If you were going to apply the DCV philosophy for energy savings, what’s a good stratgy for identifying candidates to receive CO2 sensor placement? (Assume we’re only measuring C02 for determining OA requirements) That said…
Considering all types of commerical uses in different buildings (offices, labs, paint or machine shops, kitchen, auditorium, conference room, library, gym, etc.) which makes the best candidates for CO2 sensor placement? I wager good candidates have large room volume and periods of low occupancy (the longer, the better). After surveying a mix of commercial areas, is that a good way to prioritize potential DCV candidates?
Last, say a cluster of offices has a half-used conference room? While the conference room has the lowest levels of CO2, should it be driving the OA requirements? Won’t the cluster of office spaces be suffering?
What’s the right way to judge/balance all these conditions? Hopefully, some SDopers have answers in this department. Thanks!
Thanks to the 51 viewers. I did get some pointers from a friend in the business. He admits its more of an art than an exact science. This explains why all the resources I have explored are silent about how to put the rhetoric into practice. Also, LEED is having a case of split personality asking business to save energy by cutting outside air (OA) intake while maintaining sufficient OA for good indoor air quality (IAQ)! Sheesh!
You’ve hit upon the best case for CO2 sensors, large people capacity spaces with irregular use. Conference room is the classic case.
Other ways to initiate demand ventilation would be time-of-day schedule where a space is predictably occupied on specific days or times of day(classrooms), and occupancy sensors where occupancy is regular but unpredictable(offices).
The alternative is to always ventilate for the worst case scenario which is a waste of energy.
As to LEED, they want to reduce energy use no matter the cost. IAQ isn’t LEED’s standard, it belongs to ASHRAE.
I would put the sensor in the return air airstream right before the fan so the sensor was seeing the combined space.
I’d program the controls so the (main blower) fan ran continuously during occupied periods.
You should also install a time clock that disables outside ventilation during unoccupied periods.
The alternative is to install a second sensor parallel to the ‘combined’ sensor for local issues-----like a conference/meeting area that is used frequently enough to make [local] ventilation an issue.
Batfish, pardon my lack of knowledge here on types of controllers, but is there a PLC (I assume) that can be programmed to modulate outside air by time of day schedule?
And, just to confirm my understanding: You’re saying this approach would be for offices or classrooms where occupancy, and thus CO2 level, is not changing significantly. As such, it would make more sense to montior OA by schedule.
Thanks for being so very helpful, Mr. Snarky Otto! Obviously, I am weak on controls. Let’s just ALL stay in the ignorant dark and never ask a questionn one does not already know an answer to. Yeah, that’ll get us real far! :smack:
Instead, Mr. Snarky Otto, please enlighten me on what a DDC can do for me! :o
Your OP was about CO2 sensor placement for demand controlled ventilation in commercial facilities (in which DDC systems are the norm these days). The CO2 sensors are simply control input points for a control system. So your question is about control inputs but then you later seem to have no idea about a very basic ability of a DDC system (a clock). Hence my question about why you are asking about it in the first place. You remind me of someone who would ask for help on selecting tires for your Ferrari, only to later admit you don’t have a Ferrari.
That being said, yes, time scheduling is a very basic function of a DDC system. Yes, you can program a PLC to do this (your first hint is in the name: Programmable Logic Controller), but no, you probably wouldn’t use a PLC due to cost, there are cheaper controller options.
Placement of CO2 sensors would be based on an analysis of the occupancy, and how critical the ventilation requirements area. Both Batfish and raindog have offered possible solutions, which one might be employed will depend on how critical the requirement is and/or cost of the system.
I should clarify: you probably wouldn’t use a industrial grade PLC (such as an Allen-Bradley) due to cost, there are cheaper HVAC specific controller options (such as Automated Logic).
Most commercial HVAC equipment these days comes with or have the option of some sort of microprocessor controls that may work stand-alone or communicate with a Building Automation System. The BAS, among other capabilities, can schedule the unit or zone conroller to occupied/unoccupied periods and send it setpoints. Setpoints for example would definitely include the space temperature but might include an outdoor air damper position or a minimum outdoor air requirement.
For a dedicated unit during an occupied period there might be a minimum position for the outdoor air damper. During unoccupied periods there might still be times when the unit runs to maintain a minimum or maximum temperature but the ventilation demand will be zero and the damper will remain closed.
In the case of a zone controller like a VAV that is fed from an air handler whose controller has the ability to measure in CFM the actual outside air flow then the ventilation setpoint will be continually changing as the VAV zones are occupied or unoccupied. (VAV-1 has ventilation requirement of 50cfm when occupied so 50cfm will be added to the air handler’s total requirement which is the sum of all the zones that are occupied.)
Occupancy may be determined by schedule or by an motion sensor, either way the controllers setpoints come from the BAS and its behavior comes from the configuration of the unit or zone controller.
Example 1: A classroom has a set schedule that varies over the day and from day of week. Tuesday may be different from Wednesday and certainly different from Sunday. So a Time of Day Schedule is appropriate.
Example 2: A sales office’s hours of operation are predictable but individual sales persons’ may be in and out irregularly as they call on clients or play golf. So the motion sensor changing occupancy is called for.
I don’t know how clear I’ve made that but I hope it’s helpful. For me, who works with BAS, DCV is part of automation in general. I don’t how you’d approach it with HVAC systems that aren’t already automated, but there may be some strategies I’m not familiar with.
I’ve been the end user for this type of system in the past for a commercial office building. The interface is web based where you can see a graphical representation of your building and it’s status, the parameters for all the controls, a schedule, the graphical logic programming (with real time data), and scheduling. All told, I 'd guess it pretty typical of what is available from other vendors. But as I understand the terminolgy, they are BAS specific controllers and not PLCs. PLCs certainly can be used for BAS, but overall I think that something like what I linked to (or a lonworks system) is probably a better choice. I’m sure the control guys here will set me straight.
Jinx, if you want to learn more about what controls can do, you may want to get a copy of the logic softwarehere (They used to send it out for free, they may still do so). That, along with a few books on control logic, will give you a good idea on what you can do with a DDC system.