hvac control diagrams how to make it? 5

[etchyHA]

hi every one i want to know how to make control schematic i saw one contains DI,DO ,AI,AO what is the difference i know that DI is digital input & AI analog input but i dont know for example why is the the temperature sensor is connected to AI while DPS connected to DI .please help me in any inforamtions

 

[sararamirez]

AI= Analog Input
AO= Analog Output
DI= Digital Input
DO= Digital Output

The reason your DPS is connected to a DI is that DPS is referring to a differential pressure switch, which would be simply an 'on/off' switch based on a unique pressure limit (high/low), rather than a differential pressure transmitter, which will tell you the actual pressure. Anything such as a high temperature switch or high humidity will be a DI because it has a unique limit and can be considered on or off, it won't tell you the actual temperature, humidity, etc.

When you are considering what your inputs and outputs will be, consider it from the point of view of the control system instead of the equipment. I've found this helps people unfamiliar with controls.

Hopefully this will help you.

Sara

 

[atlas06]

Here are some basics.
Typically, you want a DI if you want to start/stop a fan/pump motor. You want an AI if you want a value.

http://www.ddc-online.org/

 

[etchyHA]

thanks you sara & atlas

 

[gregha04756]

Just to expand a bit on the previous posts:
AI = Analog Input = Physical property or quantity that does not have a discrete value or state, e.g temperature and pressure. Devices such as thermistor temperature sensors and duct static pressure sensors are connected to analog inputs. Typically one must specify a scale range to convert the raw electrical signal of the sensor, to engineering units.
BI or DI = Binary or Digital Input = A physical event or status occupying one of two discrete states, usually expressed as On/Off, True/False, 1/0. A differential pressure switch, air flow switch, or thermostat is connected to a digital input.
AO = Analog Output = Continuously varying signal used to modulate the action of a final control element such as a proportional control valve or variable speed drive. Usually limited to a range of 0 to 5 or 10 VDC, or 4 to 20 mA.
BO or DO = Binary or Digital Output = Signal occupying one of two discrete states (voltages), typically used to start and stop pumps and fans, and open or close 2-position valves. Often an interposing or control relay is used to switch the device, as typical DDC controller outputs have limited current or power dissipation capacity.

When designing a control schematic, I think it's good to start with a "flow diagram" depicting HVAC equipment for a given application. A sequence of operations is one of the key documents you need to refer to when figuring out what equipment is required and the control actions that are to be performed. In my company we typically start with major units, such as air handlers, and use the sequence of operations to determine the control devices necessary. A sequence might say "Modulate outside air damper". This tells you that you need a modulating or proportional damper motor (sized appropriately for the damper) and at least an AO to drive the damper. The schematic needs to show which output on the controller, # of conductors and size, and connection details so an installer can make the necessary connections.

This is an extremely brief outline (really the tip of the iceberg). I think the link provided by atlas06 above, is a good sarting point.


Good Luck,
Greg Hansen

 

[simsd]

Please,

If you don't know this stuff, don't even attempt to start laying it out on paper. Either call a controls contractor or consultant. If not then take a class.

We see this time and time again of people attempting to do this stuff and it ends up being nothing but heartache and disaster for those involved.

Do the flow diagram and the show the sensors if you must. Then just name what you want in a list (ie: temperature, humdity etc.). Why would you want to guess at this stuff. If you don't know what analog inputs are or the acronyms for the other devices, then you are asking for trouble.

We would rather see list of "points" in a box rather than an uneducated attempt at a full control diagram.

 

[gregha04756]

simsd,

Good points.


Good Luck,
Greg Hansen

 

[simsd]

Okay,

So now that I've got that off my chest, and a few Amen's out there, let me direct you to this. Go to this thread in another talk group. This is a bunch of us who work at the design, installation and technical level of control systems. The person in the thread is asking the same question you are. The responder is directing the questioner to the Honeywell grey manual(AKA the Bible in our business). It's a great start.

http://www.hvac-talk.com/vbb/showthread.php?s=88b99bbaf33cb68f5a1f5d74b7b035ba&t=128815

Also, be careful how deep you want to go with this. I have seen many cases where the designer puts stuff on drawings and it gets installed that way. Then when it doesn't work, they change order the customer to death and say "hey that's the way it was shown on the drawings". You are opening yourself up to a lot of liability and embarassment.

Yes I'm a controls guy and have been doing this a long time. Ask the others out there who wish they'd drawn a square for a boiler and a circle for a pump, put a set point and then said "make it work" rather than try to get into drawing out all of the AI's, AO's, DI's and DO's.

And Greg is right. This is only the tip of the iceberg. We also do tons of relay logic with a lot of this stuff.

Sara also makes a very good point in that you have to also think of the drawing everythin in terms of the controller. Get the AO and AI mixed up on a valve and you'll have a valve with feedback but nothing to make it actuate.

 

[atlas06]

simsd,
You appear to be well qualified in your field and all your points are well taken and you are right, young engineers mixing it up in the AI/AO is common.

However, these same young engineers need to get educated and at least learn how to swim when talking to a contractor and trying to explain their design intent. They must know the language of control guys like youself or else they get eaten up at construcion meetings.

If one cannot control a system, he cannot design the system. Controls is a trade in itself, it's true, but untill that 15900 spec section gets removed from Master specs, engineers HVAC have to dive in and learn it (even if they have to screw up).

I highly recommend that young engineers take a controls shop drawing and sequence of operation and learn from it, take a shot at editing it, and then have it reviewed by a senior level guy and/or e-mail it to some controls manufacturers for review. with time, it'll sink in. Ask your questions now that you are young, when you get older with those Senior titles, you will not entitled to some questions.

It is not by shying away from the unknown the US has put a man on the moon.

Incidently simsd, can you give us a quick and dirty cost estimation on HVAC controls? I keep hearing $1000.00 a point for 20 years now. Others tell you it is 10% of a construction project. Our legs are getting pulled so much that we start losing confidence in controls vendors.

Thanks

 

[simsd]

Thanks Atlas,

I hope I wasn't too hard on etchyHA. I agree, with you, that the younger guys need to start somewhere. I share office space with some engineers and architects and do a lot of work with consultants. That being said, I guess I am fortunate that they give me carte blanche to do the design. Oddly enough, in addition to all of the DDC stuff we do, we also provide a tremendous amount of standard electrical control stuff.

I would love to see etchyHA take a class at the local university or college (be carefull that you don't a PLC programming course as that will completely lose you).

Oddly enough, after learning about all of this controls stuff, I ended up taking an HVAC design course in order to understand how the consultant thinks. I found that once I figured this out, it actually allowed me to understand what was really important from a controls point of view to the consultant and end user.

As to your question, the per point thing has been going around for years. Here are my unit cost rules of thumb that I usually give to a mechanical contractor:

VAV Boxes - $ 1,100 (add)$200-300 for a reheat valve.
AHU - $ 5,000.00
Boiler/Hot water system $ 5,000.00
Chilled Water system $ 5,000.00
Roof top unit $ 2,000.00 - $ 4,000.00
Front-End System $ 10,000.00 - 15,000.00

I have been using these for about 7 years and they are not that far off the money. Assume open plenum rated cable in concealed areas (ie: above drop ceiling for network and thermostats). Conduit is used on all of the AHU's boilers and chillers.

 

[atlas06]

Thanks for the numbers

Only one number is odd: VAV box thing. Why on earth would anyone charge $1,100 for controls to a VAV box that is already totally pre-wired, tested, and all you need is program and hook up a sensor (the programming part being included in your front end costs).
We figure $200.00/box when 15 plus boxes are involved.

The controls cost more than the Box itself? How many VAV boxes can you control per day? 10? 20?

If you control 10 boxes in one day, you get $11,000.00 a day? that's brain surgeon rates.

Thanks

 

[simsd]

atlas06,

I assumed it was a bare VAV box. The price I'm talking about is for the following:

1 VAV controller (DDC)with integrated actuator
1 Discharge air sensor
1 room sensor (adjustable)
1 transformer
Installation and programming of all of the above including air balancer assisting and networking from one box to the next.

I have never had controls come from the factory other than pneumatic or electric. Since the VAV controllers can stand on their own, we include programming in the VAV unit cost.

Am I missing something here? Let me know so I can help.

The front end costs are just an estimate of networking of everything into a convenient central loaction

 

[hollandhvac]

Agreed with the statement; if you are new to this, and you have to design this it is better to make de controls description of normal status, fire alarm status and the aftermath of a fire and give this to a control supplier company. I have been in this myself and it is trouble, just waiting for a desaster to happen.

 

[atlas06]

Holland,
There has never been any fees to hire a controls consultant in any bid out there. HVAC Controls are part of HVAC design, wether we like it or not.

 

[willard3]

I will also point out the no one sues the controls contracdtor when stuff doesn't work, they sue the Engineer.

Now, why should I rely on a contractor with no liability or design responsibility to design a control system? I always hear in arbitration that: "the controls contractor did it". I tell them that Engineers cannot assign design responsibility to anyone which is why they are licensed in the first place.

Learn about digital controls, it's not rocket science. Have your design reviewed by a senior Engineer and a controls contractor, it will help in the learning process.

If Engineering were easy, they wouldn't need us......

 

[nans]

Check if the below helps.....

https://www.ctrlspecbuilder.com/

 

[cme]

input (to the panel)
output (from the panel)
digital - contact closure
analog - modulation