HVAC Temperature Sensor Display and Set Points

I am designing an hvac control system for my new 4800 sf office. I am going to use a server quality PC running in a clean conditioned environment with a pared down very stable Windows 7 operating system installation. I will use USB devices to read sensors and control outputs. I am writing the controls software in VB6 as there is no need for this application to be fully web enabled. For anything needing to be done remotely, I will use something like gotomypc.

There will be a lot of analog inputs and outputs, about 16 of each plus some digital inputs for occupancy sensors and the status of various devices. The analog inputs will be for reading various temperatures and pressures and the analog outputs will control a vfd for fan speed, modulate a valve to control water flow (ground source heat pump system), and control six modulating damper motor actuators to adjust air flow in the duct system.

Although this forum is mostly about using PLC's for control systems, I believe the question I have is relevant because it mainly involves how to sense and display data for a certain situation.

At each of the six different zones of the building, I want to have a wall mounted device that shows the current air temperature at that location as well as the current user set point temperature, nothing else.

Sounds almost like a regular thermostat right? But not really, because the temperature should be derived from an analog input temperature sensor (probably rtd) that connects back to a USB device like this one http://www.mccdaq.com/usb-data-acquisition/USB-TEMP-Series.aspx

So the computer sees the temperature from this sensor and someone standing there sees the same temperature on a digital display with one decimal format like 75.4 In addition, another display just below, shows the set point temperature in similar format. There are two push buttons on the device. One raises the setpoint temp and the other decreases it. That's it. There are no mode indicators for heating, cooling or fan operation. The software at the controlling computer limits the upper and lower ranges of the set point temp so if a user continues to hold a button down too long, the setpoint temp on the display does not go beyond the allowed range.

The controlling computer then adjusts the appropriate duct damper (fully modulating) to try an achieve the set point air temperature.

So is there any off the shelf temperature sensing and dual display device such as I described above? And if not, must I cobble together an analog temp sensor and some kind of digital counter sensor to count the button pushes in either direction and pair both of these inputs (actual temp and setpoint temp) with their own digital panel meter type displays?

What is the easiest way to accomplish what I described above?

To do all this is a fairly monumental task when you are going to have to program the VB6 application as well. I find that in controls I steer away from USB hookups they don't lock in place well and vibration will often cause em to seat in and out changing their comm port assignments. However most temp controllers are setup to communicate via Rs 232. There are standalone controllers that will allow the operator to manually raise and lower desired temperature ranges as well as provide you some PI Control. They can also send a demand signal to your dampener controller. I seldom work on this type of system but I'm sure people here will be willing to recommend a few.

mordred said:

I find that in controls I steer away from USB hookups they don't lock in place well and vibration will often cause em to seat in and out changing their comm port assignments. However most temp controllers are setup to communicate via Rs 232. There are standalone controllers that will allow the operator to manually raise and lower desired temperature ranges as well as provide you some PI Control. They can also send a demand signal to your dampener controller. I seldom work on this type of system but I'm sure people here will be willing to recommend a few.

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Mordred, thanks for your comments. That's a good point about USB hookups needing to be locked in place so connections don't seat and reseat. If I stick with USB (just love the concept of being able to hot swap a computer with them), I will make sure they are very secure.

Please note though, that the device I want to buy or build in my original post is not a temperature controller. It is merely a temp sensor, two momentary push buttons, and two mini displays. The greatest distance from the PC to one of these devices is 80'. Can this be bought off the shelf or what is the easiest way to make one?

from what I've seen most temperature displays usually do not include the pushbuttons, Sounds like what you need is a temp display this is where the temperature senser would hook up to, The pushbuttons are simple enough display the desired temperature could probably be done as an output from your program.
Usually this type of application is done with pre developed temperature controllers that have the reporting capabilities to other monitoring equipment/programs.

mordred said:

from what I've seen most temperature displays usually do not include the pushbuttons, Sounds like what you need is a temp display this is where the temperature senser would hook up to, The pushbuttons are simple enough, display the desired temperature could probably be done as an output from your program.
Usually this type of application is done with pre developed temperature controllers that have the reporting capabilities to other monitoring equipment/programs.

Click to expand...

Yes, a temp display allowing a temp sensor to connect and another display that shows the PC's current stored setpoint temp as an output is what's needed. Would this extra output have to use another analog output channel or is there a simpler way for the PC to send a signal to a display that shows the setpoint temperature?

Why are you doing this instead of using off the shelf stuff? This can be quite a project, but a fun one.

I looked into doing HVAC using industrial PLC/HMI a while back and going my memory, here are some of the stuff I learned:

- Building controller are cheaper than industrial controller.
- Most VAV talks in BACnet or Lion. However I did find a few that works on Modbus.
- VAV works mostly independantly for a reason. You can't want the PC to go kaput and the VAV stuck. Are you trying to program your own VAV?
- Follow good HVAC design. It's that way because years of experience and lesson learned. ie. duct temperature should have minimal or no variation, let the VAV read the duct temp and let it do its thing. You need someway of reading the duct temperature.
- Most VAV controller can take a thermostat with setpoint.
- Think about future maintenance.

take a look at ASIC stuff, as an example.

harryting said:

Why are you doing this instead of using off the shelf stuff? This can be quite a project, but a fun one.

I looked into doing HVAC using industrial PLC/HMI a while back and going my memory, here are some of the stuff I learned:

- Building controller are cheaper than industrial controller.
- Most VAV talks in BACnet or Lion. However I did find a few that works on Modbus.
- VAV works mostly independantly for a reason. You can't want the PC to go kaput and the VAV stuck. Are you trying to program your own VAV?
- Follow good HVAC design. It's that way because years of experience and lesson learned. ie. duct temperature should have minimal or no variation, let the VAV read the duct temp and let it do its thing. You need someway of reading the duct temperature.
- Most VAV controller can take a thermostat with setpoint.
- Think about future maintenance.

Click to expand...

I am programming and controlling much more than the VAV aspect of this system. I am modulating the heat pump compressors (10-100% modulating Copeland Digital Scrolls) and installing three phase motors on the two York air handlers I will be using so that they can be speed controlled with vfds. I will be reading many temperatures and pressures throughout the system.

Off the shelf BAS systems are too closed for my purposes. I want full control of everything and will use quite a bit of high level math in optimizing the control of this system.

As the owner of Elite Software www.elitesoft.com I am well versed in hvac calculations but have very little experience in the controls side of hvac design. After looking over standard building control systems, I decided that it was worth reinventing the wheel in my case. I will learn much more and enjoy complete control down to the finest detail.

I will implement a backup simple control scheme right from the start. But I envision the ongoing growth and optimization of Elite Software's BAS system as kind of a life long hobby. I have a lot to learn, but am looking forward to it.

It may be that I can take an advanced tstat as shown here http://www.temcocontrols.com/product.php?ProductID=108&CategoryID=1 and use just a fraction of its features to get the function mentioned in my original post. Anybody have any other ideas given the way I am approaching this system?

For what it's worth:

1) that temco is a fraction of the price of industrial PID/ona/off digital controllers. Go for it.

2) I do industrial thermal sensing/control, and use quite a bit of Modbus, but no HVAC.

Oddly enough, I just talked to an HVAC engineer who had 'standardized' on Modbus RTU as his primary comm protocol, unusual in the HVAC world.

His comment was, "Bacnet was supposed to be a standard, but everyone implements it with their own proprietary quirks, so making different brand bacnet devices work together is nearly impossible."

And we had a discussion of the vagaries of Modbus as an open protocol.

I can't comment on Bacnet, I've never used it, but it was interesting what he had to say about it, having tried it and settling on Modbus.