Basic Principles of Reading HVAC Engineering Drawing Diagram

Introduction to HVAC Schematics

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Variability and Symbols in Schematics

Firstly, every schematic you look at will look completely different. The symbols used are always similar enough to recognize what they are, but they're always slightly different. However, they will all show how the chilled and condenser water system is connected and distributed around the building.

They will also show the main components, such as the valves and pressure sensors, et cetera, and they should also show which floor the components are located. Usually they will not display every air handling unit and fan coil unit that's connected to the system.

Sometimes they will show the connection to the main AHUs, and other times they will refer you to a separate drawing if you want to see this detail. If the building rents floors out to tenants, and the tenants have full control over the fit out of that area, then the drawing will just show the connection from the main risers over to the tenant space, and anything after that is up to the tenant's fit out contractors, and they should provide the landlord with a copy of their own drawings for their area.

Additionally, you should remember that these drawings do not show every bend, or the exact route through the building, they are simply a flat 2D representation of the system.

Important Drawing Details and Legend

So in the top left and the lower right, you'll usually find the drawing number, which is a unique ID for that particular drawing. In the bottom right, you'll also find the title of the drawing, for example, Chilled Water System.

You'll also find the date of production, the revision number, and other information, such as the client, the person and/or organization that produced the drawing. And most importantly on the right-hand side, you should find the legend, which lists all the symbols used as well as their title.

Key Components to Identify

Whenever I'm reading a schematic, I always start by locating the chillers, the pumps, the risers, the AHUs, and the cooling towers. This shows you how the system works, how it's all connected, what type of system it is, and where each part is located.

The schematics can be quite cramped, and you'll sometimes need to see through this information to be able to follow the system.

Reading Pipe and Valve Details

Marking	Meaning
CHW, CND	Chilled water and condenser water systems
F and R	Flow and return directions in pipes
IV	Isolating valve
NC	Normally closed valve

You can see the direction of the flow in the pipes just by these little arrows. The diameter of the pipe is sometimes listed next to the line.

Newer designs might show you the design flow rate, the velocity, the pressure, and also the pipe diameter.

Identifying Components Using IDs and Floor Lines

You'll also see these little circles around the drawings, which have numbers and letters inside. This is a unique ID for the fitting. There should be a matching ID tag fixed to the physical fitting.

You can use the number to find the correct fitting in the system and also look up details for it in the log sheet, such as the type and size.

Dotted horizontal lines from one side over to the other on the drawing mean that there is a floor there. The arrows on the pumps also indicate the direction of flow.

Component Overview and Real World Examples

Let's look around the system at each of the components to understand how it works. I've based this design off a real world example for an older constant flow water cooled chiller system, as it's easy to understand.

I'll show you some real world examples as we go through, and we can compare these to some different designs, so that you understand the differences.

Chillers and Valves

I'll just show you some examples of how chillers are illustrated on the schematics, as it really varies. The two in the top right and the one in the bottom right are all air-cooled chillers, and the rest are water-cooled.

The components of each system are similar, but as you can see, the design does vary, and not every chiller installed will use the exact same setup or components.

So we'll start with a chiller showing the condenser and the evaporator. Then we'll have flow and return lines into and out of the chiller. Then we'll find the isolating valves, which are typically butterfly or gate valves. These will be used to cut the chiller off from the main system to carry out maintenance.

You'll also find these valves and all major components, such as pumps and AHUs.

Sensors and System Balancing

Then we'll have some temperature sensors, which the chiller or BMS is connected to, and the chiller will read this and control its performance.

Then we'll have a commissioning station to measure the flow rate. This will be used to balance the system to meet the design flow rates, and also analyze the performance.

I made another video previously on how to measure the flow rate of a chiller using this exact device. Check it out, link's in the video description below.

Next we have a bypass line, and in this example, there's a double regulating valve between the two lines, but it's more common these days to have just an isolating valve here, which is normally closed. Or there isn't a bypass line at all.

These are used to either flush the system and bypass the chiller to stop dirt going into the heat exchanger during the system clean, or in this case, it's used to balance the system as it's a constant volume system.

So this can be used to simulate the pressure drop of the chiller if the chiller is isolated for maintenance.

Then because we have a bypass line, we'll also have some more isolating valves. This allows the chiller to be cut off from the system, but the bypass line can still be used.

In newer systems you will find the motorized control valve here, which is connected to the BMS, and this allows remote control.

Test Points and Gauges

Then we'll have some test points, also known as Pete Plugs, and these allow technicians to plug measuring devices in to take temperature and pressure measurements.

Then we'll have some temperature gauges to manually take readings. Over time these generally become less reliable, and the gauges often get stuck.

Then we'll have pressure gauges to allow manual readings. Again, these can become less reliable over time, and they can also become stuck.

Flexible Connectors and Pump Systems

Then we'll have some flexible connectors. These prevent the vibrations of the pumps and the chillers from running along the pipes throughout the building, and they also allow the pipes to expand and contract slightly, which takes the pressure off the connection.

We'll usually find an almost identical set upon the condenser sides, so I'll just add that in as well.

This design is for large commercial office buildings. There'll be multiple chillers. All of them will have identical pipe arrangements, so I'll just add that in as well.

Pump Arrangement and Flow Direction

Next, we'll need some pumps to push the water through the chiller and around the building. For this, we'll usually have two or more pumps in parallel, typically running in duty and standby.

Again, the triangle points in the direction of flow.

Then we'll need some flexible connectors as the pumps will cause vibrations.

Then we'll want some isolating valves, so that we can cut one or all of the pumps off for maintenance.

Then we'll have some strainers, which capture the particles and dirt in the water and prevent them from entering the pump and causing damage.

Some pump setups don't use this, and some will use just one large strainer elsewhere in the system.

Then we'll have some pressure gauges to allow manual readings.

Then we'll have a non-return valve, which prevents the pressure in the pipework from forcing the pump to rotate backwards when it's turned off.

Notice that this design doesn't have a second isolating valve past the non-return valve. In newer systems, however, like this one, you will usually find there is one.

In the real world, it would look something like this, and this is for a centrifugal pump set.

Chilled Water Circulation Loop

We know the pumps need to push water through the chiller, so we can connect the pump sets to the chiller's evaporator inlets. Chilled water is generated in the evaporator, so we'll need to connect that back around to the pumps to create a circuit, which will distribute the chilled water.

Notice there is a common header here between the flow and return lines. This separates the system into a primary and secondary loops and allows a constant flow rate in the primary side, which the chillers prefer as they require a minimum flow rate.

It also allows a variable flow rate in the secondary side, as the cooling load changes.

The chilled water is generated and circulated in the primary side loop. The secondary side loop will pull the chilled water out of the header to cool the building, and it will then dump its warm return water back into the header.

If the flow rate in the secondary side loop is low, then some of the chilled water will flow into the secondary loop, and some of it will recirculate back to the chillers.

If the flow rate is high in the secondary side loop, then all the chilled water will flow into the secondary loop and the return line will flow straight back to the chillers.

Header and Pump Distribution

Coming off the common header is another header. This allows the flow to be split, and each will receive equal distribution.

On the atlas of the header, you might find a commissioning station, or a double regulating valve. But in newer systems, this is likely to be on the return line.

From the header, outlets will have some more pumps to distribute the chilled water to different parts of the building. For example, to the east and west side.

Some designs won't have separate pumps for different parts of the building, they will simply have one pump set, which forces the chilled water around the entire building.

These pumps will distribute water off to the AHUs and fan coil units. I explained when this detail was shown earlier in the video, but I'll just briefly give you some examples of air handling unit connections, and also some fan coil units. Pause the video if you would like to look at these in some more detail.

You'll also find a pressurization unit on the chilled water side, as this is a closed system. Typically this is connected to the return line on the main circulating pumps, as this area is not affected by the pump. This unit just maintains a certain pressure in the system.

When considering the power source for the pumps in such complex systems, models like the kta19 cummins engine provide reliable and efficient operation, especially in large commercial HVAC setups.

Condenser Water Side

Remember, air cooled chillers do not require a condenser system.

We will need a set of pumps to push the water through the chiller and around the system, so we can drop these in first.

Then we'll need to connect those pumps to the condenser inlets, and another line out of the condenser and up to the cooling towers, which are located up on the roof.

So we can then connect the cooling tower to the riser via our header. This connection will need isolating valves and flexible connections.

Not all towers will use two inlets like this example, some will just have one. At the outlet of the cooling tower, we'll also need another isolating valve and another flexible connector.

Then we can connect this back to the pumps to complete the circuit for the condenser side.

As this is a large building with multiple chillers, we'll need multiple cooling towers, and in this design, we'll pipe them in parallel.

Because they are in parallel with a common supply and return, we'll typically find a balancing line to ensure equal water levels in the basins, and prevent air from being sucked into the pipework and reaching the pumps.

There might also be a bypass line with a motorized valve across the cooling towers flow and return lines. This can have various functions, such as maintaining a minimum flow rate, and also protecting the chiller from tripping on low and high pressure due to the temperature of the return condenser water being out of design limits.

For example, during start up, or a cold winter morning.

Another common version for the cooling tower you might come across will look something like this. It has the flow and return lines for the condenser water. There is a balancing line between other cooling towers.

They have a make-up line, which tops up the water if the water was to get too low. And there will also be an overflow in the drain line.

Some will have a flushing line between the flow and return lines for system cleaning. If you want to know how cooling towers work, we've covered this in a previous video. Link's are in the video description below.