My most costly mistake:Breaking part of a $900,000 house with a $0.01 crocclip - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: July 25, 2018, 10:40:39 pm
content: I don't know what I was thinking. Our upstairs A/c was working intermittently last few days. I reset the breaker and it came back to life. Its a huge house with two A/c units one up stairs in the attic and another down stairs. The up stairs goes to a condenser/fan out side and has one of those cheap mercury controls. Didn't know that was legal in the 200's or why such a huge nicely furnished hose would do marble and everything else just to put in a wasteful mechanical thermostat. I guess the type of people that buy these houses can afford to burn money I guess.Anyways one guy that lives her is mental and kept shutting off the fan to "auto" from "on". So after realizing he was too dumb to understand what that switch did I decided just to probe the pins and hard wire it on all the time. I connected to the G which is the fan to one of the common pins in the bottom right. Nothing happened. So I turned the a/c back on and nothing. No operation no sparks off the mercury and no little fluctuating voltage measurements of <0.5 volts like when I would probe random pins while the unit was working. I went to basement cycled the break and nothing. Normally it's 24 volts but couldn't find a ground to measure this when it was working thinking that the G or other areas where ground just little moments of .2 to .5 volts when the thing was running.Last year this same one did break and we replaces this big metal soup can type cap on the outside of house fan/coil unit next to the house. I suspect it broke last year because in ten years it was never serviced and the idiot that was running it would set the downstairs heat to 80'F and up stars cool to 50'F out side temp 62' to make 70' upstairs. I wonder why his electric bill is constantly in the four figures each month? Or why they would put granite and marble everywhere but not spend $30 on a digital therm?Here is picture. "g" = fan and bottom right three pins were common. G was supposed to equal fan. It's just a central A/C with natural gas heat.Do you think there was a diode or protection in the main unit this wall unit connects to and 24V blew the diode? Nothing works including A/C that I didn't touch I don't think, no sparks in the murcury as you wilge it all contacts read 0.0 not 0.3-0.4 like when working. I thought these were just realys and pretty robust things.Las summer when it broke we replaced a huge soup can sized cap right next to the motor inside the outdoor coil/fan unit. The A/cHas been run hard this year often during cold month to offset the 80'F heat ignorantly left on down stair since the whole center of thehouse is a giant room.

author: Beamin
author_rank: Super Contributor
timestamp: July 25, 2018, 10:42:41 pm
content: G is in bottom right in this picture common and those three on right, 2 with no wires to them, are common.

author: floobydust
author_rank: Super Contributor
timestamp: July 26, 2018, 04:40:57 am
content: HVAC has wacky labels for thermostats, based on wire colour code.What did you short to what? G to R? or G to B?Check the 24VAC control transformer, they are Class 2 energy limited and have an internal fuse. Any short or overload and the primary goes open-circuit and you lose 24VAC. It should be across R and B, 24VAC.R= 24VAC redB= common blueG= fan greenFan On switch connects 24VAC to G, but if you shorted G-R and then moved switch to Auto then it would backfeed into the controls.If you jumpered G to B then it would short the control xfmer.pic is a random sch of thermostat and old A/C.

author: Zucca
author_rank: Unknown
timestamp: July 26, 2018, 07:25:33 am
content: Beamin, it was soo difficult for me to read and understand it.I need to improve my english.

author: amyk
author_rank: Super Contributor
timestamp: July 26, 2018, 11:14:59 am
content: Quote from: Beamin on July 25, 2018, 10:40:39 pmThey work, and keep working reliably for decades. Much better than the "smart" crap you find all over the place now.

author: The Doktor
author_rank: Regular Contributor
timestamp: July 26, 2018, 04:06:52 pm
content: Quote from: Beamin on July 25, 2018, 10:40:39 pmI like the old mercury thermostats. Extremely reliable & maintenance free. And best of all, NOT cloud connected

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2018, 04:18:31 pm
content: Quote from: The Doktor on July 26, 2018, 04:06:52 pmUnfortunately they can't be bought any more in the EU, because they don't comply with the RoHS regulations.

author: rrinker
author_rank: Super Contributor
timestamp: July 26, 2018, 06:09:21 pm
content: Quote from: amyk on July 26, 2018, 11:14:59 amIndeed. I have 3 heat zones in my house, each has some sort of fancy=pants electronic thermostat and the morons who installed it only pulled 2 wires through, chopped off the others - so the darn things have to run on batteries all the time. Which last about 3-4 months. On the heating unit the transformer is all wired up to supply power. If I want to fix this I will have to pull all new wires into the walls. The AC on the other hand has a plain old mercury bulb mechanical thermostat which just plain works. No muss, no fuss. Set temp, and it does the business. Not sure why you would want the fan on all the time - my AC ducting is in the (hot) attic, so keeping the fan on instead of on auto would just heat the house back up and make the AC run more.

author: Tedro
author_rank: Unknown
timestamp: July 26, 2018, 07:43:02 pm
content: That's a pretty advanced house for being built in the 200's especially since the form of Electrical Air Conditioning wasn't even invented until the early 1900's....

author: CopperCone
author_rank: Super Contributor
timestamp: July 26, 2018, 08:23:00 pm
content: how do you even outfit a teepee with air conditioning? put it on a cave entrance?

author: SeanB
author_rank: Super Contributor
timestamp: July 26, 2018, 08:30:07 pm
content: There are plenty of retrofit thermostats that only need a 2 wire connection, they get power by simply dropping 3V across the unit when on, and when off they of course drop the full 24VAC. They work well with older AC units that use a 24VAC relay.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 26, 2018, 10:31:41 pm
content: It's also possible to modulate a high frequency carrier to send more data than could ever possibly be useful in a residential HVAC application.

author: Tedro
author_rank: Unknown
timestamp: July 26, 2018, 10:55:31 pm
content: Quote from: NiHaoMike on July 26, 2018, 10:31:41 pmHigh Frequency Carrier pigeon?

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 26, 2018, 11:44:25 pm
content: Something like DSL or Homeplug but much lower bandwidth. Apparently, quite a few mini split HVAC systems work that way to cut down on the number of interconnect wires needed.

author: amyk
author_rank: Super Contributor
timestamp: July 27, 2018, 02:34:11 am
content: Quote from: Hero999 on July 26, 2018, 04:18:31 pmThe replacement is a bimetallic strip. Just as simple and reliable.

author: Beamin
author_rank: Super Contributor
timestamp: July 28, 2018, 08:23:28 pm
content: I got it fixed. That diagram makes perfect sense and why I was getting weird readings with the meter; I had it on DC.I thought it was DC and it just clicked on relays.Turns out it was a 5A automotive fuse. Unfortunately due my limited mobility I had to hire an guy of craig's list for $50 to fix it. He was nice and admitted that I knew more then him so he went up into the attic and described what he was seeing as he took it apart. So after he spent about 2 hours up there (was hot and didn't have the proper tools to takes the nuts off so a few breaks between) he found the fuse and replaced it and now it works fine. Since the home owner is a pile of shit I'm going to charge him 400$ to fix it.Why would they send AC to the controls and not just have DC click on relays? This is an 8 year old unit.

author: Beamin
author_rank: Super Contributor
timestamp: July 28, 2018, 08:29:40 pm
content: Quote from: SeanB on July 26, 2018, 08:30:07 pmSo they send 24VAC up to the relay and the relay has a rectifier in it? Seems backwards. Or else it would be a 24hz spark gap transmitter like I made as a kid much to the annoyance of the people on the phone and watching TV.Also you could hear my voice through the TV when I was on the CB. I think you could actually make out what I was saying. The picture would also get messed up. We had analog cable, no box, at the time and my CB had a huge 1/4 wave antenna running more then 4 watts.

author: Nusa
author_rank: Super Contributor
timestamp: July 28, 2018, 09:05:49 pm
content: Quote from: Beamin on July 28, 2018, 08:29:40 pmRelays can be had with either AC or DC coils. No rectifier required or desired to use AC with an AC coil.Don't confuse voltage and frequency. There's usually a 120V->24V transformer that supplies this circuit, and the frequency will still be 60 Hz.

author: tooki
author_rank: Super Contributor
timestamp: July 29, 2018, 01:09:05 am
content: And using AC means the contacts in the thermostat itself would last much longer, since there's less arcing. I'm talking out of my ass here, but I assume that with a bimetallic strip (the kind that switches directly, not using a mercury tilt switch) that it could open quite slowly, where DC could produce a prolonged arc.

author: amyk
author_rank: Super Contributor
timestamp: July 29, 2018, 02:22:07 am
content: Quote from: tooki on July 29, 2018, 01:09:05 amThere's a snap-action mechanism (which also provides hysteresis).

author: Beamin
author_rank: Super Contributor
timestamp: July 29, 2018, 09:57:14 am
content: Quote from: Nusa on July 28, 2018, 09:05:49 pmI think I know the difference in frequency and voltage. I wrote 24 instead of 60 by typing too fast.How would a coil work with AC to hold a contact open? It would keep opening and closing at 60 hz. They would need at least one diode using the mechanical nature of a "soft/weak" spring to not react fast enough to the 120 hz bumps of the voltage spikes to open/close.I'm sure they have a reason to use AC.I don't think it's to save the contacts as 24 volts doesn't make much of a spark especially at low current. The only sparks I ever see are the little ones in the mercury tube. You would think they would glow when you put high voltage through them but they must have too much air.

author: iainwhite
author_rank: Unknown
timestamp: July 29, 2018, 09:37:21 pm
content: Quote from: Beamin on July 29, 2018, 09:57:14 amAC relay coils use a thing called a 'shading coil' - a secondary winding that creates a magnetic field slightly out of phase with the main field, such that the magnetic field does not collapse when the AC passes thru zero. (the shading coil is not connected to the main windings - it is powered by an induced current from the main field)A similar principle applies to a Shaded-pole motor.

author: Cubdriver
author_rank: Unknown
timestamp: July 29, 2018, 10:15:58 pm
content: They use AC for a number of reasons - it's easily obtainable from a transformer, doesn't require rectification or filtering, is easier on contacts and is backwards-compatible with old systems that have been around for years.-Pat