Relay board, pretty sure can't do this.

[DavidB]

Haven't seen a SPST relay in long long time, and didn't catch that the relay board had them instead of SPDT. I was going to use this to control my Thermostat. But I want the Thermostat inline still incase the computer fails or relay board fails. So like for winter have the Thermostat set to 40F just incase. I had it all drawn out with SPDT relays until I realized my mistake before actually starting.

Now, with the picture below, this would all work fine if I made sure the Thermostat was changed to not come one at for example until 40F. So likely hood of it supplying 24vac is low. But what if I forgot to and the relay board and the Thermostat came on. I haven't been in school in awhile, so forgive me if my theory is off. But I don't see any good coming of this if the thermostat turns the blower on and the relay board does also. Even if they are in phase (the 24vac is from the same source (ie the furnace)).



If I am wrong let me know. But I think I am going to have to use the relay board to control other relays to do the job of keeping the Thermostat in-line, but switch the blower, ac, heater. My initial design left 24vac going to the Thermostat, but switched to NO contacts to supply 24vac to the furnace relays and leave the NC to the thermostat, just incase relay board or computer failed. But without the NC contacts can't do that.

I designed another way to do it with the relay board, but that involves keeping two of them switched on all the time, I prefer to keep NC as the default so don't really want that either. That way if the relay failed the thermostat still controlled.

Not wanting to damage my furnace transformer during the winter so thought I better ask.

Thanks, Dave

 

[ericvic]

Dave,

It shouldn't hurt to wire the Relay and your thermostat in parallel. If they both come on it will just be completing the circuit through two paths. You will still be supplying the blower relay with just 24vac.

Eric

 

[DavidB]

Thanks for some confirmation Eric. I repair systems that are already designed and working but don't get down to component level much anymore, so have questions when designing my own sometimes. Haven't really done any hobby stuff for quite awhile until starting this 1-wire project. In that case it looks like the SPST relays will not change my circuit much and can still keep the Thermostat in play.

I'll see if I can whip some test circuit up at work if have some spare time.

 

[sda]

I'm going to use a 3PDT switch between the original stat, the 1wire relay, and the HVAC unit. If something goes wrong with the 1-wire setup, just flip the switch and you're back in business with the original stat.

 

[DavidB]

I'm going to use a 3PDT switch between the original stat, the 1wire relay, and the HVAC unit. If something goes wrong with the 1-wire setup, just flip the switch and you're back in business with the original stat.

Glad you chimed in here buddy!

So what happens if your gone for week or so and can't flip the switch (assuming your talking a manual switch)? Wouldn't it be better to have the thermostat on ready standby by either using the above circuit or SPDT to have the NC connection for the thermostat? Or maybe use these: Construction Thermostats They would probably only be good for an extra backup or if you wanted remove the thermostat out of the picture and just use the relay board though.

 

[huggy59]

You'll want to test everything in a lab before wiring it up for good. I still have some questions. For example, feeding 24VAC back to the thermostat over that fan relay feed might cause issues inside the t-stat if it uses something other than an actual relay or if other leads are tied to the fan output. Might not, probably won't, but what if it does?

You could easily build an intermediate relay that is controlled by the board SPST relay - make it a DPDT or even a 3PDT, etc. Just need that relay and a power source for it. Then you have what you originally wanted, I believe.

 

[sda]

The point of the switch is to completely isolate the regular stat from the 1-wire stat. The existing stat can be left exactly where it is on the wall and quickly enabled if needed. For example, if we go on vacation for a week the MIL might house sit. No point in explaining the system to her, so flip the switch and she can use the regular stat.

So what happens if your gone for week or so and can't flip the switch (assuming your talking a manual switch)?

Taking the 1-wire thermostat out of the equation ... What if you're gone for a week and your regular thermosat dies? What if you're gone for a week and the power is out? I guess the house gets colder or hotter than the cats would prefer.

Wouldn't it be better to have the thermostat on ready standby by either using the above circuit or SPDT to have the NC connection for the thermostat?

Could work. You'll have to be careful because of how the regular stat works.

I like the idea of the backup thermostats to make sure the house doesn't get too cold or hot in case the 1-wire stat or controlling computer crashes. I was already going to put in limiter stats to prevent excessive heat/cool if the 1-wire stat went crazy. The heat will cut off if the return air is above 80 and the cool will cut off if the return air is below 65. I'll need another pair that turns heat on at 50 and cool on at 90.

I still have some questions. For example, feeding 24VAC back to the thermostat over that fan relay feed might cause issues inside the t-stat if it uses something other than an actual relay or if other leads are tied to the fan output. Might not, probably won't, but what if it does?

Read this, it explains it in detail.
http://www.high-performance-hvac.com/therm...tat-wiring.html
If you have a gas/elec heat and A/C, the 1-wire thermostat has to turn on Y+G for cool or just W for heat. If the 1-wire stat has "fan only", then it can't go back to the regular stat (or your compressor will kick on).

I want my setup to be in three parts. 1) The existing system, which consists of the thermostat and the HVAC unit. 2) Limit controls. 3) The 1-wire stat. The limit controls would work if either stat is in use. I also want to be able to switch out the extras, #2 and #3, if there's problems, to troubleshoot, or in case I have to call a repair grunt.

 

[DavidB]

Yes, I was concerned about the tstat and the feedback also. I think putting another relay to isolate the tstat when then 1-wire is controlling is the correct way to go with this. That was my original plan until I found my booboo on the relayboard relays. So I was just looking for possible alternatives, but looks like I need to add another relay or so. Will have to see what I have around or order one.

My original plan was to use one relay for 24vac, but still have it tied to the tstat. Then switch the 24vac to each NO contact on the Y,G & W relays (that used up the 4 relays on the board), then switch the appropriate relays on Y & G for cooling, W for heat. Then incase of 1-wire failure the tstat was still inline with the NC contacts. But don't have NC contacts with this board, so that went out. I like the idea of adding the limit switches incase 1-wire went crazy or something also.

So I am going to add another relay or so to my design.

I also want to be able to switch out the extras, #2 and #3, if there's problems, to troubleshoot, or in case I have to call a repair grunt.

Yes, my original design didn't use the G contact from the tstat, I was going to turn it on myself at the same time turned Y on. But if you need to troubleshoot or call a tech out, yes, depending on the problem, he will want to turn the fan ON at the tstat to see if works that way or jumper it to 24vac to see if tstat broke. So it would be better to keep it in place on the design to save possibly having to remove wiring because he won't understand your 1-wire setup.

What if you're gone for a week and your regular thermosat dies? What if you're gone for a week and the power is out?

I'm not sure how often they go bad, I have seen them not keeping the temp correct exactly, but not go bad. They seem pretty stable most of the time. But that is just what I have seen. Well, can't do anything if power goes out. I was wondering more on the case if 1-wire went bad, but pretty much the same thing I guess as losing power or tstat going bad. But that would be a good case for the limit switches.

Thanks for the feedback on this guys. Always nice to bounce ideas off others that have a clue.

 

[sda]

I'm not sure how often they go bad

I'm sure with all the cheapo electronics in them that they do go bad now and then or get a surge through the 24vac. I've only had one stat go bad in the last 30 years. It was about 20 years ago, an old thermostat that used a piece of wire for the heat anticipator resistance and the wire broke. I remember that because we came home from the outlaws on Christmas day and our place was stone cold. Didn't know enough back then to just jump R+W.

Always nice to bounce ideas off others that have a clue.

Clue? That's one board game I don't have.

 

[DavidB]

Clue? That's one board game I don't have.

<_<

 

[sda]

FWIW, I've given up on the idea of using extra thermostats for backup and safety. After I drew it up, it would take 4 separate stats - because I don't want to be switching them between heat and cool. Ideally, you'd need a non-programmable digital stat that runs on 24vac - because I don't want to be changing batteries. Even then it wouldn't work because as stupid as stats are, some have things like 5 minute compressor delay and heat cycles per hour. A truly dumb mechanical stat might work, but the bimetallic coils are slow and inaccurate. So the new plan is to use a PICAXE to sense the temperature and heat/cool/fan signals and control them as needed. One temp sensor, 3 inputs, 3 outputs, and some code.

 

[DavidB]

Ya, I decided to go with a 24vac 4PDT relay. I have an extra 24vac transformer from aprilaire install that not using (the current sense relay on the fan stopped working, so I just moved the humidifier solenoid to the main 24vac xfmr to the heater relay from the tstat. Then it comes on when the heater starts, if the humidistat let's it.)

Anyway, I've already determined the current use I will need and have enough to control the 4PDT relay and the all relays on the board (ya, going to develop my DC from the 24vac to run the board). So I am going to run my 24vac from the main hvac xfmr to one contact and the thermostat wires to the other 3. Then when the 1-wire decides to turn on cooling/heat, it will first turn on 1 relay on the board, which will turn on the 4PDT, that will then deliver 24vac from the main hvac xfmr to the other 3 relays on the relay board (which will be turned on either for heating or cooling), but the main thing is that the 4PDT will also take the thermostat out of commission at the time the 1wire is controlling the hvac. So if the relay board fails or the 4PDT fails the tstat can still work. Of course this doesn't take into account every scenerio and some will have to be done in software.

With the software, I will check that the furnace isn't on, but looking at it's temp sensor (that is just to make sure that I didn't forget to turn the tstat down for winter or up for summer). I haven't ordered the parts yet from Digikey, so might think of something else need to incorp before then.

I'm also ordering some parts to see if can count the wheel turns on my electric meter. Going to do that before buy a counter board.

 

[sda]

Ya, I decided to go with a 24vac 4PDT relay. I have an extra 24vac transformer from aprilaire install that not using (the current sense relay on the fan stopped working, so I just moved the humidifier solenoid to the main 24vac xfmr to the heater relay from the tstat. Then it comes on when the heater starts, if the humidistat let's it.)

Anyway, I've already determined the current use I will need and have enough to control the 4PDT relay and the all relays on the board (ya, going to develop my DC from the 24vac to run the board). So I am going to run my 24vac from the main hvac xfmr to one contact and the thermostat wires to the other 3. Then when the 1-wire decides to turn on cooling/heat, it will first turn on 1 relay on the board, which will turn on the 4PDT, that will then deliver 24vac from the main hvac xfmr to the other 3 relays on the relay board (which will be turned on either for heating or cooling), but the main thing is that the 4PDT will also take the thermostat out of commission at the time the 1wire is controlling the hvac. So if the relay board fails or the 4PDT fails the tstat can still work. Of course this doesn't take into account every scenerio and some will have to be done in software.

Possible issue.

If you only energize the 4pdt when the 1wire stat is running, its quite possible (probably guaranteed) that somebody will fiddle with the existing stat and the 1wire stat will always be satisfiied. For example, 1wire stat is set for heat @ 72, and somebody ups the existing stat to heat @ 75. House warms up to 75 and stays there. Since the 1wire sees the temp is 75, there's no reason for it to energize the 4pdt (disabling the existing stat) in combination with not energizing the heat relay (turning off the heat).

But if you energize the 4pdt constantly, the existing stat would be disabled and would act as the backup as intended in case of 1wire stat failure.

Are you using the old 24vac Aprilaire xformer and parts to get the 12vdc for the relay board? (or did I miss something)? Have you considered tapping the 24vac HVAC power instead of adding a xformer? The HVAC xformers are usually at least 40VA, which is about 1.66A at 24vac. If you need more power, you can replace it with a 75VA, ebay $10-$20.

I'm also ordering some parts to see if can count the wheel turns on my electric meter. Going to do that before buy a counter board.

Been doing that solidly for a couple of months now. "Great Success" as Borat would say. I'll see if I can post the circuit, pics, and notes on my website this week.

 

[DavidB]

Possible issue.

If you only energize the 4pdt when the 1wire stat is running, its quite possible (probably guaranteed) that somebody will fiddle with the existing stat and the 1wire stat will always be satisfiied. For example, 1wire stat is set for heat @ 72, and somebody ups the existing stat to heat @ 75. House warms up to 75 and stays there. Since the 1wire sees the temp is 75, there's no reason for it to energize the 4pdt (disabling the existing stat) in combination with not energizing the heat relay (turning off the heat).

But if you energize the 4pdt constantly, the existing stat would be disabled and would act as the backup as intended in case of 1wire stat failure.

Yes, that is true. I was mainly thinking that if the furnace was on, by looking at the plenum temp, to not turn the gas relay off to just turn my 1wire on and do what it was already engaged in. So that leaves two things to look at. Check to make sure furnace plenum isn't high (gas relay probably on) and then decide on the issue you bring up. I will have to think about that. I am thinking always turn the 4PDT unless for some reason the plenum is hot. Something to think about. Thanks.

I already had a glitch in the matrix since restarted the rrdtool database 31st. Just one time and a few sensors. So have to edit my script for reading sensors to do a little error catching and reread the sensors if happens. One of them was the plenum temp sensor. So I will have to add this error catching into the hvac control script also. So good thing working on it now.

Are you using the old 24vac Aprilaire xformer and parts to get the 12vdc for the relay board? (or did I miss something)? Have you considered tapping the 24vac HVAC power instead of adding a xformer? The HVAC xformers are usually at least 40VA, which is about 1.66A at 24vac. If you need more power, you can replace it with a 75VA, ebay $10-$20.

Actually there is a Aprilaire xfmr on there already. I took it out of action cause the current sensing relay was always on and the humidifier would always run even when blower wasn't on. Hooked it up to the heat contact, so when the furnace comes on it will turn on if the humidistat lets it. Ya, the main xfmr should be enough, but since the other is there figured I would use it for the 24vac 4PDT relay coil and put a half-wave rectifier/filter/regulator on it to give me 12vdc to feed the regulator on the relay board.

I'm also ordering some parts to see if can count the wheel turns on my electric meter. Going to do that before buy a counter board.

Been doing that solidly for a couple of months now. "Great Success" as Borat would say. I'll see if I can post the circuit, pics, and notes on my website this week.

Oh, yes!! Circuit and pics would be very helpful. I was going to order some parts tomorrow. Going to use this infrared circuit (2nd one) 2nd Circuit and use the opamp output to turn a 2n3904 on as a switch (open collector) to send to the counter board. Most circuits I see seem to be pulsing the counter high. But from what I read on the specs, the counter should be pulsed low. So going to tie the counter inputs to the collector and turn it on to make a low pulse. Got to go to work right now. Will check this again later.

 

[sda]

I'm also ordering some parts to see if can count the wheel turns on my electric meter. Going to do that before buy a counter board.

Been doing that solidly for a couple of months now. "Great Success" as Borat would say. I'll see if I can post the circuit, pics, and notes on my website this week.

Oh, yes!! Circuit and pics would be very helpful. I was going to order some parts tomorrow. Going to use this infrared circuit (2nd one) 2nd Circuit and use the opamp output to turn a 2n3904 on as a switch (open collector) to send to the counter board. Most circuits I see seem to be pulsing the counter high. But from what I read on the specs, the counter should be pulsed low. So going to tie the counter inputs to the collector and turn it on to make a low pulse. Got to go to work right now. Will check this again later.

Got tied up with work the last couple of weeks. Web site is up and there's some info on the electric meter reader there.

http://www.ludditelab.com
1-wire projects -> electric meter reader