Gas Boiler Firing Detection - by various means, some good some not!

Read the “gas flame by deltaT/electrical power posts” with interest so though to post what I have knocked together and learned this winter.

When working at home the TRV’s are set very low in the rooms am not using (Pegler Terrier programmable TRV’s I run round turning them down to 10degC when everyone leaves for work/school and close the doors. The TRV’s revert to program at 16:00).

Sat in my office I am aware the pump in main house - under boiler control- runs intermittently for sometimes as little as 5 minutes, so I think the boiler is cycling relatively fast, but I can’t see if the boiler is firing.

My boiler is in the garage, accessible by only going out the main house so I needed something to monitor it.

Attached shows the setup,it’s a RasperyPi zero and some DS18B20 sensors on the feed/return pipes, the PI reads the sensors and then makes a http:// call to the emonPI, which logs to a feed. The loop waits 60 seconds between readings, so the actual frequently is probably closer to 63 seconds, but for temperatures it’s fine.

The challenge has been working out if the boiler is firing, there is a nice green light on the front that comes on with the burner, you can hear the fan spin up and then the light comes on. I made an optical pickup from a LDR and hot glue, a quick resistive divider and digital conversion with a 4093 Schmit trigger and it feeds to the PI ZeroW’s GPIO pins. On the bench it works perfectly, however not on the boiler, where it would often indicate firing when it was not!

After much investigation the reason is there are too many light leaks around the knob above the green light. If the garage light is put on, or even the door open and some sun low in the sky enough light passes through the gap between knob and casing. Looking closely it’s not a LED behind the green indicator so I suspect it’s got some sort of perspex “light pipe” between a LED on a PCB and the indicator, that catches the light leaking in.

Given up for now - I think I will go the power monitoring route. Feedback on the points below very welcome…

Working out the boiler power…
The junction box is in the airing cupboard so I can put a CT on the boiler feed but the boiler also controls the pump. I also have trace heating on some pipes in the garage with it’s own outside thermostat, Trace heating is 5-30W when running. Thus the boiler power is boiler+pump+trace heating. The total load is around 170W when everything is running.

There’s a range of ways to solve this, power the pump via a relay switched form the boiler and rewire the trace heating are perhaps the “hardware” fixes, but has anyone any other ideas?

Measuring the Boiler Power
The airing cupboard is some way from the main emonPI, I could perhaps use an emonTX but they are 6 channels now and I only need one or perhaps 2, or are the old ones still for sale?

Last year when there was no emon’s in The shop I made a simple monitor for my solar immersion divert that just measured current with a CT, ArduinoNano and PiZero, this works very well, the diverter power factor changes with load, but it’s one device so I just made a calibration curve for current → power.

I am not sure I could do the same as the power factor of the total load would depend on which bits of the system are running; trace heating is restive, but the pump, boiler (and the components within it) I have no idea?

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What is this doing?


What do you want to achieve? Reduce the cycling and save energy?

If so, I’d take control of when the boiler can come on. As you say, put a relay in a suitable point in the wiring, so you can control it coming on or not. If all the TRVs are turned down to effectively off, do you basically want the heating to go off and stay off? What is telling the boiler to come on?

If you basically work in one room, get an oil filled radiator and just use that - probably cheaper than the gas boiler (I have this setup to come on when I’m in the room and off when I’m not). Just watch the CO² levels in the room if all shut up.

Very observant… It’s not actually part of the question but the mysterious other thermostat you pictured is part of the freeze protection on the system. it’s on the return flow to the boiler set to open at ~ 30 degrees there is another “frost stat” you can’t see in the photo on the wall set to close at 3 degrees, both are in series and then in parallel with the other zone valves that call for heat from the house.

So on a cold night assuming the insulated house is not calling for heat. Garage temperature drops to 3 degrees, risk of pipes freezing, frost stat closes, boiler gets call for heat, pump starts, boiler fires and pumps round a loop through the bypass valve in the airing cupboard in main house until return temperature reaches 30 degC. when the thermostat on the return pipe opens, call for heat stops, burner stops.

Together with trace heating on cold water/condensate pipes such freeze protection systems are relatively common round here, -15degC is not unheard of.

Back to my original post - what am I trying to achieve? an understanding of what my boiler is doing! I have various plans into the future, better zoning (evohome/tado or do it yourself) to replace the 15 year old Pegler TRV’s, perhaps something to turn the flow temperature knob lower to improve my efficency , and increase it the hot water cylinder needs heat, to prevent legionella. And at some point a heat pump, although I am in no rush for this reading some of the stories I read on here and the cold nights we can get!

For all the above - which are no more than ideas at this stage - it would be good to have some baseline data. When the boiler is actually firing (or even better it’s power) is a key part of this. It just seems harder to get than I thought!

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so measuring the boiler Flow an Return temps would be good. The calculated diff between them, will be noticeably higher when the flame is on.
An emonTH from the shop here, and buy 2 of the temp sensors-on-wires offered on that page. (“Encapsulated DS18B20 temperature sensor”)

You will need to do some software programming, to upload firmware to the emonTH to allow multiple External temp sensors, it can handle 4 or more

I use an EmonTX at the boiler to watch flow temperatures and calculate gas usage from the CT on the supply cable to the boiler. Logging is more often than 63 seconds.

And elsewhere on the system I monitor the temperature around the bypass valve using an EmonTH with four sensors.