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Discrete Inputs and Outputs for HVAC system

04/15/2020 10:29 PM

I'm building a HVAC system with 6 independent zones, controlled by 6 independent thermostats. This requires me to control the fan speed in 6 increments. The fan speed is determined by 6 separate dry contacts on the fan board.

So, if only 1 damper (any one of the 6) is active, the output needs to activate relay #1. ANY combination of 2 dampers need to activate relay #2 and de-activate relay #1. And so on for all 720 possible combinations. I was thinking about using LM3914 but since the inputs are discrete there should be an easier way? There must be an example out there but i cannot find it.

One of the issues is that the HVAC system design should be so straightforward that maintenance in 30 years from now shouldn't require any understanding of ancient integrated circuits. Something using only standard relays, resistors, maybe some zener diodes would be nice.

For concerned HVAC people out there: it's a hydronic heat and cooling system which enables me to use very low - or for that matter: any airspeed in the air handler. No risk of freezing up or over-cooking anything.

Thanks for your time and help,

Paul

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#1

Re: Adding discreet inputs and generating discrete outputs for the total level

04/15/2020 11:05 PM

A six input analog adder or summing amplifier where each zone input will add 0.5 VDC to the output.

https://en.wikipedia.org/wiki/Operational_amplifier_applications#Summing_amplifier

https://www.analog.com/media/en/training-seminars/tutorials/MT-214.pdf

Whose output is connected to six window comparators where Relay #1 is activated at a voltage between 0.4 and 0.6 VDC, and Relay #2 is activated at a voltage between 0.9 and 1.1 VDC and so on.

https://en.wikipedia.org/wiki/Window_detector

https://www.analog.com/media/en/technical-documentation/data-sheets/184123f.pdf

. . . . Or you could program an Arduino with an I/O shield.

Good luck with your quest.

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#3
In reply to #1

Re: Adding discreet inputs and generating discrete outputs for the total level

04/16/2020 12:00 AM

Or another window comparator I have used in previous designs was the LTC1042.

https://www.analog.com/media/en/technical-documentation/data-sheets/1042fa.pdf

. . . but this one needs an external reference whereas the LTC1843 has an internal reference. You can use a single reference and use a string of six resistors to generate your different reference voltages for your window comparators.

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#2

Re: Adding discreet inputs and generating discrete outputs for the total level

04/15/2020 11:07 PM

How about an Arduino?

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#4

Re: Adding discreet inputs and generating discrete outputs for the total level

04/16/2020 2:43 AM

How about a small Programmable Logic Controller?

(Not an endorsement - other brands are available).

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#5

Re: Discrete Inputs and Outputs for HVAC system

04/16/2020 1:06 PM

Here's a possible solution. You have a Read-Only Memory (ROM). The address lines are logic levels controlled by the damper positions, 0 = closed and 1 = open. The data lines are connected to drivers for the motor relays 1 - 6.

The ROM is programmed so that addresses with one '1' output = 0x01, with two 1's as 0x02, ... with six 1's as 0x06. It only requires 64 bytes of memory.

Contents of ROM:

Address - Data

000001 00000001

000010 00000001

000011 00000010

000100 00000001

000101 00000010

000110 00000010

000111 00000100

001000 00000001

001001 00000010

001010 00000010

001011 00000100

001100 00000010

001101 00000100

001110 00000100

001111 00001000

010000 00000001

010001 00000010

010010 00000010

010011 00000100

010100 00000010

010101 00000100

010110 00000100

010111 00001000

011000 00000010

011001 00000100

011010 00000100

011011 00001000

011100 00000100

011101 00001000

011110 00001000

011111 00010000

100000 00000001

100001 00000010

100010 00000010

100011 00000100

100100 00000010

100101 00000100

100110 00000100

100111 00001000

101000 00000010

101001 00000100

101010 00000100

101011 00001000

101100 00000100

101101 00001000

101110 00001000

101111 00010000

110000 00000010

110001 00000100

110010 00000100

110011 00001000

110100 00000100

110101 00001000

110110 00001000

110111 00010000

111000 00000100

111001 00001000

111010 00001000

111011 00010000

111100 00001000

111101 00010000

111110 00010000

111111 00100000

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#6
In reply to #5

Re: Discrete Inputs and Outputs for HVAC system

04/16/2020 1:35 PM

Correction:

"The ROM is programmed so that addresses with one '1' output = 0x01, with two 1's as 0x02, ... with six 1's as 0x06. It only requires 64 bytes of memory."

should be:

"The ROM is programmed so that addresses with one '1' output = 0x01, with two 1's as 0x02, three 1's as 0x04, four 1's as 0x08, five 1's as 0x10, six 1's as 0x20. No dampers open would have 0x00, blower motor turned off. It only requires 64 bytes of memory".

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#7

Re: Discrete Inputs and Outputs for HVAC system

04/16/2020 7:37 PM

Wow, I am impressed by the detailed responses to my question. In less than 24 hours there are already a number of viable solutions.

BRAVE SIR ROBIN: I like your approach and the voltage window comparator is a good one and it uses standard components. It is very similar to the LM3914 solution - but with better control.

SOLAR EAGLE: yes, very tempting to use this project as an excuse to finally build my own Arduino. The problem would be obsolescence of that hardware. I would like to be able to repair the HVAC system in 20 years from now and that might become a problem. Nevertheless: it is very tempting to expand the scope and introduce other functions (like keeping time records of damper values, temperatures, airflow, energy use, and so on). I will probably end up in the AA (Arduino Anonymous). For now, I'm staying on the wagon.

PW SLACK: Seriously, I looked at these PLCs and I can get one of those used for $ 25. However, a reliable PLC (over 20 years) is very expensive. They all have their own programming foibles.

RIXTER: your solution stimulated me to go back and try to solve this puzzle with a “basic logic solution”. However, your solution is quite elegant.

So, I figured out the “basic logic solution” that I could build with just two types of components: a NO optocoupler and a NC optocoupler. However, since the project has gained another day in its life, the number of dampers has - of course - gone up to 9 and my “basic logic solution” would require 512 optocouplers. That is a heating system in its own right.

I think for now I’m going to try the LM3914 solution and buy a bag extra parts for future replacement. I’ve just ordered the HVAC hardware and that will take a few months to install. I will get back to this to report the end-result (or end-disaster).

Thanks again, I just love this website and the people that make this website such a great help!

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#8
In reply to #7

Re: Discrete Inputs and Outputs for HVAC system

04/16/2020 9:40 PM

Welcome aboard. There are a lot of good people on this site who are willing to share what they know.

Cheers!

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#13
In reply to #7

Re: Discrete Inputs and Outputs for HVAC system..... Crossbar Diode Logic

04/17/2020 6:57 AM

Crossbar Diode Logic

One can trivially implement or's and and's with crossbar logic. You can implement non-inverting outputs(buffers) and inverted outputs with your optocouplers driven by the output of crossbar lines. Diodes might be tiny, extremely cheap, signal (so low power) diodes such as the surface mount 1N4148 or similar. Crossbar lines driving optocouplers can be used to drive derivative crossbar lines within the same crossbar array. Thus, you can build up any arbitrary logic you wish with a little care to deal with race conditions. With good physical design the printed circuit board could be universal, requiring a single step and repeat pattern for a crossbar line. Optocoupler output could be implemented with discreet jumpers making the levels of logic explicit and field modifiable. Spare logic function crossbar lines could be populated in anticipation of requirements. There are lots of diodes in this design but their resource demands are small and they allow you to reduce the optocoupler count profoundly over an all optocoupler design.

The generic approach to this hardwired random logic engine can be understood with minimal technology specific detail such that documentation could actually be etched right on the printed circuit board. Clever (perhaps truth table) symbology could make it adequate for an extra-terrestrial to understand with brief study.

If you have a pick and place device, the PCB diode placement can be less labor intensive than a by-hand process. Crossbar encode/decode logic is well established in legacy keyboard designs.

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#16
In reply to #7

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 2:05 PM

Thanks for sharing this with us. It's fun to play around with these design problems.

I'm curious. If you have 9 dampers now, does your motor have contacts for 9 speeds or still 6?

One thing you might want to think about is flexibility, not having a major redesign if the number of dampers changes, for example.

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#20
In reply to #7

Re: Discrete Inputs and Outputs for HVAC system

04/28/2020 10:22 PM

Explain to the dumb ole Mech Engr why a passive voltage divider won't work with your LM3914. In the sketch below, S1 through Sn are dry contacts from each zone. Then the active 1/R1-n's sum and divide with Rb to provide 0 - 5v to pin 5 SIG. You might want the R's to be trimmable so some zones could be biased relative to the others.

Can the unused LM3914 outputs be tied together so your fan doesn't turn off on overrange? If this would work, you might want jumpers across S1 - Sn so the thing could be tuned from the control panel.

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#9

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 1:54 AM

Ho are you going to control the compressor for the broad demands?

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#10

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 2:20 AM

ANDY, the compressor is actually an air to water heat pump that feeds either hot (during winter) or cold water (during summer) into a tank. The airhandler takes either cold or hot water, when it wants and how much it wants.

The heat pump is only controlled by the tank temperature.

Normal A.C. units have a evaporator in de airhandler which would freeze up if not enough air passes over that coil.

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#11
In reply to #10

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 4:11 AM

That is a great way to do it I installed Ground source until last year, I think 78 is long enough to work out there, I'm now teaching HRA, in a local college. I like all the answers you received on the controls. Now you are going to have trouble deciding which one to use.

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#12

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 5:31 AM

Use a small PLC and that way you future proof the design because the PLC manufacturer looks after the upgrade path and backwards compatibility.

I have seen the path where people use Arduino or Raspberry Pie but get someone called out who has no idea of them and you are asking for trouble, no stick with a small PLC and if you like you can have a small operator interface display for trouble shooting.

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#14

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 10:48 AM

Does the fan need to be controlled directly by the dampers or can it be controlled by a static pressure sensor in the duct? If you can just maintain constant static pressure by varying fan speed it should work for any combination of open or closed dampers.

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#15
In reply to #14

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 11:26 AM

TROY the pressure sensors are not very accurate and the air pressure tends to fluctuate because of doors opening, fireplace drafting, filters filling up and so on. So, the fan would switch all the time.

In this case, it is much better to use a feed forward control since we know exactly how many dampers are open. The fan has an ECM motor and allows for rather precise airflow control. so, 1 damper open? The ECM tells the fan to deliver 300 CFM. And so on: 2 dampers open? Deliver 400 CFM.

It will be all very stable and independent of external influences. The fan reports back the actual flow based on measured motor rpm (highly accurate) and current (highly accurate). Adjusting flows per register becomes a breeze (yes, pun intended). Don't even need a mechanical flow gauge (not very accurate) or pressure gauge. No drilling holes anywhere. No sophisticated PID controllers needed either.

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#17
In reply to #15

Re: Discrete Inputs and Outputs for HVAC system

04/17/2020 11:05 PM

Paulnederland,

I think you are dismissing the simplest and most long-lived solution too quickly. A differential pressure reading as an input with a design setpoint should be explored. Depending on sensor locations you should have minimal fluctuation because of the various outside influences you mentioned. Then there is no real need for inputs from the local thermostats back to the central unit, which will result in lower maintenance costs, lower installation costs, and simpler troubleshooting in the future. Furthermore, this type of approach can be scaled up/down as needed, without any need for programming or other higher-tech (compared to regular service tech) skills.

The beginning point for this would be to determine a graphical or mathematical relationship between the differential pressure needed to give a constant air flow at a selected open register and the number of open registers.

I could be wrong, but it is worth investigating. --JMM

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#18
In reply to #17

Re: Discrete Inputs and Outputs for HVAC system

04/28/2020 2:27 PM

Hi JMM,

It is interesting to consider the system (design to handle to maximum airflow) and see what it does under low flow situations. As an example, the filter takes a huge part of the "pressure budget" in the design airflow but it disappears at the minimum flow. Same thing for the supply branch and return branch.

So, let's take one register. We need an airflow that is not too high (generates too much noise and discomfort) and not too low (doesn't heat the room). The air pressure behind the fan must be different when the system feeds all 9 zones or just one. So, air pressure is not a good parameter to determine the fan speed settings.

If all registers are about the same and ducts are right-sized, and all zones are sort of the same (within an acceptable range), then the number of zones in an ON state is the best feed-forward input to determine fan speed.

My system has 9 independent dampers and 6 independent fan speeds. In most cases (about 75% of the time) grand mother wants her room at 75 F while I am not using the heater at all. in 90% of the cases, only 2 or 3 zones are active. In 5% of the cases we have a big party and need all zones.

The system design uses the following air flow versus zones:

Active Zones --1----2-----3-----4----5-----6----7-----8-----9

Total Airflow 300--300--360--480--600--720--720-1100-1100

Airflow/zone 300--150--120--120--120--120--103--138--122

Notes:

I have only 6 speed settings

300 CFM is the lowest flow I can get.

1100 CFM is the maximum flow of the system

The ECM fan control measures fan speed and current and calculates airflow in CFM and the off-set tot the desired flow. Even variations in filter conditions, opened doors or even windows (it happens), the control tries to reach the desired flow.

The beauty of hydronic system is that an enormous energy is available for instant release, only limited by the (oversized at 6 tons) heating coil. Oversizing a coil is cheap. The tank is fed by a relative small heat pump (2.5 ton) that runs at high efficiency because it runs very long without interruptions. It turns on at the right time a day (9 AM when in heating mode, 9 PM when in cooling mode). The outside temperatures further improve efficiency. This should fully compensate for the efficiency loss because to the use of water as an intermediary.

The hydronic tank system allows for very rapid heating. I come home, turn the heat ON in one or two rooms and 10 minutes later, the rooms are at temperature. Water temperature is controlled by the air return temperature and adjusted in a few steps to ensure the room temperature doesn't over-shoot.

So, it turns out that variable airflow control fed-forward by damper activation requires hydronics that enable high comfort, extremely rapid temperature adjustments and high efficiency.

It's a bit off-topic on this threat but, this is why i'm doing something unconventional - and this is why i need a robust conversion of number of zone dampers open to desired air flow settings.

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#19
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Re: Discrete Inputs and Outputs for HVAC system

04/28/2020 3:06 PM

Assuming the filters are on the return side, they should not affect the static pressure in the supply duct no matter how dirty they get unless the get plugged beyond the point where you can compensate with fan speed but that issue would be even worse if just controlled by number of open dampers.

If one knows how much static pressure is needed to produce a given flow through a given register, all you need to do is run the fan at a speed to maintain that pressure on the supply side. The return side can be ignored completely. A decent magnehelic that has been properly installed is very accurate.

I have seen hundreds of commercial and industrial systems controlled in exactly this way, some with several hundred diffusers of different sizes throughout the system (usually medium pressure duct with VAV boxes and reheat coils to control individual zones or rooms).

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#21
In reply to #18

Re: Discrete Inputs and Outputs for HVAC system

04/29/2020 10:32 AM

Paulnederland,

Your reply and thinking behind it deserve accolades. I am fully in favor of anything we can do to reduce energy demand while keeping the overall cost reasonable. I wonder, however, if you have access to the air handler's fan curves. I believe these are not a single graph, but a family of them to relate current, air flow, and pressure drop. Thus your reliance on the ECM control's ability to determine air flow is questionable.

It is a relatively trivial problem for HVAC contractors to determine the proper duct size for a given air flow, length, and pressure drop. The system can be designed with 9 individual ducts from a single plenum, which gives identical air flow to each open register at the designed pressure in the plenum. Or you can design with a somewhat oversized supply duct and branches off it so the pressure drop down the length of the supply duct is relatively small compared to the pressure drop down the branches.

With either of these approaches, if you are measuring the differential pressure between the supply air into the air handler and the discharge air at a nearby point on the plenum or duct, you are getting the desired air flow through the system to each open register.

You raise the valid concerns regarding dirty filters, closed doors, open windows, and so on. Windows have to be ignored because when they are open the system has lost efficiency. Doors can be ignored if you have adequately sized return air ducts from each room. Dirty filters can be ignored if you are taking your supply air pressure from a location after the filter and you have planned on a filter that is adequate for the system (not cheap 1" ones), and you use a dirty filter alarm.

Another post mentioned the accuracy and reliability of a Magnehelic type control, and I can endorse that.

I suggest that the above approaches are more than adequate for your system, and follow the KISS principle.

--JMM

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#23
In reply to #21

Re: Discrete Inputs and Outputs for HVAC system

04/29/2020 4:48 PM

JMM,

PSC motors have fan curves. ECM driven fans have an operating map. You can ask for a particular air flow and the system manipulates RPM, and electrical parameters to achieve the air flow set point. My air handler has an ECM motor and I have the operating map.

It's like driving a car. The PSC has fixed throttle settings and your speed is determined by the slope of the hill. The ECM is like cruise control: you ask for speed and it maintains it, no matter what the road looks like - within certain limits.

The lay out my house does not allow me to have 9 separate ducts. There is an opening in the foundation of round 12 inch. I cannot fit three equivalent pipes of 8 inch in there. Had to stay with 12 inch and branch out later to three rooms - each with their own damper.

For the highest airflow, the drop in air supply and air return is about the same as the drop in the register-branch. At the lowest air flow, that air supply branch and air return branch pressure drops to almost 0. That means that if I keep pressure constant, the airflow at the register would vary by a factor of Sqrt(2). That is too much. I need the flow to be below an upper limit (to keep noise and draft down) and above a lower limit (to ensure the room gets enough warm or cold air).

The overall system is actual very straight forward: every zone has its own damper and the thermostat directly controls that damper; there is no need for centralized damper control that handles 9 zones ($ 800). My control for the air handler is a simple system ($ 40) that selects the correct fan air flow. This system has LED's that make trouble shooting done in a blink of an eye - literally. My air handler reports air flow in real time with some accuracy and a stable control that is not easy to achieve with a pressure sensor.

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#22

Re: Discrete Inputs and Outputs for HVAC system

04/29/2020 1:41 PM

Why don't you let the dampers do their thing and control the fan on static pressure, as VAV systems have been done for many years?

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#24

Re: Discrete Inputs and Outputs for HVAC system

04/29/2020 5:11 PM

Thank ALL for the inputs, suggestions and good guidance. Some of the sub-threads went into HVAC design issues and that was also helpful.

I eventually decided to go with the approach that BRAVE SIR ROBERT suggested - after I found a cheap and simple 8 channel voltage comparator with the adjustment capabilities that will enable stable controls and flexibility in case I expand the system. I needed only 6 channels. The spare 2 channels will be used for temperature control. I just need to build a simple voltage divider. My initial solution, using LM3914 would have worked but the standard 8 channel voltage comparator allows me to exercise my laziness; no soldering needed. The LM3914 would have had some undesirable limitations too.

Today, I get delivery of the air handler and I promise that after installation, I will report back on the findings. Might be a while.

Thanks again for all the help. Great people, great website, great help.

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#25
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Re: Discrete Inputs and Outputs for HVAC system

01/18/2021 9:13 AM
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