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Yet another HVAC monitor question


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#1 SethP

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Posted 22 January 2008 - 03:56 PM

I was poking around my furnaces with a multimeter, and the thermostat side of the transformers are not connect to ground. I get around 24 volts AC on the transformers themselves, but zero when checking either output side against ground.

Can I just connect one side of the transformer to ground? Is there anything I need to be aware of?

Related: Can I connect the 1-wire ground to house ground? Then I only have to run a single wire from the furnace.

Thanks!

Seth

#2 Timoh

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Posted 22 January 2008 - 09:04 PM

What do you mean by output transformers?

Typically a HVAC will have a single 24vac transformer for providing a voltage to work your thermostat. The thermostat will energize relays with the 24vac when the function that that relay controls is required. So if the thermostat calls for the fan, the relay that controls the fan will be supplied with 24vac. This will close a 120 circuit to the fan motor. Like wise for if you use a heat pump or AC. 24vac will be applied to a relay that controls or 120 or 240 supply to the compressor.

Are you sure you are not confusing transformers with relays?

If you are looking at monitoring HVAC, just look for the 24vac signals coming out of the thermostat going to the relays.

I don't know anything about 1-wire, but I would be surprised if you could use your house ground. I suspect the the 1-wire bus requires a fairly "stable" ground. The different electrical characteristics of the ground vs the bus path might cause problems. + any noise that might be on the ground from motors, fluorescent lamps, etc might cause issues too.

Tim

#3 SethP

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Posted 22 January 2008 - 11:49 PM

I can see that I wasn't clear. My heat furnace has 6 thermostats connected to it. Each one wires into a box that contains both a transformer and relay. Each transformer has an "input" side of 120vac, and an "output" side of 24vac. The HVAC monitor has an input to measure the voltage. To connect it, you must connect one wire to the ground terminal on the HVAC monitor and one to the voltage input.

I can measure 24vac across the transformers on the furnace, but neither side of the 24vac is connected to ground -- if I measure between either output wire and ground, I get nothing.

So, how do I wire this into the hvac monitor? I assume I need to ground one of the output wires (24vac), also connect that ground to the hvac monitor ground? Does this make sense? If so, that answers my other question -- I have no choice but to connect the HVAC monitor ground to house ground.

Thanks!

Seth

What do you mean by output transformers?
Tim



#4 nachbar

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Posted 23 January 2008 - 09:17 AM

So, how do I wire this into the hvac monitor? I assume I need to ground one of the output wires (24vac), also connect that ground to the hvac monitor ground? Does this make sense? If so, that answers my other question -- I have no choice but to connect the HVAC monitor ground to house ground.


I just received one of the HVAC monitors yesterday. They have a common ground for the four input circuits, and the ground connects to the one-wire ground. There is also a connector for the optional +5 volts DC for the one-wire system, and a connector for the one-wire data line. In addition, there are two RJ-45 connectors that you can use instead of the terminals for the one-wire connectors.

So, you are correct that you will have to connect one side of each of your 24 VAC transformers 1) together and 2) to the one wire ground in order to use this device. That should probably be safe as long as the 24 VAC circuits are not grounded on either side anywhere else. Because you tested with your voltmeter and could not see a voltage between EITHER wire and ground, the 24 VAC circuits do not appear to be grounded.

Note that, if you connect to ground at TWO places, you can create a ground-loop, which can generate a current and cause problems. Also, be aware that, as it is, if someone contacts one of the wires in your 24 VAC circuits or your one-wire circuits, no current will flow through them. However, with one side grounded, the potential would now exist for them to get a current from your 24 VAC circuits. Of course, if there is a fault in one of the transformers and the 24 VAC circuits become connected to the 120 volt lines, suddenly all of your one-wire circuits will become live, which you won't know until somebody touches them.

Another option might be an opto-isolator connected to the 24 VAC circuits, with the HVAC monitor connected to the output of the opto-isolator. However, the HVAC monitor is designed to convert AC to DC and cut the voltage to 9%, to reduce the voltage so the DS2450 can read it. You could, however, get the monitor kit and put it together yourself, changing the resistor configuration to read the voltage from the opto-isolator (and hobbyboards.com might be willing to sell you an HVAC monitor with that configuration).

I wouldn't try using the house ground as one of the wires in your "one-wire" system. Having the second wire in the same cable reduces the effect of electromagnetic fields that your cable travels through. Dallas Semiconductor has some very clear monographs about how one-wire works, and how to make it more reliable. I have been using CAT-5 cable, which is cheap, durable, and easy, and that has been working great.

It goes without saying that I TAKE NO RESPONSIBILITY FOR INJURY TO PERSONS OR THINGS RELATED TO YOUR FOLLOWING THIS ADVICE. I AM NOT AN ELECTRICIAN, AND THERE MAY BE NATIONAL OR LOCAL ELECTRICAL CODES THAT FORBID CONNECTION BETWEEN GROUNDS OR OTHER WIRES. ELECTRICAL CIRCUITS CAN BE DANGEROUS; GET HELP IF YOU ARE UNSURE.

Edited by nachbar, 23 January 2008 - 09:19 AM.


#5 Timoh

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Posted 23 January 2008 - 10:52 AM

I'm not so sure that tying outputs of multiple transformers to a common ground is a good idea. While I can't think of any reason other than those noted below not to do it, it just doesn't feel right.

Taking the outputs & isolating them in some fashion is what I would do. I would pickup some 24vac relays, wire them to your transformer 24vac and then run a separate common 24vac through the relay contacts and onto the monitor board.

Not the easiest, but on the flip side you are not changing how your furnaces are wired.

Tim

#6 SethP

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Posted 23 January 2008 - 01:35 PM

Thank you for all the good feedback. To clarify my question about connecting the 1-wire ground to house ground: I use only cat-5 cables for my 1-wire devices; I was just wondering if there was going to be an issue connecting my existing 1-wire ground to house ground. I am not looking to use the house ground in place of a proper ground wire going to each device. The question answers itself if I connect the HVAC monitor to ground.

I do like the idea of wiring in a relay or opto-isolator. With that, I can just use my DS2406 inputs. That also potentially gives me more channels -- ideally I want to monitor the thermostats, as well as actual furnace on-time.

Does anyone know of an opto-isolator that will take 24vac as input? I'm checking into relays as well.

Thanks!

Seth

#7 nachbar

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Posted 24 January 2008 - 02:08 AM

Does anyone know of an opto-isolator that will take 24vac as input? I'm checking into relays as well.


I would try the Clare solid state relays, on page 130 of the Jameco catalog. They take about 5 mA of current through the built-in LED. You can just use a resistor to get the required current from whatever voltage you have as input. About $4 for a two-relay chip.

You can get the full spec sheet from either the Clare or Jameco websites.

#8 nachbar

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Posted 24 January 2008 - 04:54 PM

So, you are correct that you will have to connect one side of each of your 24 VAC transformers 1) together and 2) to the one wire ground in order to use this device. That should probably be safe as long as the 24 VAC circuits are not grounded on either side anywhere else. Because you tested with your voltmeter and could not see a voltage between EITHER wire and ground, the 24 VAC circuits do not appear to be grounded.

Note that, if you connect to ground at TWO places, you can create a ground-loop, which can generate a current and cause problems. Also, be aware that, as it is, if someone contacts one of the wires in your 24 VAC circuits or your one-wire circuits, no current will flow through them. However, with one side grounded, the potential would now exist for them to get a current from your 24 VAC circuits. Of course, if there is a fault in one of the transformers and the 24 VAC circuits become connected to the 120 volt lines, suddenly all of your one-wire circuits will become live, which you won't know until somebody touches them.

It goes without saying that I TAKE NO RESPONSIBILITY FOR INJURY TO PERSONS OR THINGS RELATED TO YOUR FOLLOWING THIS ADVICE. I AM NOT AN ELECTRICIAN, AND THERE MAY BE NATIONAL OR LOCAL ELECTRICAL CODES THAT FORBID CONNECTION BETWEEN GROUNDS OR OTHER WIRES. ELECTRICAL CIRCUITS CAN BE DANGEROUS; GET HELP IF YOU ARE UNSURE.


In thinking about this some more (and again, I am planning for my own installation) I think that, if I did not use the optoisolator, I would put a 100,000 ohm (100K) resistor between the ground of the HVAC monitor and each of the grounds of the 24 volt control circuits. That should have minimal effect on your reading (the monitor already puts 1.1 Mega-ohms in that circuit, so the additional 100K should only reduce the reading by about 9%), and it will provide insurance against significant current traveling through your connection (even at 120 volts, a max of 1.2 mA and a max of 0.14 watts)

I haven't tested that configuration, however.

Edited by nachbar, 24 January 2008 - 04:57 PM.


#9 SethP

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Posted 24 January 2008 - 11:17 PM

I'm looking to monitor both the thermostats and the furnace on/off. The furnace has a simple thermostat input, so I took it apart to check it out. It has a 24vac transformer, relay, and line voltage coming in and out. The transformer & relay are on a circuit board, and there is no way I can get to anything without soldering, and I don't want to do that. I could theoretically splice something into the line voltage, but there are no punchouts, and I don't want to do anything really outside code. Years ago my dad wired a standard outlet into the line voltage that controls his furnace, and plugged in an analog clock. He could then very easily determine how long the furnace was on.

In my case, the only thing I have easy access to is the thermostat line, as the terminals are on the outside of the control box. There are two screws; connect a wire between them and the furnace tunrs on. The thermostat relay connects these two terminals. I tried taking two 12 volt relays in series, hooked them up to power, and neither tripped. I figured I could try disconnected one thermostat wire and connect it through a relay, but that likely won't work.

I think my only real option is to try to sense the current running through the thermostat line, so I ordered a CR Magnetics 3110. It will probably need 20+ wraps of the thermostat wire, since it's 24vac and low current, but I'm ok with that. I'll let you know if it works.

Another possibility: Wire a light bulb in series, as long as it doesn't prevent the relay from tripping. Put the bulb in a box with a photosensor -- instant photocoupler.

Hope this makes sense.

Seth

In thinking about this some more (and again, I am planning for my own installation) I think that, if I did not use the optoisolator, I would put a 100,000 ohm (100K) resistor between the ground of the HVAC monitor and each of the grounds of the 24 volt control circuits. That should have minimal effect on your reading (the monitor already puts 1.1 Mega-ohms in that circuit, so the additional 100K should only reduce the reading by about 9%), and it will provide insurance against significant current traveling through your connection (even at 120 volts, a max of 1.2 mA and a max of 0.14 watts)

I haven't tested that configuration, however.



#10 BraveSirRobbin

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Posted 24 January 2008 - 11:24 PM

If you have access to the coil of the thermostat relay, but don't want to disturb any of the wiring already in place, try placing a DS10a sensor near the relay coil. When the relay turns on the magnetic field may trip the DS10a.

I read THIS post on the HomeSeer boards where this methodology was suggested for monitoring a doorbell ringer. I myself never tried this and don't know if this will really work for your situation.

Edited by BraveSirRobbin, 24 January 2008 - 11:27 PM.


#11 Timoh

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Posted 25 January 2008 - 10:51 AM

Sensing the magnetism of the relay... Nice idea.

It reminded me on something I saw on my new favourite site... Am magnetic sensor. http://www.phidgets....product_id=1108

It'll definitely be more sensitive than a regular alarm window sensor. I haven't tried the sensors yet, but plan to do so soon. They output 0-5v, so should be fairly compatible with any analog input.

Thinking about it you could probably use a temp sensor on the duct or pipework. If it's relatively warm, furnace is on!

Tim

#12 SethP

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Posted 25 January 2008 - 01:03 PM

Interesting...I have a pile of old magnetic reed switches, so I'm definitely going to try that. I'm giong to get some pictures up here, too, so everyone can see what I'm talking about.

I thought about a temperature sensor, but it won't be precise enough -- I want to measure the actual time the furnace is running. During the day, it's heating the whole house, so it will cool much faster than at night, when it has less work to do. I'm logging all of this to a database so I can calculate proper setbacks to use at night, taking into account outside temperature. Plus, this is just a great way to use up all my free time :-)

Seth

If you have access to the coil of the thermostat relay, but don't want to disturb any of the wiring already in place, try placing a DS10a sensor near the relay coil. When the relay turns on the magnetic field may trip the DS10a.

I read THIS post on the HomeSeer boards where this methodology was suggested for monitoring a doorbell ringer. I myself never tried this and don't know if this will really work for your situation.



#13 SethP

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Posted 25 January 2008 - 01:55 PM

Here are some pictures. I apologize for the grainy quality...the room is dark and I only had my phone. The first is the furnace itself:

Posted Image

The transformer is on the lower left, relay upper center. On the upper right you see two coiled wires -- these go to the thermostats. When they are connected together, it trips the relay, and the furnace goes on. I was thinking I may be able to use a current transformer on these to detect when it goes one. In the middle you can see the line voltage wires -- white, black, red. In theory I could tap off the red, which goes hot when the relay trips, but I'd like to stay away from that. The yellow wires on the lower right are connect to what appears to be a photosensor in the furnace, so that this unit can detect when it's on. I did lift up this circuit board, but there's no way to attach anything underneath, except by soldering, and I don't want to do that.

Here is one of six thermostat controllers:

Posted Image

Similar setup: Transformer on center left, relay top right. The two screw terminals above the transformer on the left go to the thermostat -- connect those, and it trips the relay. The two screw terminals on the lower right go to the furnace itself -- they are connected when the relay trips. In addition, the relay connects the two red line voltage wires on the lower middle, which power the circulator pumps.

In both cases, the only "easy" thing to tap is a line voltage wire. In the thermostat controller it's much easier since there is a screw terminal. On the furnace it would be a bit more complicated.

I did try a magnetic reed switch all round the relay coils, including a wide gap one used for garage doors, but it didn't work. A sensitive hall-effect device might work, so I'll see if I can get my hands on one.

This has turned out to be a lot more complicated than I anticipated. Of course, so has everything else HA-related...

Seth

#14 sda

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Posted 26 January 2008 - 12:32 AM

I'm not so sure that tying outputs of multiple transformers to a common ground is a good idea. While I can't think of any reason other than those noted below not to do it, it just doesn't feel right.

I did a little experiment with a couple of 24VAC transformers I have for a project. I connected one pair of leads together (yellows) and got 0V across the other pair (blues). I connected the blue-yellow together and got 56VAC across the other blue-yellow pair. It makes sense that the transformer passes the AC waveform through it and using opposite secondary leads inverts that waveform. These are matched transformers so there might be different results (more than 0V) with any old pair of transformers even connected on the same AC cycle. Yeah, probably not a good idea to connect multiple transformers.

#15 SethP

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Posted 31 January 2008 - 09:04 PM

Problem solved!

I got 3 CR Magnetics CR3110 current transformers. I connected ran a wire in series with the 24vac thermostat lines, and 5 wraps around the CR3110; one with 8 wraps; more on that later. On the other side of the CR3110, I have a bridge rectifier, 10k resistor, and 4.7uf capacitor. Two of them I'm running into DS2406 inputs, and one into the Hobby Boards HVAC monitor -- the one with the 8 wraps.

The HVAC monitor shows 0.26-0.27 volts when the thermostat is on, so it's very easy to monitor. With the DS2406 inputs, the 10k resistor to ground keeps the inputs low when the thermostats are off, and the 2.5 or so volts coming off the current transformer are plenty to turn on DS2406 input.

I have one of these on each of my two furnace burners so I can monitor total on time, and one on the main house heating zone. My plugin shows live status in Homeseer, and logs all on-times and durations to a database.

With this setup, all you need is a 1-wire input: DS2405, DS2406 or DS2408. Since I have the HVAC monitor, I have one monitor going there so I can ensure my plugin fully supports it.

The CR3110 units are about $15 apiece, plus another $2 in parts each. Unfortunately, the $ add up, but the data I'm generating should allow a quick payback. I'm going to order a bunch more so I can monitor 4 more heating zones, plus a few other things.

So far I'm very happy with the setup, and everything is on the low-voltage side of things, which I certainly prefer as well.

Seth




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