Controling tempature in a residential area

Joe Boxer · Dec 2, 2007

Hello

I have done a lot of programing with AD and some Allen Bradley in the past but have nothing anything for last couple of years due to a disability

Now looking to get my mind right again by designing a program using AD to do some home automation

I want to start with the HVAC in the house by zoning different parts of the house using thermocouples in different rooms. then have the PLC TO control dampers in the supply air vents.

Plan to use a DL205 with a 250 CPU will have 24v outputs to operate the dampers a nd the HVAC system. Use a touch screen to input the data.

My biggest problem is how to do the program using thermocouples to provide temperature inputs. Done analog programing in the past but doing thermocouples is not in my knowledge.

Anyone have any information out there?

John Soltesz · Dec 2, 2007

Joe Boxer said:
Hello

I have done a lot of programing with AD and some Allen Bradley in the past but have nothing anything for last couple of years due to a disability

Now looking to get my mind right again by designing a program using AD to do some home automation

I want to start with the HVAC in the house by zoning different parts of the house using thermocouples in different rooms. then have the PLC TO control dampers in the supply air vents.

Plan to use a DL205 with a 250 CPU will have 24v outputs to operate the dampers a nd the HVAC system. Use a touch screen to input the data.

My biggest problem is how to do the program using thermocouples to provide temperature inputs. Done analog programing in the past but doing thermocouples is not in my knowledge.

Anyone have any information out there?

This sounds just like a project one our younger EE's wanted to do on a new house he was building. He got overwelmed with all the other stuff that needed to be done so he never got as far as the design. He was going to use AD also. We did look into some devices and one was temperature sensors. I would stay away from thermocouples. An inexpensive one would not have the accuracy out of the box. Go with an RTD. An RTD would be much closer in accuracy from one to another and if you don't use a transmitter you could run the correct wiring direct to the PLC input card. A thermocouple needs T/C extention wire from the T/C to the PLC input card or a transmitter that would just need a twisted pair. Add at least $90/point for a cheap transmitter. Do a search into the differences between a T/C and RTD. We get higher grade T/C's and there still can be a difference of 3°F from nominal, cheaper grade easy 6°F. Not good for a house. I think he may have found an RTD in a nice looking room enclosure.
The programming isn't any different once you get the signal into the PLC. If it is a T/C or RTD card each will have specifics to correctly convert the sensor signal to a real temperature number. Once you have that number, your process variable (PV), you just use it in comparitors to do what is needed. If PV1 is above SP1 open damper1. You could do that for each room and if no room requires heat/cool shut down A/C or heater. Lots of options.
Good luck on your project.

Joe Boxer · Dec 2, 2007

my plan is to take temperature readings in the main part of the house, to control on/off part of the HVAC system in parts seldom used try to maintain them at a set temperature

think will need a pressure device to monitor the air flow in the main trunk maybe compare air pressure of the return to the supply air pressure monitor when one needs to change the filter.

want to be able to use a touch screen monitor temperatures in each room, outdoor temperature, and the temperature of the pool. be able to change temperature by room

whole plan is eventually set up sensors on doors and windows for intruder alarm. Set all the lights in the house to be controlled by the touch screen

yes I realize way to much time on my hands, but after 2 years sitting in a wheelchair and watching law and order on TV. that there has to be something more productive to do with my time

Lancie1 · Dec 2, 2007

Hello, Joe,

I am glad that you are feeling like tackling some jobs again.

I have gotten involved with a couple of air conditioning projects on this forum in the past. One of the big difficulties in using a PLC is that most HVAC systems use 24 volts all right, but it is 24 volts AC, not DC. There are not too many PLC's that can accept that voltage directly as an input. There are some that will allow 24VAC as an output.

I thought it was a smart idea to make the design fail-safe, due to the fellow's inexperience in working with PLCs. My soultion was to keep the normal 24 VAC control thermostat, then use relays to intercept that signal between the thermostat and the HVAC unit. This allowed the HVAC to continue in normal mode and keep running, even if the PLC was dead, not in run mode, or disconnected.

It must have worked okay. He sent me donation credits for this church that had the 11 HVAC units.

leitmotif · Dec 3, 2007

Joe Boxer said:
my plan is to take temperature readings in the main part of the house, to control on/off part of the HVAC system in parts seldom used try to maintain them at a set temperature
Joe
in initial post you mentioned air dampers. So I assume you have forced air. All is well and dandy IF you have enough rooms calling for heat - when it is only one or two you will be choking the discharge or suction of the blower. Of the two the suction is preferable - it unloads the blower and you will not get duct rumbles. I would think it best to have a temp sensor for each room and have each room stand alone - not referanced to main portion of house.

think will need a pressure device to monitor the air flow in the main trunk maybe compare air pressure of the return to the supply air pressure monitor when one needs to change the filter.
I would measure the dP directly across the filter itself. Then maybe another pressure transmitter on the outlet of the furnace.

want to be able to use a touch screen monitor temperatures in each room, outdoor temperature, and the temperature of the pool. be able to change temperature by room
Outdoor temp will allow you to use a temperature setpoint (or maybe damper opening) for each room based on outside temp.

whole plan is eventually set up sensors on doors and windows for intruder alarm. Set all the lights in the house to be controlled by the touch screen
I would leave at least one light in each room in case you have to work on PLC or in case of emergency (battery backup??)

yes I realize way to much time on my hands, but after 2 years sitting in a wheelchair and watching law and order on TV. that there has to be something more productive to do with my time

Gee you could become a lawyer??? Nahhhh I think your project is a better idea and probably more fun.

Dan Bentler

TConnolly · Dec 3, 2007

Home thermostats incorporate something called a heat anticipator. People are surprisingly sensitive to heat variations in a home and what happens is that people feel comfortable before the temperature sensor actually registers that the air temperature has reached the comfort value. So a very small current is passed through the bi-metallic spring to warm it up a little in antipation of reaching the desired temperature. The heat anticipator can be adjusted by a service technician. Modern digital thermostats adapt to anticipate the heat and don't need adjustment. In any heating control system that you design you should build in some kind of anticipator, otherwise your room will feel either drafty or too great of a hot/cold swing to be really comfortable.

AJZ · Dec 3, 2007

I have similar plans using a micro controller. Two things to watch for are:

1. Prevention of short cycling of the furnace or AC. AC needs to be off for at least five minutes before starting again.
2. Prevent overheating of furnace or freezing of AC evaporator when the air flow is too low. I am planning to install two additional sensors in the furnace. One measuring the temperature of the air entering the furnace and one measuring the air coming out of the furnace. If the temperature rise is too much or too low, turn off the furnace or ac.

John Soltesz · Dec 3, 2007

Joe Boxer said:
my plan is to take temperature readings in the main part of the house, to control on/off part of the HVAC system in parts seldom used try to maintain them at a set temperature

think will need a pressure device to monitor the air flow in the main trunk maybe compare air pressure of the return to the supply air pressure monitor when one needs to change the filter.

want to be able to use a touch screen monitor temperatures in each room, outdoor temperature, and the temperature of the pool. be able to change temperature by room

whole plan is eventually set up sensors on doors and windows for intruder alarm. Set all the lights in the house to be controlled by the touch screen

yes I realize way to much time on my hands, but after 2 years sitting in a wheelchair and watching law and order on TV. that there has to be something more productive to do with my time

You may want to use a differential pressure (dP) transmitter across the filter to determine the need to replace the filter. We do that on our large systems. Now, if you are limiting the air flow you may not get a proper reading. We have 11 make-up air system heaters on one building with variable speed drives to maintain a specific building pressure. The OEM tried to monitor the filters but with the variable speeds you could only do it at max speed. Just something to keep in mind. Check with omega.com for some inexpensive sensors.
I think a lot of us would like to do something like this.

Lancie1 · Dec 3, 2007

...but after 2 years sitting in a wheelchair and watching Law and Order on TV, that there has to be something more productive to do with my time.

Who knows, being familiar with Fred Thompson on Law and Order might turn out to be valuable. Ole' Fred has never played a role where he said things that he didn't actually believe. For me, his greatest role was playing himself in Marie. Marie Raghetti's life expectancy wasn't worth 2 cents until Fred got involved. I hear Fred is looking for campaign workers all over the country.

Joe Boxer · Dec 3, 2007

the part having a coundry about about 2 years ago put in a brand new Lenox system its a 12 seer conditioner and a 2 stage gas furnace. Know the thermostat now controls the low stage when more heat is required switch to the high stage.

know that I will need to design this in the PLC program. I don't see any difficulties but maybe just being optimistic.

Already figured on the built in heat anticipation in the current t/stat figured with the PLC would have it automatically adjust the user set temperature by a couple of degrees and will put in 5 minute time delay to prevent short cycling.

Also think this will be good to get my mind working and have the house on the market, would think done right looks good would be a good selling advantage, plus we are planning on building a new house next that I will be able to incorporate the PLC into the house design so if everything goes right will have all the bugs worked out

Joe Boxer · Dec 3, 2007

Ok here is question was looking at the RTDs was wondering if any suggestions of placements of the probe in a room. preferably on a wall, and a way to camouflage it?

also planning on using a AD 205 PLC with a 250 processor, was looking at the rtd input card looks like maximum of probes is 4, so can you use multiple cards in a rack?

John Soltesz · Dec 4, 2007

Joe Boxer said:
Ok here is question was looking at the RTDs was wondering if any suggestions of placements of the probe in a room. preferably on a wall, and a way to camouflage it?

also planning on using a AD 205 PLC with a 250 processor, was looking at the rtd input card looks like maximum of probes is 4, so can you use multiple cards in a rack?

You might do some research in the HVAC world as to proper position of the sensor. I'm sure there are good guidelines for that. Also look for designer covers for the sensors.
I've used the (as TI)AD405 and 305's and I believe the 205 has a means for additional cards. Whether a real rack or just adding a card on the end of the stack to some kind of limit. Their web site has tons of info. The catalog has good information. Or just call them; they seem to be quite helpful. I have used their touch screens and they do a nice job for the cost. The older ones were a little dim, but that was 6 or more years ago.

harryting · Dec 4, 2007

My 1.5 cent...

This in reality is a lot more difficult than it seems. One the reason that has been already mentioned is that the BMS and PLC world diverged many years ago and two world has quite a different standard. Using PLC will work but it cost a quite bit more.

I did find one vendor that sells a damper controller that utlize modbus protocal. the issue I had with it is that it need a gateway to program it which cost about $1k, I think. Of course, it would be a lot cheaper if you can find just a open/close damper.

A good resource is the Controls Forum at hvac-talk.com

Joe Boxer · Dec 4, 2007

harryting said:
My 1.5 cent...

This in reality is a lot more difficult than it seems. One the reason that has been already mentioned is that the BMS and PLC world diverged many years ago and two world has quite a different standard. Using PLC will work but it cost a quite bit more.

I did find one vendor that sells a damper controller that utlize modbus protocal. the issue I had with it is that it need a gateway to program it which cost about $1k, I think. Of course, it would be a lot cheaper if you can find just a open/close damper.

A good resource is the Controls Forum at hvac-talk.com

realize there are cheaper systems out there, do appreciate the suggestion, but really want to do this with a PLC.

More for the challenge of designing the program, the mental part, ever since becoming disabled the physical part of doing anything has to be done by a contractor. Will just need somebody to do the wiring and install the dampers. Just need to do the actual program, then to over see the wiring.

Money is not a problem, just the actual boredom of just sitting around. Have the direct soft software and the software to do their touchscreen design. know I have the experience to do the programming, done a lot of analog programming in my previous life, just the RTD and thermocouple programming is a new ball game.

danw · Dec 4, 2007

You've got your goal set on doing this, so I'll chime in and advise you to stick with thermocouples, versus RTDs.

Thermcoouples are much less expensive than RTDs. Yes, the
ANSI spec shows that a standard limit of error T/C can have 3° or 4° error, but that is not typical, particularly when the T/Cs have not been subjected to high heat (>600°F). The easy work around is to hang a thermometer that you trust next to the TC and compare the thermometer's reading with the PLC reading, then either add or subtract an offset/bias value or put the offset value in as part of a calibration. The offset will not change over time because the TC isn't being stressed at room temperatures, like it is in high heat.

You can make or fabricate your own TC from TC wire, stripping the insulation and twisting the bare ends together. PT100 RTDs are purchased in a sheath, and just cost more.

Yes, you have to run TC wire through from the measuring point to the PLC, but presumably you'll have to run copper wire for the RTDs, so it's running one or the other.

Yes, the TC wire is TC wire, but the RTD extension wire should be twisted pair, not romex, so the cost difference isn't huge.

Dan

Controling tempature in a residential area

Joe Boxer

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