Parallel mains switches for boiler control - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: November 19, 2019, 01:43:44 pm
content: I have a new boiler which has a 7 day timer stuck on the wall in the boiler cupboard.I want to additionally add a relay to control it independently.Previously with my older boiler I put my relay inline with the day timer, so the day timer had to be on for the relay board (SOnOff) to have power. The relay was setup to go ON when it was powered and this provided transparent control using the 7 day timer, should I need it.I'm not sure I want to do this again. I want to run the 240V mains output the boiler provides and send the "Live return" back for the heating control from my relay. This would mean I have two return paths for the "live return", one from my relay, one from the 7 day timer.In theory there shouldn't be an issue if I have a live return from both the 7 day timer and an additional relay. If one relay is off and the other on, it will just energize the disconnected pin on the other relay. If both relays are on then both relays will be at "live" / line voltage, but that's fine too.Am I missing anything here? So two parallel circuits:Mains Out -> 7 day timer -> heating LRMains Out -> Smart Relay -> heating LRNote, the SOnOff is the more recent version after CE certification and it will (I hope) be mounted in a fire safe box on the wall with proper strain relief glands to prevent accidental exposure to the mains should it get knocked off the wall etc, though unlike as it's in a cupboard. Should also help contain any potential fire if it were to fail.

author: paulca
author_rank: Super Contributor
timestamp: November 19, 2019, 03:26:32 pm
content: I could get unlucky and one of the two relays switches it's output to neutral when it's off, that would then blow the fuse.Maybe putting the two in series again is the safer option.

author: Docara
author_rank: Regular Contributor
timestamp: November 22, 2019, 10:45:06 am
content: HiCould you explain a little more why and what you are trying to do.Are you trying to have an additional control to turn the boiler on when required? If so get a new timer with a 'Boost' switch at least then you could parallel up the switch. The beauty of this is with all the boost controls I have dealt with 1 press = 1hr, 2presses = 2hr and up to 3 then off in a cyclical manner.

author: mikerj
author_rank: Super Contributor
timestamp: November 22, 2019, 03:11:41 pm
content: Providing both the timer and your extra relay are both SPST devices (i.e. they both either make or break the circuit rather then e.g. switching between live and neutral) then paralleling the switches is fine.

author: paulca
author_rank: Super Contributor
timestamp: November 23, 2019, 12:01:20 pm
content: Quote from: mikerj on November 22, 2019, 03:11:41 pmYes, this is what I thought, but one of the switches (the 7 day timer) is a sealed unit and asides testing for continuity to neutral while it's off I can't really test this.I figure it's quite unlikely they have DPST switches which switch the output to neutral, but the results if they do would be a bit nasty. The last timer switch I had, found the manual for it, actually switched the live between two outputs so you have Live out ON and Live out OFF. Given that these are all meant to mount to an industry standard back plate, it's likely they all switch the same way.I'll see if I can google up the manual for this one and maybe do some careful testing. Slightly annoyingly the boiler is on the main plug ring circuit, so to make it safe I have to take out most of the house power

author: MarkF
author_rank: Super Contributor
timestamp: November 23, 2019, 12:18:48 pm
content: Quote from: paulca on November 23, 2019, 12:01:20 pmActually, they do here in the states at least.https://www.lowes.com/pd/Hubbell-30-amp-Double-Pole-Ivory-Toggle-Industrial-Light-Switch/50179199The switch disconnects both lines (neutral and live).Do not confuse them with a 4-way switch that swaps the line pair when switched.There are also 3-way switches that switch one of two lines to a third output line.

author: paulca
author_rank: Super Contributor
timestamp: November 23, 2019, 12:23:41 pm
content: I found the installation manual and it looks good, standard back plate. Switch is between two live outs.https://www.ephcontrols.com/wp-content/uploads/2017/05/20180621_R17_InsIns_JW-1.pdfIt also gives me an easier access to wire my relay to as I have mains power on that back plate and access to the live return terminal for the boiler.So if I connect the SOnOff to the L and N for power and bring the output live back to pin 4 then I should be safe to have either or both relays closed.This is only phase 1, I'm investigating an interface card for the EMS data link to the boiler as this will give me access to monitor the various boiler metrics and automate the heating flow temp. ie. backing the temp off as the target temp is approached in the house rather than switching the boiler on/off. Need to do a lot more research before I'll feel safe connecting one of the interface boards to the boiler and there is a remote but present chance you can send erroneous messages to the boiler and damage it.

author: sokoloff
author_rank: Super Contributor
timestamp: November 23, 2019, 12:32:15 pm
content: If your boiler has “outdoor reset” (almost all do or can support now), configuring that is likely simpler and “better” in terms of support, safety, and serviceability than trying to roll your own.Principal is that outdoor temp is an excellent proxy for heat loss, which is an excellent proxy for heat load demand, which drives the boiler circulation temp target, saving money and adding comfort. Whether the boiler operates bang-bang or with variable output to maintain that target temp doesn’t matter as much as our intuition would suggest, provided there’s enough mass in circulation to stop really short cycling of the boiler.

author: paulca
author_rank: Super Contributor
timestamp: November 23, 2019, 12:57:11 pm
content: The boiler does have an outdoor probe, but it's not installed. It also has a blanking plate for the electronic control system, which is not installed. The reason for this is because of cost and 'proprietary lock in'. If I was to request this when it was installed I would have received 2 Worcester Bosch Wifi thermostats. Expanding the system would be quite expensive as EMS/Wifi thermostats and radiator valves are very over priced. You also get forced to accept how Worcester decide to control the heating with little to no flexibility if you disagree with this.I am also somewhat at odds with modern "smart" heating controls, they are typically steered towards convenience and aesthetics, as well as over priced with non-standard interfaces, meaning once you choose your brand you are stuck with it and they can charge you whatever they like for adaptions. Then there is the privacy, data-mining, "cloud" reliance and all that nonsense to consider.A primary example of this is digital set-point radiator valves. I don't care how many times someone tries to explain these or sell these to me, they are not worth it. Well, they are not worth it if you are prepared to go the extra mile and install actual temperature probes in rooms away from the radiator. Putting the temperature senor beside a radiator, actually attached to it, is just ... em... dumb. The only good reason to do this is to save on having to install a proper temperature probe elsewhere. In order to modulate the room temperature they need to use complex machine learning style routines switching the radiator on and off and watching the temperature gradients to try and "deduce" the room temperature while the temperature probe itself is completely blinded by the hot radiator beside it.These types of system, including TRVs are notorious for being bad at heating up a room/house and tend to modulate a lot before eventually stabilising. So they are better suited for having the heating running 24/7. If you switch the heating off during the day while you are out and the house gets cold, when you come home and the boiler comes on the radiators get hot, the TRV shuts off, long before the room gets warm, they osculate on and off for hours before the room temp stabilises. It's extremely annoying when you feel the room is still cold but the radiator valve has shut off as "it" got warm.As such I intend to replace the TRVs with electro-thermal radiator valves in each room I want to control, with a temperature probe in a suitable location in that room. Each of these forms a demand zone. Obviously if the target temp is not reached the radiator will be on. Then taking the largest differential between actual and target for any zone (and the outdoor temp) I can choose the heating loop flow temp. So when I come home in the evening the heating will have gone on, high temp, radiators on and as the target temp gets approached various radiators will start to shut off and the boiler will be backed down to a lower temp.It's been a project I have been working on for a long time, been designing it for years, but can only now start to put it into action as I now own the house, previously renting so couldn't do much.Two of the hardest parts of the requirements I have set myself are 1) Fault tolerance / Fail safe and 2) trying to maintain transparency for manual control. Both leading to ease of use and allowing the system to still be operational if my hardware or software has a failure, such as the Wifi going off. In such a case the automated heating will default to off, but still allow me to control it manually.