I think he planning on using more than one photo(device); one per LED was my impression. And not opening or tampering with the device was my impression. I've rolled my own photocoupler in the past by putting a LED and a photodiode together end-to-end inside a heat shrink tube. Without opening the case, just glueing/taping photo-somethings to the case, it might present a small challenge to detect whether an LED is actually on, or if the one next to it is on - there will be some "cross-talk." So it may make more sense to use photoresistors connected to a level detector. This would be super simple to do inside an arduino using analog inputs and empirically derived values to differentiate between LED-ON and LED-(neighbor)-ON (OFF).

Using an arduino or similar uC, it would be trivial to program in recognition of all these binary states and unique actions for each/any of interest.

Let's wait for the TS to respond. My impression from his first post was that he wanted to use one LED. Using an optocoupler rather a phototransistor is a bit of a moot point to me. Using a photocell or LDR is not quite the same. My point was and is that simply monitoring one LED is not likely to work well.

 

A totally different approach would be to have a thermostat sensing the temperature of the exhaust gas going up the stack. But if it is a forced draft heater that may not work. Otherwise, as long as the vent air passing is warmer that the ambient, the thermostat would stay open, when the heat stopped the thermosta contacts would close and sound the alarm, or whatever. quite simple and very reliable and totally separated from the water temp control. Of course, if you have no hardware to play with it might cost a bit. I believe that there are temperature alarm thermostats marketed for other purposes that could be adjusted to work for this application.

 

A totally different approach would be to have a thermostat sensing the temperature of the exhaust gas going up the stack. But if it is a forced draft heater that may not work. Otherwise, as long as the vent air passing is warmer that the ambient, the thermostat would stay open, when the heat stopped the thermosta contacts would close and sound the alarm, or whatever. quite simple and very reliable and totally separated from the water temp control. Of course, if you have no hardware to play with it might cost a bit. I believe that there are temperature alarm thermostats marketed for other purposes that could be adjusted to work for this application.

Source: Service manual
• Flue gas temperature in blower assembly is excessive, pressure switch circuit is open due to vent temperature limit switch activating (see wiring diagram page 7). Determine cause - flue baffle/restrictor missing/damaged or not installed correctly & make repairs.

It is forced. Isn't that why we have service manuals?

 

OK, I see that it is a forced draft water heater. I did not see that in the earlier posts. But now it still would work if there is no damper that shuts off the vent when the burner is not on or when the fan is not running. So it would make the implementation of a thermostat a bit more challenging. An easier option then would be to epoxy a small disc type thermostat to the side of the pressure/temperature relief valve on the side of the unit. Then as long as the water was fairly wam it would be off, and only switch on when the water was cool. The same principle but possibly even a bit simpler.

 

OK, I see that it is a forced draft water heater. I did not see that in the earlier posts.

Post #4. Plain as day. I can understand you ignoring datasheets and manuals, but that is not a legitimate excuse.

The TS posted about this time yesterday and has not responded since then. Maybe we should wait for his response. At least, I will.

 

Let's wait for the TS to respond. My impression from his first post was that he wanted to use one LED. Using an optocoupler rather a phototransistor is a bit of a moot point to me. Using a photocell or LDR is not quite the same. My point was and is that simply monitoring one LED is not likely to work well.

That was based on an assumption of wanting to know what failure had occurred. After a bit more thought, maybe monitoring just the left most LED would work. That comes on with any fault condition or low temperature (assuming it is on with temperatures ≤70°F). Apparently, there is a "vacation" setting, and one would probably want to disable the warning when setting that condition.

A phototransistor might work and could act as a switch. Other sensors (e.g, LDR and photocell) might also work. My choice to start with would be the phototransistor.

What type of signal does the TS want? Does he want the signal only during certain times? Let's say the regular hours of sleep are 10 PM to 5AM. In that case, he may want the signal to work only between 4 AM and 10 PM. One could use an ordinary light timer for that.

 

I rather like the idea of a thermistor on the hot water pipe as it's relatively simple to monitor.
If you mount it just where the pipe comes out of the top, it should be a fairly good indicator of the tank temperature.
The downside is there will be some significant delay between the failure and the detection, depending on how fast the tank temperature drops.

 

I rather like the idea of a thermistor on the hot water pipe as it's relatively simple to monitor.
If you mount it just where the pipe comes out of the top, it should be a fairly good indicator of the tank temperature.
The downside is there will be some significant delay between the failure and the detection, depending on how fast the tank temperature drops.

I suggested a THERMOSTAT because it is very simple to interface to whatever signaling device is used. In addition it does not require as much calibration. There is a question of how long the water takes to cool off when the system shuts down. That will have an effect on most methods of checking.
Another thought is that if the system is smart then the forced draft blower will also not run when it is shut off.
So if that is the case, then it would be a fairly simple matter to know how often the forced draft blower runs, and have the control that powers the blower reset a timer set for a bit more time than the interval between heating cycles. When the system shuts down, the timer would time out and trigger the alarm signal. So all that is needed is a timer that can reset electrically, and has a switch for when it times out.

 

fairly simple matter to know how often the forced draft blower runs,

I would think that would greatly depend on the hot water usage.

 

I would think that would greatly depend on the hot water usage.

I would think that would greatly depend on the hot water usage.

Certainly the frequency of the blower and burner operating will depend on the water usage. THAT is the reason that my post stated that the time between cycles when no water is being used should be used to determine the timer interval, so that if there was a delay beyond that time it could be presumed that the system had shut down.
This would be identical to the watchdog timer used in many processor controlled systems, except with a longer time period. Any water usage would reduce the time and so the timer would still be reset before timing out, and never generate an alarm
Evidently my explanation was not clear enough.

 

Evidently my explanation was not clear enough.

It was quite clear.
I would suspect the maximum time interval would be in the neighborhood of 12Hr or more.
But that may still be sufficient for the TS's needs.
Probably still faster than using a thermistor on the hot water pipe.

 

I did not think it would be anywhere nearly that long a time interval.

And another thought, given what the manual states about reasons for shutting off, is the question of WHY it happens. I did fix a furnace that was failing to run because the home owner put a plastic bug screen over the exhaust discharge tube. So after cheching the furnace systems very completely I looked at the outside vents, because there was nothing else to check. I removed the plastic and by the time I got back to the furnace it was running and heating the house.

 

I suggested a THERMOSTAT because it is very simple to interface to whatever signaling device is used. In addition it does not require as much calibration. •••
Another thought is that if the system is smart then the forced draft blower will also not run when it is shut off.
So if that is the case, then it would be a fairly simple matter to know how often the forced draft blower runs, and have the control that powers the blower reset a timer set for a bit more time than the interval between heating cycles.

Already been suggested. See post #10

Here's one possible solution: Since the hot water pipe is always warm or hot why not sense the temperature. If the heater has shut down you'll get an indication simply by detecting a drop in water temperature. •••

Another approach would be to put a photo transistor directly over the LED you want to detect. If it's like mine, flashing once every three seconds you can build a circuit where the flash of the LED resets a timer. The timer will detect periods of longer than 30 seconds without a flash and can trigger an alarm. Since mine would reset every three seconds, I would know in less than one minute of failure.

Granted, I suggested a self resetting circuit that times out after a period of time but using the flashing LED as a signal that all's well. My LED flashes once every three seconds. The forced combustion air you suggest is different than the LED but the same principal applies - if something that happens on a regular basis while the heater is operating then it can be detected when a "Longer Than" time period fails to reset the monitoring circuit.

In other words, already said that. Glad you like my idea well enough to reiterate it.

 

I did not think it would be anywhere nearly that long a time interval.

It depends upon the heat loss of the heater when hot water is not otherwise being used and, if it has a reasonably well insulated tank, it could be a long time.

 

The big deal is that on a water heater with a forced drf blower it is a mains powered system and so none of the LEDs need to flash, which is done mostly to save power. And probably it uses an igniter instead of a pilot light.
My reason for suggesting thermostat on the overtemp valve port is that it would cool off faster than at the top of the tank.
But the real fix will be stopping the problem, which seems to be related to an inadequate stack air flow. That could be a dead rat in the vent pipe, or some other rodent that breathed to much stack gas.

 

You quoted Crutschow

If you mount it just where the pipe comes out of the top, it should be a fairly good indicator of the tank temperature.

So I assumed you meant the hot water out-feed pipe, same as I had mentioned earlier on. This is the first time I've heard anyone mention the pressure relief valve on the side. It's another viable approach, but being lower in altitude (not at the top of the tank) the temp will be cooler than the out-feed. It could be possible that the temperature of the room could affect the sensitivity, even at the top of the tank. So I'm not so confident it would work, but I suspect it would. As I said earlier on, my hot water out-feed pipe was at 100˚F and had not been used over night. So my standard gas fired burner warmed the water enough to be that temperature by morning. I don't know if the heater came on during the night; might have, I don't know. But at the time of taking that measurement nobody had been using any water.