BMS inputs & outputs

Written by HVAC Guy

This blog will give you an honest insight into the day-to-day process of being a BMS Engineer. I will be talking about all different aspects of the job from software strategy planning to on-site hiccups and frustrations.

17 July 2021

This is BMS 101, if you can clearly identify your inputs and outputs then you are going in the right direction.

A key thing to understand in the world of BMS is the difference between inputs and outputs, often referred to as IO.

Most of you will probably be thinking that it’s obvious, and it is when the penny has dropped, but think back to your early days, was it so obvious then?

All Building Management Systems are made up of points, whether they are physical or not doesn’t really matter, they are all either inputs or outputs in one way or the other.

When I look at starting a new project, I always look at breaking it down, working out the points, and keeping it as simple as possible.

Once you can fully break it down into simple parts then you can start to put the design and the software together to make it work. Too often things are made more complicated than they need to be by adding words and phrases that can be unnecessary and even intimidating to new engineers. Don’t get me wrong, once you fully understand the process this is how you will want it, but in the early days, it can be made much easier by breaking it down into simple parts.

What is a point?

A BMS point can be almost anything to do with the project. Traditionally points are the physical inputs and outputs which are wired to and from the controller. These days a lot of the points within the project can come from software logic and integration networks, we are no longer limited to the IO physically connected to the controller.

An example of an integration point would be a supply air setpoint for an Air Handling Unit. Traditionally this would be dealt with in the controller and the IO associated with the AHU would be wired back to the BMS controller and operated directly.  E.g. BMS monitors the Supply air temperature against the supply air setpoint, if the temperature drops below the set point the heating valve is opened. This is oversimplifying it, but you get my point.

These days the AHU’s tend to come with packaged controls that look after themselves, almost! Putting it simply the BMS sends a set point to the AHU via a network. The AHU packaged controls receive the setpoint and acts accordingly.

Why is it important to understand the points?

I will try to answer this question from every angle, starting with the Sales Engineer.

It is important to understand the points within the project as early as the tender stage. Because if not, how can you select the correct controller? If the Sales Engineer does not recognise the points required, then how can they possibly quote for the correct controller and ensure that they have enough points/licenses available to get the job working? In short, they can’t. But it does happen, that’s what the DIM4 modules and the like were invented for isn’t it?? Sarcasm aside, we will cover these type of modules in a different post.

Costs can soon rack up if the points have been calculated incorrectly and additional hardware is required further down the line. On top of the hardware cost, there are sometimes license costs associated with purchasing BMS controllers. The licenses come in different forms depending on the hardware you are buying, some hardware manufacturers charge per point, others charge per set amount of points. It is worth noting that not all BMS manufacturers issue license costs for the hardware.

Moving along the project timeline, an order is received and now the Project Engineer is in charge. Why should they care about points? Well typically, they need to create the documentation/strategy to build the control panel and complete the electrical installation. Most of this is built around the points! It is standard practice to put a points schedule together for any BMS project. And whilst this is a paper exercise that in theory could be completed by anyone, it helps if some thought is put into it and a standard is followed.

I am reminded about a software engineer I came across on a large refrigeration job, we were working together to integrate the BMS and refrigeration plant at a large distribution centre. It soon became pretty clear that he knew very little about the Refrigeration plant and the general operation of it, he had simply been employed to write the software and make it work. And he was great, he was obviously surrounded by engineers who knew the practical side of things, but his brief was made up of simple statements something along the line of: if X and Y are true then operate Z. This was an example of somebody really understanding the points within their project, so much so that they are happy to write a description of operation for a software engineer without even mentioning the plant!

Now the job has been designed and the panel is on-site, up steps the commissioning engineer. It is their job to ensure that all the points are correctly installed into the control panel, set to work and operate as required. To do this you would be required to follow the control panel drawings, site drawings and also the points schedule, amongst other documentation. If you had no knowledge of the points how would you know where to start? If you didn’t know that the boiler was an output which was to be operated how would you know to test it correctly?

Often each point is checked by the commissioning engineer to ensure it is installed correctly and wired correctly, these are commonly referred to as the point-to-point checks. Once you can confirm the point-to-point checks you can then start to set the system to work in automatic mode.

Finally, the service / call-out engineer. It is not uncommon to pick up a new service contract or call-outs in a new location with absolutely no information, no panel drawings, no points schedules, nothing! How can you possibly start to service the BMS without this information?

Start with the points. It is easy to look around a plant room and see that 5 sensors are installed, but how do you know you have found them all. A good indicator of this would be to look in the panel to check out how many sensor inputs are wired. There may be more sensors wired than you have found, one of these sensors may be the reason the plant isn’t working correctly. Now you know you have to find 2 more sensors, without having a little bit of knowledge about points you could’ve cost yourself a few return visits.

Different types of I/O

There are 4 types of IO to be aware of on a building controller, 2 types of inputs and 2 types of output.

  • Analogue inputs
  • Digital inputs
  • Analogue outputs
  • Digital outputs

Inputs are points that are not affected by the controller, they are simply read and used if required.

Outputs are points that are directly affected by the controller. The controller will command outputs to either operate or not based on certain conditions.

Analogue inputs

These types of points are typically sensor inputs, but not always. An analogue input is any input where the value is variable and not just true or false.

A 10k temperature sensor is an analogue input because the controller will read the resistance from the thermistor and convert it to a real temperature value.

Other examples of analogue inputs include:

  • Duct pressure sensor
  • Room humidity sensor
  • Water level sensor
  • Water flow sensor

Digital inputs

These types of points are typically status inputs such as a fault monitoring point. A digital input is a point the controller reads which can be one of two states; it is either true or false. Sometimes different terminology is used but it all means the same thing; on / off, 1 /0, open/closed.

The fire alarm input on a building management system would always be a digital input, this is because the fire alarm is either activated or it is not, it has to be one of two states.

Other examples of digital inputs include:

  • Boiler fault status
  • Air differential pressure switch
  • Water tank low-level switch
  • Hot water tank high-temperature thermostat

Analogue outputs

These types of points are typically used to control variable output devices such as a control valve. An analogue output is a point that the controller commands across a variable range such as 0-10Vdc.

The heating coil valve actuator on a supply air unit would be an analogue output because the valve needs to be closely controlled to maintain the desired set point. The valve can be controlled anywhere between fully closed and 100% open.

Other examples of analogue outputs include:

  • Boiler burner control
  • Temperature output to 3rd party system
  • VAV control
  • Heat recovery damper control

Digital outputs

These types of points are used to control anything that is either on or off, true or false, open or closed etc. It is a point which the controller commands such as a pump enable signal.

A fixed speed heating pump would be classed as a digital output because when required the pump is switched on, when it is no longer required the pump is switched off. It is always either on or off.

Other examples of a digital output include:

  • Standard lighting control
  • Heating boiler enable
  • Extract fan enable
  • Instant hot water heater
Identifying the I/O

Below is a typical line from a BMS description of operation:

“The hot water control valve will be opened whenever there is a heat demand from the control thermostat installed within the storage cylinder. The valve will be immediately closed if the high-temperature thermostat is operated.”

The above will probably make sense to most people reading this, but could you write software to make this happen?

Lets take a closer look at the above statement and break it down.

“The hot water control valve will be opened”. Here we are talking about an output, we are giving this a command to open and close based off the conditions stated.

“Heat demand from the control thermostat installed within the storage cylinder.” – This thermostat is acting as an input because the point will be monitored by the controller, we cannot command the thermostat, it is read only.

“If the high-temperature thermostat is operated.” – Here we are again talking about affecting the operation of the valve based on a condition, the thermostat is an input.

So let’s look at it again now we have identified what the inputs and outputs are.

“The hot water control valve [1] will be opened whenever there is a heat demand from the control thermostat [2] installed within the storage cylinder. The valve [1] will be immediately closed if the high-temperature thermostat [3] is operated.”

In short, we have a single output which is controlled based off two inputs.

Provided you are familiar with basic “and/or” logic you can now make this work.

Output [1] = True if input [2] = True and input [3] = True.

(We are assuming that the high-temperature thermostat has been configured for fail-safe operation, hence true meaning ok and not fault).

The above example is clearly very simple but ultimately every operation within the Building Management System is a product of different inputs changing state in order to provide the controller with information to operate an output.


Know your points!

The key to truly understanding how the system should operate is by stepping back and working out the points first.  As demonstrated previously; whether it be at tender stage or as far on as servicing years later, knowing and understanding the points is going to go a long way towards a successful handover of the project or servicing of the system, it applies at all stages of the Building Management Systems life cycle.

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