Switching boiler with a 5V / 240V relay - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: July 08, 2019, 02:18:19 pm
content: I want to use an ESP32 to switch my oil boiler on/off.From the wiring diagram of the heating controller it simply switches the mains to the boiler spur in the garage. Boiler is 240V.From the various datasheets and manuals I can find for the boiler the start up currents for the various motors is quite low, in the order of a few amps. In one I found "Electrical charateristics under extreme conditions" which listed the maximum current draw of 3A. The burner unit has a time delay 6A fuse.Given this I should be fine placing my opto-isolated relay in series with the controller as is suggested for wiring room thermostats.Can anyone see a problem with this?Personally I'd like to measure the startup current, but I see problems. It's mains and AC so the only thing I have that would register AC current is my multi-meter and I don't fancy putting that in series in the mains. Also, even with that the startup current, most likely for the draught fan or fuel pump might be a short spike and missed by the meter.

author: paulca
author_rank: Super Contributor
timestamp: July 08, 2019, 02:24:33 pm
content: I am thinking of switching with one of these:https://www.ebay.co.uk/itm/3-3V-5V-10A-Relay-Module-for-Arduino-Nano-Due-Raspberry-Pi-ESP8266-Optocoupler/272272819712?epid=4021131680&hash=item3f64b98600:g:3W4AAOSwMNxXXo16

author: capt bullshot
author_rank: Super Contributor
timestamp: July 08, 2019, 02:45:13 pm
content: Basically, there's two or three issues to consider:- does the relay board provide sufficient isolating space between mains related circuit and your EPS32 circuit - can't tell from the ebay pictures- can the relay stand the normal operational current of the boiler - apparently yes- what does the inrush current of the boiler look like and will the relay stand this?Especially the third one can cause some headaches: For example, an ordinary SMPS (e.g. computer power supply or laptop brick) has a nominal current widely covered by the relay rating, but the relay contacts weld (to a continuous short) after a few cycles. This is due to the inrush current of these supplies, charging their input bulk capacitor. Up to a certain size, these inrush are worse than motors up to a certain size. So you should go and check your boiler for any signs of SMPS units. If it's only the motors and no VFD or other stuff, you're probably fine.

author: paulca
author_rank: Super Contributor
timestamp: July 08, 2019, 02:59:42 pm
content: I looked at a represetative thermostat device:https://www.plumbworld.co.uk/documents/1-honeywell-dt90e-digital-room-thermostat-data-sheet.pdfIt seems they rate for up to 8A, but they do note for higher than 3A they have a high energy mode on the relay. Given my relay power will come from an ESP32 I will need to check the datasheet for the relay I use to check it's coil power requirments to ensure it can break whatever the boiler pulls.On inrush current, all I did was check through any manual or datasheets I could find. None seemed to suggest high inrush currents. The burner unit is mostly mechanical with a small electronics control box. I had said control box replaced this morning, so I can open it and look for any capacitors.The boiler does have a 4uF capacitor, which I believe is for the HV electrode transformer (to create the ignition spark).

author: capt bullshot
author_rank: Super Contributor
timestamp: July 08, 2019, 03:15:29 pm
content: QuoteInteresting setting and interesting description: One could figure they're underpowering the relay on purpose, knowing this might fail at nominal load so they provide this setting to drive the relay at its nominal rating. Looks kind of dodgy to me without knowing the details behind.Anyway, as long as you use the relay at its nominal ratings (coil and contact), there's no need to increase the drive power. In case of SMPS inrush, this wouldn't save the relay contacts from welding.BTW: one can do quite a lot of interesting "tricks" with relays, regarding the power required to close the contact and to hold the contact closed, and to reduce the SMPS inrush current (by closing the contact near voltage zero-crossing, but that's real tricky and could be counter-productive if your load is a transformer or motor).

author: floobydust
author_rank: Super Contributor
timestamp: July 08, 2019, 04:07:29 pm
content: I would just use an opto+triac in series on the 24VAC control rung.A few thing to consider. It depends on your boiler.If you pull power to the boiler, if can interrupt a cool down cycle, where the fan is cooling the heat exchanger/doing exhaust after the burner switches off.It can interrupt a purge cycle, where the boiler was trying to light (fuel on+ignition) but did not succeed. It can leave a fuel build up, and in the case of natural-gas fired boilers it's an explosion hazard. They time about 30 seconds (depends on how many BTU) before another attempt. This is if you have no pilot light. The gas build up from the first attempt to light up remains and next time you switch power on, it can get ignited.A hydronic boiler has an interlock with the circ pump, so boiler off = pump off. If the burner was just on, the excess heat is not moved away and circulated, so you'd have local boiling in the exchanger.You may want an old mechanical thermostat for freeze protection, if the ESP8266 doesn't work the building temp can drop in winter and pipes freeze.ESP8266 control is not reliable, most people have no loss of WiFi (recovery) code and they can hang doing the web tasks. The code examples on the web are pretty terrible.

author: paulca
author_rank: Super Contributor
timestamp: July 08, 2019, 07:42:01 pm
content: On hard power switching the burner. I checked the boiler installation instructions and it takes a straight mains feed, so when it's turned off, it's turned off. I have read elsewhere you 'can' wire your heating water pump separately and have it run for several minutes after killing the boiler, but I don't think it's necessary. There should be some convective flow anyway. A previous house I lived in had a coal fired back boiler and the heating pump was optional, without it the radiators still heated, just much slower.It's a kerosene burner, so unburnt fuel from a failed ignition shouldn't be an explosion risk it will burn off the next time the boiler gets up to temp.The gas combi boiler I had in my last place had an anti short cycle to allow a failed ignition to disipate out the flu and it would continue to run this for a minute after before turned off. (Unless you pulled the mains feed).So I think I'm good on those fronts.For Wifi failure I can handle that in software. For software failure I could do something in electronics, but the relay being 5V high to close is a start, meaning if power fails to the ESP the heating goes off. If software failed, locked up, however with the 5V stuck high that might be more tricky and run the heating constantly. I could instead send a pulsed signal to electronics that would only close the relay if there is a pulse, a constant 5V would open the relay. As to designing that, not sure, but current across the secondary of a small coil/xformer might allow me to charge a capacitor to hold the relay coil closed, if that pulsed 5V become a solid DC 5V the xformer would not transmit current and the relay would open.That only leaves the anti-freeze mechanism and as the current system does not provide such a feature and in this part of the world the heating loop contains anti-freeze I should be fine. I can rely on software, noting the garage temperature and run the heating if it's below freezing. This will be the coldest part of the house and "unheated" although it does contain the boiler, so running the boiler should, maybe, help keep it above freezing. It does contain the laundry equipment which is a consideration for freezing. An oil filled electric radiator might be a better option for the garage.