Controling tempature in a residential area

[Lancie1]

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?

Any type of ventilated box will work, maybe an old thermostat with guts removed, or a small electronic project box from Radio Shack or Allied Electronics, or you can have your own custom made from wood, metal, or plastic.

...Also planning on using an AD 205 PLC with a 250 processor, and was looking at the RTD input card. It looks like the maximum number of probes per card is 4, so can you use multiple cards in a rack?

Yes, as many cards as the rack has room for. If you get the DL-205 9-slot rack, with a 250 CPU, 2 digital Input cards, 2 digital Output cards, then you would have room for four of the 4-channel Thermocouple or RTD cards (16 total temperature inputs). Normally I would say that you need about twice as many digitial inputs as ouptuts, but I assume that most of your inputs will be the termperature measurements.

I agree with Dan. Thermocouples would be much cheaper to buy (or make), and will provide enough accuracy for HVAC.

I have welded up many thermocouples using a homemade rig. Many thermocouples used to be made this way. Use a fused circuit, wear gloves and goggles, and don't try this if you are not agile with steady hands.

Pour a half-inch of mercury in the bottom of a quart mason jar, then add 2" of mineral oil floating on top of the mercury. Twist your bare T/C wires tightly together for 1/8" to 1/4" length, forming a small ball at the end. Wire-nut both the other ends to the hot black wire in a 120 volt cord, drop the 120 volt white neutral into the mercury near one side of the jar, apply power, then lower the T/C wire ball into the mercury (about 1.5 to 2" away from the neutral wire). The small arc will blow away the mercury, killing the circuit. It will weld, then you pull it out, with the oil quenching and protecting the weld from oxidation. It works every time.

Be warned again, the lowest AC voltage you can input into the DL-205 is 110 volts AC. You WILL NOT be able to connect it to your existing 24 Volts AC HVAC control circuit (thermostat circuit) without using a relay. See this list:

http://web1.automationdirect.com/adc/Shopping/Catalog/PLC_Hardware/DirectLogic_205/AC_I-z-O

If you do want to tie to your existing controls without relays or diodes, you can actually get a DL-105 that has 6 inputs that accept 24 VAC. The model F1-130DA is one such, and has 10 inputs, 8 outputs, and a built-in 24 VDC auxiliary power source:

http://web1.automationdirect.com/static/specs/f1130da.pdf

If you want interface directly to 24 VAC with many Inputs and Outputs, then consider buying an Alerton Bactalk VLC (a PLC made for HVAC control systems), or one of the other Building Management System controllers. Learning to program and set up one of these systems would be a valuable trick in your tool bag.

http://www.alerton.com/Products/BACtalk/Field_Controller_Level/VLC-550.asp

 

[Joe Boxer]

Thank you for the input, was very helpfull

now do have a really stupid question about thermocouples. this is probably the only item never messed with. Understand the part where you twist the end to sense the tempature and how to tie them into the plc input card, but like when I run the lead down inside the wall, and bring the end out about 5' up from the floor, is how much of the tip do you bring out to be exposed? know sound stupid, but was wondering

also is there any sample AD code showing thermocouple input, and logic. done a lot with analog inputs as well as outputs is there any similarity to this code compared to thermocouples code?

hope I asked his right but since its after midnight I blame brain fatigue

 

[danw]

The junction of the two wires is the sensing point of a T/C. Being an exposed junction, it will have a fast response. That's not always a plus. If you open a door in the winter time (in a northern climate), the in rush of cold air will be seen by an exposed junction T/C almost immediately.


The more you hide the bare junction the more thermal response lag you get.

I like Lancie1's idea of hiding a TC in a gutted thermostat. Thermostats have openings to allow the guts to sense air temperature, but stats look like they belong on a wall.

Dan

 

[Peter Laing]

Hi Joe,

I am well into a similar project.

It's been a few years in the making. I planned on doing it about 10 years ago, but never got aroun to it until this year.

A SLC5/03 in 7 slot rack is in the furnace room. There are quite a few analog temperature sensors (LM35s), which connect to a NI4 module via several 16-1 analog multiplexers (ADG507s). A thermocouple (via a AD595) monitors flue temperature. There is a RIO scanner in the rack, which interfaces two drops of Flex IO. An outside temp sensor provides "outside reset" for control.

We have hot water radiant floor heating, with a Viessmann Atola boiler. The original controls have been replaced by the PLC, except for a pair of hard wired aquastats on the boiler.

A PID loop to controls a 4 way mixing valve to set the floor loop temperature. Individual relays control zone actuators.

A VB program, on top of a MySQL database monitors and provides a setup interface. See attached pictures for screen shots.

If anyone is interested I can elaborate further on this.



Cheers,

Peter

 

[akreel]

I hear Fred is looking for campaign workers all over the country.

Funny you should say that. A friend of mine moved out to DC to help out on the campaign.

 

[CroCop]

The junction of the two wires is the sensing point of a T/C. Being an exposed junction, it will have a fast response. That's not always a plus. If you open a door in the winter time (in a northern climate), the in rush of cold air will be seen by an exposed junction T/C almost immediately.


The more you hide the bare junction the more thermal response lag you get.

I like Lancie1's idea of hiding a TC in a gutted thermostat. Thermostats have openings to allow the guts to sense air temperature, but stats look like they belong on a wall.

Dan

Software could compensate for the exposed TC if Joe wanted it to.

 

[Peter Laing]

Just a follow up on temp sensors...

I've been looking for an easy way to weld TCs, but I think I'll pass on the "mercury in a bottle" method. I'm sure it works well, but is it worth the hazard.

I've been using National LM35's for years now... Epoxied to a magnet on the oil pan of my '88 Taurus way back in '89. Son still uses the car, the oil temp sensor still works fine, after some 19 years.

The only thing that you have to watch for on a the LM35 is cable capacitance. For any cables longer than a few meters, you need to use a RC snubber across the output. Something like 100R in series with 1uF. If you don't, it'll oscillate on you.

For temp control "anticipation" (PID rate) you want to have high resolution. That's a nice thing about the analog sensor ( as apposed to a digital one). I've used an op-amp to give a X10 gain, which gives an output of 2.0V at 20C. Couple that to a 16bit analog input, and you have 1/100degC resolution.

I solder pigtails on each lead, heatshrink individually, then over all three. One more heatshrink over the whole thing does it. Use epoxy for an ugly location.

I've fitted them in existing thermostat boxes. That works fine too.

Another trick is to epoxy one to the back of a small heatsink, such as one for a 14 pin DIP IC.
The heatsink is mounted exposed on a flat bakelite switch plate cover. Doesn't look too bad, and works great.

For about $2.50 each or so, they're hard to beat.

Cheers

Peter

 

[OkiePC]

Any errors you encounter with the thermocouple inputs can be corrected with scaling controls on the "engineering page" of the HMI.

That is a perfectly practical application of code to hone your programming skills, and apply indefinitely.

I am most curious about your damper actuators. Are they going to be controlled by digital outputs?

Paul

 

[Joe Boxer]

Any errors you encounter with the thermocouple inputs can be corrected with scaling controls on the "engineering page" of the HMI.

That is a perfectly practical application of code to hone your programming skills, and apply indefinitely.

I am most curious about your damper actuators. Are they going to be controlled by digital outputs?

Paul

was planning on digital outpus for the dampers, then use ice cube relays to control the hvac equipment control relays.

 

[OkiePC]

So, you'll have an open and a close command?

What are the actuators you are using?

 

[Joe Boxer]

So, you'll have an open and a close command?

What are the actuators you are using?

yes planning a straight on / off for the dampers

not planning on using any type of analog in the program, just thermocouple temperature input so I can control the dampers

actuators i take you mean the dampers, thinking of using the basic 6" 24v. motorized damper

 

[danw]

I'd recommend that whatever the current heating/cooling system is in place in your house be left there, just for resale purposes. Many buyers nowadays hire an inspection service and HVAC controls that are way out of the ordinary are likely to raise a red flag.

My Dad has had experience with such. He retired 25 years ago when he moved to Arizona and built a solar house. A former work colleague had moved there a year earlier and had drawn up the plans and had built two such houses, one for himself, one to sell. The colleague built his own circuit boards for heating/cooling control, using TTL and analog chips. My Dad did the heating/cooling control in his house with standard thermostats, several relays and a toggle switch. Not because he couldn't do it the other way, but he prefers the simplicity and reliability of relay logic.

The custom circuit board system had persistent reliability problems due to lightning strikes taking out the analog temperature sensor circuits (the T/C wire runs were long antennae). The owner who purchased the for-sale solar house hired my Dad to replace all the special circuit boards with my Dad's thermostat/relays/toggle switch components about 2 years after he purchased the house, due to the unreliability of the custom system. That house has sold twice since without problem - the heating/cooling system looks ordinary with its thermostat on the wall and a toggle switch labeled 'summer/winter'. The small panel that hosues the relays has a wiring diagram pasted on the inside of the door that looks like an appliance wiring diagram that any heating/cooling guy can figure out.

The colleague's house recently went on the market, but according to the real estate gal who called my Dad about changing out the heating controls, potential buyers are wary of the maintainability of the heating/cooling system, having gotten reports from the instpection services that point out that the whole thing is a one-off, custom system with who-knows-what heating cooling controls. Granted, this is not a hot market for real estate in Arizona, but I can understand the reluctance when there are other properties for sale . . .

Now, I'm not saying a PLC won't be reliable and that relay logic is a better way. I am saying that a PLC is 'custom' to the extreme. Having a PLC for heating control would mean that an ordinary property owner could not call any of the heating guys in the yellow pages to fix his heating or cooling. In fact, it even rules out the commercial HVAC guys, too, because they don't use PLCs either. It would take someone with very specialized knowledge AND the development software/cable/etc.

But, if you keep that heat/cool thermostat on the wall with the wires disconnected, so you can re-connect it whenever it is you decide to sell the house down the road, then you can take your rack & I/O and what-not PLC controls with you. Your house buyer can be blissfully ignorant of the great control scheme you had, because the thermostat does its thing and ignorance is bliss, isn't it?


Dan

 

[Joe Boxer]

But, if you keep that heat/cool thermostat on the wall with the wires disconnected, so you can re-connect it whenever it is you decide to sell the house down the road, then you can take your rack & I/O and what-not PLC controls with you. Your house buyer can be blissfully ignorant of the great control scheme you had, because the thermostat does its thing and ignorance is bliss, isn't it?
Dan

Thats a good idea, I tend to see this being so cool, that others would be impressed. Where potential home buyers could see it as being a head ache to have deal with.

I have already considered still using the original thermostat to control the system, and use the PLC to monitor room temperature, and control the dampers.

I figure that as rooms come to set temperature, closes the damper. Causing more air flow into the rooms not to set temperature. Where the original thermostat still controls the main part of the house.

Then monitor air flow, make sure not over loading the system, and also warn you its time to change the air filter.

not sure this will work, I'm basing my theory on my first choice of career as a HVAC technician, took 2 years in technical school, then 4 years working for a contractor started as a installer, then service work. did this then went onto working maintenance in a cold storage since then became a industrial electrician. Then onto doing PLC work for the last 15 years.

 

[John Soltesz]

Thats a good idea, I tend to see this being so cool, that others would be impressed. Where potential home buyers could see it as being a head ache to have deal with.

I have already considered still using the original thermostat to control the system, and use the PLC to monitor room temperature, and control the dampers.

I figure that as rooms come to set temperature, closes the damper. Causing more air flow into the rooms not to set temperature. Where the original thermostat still controls the main part of the house.

Then monitor air flow, make sure not over loading the system, and also warn you its time to change the air filter.

not sure this will work, I'm basing my theory on my first choice of career as a HVAC technician, took 2 years in technical school, then 4 years working for a contractor started as a installer, then service work. did this then went onto working maintenance in a cold storage since then became a industrial electrician. Then onto doing PLC work for the last 15 years.

You might also consider the dampers to fail open. If there is a total failure (more for future owners) they might not notice it not working efficiently and not cuss out that damn designer. Also consider if the PLC is shutdown that it will revert to simple controls incase the PLC looses it's brains. If you are gone who will be able to reload the program?

 

[Joe Boxer]

You might also consider the dampers to fail open. If there is a total failure (more for future owners) they might not notice it not working efficiently and not cuss out that damn designer. Also consider if the PLC is shutdown that it will revert to simple controls incase the PLC looses it's brains. If you are gone who will be able to reload the program?

plan to use ice cube relays to feed the dampers, so when the PLC sends an output to close the damper, will turn on the relay, in turn the N/O contacts of the relay will power the damper to close.

if the the PLC fails all ice cube relays become deengerized then any N/O contact would be open, and in turn dampers will all be open.

will also put in a switch to go from auto to manual. basically take the PLC out of the loop.

plus any problem with the dampers, any HVAC technician could handle