Allen Bradley HVAC

I'm doing my home automation control with AllenBradley (AB) controls.

I want to add all new furnace controls to the HVAC system(s). The ones on there are 1980's era. The current thermostat is a Rockwell design. It is not accurate, even after routine calibration.

It's a natural gas furnace but it also has cooling fans and a couple blowers with AC motors.. that probably could be servos or industrial drives.

Anyone got ideas that would work?

Easiest for me would be SLC500 (1746), Controllogix, MicroLogix 1500.

I want to have AB Panelview 1000+ or Panelview 400's.

I would do a FANUC 90-30 only if a suitable system couldn't be put together.

I have a friend that successfully integrated PLC HVAC into his home automation.. but his is an electric furnace.

Greg

I do not recommend that you mess with the furnace controls. Leave that to the furnace manufacturer. Especially on a gas furnace. Same goes for the AC. Compressors have controls built in to prevent compressor short cycling, low and high pressure, etc. Unless you are an HVAC expert then chances are very very good that you'll just end up buying a new compressor or two before you get it right - even if you are good with controls. Leave the manufacturer controls alone and furnish the on/off signals to the respective units.

If you want the PLC take over thermostat functions just be aware that a thermostat is not a simple on/off temperature switch. A typical home thermostat contains something known as a heat anticipator. The furnace provides a small current while it is running that ever so slightly heats the bi-metal spring in the thermostat. This compensates for the lag between when the air in your house reaches temperature and when that temperature increase actually warms the thermostat bi-metallic spring. The heat anticipator is adjustable but that is rarely needed since furnace makers have been doing this for decades and figured it out a long time ago. Modern digital thermostats start with a general heat anticipator and "learn" to better compensate for the lag, improving the overall comfort level. If you wanted to use your PLC to perform thermostat functions you will need to build your own heat anticipator.

A Micrologix will be the least expensive and is more than capable. If you use a Micrologix 1100 then the programming software is free.

If you want to save money on the HMIs then take a serious look at the Redlion HMI products. They are a strong favorite of many on here, myself included. Automation Direct also has some HMI products play very well with the Micrologix PLCs, but I spend part of the last week futzing around with an Automation Direct C-More and I have to say it only made me like the Redlion G3 even more. There are quite a few here who really like them though.

Alaric has given you solid advice. I am a seasoned HVAC guy and WILL NOT BUILD RESIDENTIAL GAS CONTROLS. I have built gas controls for other systems like heat treat furnaces, pre forge systems,etc. I DON'T BUILT RESIDENTIAL GAS CONTROLS. You can build a thermostat what will kick the off the shelfs as$. I have a stat in my house that does all kinds of control features on it.

If you have specific plans or wants let us know and we can try to help you with a system.

After really reading your post I have a couple of questions that might spark a few thoughts for you.

Is this system going to be changed out to a new gas furnace? I can only ASSuME you are going to use the existing 80% non induced draft furnace. Standing pilot? Let's think efficiency.

If you put a drive on it how would you run it? Constant torque or speed? Variable speed furnaces are out on the market. One company I have used had dp sensors to control the air flow. You had to run the furnace with the return door off to set the supply baseline. Then close the return door to get a system baseline. The controller l would ramp up the blower to try to keep the 400 cfm per ton needed for proper air flow and temperature exchange. We would reduce the speed to 30% of base speed to keep air circulating and filtered.

You said you have a RA stat now. Why do you think it is drifting? What type is it? Is the drift in the device or in the old thermocouple card? How do you plan on capturing the temperature throughout the home and outside?

Have you thought about zone controls?

Sorry if I overwhelmed you with questions. This is an area that I know alittle about. I might be able to give something to someone on this forum instead of always asking and taking.

The problem I am having is not with the gas supply, gas flow, but that the controls, including the intermitent pilot light controls allow a yellow/orange flame condition.. which makes me think its letting too much gas past the thermocouple, etc. into the burner chamber, heat exchanger, etc. The gas company tested the supply and flow, even after placing the meter outside.. which actually made the problem worse.

The furnace manufacturer, Builtwell, I cannot seem to locate.

The honeywell part is a VR8440M3006, 24v dual gas valve for intermittent pilot light. Honeywell VR8304M3509 replaces VR8440M3006. I'm thinkin about replacing with the new part since the old is nearing 30 years old.

The gas company doesnt want to touch the furnace. The area repair companies don't seem to know much about the gas valve control or the furnace or even the blower controls for the furnace fan.

I would welcome a replacement for the horrible control the off the shelf thermostat provides. Even after some calibration, the 60 degree setting is 55 degrees and the furnace cycles a bunch of times to get there.

The worst problem is the inacuracy and the yellow flame.. which I know should be at worse a blue flame with a yellow tip..

and this with a new thermostat, thermocouple, new pipe, new heat exchanger, new meter, etc..

Greg

I don't see what the big deal is as long as you keep the critical control elements hardwired and bulletproof. Maybe it's that all my experiences with home HVAC has been at least 20 year od technology, but I haven't dealt with one yet which I wasn't certain I could improve on the controls.

Local codes and especially enforcement practices play a critical role in the decision.

I have some of the parts to add duct and duct booster blowers to my old 1978 system, as well as some fresh air recirc from two edges of the house, and have contemplated a VFD for the existing main blower. I had to put a $178 single phase motor on it once already and blow the crud off of it...it's about time to yank it out again...3 phase and VFD would be nice to end the startup whine and the sudden start noise of the 1 phase motor and squirrel cage. I have updated the intake with extra volume and pre-filters, and IIRC there is a n air proving switch that works.

If only I had an Allen Bradley budget, I could modify the widgets I am building for my HVAC HMI I am finishing up at work and have better safety, diagnostics, and most of all comfort and efficiency.

If you are experienced and qualified and can do it safely within your law, then go for it...yes it's rare (perhaps the 1st time) but I humbly disagree with Alaric.

One day, I will crawl up in that dusty attic and install the ducts to my room addition, and some hand lever operated fresh air and recirc controls, but I am holding out for a cheap PLC and a G3 so I can put the logic controlling all of it and my duct booster fans on something smarter than my GE hardwired controls. Mine's all electric, so it's much easier to keep simple though...somewhere I have a jpg of the electrical dwg from that thing...it's a fine example of barely good enough...

For gas, definitely use the right burner control timing relays for your ignition system, and let the PLC supply the enable and monitor the status with digital inputs.

Paul

I have 2 people willing to do the gas side, but not the control side. I would include a safety PLC so that if the main PLC quit running or errored, the other would take over.

A friend of mine actually does work for the gas company.

I'm not exactly new to this idea though, I have built stamping press controls and am still working on building and upgrading that control. (a ControlLogix to a dual slaved Micrologix 1500 with the 6556 Clutch/Brake control (1746 type modules) and PanelView 1000+.

I have also built a FANUC 90-30 system and have some experience with FANUC PMC's for CNC. I also have experience with Mitsubishi drives, servos, some Siemens controls, etc.

I just hope I'm right about the flame quality from the burner(s). Last I figured I waste about $100 worth of nat. gas every week..

Greg

The old furnace is huge.. 360 Max ^2 in. pipe take off. 32,500BTU per hour output from 20,000 BTU input.

I might replace the furnace.. but unless anyone's got $5k-$10k to donate I can't at the moment.

Still the gas company doesn't wanna touch it due to its age.

Greg

Let's start looking at why the flame color is odd to you. Does it look good until the blower starts or from the beginning it looks bad? What is the line pressure before the gas valve opens and after it opens?

Safety PLC is overkill for HVAC. OEMs only use safety gas valve. They don't use redundancy for any of the electrical systems.

gbowne1 said:

The old furnace is huge.. 360 Max ^2 in. pipe take off. 32,500BTU per hour output from 20,000 BTU input.

I might replace the furnace.. but unless anyone's got $5k-$10k to donate I can't at the moment.

Still the gas company doesn't wanna touch it due to its age.

Greg

Click to expand...

That is a pretty good old gas furnace to give you more output btus than input. Keep it.

There are kits available to convert your old standing pilot to auto ignition but the $$$ would be better spent on even the lowest end 85% furnace. Yours is maybe 60% if evereything was in perfect working condition.

I guess you kinda missed the boat on the $1500 tax credit. I had a 95% installed late December and my house has never been so comfortable.

I've often considered adding PLC to my system but only to force a recirc cycle (fan only) when there is no call for heat or cool after 30 minutes or so. I do have a smart relay laying around. Hmmm.

Shawn buy a state for $125 that will do it for you.by the time you count your time at nothing you will be money ahead.

OkiePC said:

If you are experienced and qualified and can do it safely within your law, then go for it...yes it's rare (perhaps the 1st time) but I humbly disagree with Alaric.

Click to expand...

Actually Paul, you're not really disagreeing with me, except maybe for the gas controls part. I said "unless you are an HVAC expert..." and you said "If you are experienced and qualified..."

I just can't recommend to someone I don't know anything about that they do anything at all with residential furnace controls, and pass on a warning that they don't assume that compressor control is just starting/stopping the compressor motor, and that they can very quickly ruin a compressor.

I'm not comfortable working with gas furnace controls myself but I'm confident with refrigeration controls. I installed commercial AC systems for my first couple of years of college and have done controls on several industrial systems so I'm comfortable working with them. I've been called upon a few times to fix improperly controlled compressors, cases where I think the original control programmer thought "its just a compressor, how hard can it be..." and didn't bother to take the time to find out how to control a compressor and decided to go at it his way. BTW, compressor control isn't hard, I've covered it in a few posts that can be found with a search.

In the residential HVAC industry there is a tremendous amount of room for improvement. For twenty years I've been wishing the residential compressor manufacturers would come out with variable displacement compressors and proportional unloading valves and compressors suitable for use on a VFD. Those are just starting to appear on the market and are IMO long over due. I agree that the typical home HVAC system can be dramatically improved upon. As far as the rest is concerned, a good controls guy can dramatically improve thermostat function, air circulation, zone control etc.

Greg, after reading the rest of your posts here it really seems to me that the best money you can first spend is to update that furnace with a modern unit. While that may be more money than you might spend on controls it is the biggest bang for your buck. Most of the money you spend on controls at this point won't get you any significant improvement compared to the outlay. Basic triage, stop the hemorrhage (the large natural gas waste) first, then with the money you save you can make some later improvements.

The age of the furnace may not be an issue. Some of the old ones are nearly as good as new. Seems to me that if the heat exchange materials are intact and not thin you may be OK.

I have doubts about running variable speed on fan for "house air side" of heat exchanger. You may create hot spots in heat exchanger if you run fan too slow.

As far as pilot flame visual appearance it would be better if it is blue of course. I would put a pressure gage in gas line feeding the furnace and one downstream of the regulator (if there is one) feeding the pilot lite circuit.

Why did the gas company replace the meter? Or did they replace the regulator also. Did they replace it cause they had no idea of what to do but had to do something to make the customer happy?

If you are sure on the numbers ie 20K BTU input and 35 K BTU output please
1. Have these numbers verified by independent testing lab
2. put the furnace on a truck and ship it to me.
I think you have them backwards.

Dan Bentler