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Your HVAC system (Heating, Ventilation, and Air Conditioner) plays a critical role in keeping your remote locations online. It fights the damaging effects of an outside climate that may be too hot, too cold, too humid, or sometimes even not humid enough. It also keeps the build-up of heat from your racks of computer equipment in check. If your HVAC goes down, it generally isn't long before you'll encounter several serious problems that can take down a good portion of your network.
This is where an RTU (remote telemetry unit or remote terminal unit) comes in. Component of SCADA systems, this electronic device monitors your HVAC for failures. This happens both directly (most HVAC units will self-report at least a few different types of failures by latching a relay contact) and indirectly (monitoring the temperature and humidity ranges that the HVAC system controls).
Guides for HVAC Monitoring System
An RTU can also monitor many other pieces of gear simultaneously. A complete discussion of this monitoring device is outside the scope of this article, but you do need to realize that your HVAC remote monitoring needs must justify only a small fraction of an RTU purchase. You're going to get a lot of other value out of a deployed RTU.
How big should an RTU be to monitor your HVAC? As described above, your choice is dependent on other things at the remote site that must be monitored. However, if we assume for a moment that you only had to monitor HVAC, we can list the important elements for monitoring just that equipment. You can then add to the spec for any other equipment that must be monitored.
Most HVAC units will output alarms via relay contacts. At a minimum, you'll need a few discrete contact closure inputs to accept these alarms at your RTU. Sometimes, the alarms will be named ("Running", "Failure", etc.) and other times they may simply summarize several alarms with a given severity ("Critical", "Major", "Minor", "Status").
In case your HVAC is unable to self-report alarms (or to get more detail after it does), you need the means to measure temperature and humidity directly from your RTU. While it is possible to use a few more discrete inputs and discrete temperature sensors, these tell you only whether the temperature is above/below a critical value (think of your home thermostat to understand how this works).
You're much better off with analog sensors for these 2 values. You'll need 2 compatible analog inputs for these sensors (0-5vDC or 4-20mA are the two industry standards). Or, you can purchase an RTU with a built-in temperature/humidity probe. This should be on a lead so you can position it within your site, but it will have the advantage of being powered directly from the RTU HVAC system (no external power source or wiring required).
This is true for all RTU purchases, but it's worth mentioning again in an HVAC context. You never want to be the guinea pig for untested data logger technology. Are you the first buyer of this RTU?
Ask to see testimonials and case studies from previous satisfied customers. Remember, it's OK (even preferable) to have a proven manufacturer craft a customized RTU for your exact spec, but it had better be made from proven RTU components.
Now you know some of the key RTU/HVAC considerations, and how to select the right monitoring device for your Heating, Ventilation, and Air Conditioning systems. You know what it really takes to monitor and control effectively.
Don't accept anything less from a vendor. Your HVAC, and the field devices that it monitors, is depending on you.