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11 points every engineer and contractor should know about control logic theory - sample slides.  For additional support visit our visitor services page.

Our integrated design program has over 2100 slides illustrating architectural, interior design and HVAC engineering principles which contribute to indoor environmental quality and energy allocation for conditioning the occupants and building.

The following course materials on control logic theory are samples from the lecture and based on a Steven Covey principle of "Begin with the End in Mind". They are a very small but important sample of the Covey principle and are provided here to give you an idea of what kind of materials we'll be discussing during the program.

The course is also registered with AIA and participants can earn up to 21 Learning Units.

For more sample slides visit our list of training modules.

Figure 1: You can both create and solve a lot of problems with building controls but many of the hardest thermal comfort problems can be solved with intelligent building solutions including the generous use of insulation, caulking and lower window to wall ratios using higher performance fenestration systems. When it comes to thermal comfort problems the first solution can always be found in the building enclosure.

Figure 2: By default industry still tries to solve thermal comfort problem by focusing on the building.  Again we don’t feel the heat loss or gain from the building we in fact feel the heat loss and heat gain from and through our skin (1) and so our focus must be first on the needs of the occupants by asking what is the combination of mean radiant temperature and dry bulb temperature which encourages body heat loss for summer time cooling; and suppresses body heat loss for winter time heating. This combination of mean radiant temperature and dry bulb temperature is called the “operative temperature” and is the control metric used by indoor climate engineers adhering to ANSI/ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy.

Figure 3: Thermal comfort controls are a proxy for the occupant experience…you can consider them to be your thermal ambassador to the HVAC system. At the grade of Type 1 and Type 2 building categories, changes to the outdoor conditions are readily experienced by the indoors due to the inefficiencies of the enclosures, ergo it’s very important to have controls which can feed changing outdoor conditions to the system controls. These devices are called 'feed forward' controls and are known frequently as weather compensators, indoor/outdoor controls or reset controls. Without these types of controls the room thermostat and system controls can only respond to what has already happen instead of having a “heads up” of what is presently occurring and anticipating pending change by adjusting up or down, the fluid or air temperature in the system.

Figure 4: The typical control loop has applications across many processes including HVAC controls. It is a way of illustrating each of the “players” and a means of describing who does what, how, when and why. The control loop is of great importance to manufacturers and control engineers and technicians who take on the responsibility of designing appropriate systems. Typically beyond the scope of “retail” knowledge, it’s enough that consumers should know that HVAC related trades such as plumbers, pipe fitters or sheet metal contractors are not by default “control technicians” unless trained in controls. The most common mistake by the unskilled is failing to understand that just like people; the components in a control loop also have different personalities - a mismatch will inevitably lead to problems. In control logic, personalities are described by such terms as authority, speed, proportionality, torque and rate amongst many other terms. In our course we cover all the players and terms and teach students about the various relationships to help them become better control technicians.

Figure 5: When it comes to high tech. versus low tech., I can share with readers that I have been up the technology ladder and experienced the wailing and gnashing of teeth and have climbed back down. With today’s Type 3 and Type 4 housing technology we don’t always need zippy stuff; and a decade ago I for one resurrected the use of non-electric thermostatic control devices (shown above) for the control of space temperatures. Why? Because they were the first wireless thermostat, they require no electricity, are fully modulating, work on both radiant and dry bulb temperature, are easily understood by the lay person and the good brands are bullet proof. Die hard control connoisseurs find them so simple it insults their intelligence which is how I now judge the usability of technology…if the aficionado of controls hates it – in all likelihood it is good for the average Jill and Jack consumer.

Figure 6: Ok so not all control stuff can be simple so those working on upgrading your knowledge will learn the various control logic working away in behind all those fancy user interfaces. I’ll also show you how they can help and hinder the control outcome.

Figure 7: One of many illustration from our P+I discussion will show students how error elimination is achieved and how this benefits the control outcome but also how valve authority (β) fits into the control signal logic and how improper valve sizing can partially destroy the benefits of various control logic.

Figure 8: Shown above are the three traditional control logics and their respective outcomes from incorporating additional algorithms to influence the control point.

Figure 9: Previously I mentioned the terms weather compensators, indoor/outdoor controls or reset controls. Shown above is an illustration of the logic used in these devices. Spend some time studying the “geography” of the slide and note the relationships between outdoor temperatures and fluid temperatures based on the various “sloped” lines called "reset curves". When using these controls for optimizing boiler operation and also for thermal comfort the designer will specify the “curve” best suited for the combined characteristics of the building and HVAC system.

Figure 10:  Fuzzy logic…the perfect descriptor of human logic. Where other processes see the world in ones and zeros, wet or dry; or black or white - fuzzy logic sees in-between 1 and 0…it sees damp, grey’s and maybe’s. I love fuzzy logic; there is something holistic about working in a control world without right or wrong answers and I love it more because it drives those who can’t get beyond absolutes absolutely bonkers.

Figure 11:  So how does fuzzy logic work? Well fuzzy logic corresponds a control signal to the degree or strength of memberships in stated categories. For example, if the measured temperature is close to setpoint (ex. 20C (68F)) established as "comfortable", it has very strong membership (1 at A3) resulting in mid range control output (12ma of a 4-20ma signal). If the measured temperature is below setpoint at 19.5C (67F) , it will be recorded as having a weak membership in cool (.25 at A2) plus a strong membership in comfort (.75 at A1) and a corresponding output of 14ma - translation: valve needs to open slightly. If the measured temperature is above setpoint at 20.6C (69F), it will be recorded as having a weak membership in warm (.40 at A5) plus a strong membership in comfort (.60 at A4) and a corresponding output of 10ma - translation: valve needs to close slightly.

So there you have it, a few sample slides from our control logic lecturer...just a hors d'oeuvre from our library of over 2100 slides addressing a small but important element of integrated design and radiant based HVAC systems. In the program we will get into this and a whole lot more? How much more? Well just follow the links to the other parts of our website and you’ll get a feel for the scope of materials that we’ll be covering.

See you soon.

Robert Bean, R.E.T., P.L.(Eng.)
Registered Engineering Technologist - Building construction (ASET #8167)
Professional Licensee (Engineering) - HVAC (APEGA #105894)
Building Sciences / Industry Development
ASHRAE Committees: T.C.61. (CM), T.C.6.5 (VM), T.C. 7.04 (VM), SSPC 55 (VM)
ASHRAE SSPC 55 - User Manual Task Leader

Note: The author participates on several ASHRAE and other industry related committees but be advised the materials and comments presented do not necessarily represent the views of these societies, only the president of the society or nominated representative may speak on behalf of the organization.


1. Though humans also exchange heat and sense temperature via the respiratory system the skin is the dominant surface for thermal sensation and heat exchange.


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