s weird is refrigerant can be in multiple states at once, think of it like taking a shower…sure, there is some true water vapor, but, there is also probably a lot of mist floating in the air. That mist is really many small liquid particles of water. This is the same two-phase matter that exists in your HVAC in your condenser and evaporator.

The actual temperature, is measured by literally sticking a thermometer in there and reading off the measurement. But if the refrigerant is all liquid or all vapor, it is considered “saturated”. The saturation temperature tells you, given the current pressure, at what temperature the fluid changes phase, boil to vapor or condense to a liquid. Sit right there, and you’ll be in a misty-vapor state, above that you have a gas, below that you have a liquid.

Saturation Temperatures

If you were to take a vapor that is already saturated and heat it MORE, that will create superheat. If you take your saturated liquid and COOL it even more, it will give you subcooling.

$T_{vapor}-T_{sat}=Superheat$

$T_{sat}-T_{liquid}=Subcool$

Superheat

Superheat is Important

Superheat is really important, it tells you how much headroom you have before your vapor will start turning into a liquid.

Remember, YOU CANNOT COMPRESS A LIQUID, it’s a great way to kill a compressor. Superheat tells you how safe you are, but make sure you have enough superheat because no measurement is perfect.

Subcool

The expansion valve is designed to meter a liquid, it might behave abnormally if there is vapor entering the valve, this is called Flash Gas. Although subcooling is still an important control point, your unit will at least survive some vapor in the expansion valve.

Charge Indication

If you are meeting your target superheat temperature (the most important), and your system is at a steady state, you can find out if you are over or under charged with refrigerant:

• Low Subcool: System is undercharged and cannot reject enough heat from the condenser
• High Subcool: System is overcharged and you are losing efficiency on your condenser

Wrapping that up…

• Proper superheat protects your compressor
• Proper subcooling keeps your expansion valve working properly
• With proper superheat, subcooling indicate your charge balance
• If you KNOW your charge is correct, subcooling can tell you if you have condenser headroom

Primary Control Elements

Compressor

The compressor is the most active component, it will consume the most electricity in your system, but it is also what literally drives the rest of the work. That energy is used to compress the refrigerant, increasing its pressure and temperature ($PV=nRT$). This pressure differential keeps refrigerant flowing and the higher temperature allows the refrigerant to reject heat outside, even if it’s a hot 90 degree day.

Liquid Slugging

It’s also important to note that your refrigerant should be a vapor before getting to the compressor or you’re in for a bad time! This is called liquid slugging. You cannot compress a liquid and nothing good comes from trying.

Expansion Valve

Out of Order!

I will discuss the expansion valve next, and I want to note that this is out of order, but helps to understand the dynamics of the system.

The expansion valve is not PHYSICALLY the next component in the HVAC, but it is the most important regulating component in the system. While the compressor is targeting the cooling you need, as little as necessary if possible, the expansion valve works to ensure the rest of the system stays in balance at that desired operating load. To do this the valve will open and close to maintain an optimal compressor suction superheat at the compressor. Physically it is expecting a liquid refrigerant, and that passes through a narrowed opening, or orifice, suddenly dropping the pressure by opening to a larger volume, and lowering its temperature significantly. The refrigerant exits as a cold mix of liquid and flash gas ready to vaporize as soon as it absorbs heat from your home.

Flash Gas

Having flash gas at your expansion valve is like trying to drink soda that you’ve shaken up, it makes it hard!

Secondary Elements

Condenser

Now let’s look at the Condenser.

The compressor makes the refrigerant a hot gas at high pressure, importantly hotter than the outdoor air. Because of that, the condenser can get rid of your indoor heat by sending all that extra energy from your home outside. This referred to as rejecting heat because energy always flows from hotter to colder fluids. The other goal of condensing relates to the expansion valve. Since it can only effectively control liquid flow, the condenser needs to full condense and subcool the refrigerant so the expansion valve can do it’s job reliably.

Ineffective Condensing

The condenser can be ineffective at it’s job meaning it does not fully condense creating flash gas. Causes of this are, the unit being undercharged, the condenser clogged with dirt and debris, or running on a day that is too hot, where the refrigerant-to-outside temperature delta is too small. What this does in practice is create flash gas bubbles at the expansion valve. Like trying to drink overly carbonated soda through a narrow opening, the expansion valve can’t accurately meter the flow when vapor is present. Refrigerant backs up on the high side, pressure rises, and the compressor works harder while efficiency drops. This can even cause your system to shut down or damage your compressor if it gets out of hand and pressure builds too high.

Condenser Fan

The condenser fan can be used as a secondary control to help maintain head pressure. This can help keep your compressor delivering the correct cooling while keeping the refrigerant flow stable. As an example, during a cold autumn day when it might be hot inside, maybe you’re having big party, ovens roaring and you have lots of guests. Your HVAC needs to work hard, but the condenser is too effective which can lead to a low head pressure, in this case the condenser fan can reduce it’s speed to maintain your needed heat rejection and stabilize your pressure.

Evaporator

After the expansion valve there is cold refrigerant, in either fully liquid or a liquid-vapor mixed state. This cold refrigerant is what cools your home, as heat is transferred from your indoor air to the refrigerant, in the process causing it to boil and reach a superheated temperature. The expansion valve will regulate the performance of the evaporator coils, while the blower fan will be focused on maintaining indoor air quality and pressure.

HVAC Is Like a Train Line

Before I deep dive into the system, let me pose the design of an HVAC as a train line…bear with me:

• Passengers represent the heat energy
• The passenger train cars are like your refrigerant, it’s goal is to move passengers on a looping track
• When your HVAC is cooling, you are trying to move passengers from the indoor station to the outdoor station
• The indoor and outdoor stations are like your evaporator and condenser, letting passengers(heat) in and out of your system
• The compressor is engine driving the train forward
• Finally, the expansion valve is like the turnstile, only letting a certain number of passengers on at the indoor station so that you don’t overcrowd your train

Applying It

It’s a hot day and you need your HVAC to run at full speed, the train needs to pick up as many people as it can, but the outdoor station isn’t letting everyone off fast enough people are still on the train, your condenser has reduced efficacy. As such, your turnstile (expansion valve) needs to limit the number of people who get on at indoor station so that your train doesn’t overcrowd. Your train isn’t running as efficiently as it can, but it does keep running. In the perfect world, everyone who needs to go from indoors to outdoors can get on the train, and the turnstile can let them all through fast enough. The system is in balance.

Analogies

• A fixed speed compressor has a fixed number of seats, a variable speed compressor can adjust it’s train size to fit the passenger need…running fewer train cars (lower compressor speed) will increase efficiency…a shorter train is also easier for the outdoor station to unload quickly keeping things running smoothly
• The number of train cars available is like your charge. Overcharged and plenty of people can ride, but your outdoor station is easily overwhelmed. Undercharged and you don’t have enough trains to move the people you need to.
• If you can’t let enough people through your turnstile (EXV maxed out), your train will run with empty seats, wasting energy, in this case, your superheat climbs because there isn’t enough refrigerant absorbing energy
• If your turnstile is closed as it can get, but the train is still overfull, something is wrong at your condenser, not everyone can get off at the outdoor station. If you keep this up liquid makes it into the compressor and your train breaks down. Not a good day.