Monitoring Pool Gas Valve State

I currently have running a version of this pool temperature monitor and I want to go a step further and monitor when I am consuming gas in my heater. To do that, my pool heater documentation indicates 2 test points that will have approximately 220mVDC between them when the valve is operating.

Essentially, I want to use my Photon as a volt-meter across these two points, but I don’t know which ports to connect to. Is it as simple as connecting the two test points to A3 and GND and reading the voltage at A3? (A0, A1 and A2 are my thermistor ports).

1 Like

@eric.spaeth, do you have a link to that pool heater documentation? The way the voltage is presented (ground referenced or not) is important to how you connect the photon and whether isolation is required or not.

Section 10.4.1 on p30 is where I’m quoting the voltage reading. Troubleshooting schematic is on p33.

It looks like these are thermocouple voltages that are part of the safety system of the heater. I would be very hesitant to add my own circuitry to them.

I would try to measure something easier and safer to test, like the output temperature of the heated water or the electrical current being used by the pump.

If you are set on measuring when the heater flame is on, I would add a thermocouple sensor that is completely separate from the internals of the heater.

1 Like

@eric.spaeth, 10.4 states:

These procedures require a voltmeter with a minimum 0-1000 mVDC range

So that is good since the Photon can handle up to 3300mVDC on an ADC input. What is interesting is:

The Lite2 model LG heater does not require an external source of electrical power to operate. The power for the gas valve and safety circuit are generated by a thermopile. The thermopile generates a millivolt signal when heated by the pilot flame. Figure 17 shows the internal electrical wiring diagram and schematic of the heater. I would hesitate connecting the Photon GND to either of the test points.

@bko, would a differential op-amp (powered from Photon’s 3V3 and GND) between the test points provide enough isolation and remove the need for a common ground?

“+1” for @bko 's suggestions.
Too many things can go wrong.
Measure Temp independent of the gas safety equipment instead.

Later, you could add temp sensors to both the Supply and Return Water lines and calculate the actual BTU output your heater is providing. You would need a pressure sensor (to know what the operating pressure of your sand filter is), and the pump curve from your pool pump to know the Gallons Per Minute water flow (easy to find on the net).
BTU = 500 x GPM x Temperature Change.
The bonus to having a pressure sensor is your Photon can tell you when to backwash your pool filter.

Folks, this is what the OP asked for. The heater already has safety equipment. He just wants to measure the time the valve is ON to approximate gas consumption.

As @peekay123 said, I don’t really care about what the temperatures are in the supply and return piping. Yes, that might be useful information to have at some point, but it also requires probably ~$100 in sensors to ESTIMATE what I’m hoping can be directly measured for the cost of a 22ga wire pair: when the gas valve is open. This is what costs me money. Since I can measure this voltage with a multi-meter (presumably safely), my question is basically “how can I turn my photon into an always on multi-meter?”

The heater has no electrical connections to mains power. It is like a typical hot water heater with thermocouples for safety to detect pilot on and main burner temp that generate a very small amount of electricity that is used to control electromagnetic valves directly.

If you use the test points to measure the voltage going to gas valve, you better make damn sure that when your circuit fails, it fails safely.

Consider for instance the case where the Photon circuit power fails, even temporarily:

  • Does the heater continue to function normally with the “dead” Photon on it?
  • Does the load presented by the “dead” Photon cause the temperature of the set point to change?
  • Does the Photon drive the input pin as an output even briefly when it powers back on?

My advice to avoid measuring the actual thermocouple used to control the device and add your own that is independent of the safety system.

1 Like

OK, I understand your concerns here, @bko. I’m not really keen on measuring the output water temperature of the heater, because that won’t react until the heater comes up to temperature. I’m partly interested in determining savings/efficiency based on how wide my set-point tolerance is, for which I can’t use temperature data in the water because thermally it’s downstream of the gas burner and heat exchanger.

I think I’ll look into either some sort of IR sensor to detect the flame (but somehow positioned so it won’t see the pilot) or possibly look into an acoustic sensor to detect the very noticeable sound difference between heater on/off. I’d be open to other ideas of how to accurately determine when gas is being consumed.

Note, the pump is on a mechanical timer, but the heater is not always on when the pump is on, so measuring pump on time, while helpful in another sense, doesn’t help quantify gas usage.

If the Heater vibrates when running then you could detect that to know when the heater is on or off.

Here is a video where a guy used a Peiazo speaker to measure when his dryer was ON or OFF.

A replacement hot water heater thermocouple is only $5-10 in any hardware store. You would have to figure out how to mount it, but that is just some sheet metal screws etc.

Not very expensive for some peace of mind.

Just brainstorming here, but how about simply measuring the exhaust temp from the outside of the metallic exhaust stack ?
Easy to wire and you never open the heater’s enclosure.
You may be able to correlate the gas usage to the temp graph. The stack temp will rise and fall pretty quickly following the modulating gas valve.


Hmm, I like that idea. Especially since that temperature might even be low enough to use a spare thermistor to capture.

I just skimmed through the Jandy Heater Manual you linked to.
Since the Heater has an automatic flow control valve, the burner probably doesn’t modulate at all.
I’m thinking if the supply water is below the Set-Point Temp, the burner fires at full capacity, until the supply water reaches the Set-Point Temp. It’s either ON or OFF.
I’m sure you already knew this… but I didn’t (I run a solar heater on roof)

I’m guessing the flow control valve directs the proper flow rate through the heater to maintain a Temperature Rise in Table 6 ( 27-40 degrees F). This heated water is mixed with the Bypass water for a final temperature rise of only a few degrees for the entire flow rate.

This means you have a decent shot at measuring the Exhaust Stack Temp to confirm it’s relatively stable while the heater is firing. A Stack Temp may be as good as an On/Off signal. You could read your Natural Gas Meter before and after a 1 hour test and know how much gas the burner actually uses per hour, assuming no other major Gas Appliances run during that hour. I think this will remain constant.

Or calculate based on the heater size.
1,032 BTU per cubic foot of Natural gas (cf)
Natural Gas is billed as ccf (100 cf) so a 125,000 BTU heater would burn 1.21 ccf/hr = $9/hr.
But just like any other device… it rarely matches the actual rating. I wouldn’t be shocked it it was less than half the rating.


This is measuring the direct exhaust from the heater (before it has a chance to mix with any ambient air). Very responsive. I take data points every 1min for this chart. It cycles quite a bit more than I was expecting, which leads me to question how I can minimize that.

@bko (or anyone else), would you have any concerns adding a relay to the safety circuit that I could use logic to control when the heater could be on, independent of the pump? I’m thinking I would watch the exhaust temperature and when it started cycling on/off, I would know it was maintaining the pool temp and I could disable the circuit for a period of time to reduce the inefficient cycling while allowing the pump to keep circulating.

Wow, that’s an interesting Graph Eric !
From what I remember from reading your Jandy Manual, that’s not at all how I expected it to operate.

What BTU rating is your heater?
Is there any chance your pump isn’t moving the minimum GPM for that Heater?
That could be as simple as the heater’s bypass valve stuck.

Does your Filter Valve have a position to bypass the sand (to move as much water as possible) ? That would be an easy way to temporarily test the flow-rate question.

Is your pump a multi-speed or have a VFD ?

I don’t see any way that the supply water Temp could be rising and falling that quickly to cause the heater to cycle off. Something else is going on.

A few other ideas:
The secondary gas regulator may be starving for fuel causing a shutdown. Or it may be running higher than 10" W.C. and producing too much heat for the exchanger. Or my first thought, the exchanger isn’t getting enough cold water through it (bypass valve) causing a high temp shutdown of the heat exchanger.

125,000 BTU/hr pump on a roughly 10,000 gallon pool. 1HP constant speed pump. No, I cannot bypass the sand (I can bypass the heater, but that doesn’t help us here).

Last summer I had the heat exchanger apart to descale it because I was getting knocking. I recall looking at the bypass valve (which was basically just a piece of plastic on a spring) and thinking how flimsy it looked. So it’s possible it’s letting too much through. You reminded me that it’s time to purge my sand filter. I’ll do that and see whether it has any effect on tomorrows heating cycles.

I was assuming the bandwidth was just too sensitive, but now that you mention a high temp shutdown that seems plausible.

I looked back at your manual. The Troubleshooting sections says the knocking you heard was from Too Low of a water flow through the heater. It also says that Short Cycling is caused by low flow. I’m betting the temp limit switches are shutting down the burner because you don’t have the proper gpm moving through the exchanger to remove the heat.

The burn times should increase after you backwash your pump. If the heater runs better, but still cycles too much you can look back at the “automatic” bypass valve and try to get more water going through the heat exchanger and not bypassing it.

Back to your Control Question: The On/Off switch by the Target Temp POT simply breaks the circuit of all 3 limit switches. It seems adding a relay in-line would be a safe way to operate the Heater (via Photon) without touching the existing safety devices or control scheme. You would never be able to turn the heater ON unless it was otherwise ready to Fire, but you could force it OFF. Hopefully someone else will confirm.

1 Like

Yeah, the amount of scale that was plugging the headers of the heat exchanger was quite impressive last summer. If the issue is only back flushing the filter I’ll be surprised because normally when that happens, the heater stops firing completely.