Baxi 836 boiler
- Thread starter Jenny3
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Receiver would be replaced, boiler side termination need not be disturbed. The wireless unit would be pre paired. Intelligent thermostat which present is, makes no odds in this case as 'intelligence is at the thermostat not at the boiler and boiler control method remains the same
How robust. Relay in either case will be rated at 5amp if lucky. Try a test of putting 13 amp fuse and shorting the switching terminals- see what happens. Fuse will remain intact but timer/ programmer/ robust switching unit/ intelligent unit will all croak it.
There is no need or requirement for this
Method of switching makes above irrelevant when boiler control is concerned. To think of above is overcomplicating the issue when an on/ off switch would suffice, be it then the boiler control becomes manual (in fact a lot of elderly folk just switch the boiler on manually and switch it off when they get to hot- an on/ off switch plate would be more at home with such users so capacitance, resistance, reactance, switch spikes are of no importance)
All they quote is current rating of about 3amps
Some folk do not know a 13 amp fuse in the spur unit need to be replaced for a 3 amp fuse. There are so called heating engineers whose vans and letterheads proclaim super efficiency and professionalism, yet unable to wire a boiler so some boiler come with lead already connected and they still make a hash of it; what makes you think they will be able to make sense of over complicated scenario where load matching is required with factor correction
But the majority of it is based on the knowledge and skill acquired in a 50 year career in electronic engineering.
"" a relay is just a switch operated by current in it's coil "" How many ties have I heard that.
It is far more complicated than that. https://www.machinedesign.com/archive/article/21814917/tips-for-selecting-relays
A relay designed to switch 230 Volt AC but used to switch 24 V DC may have a short life due to prolonged arcing when the contacts open. ( when switch AC the arc will extinguish within 20 mSec ( at the zero crossing point of the AC waveform )
Back to possible reasons for the OP' problem of failing Nests
The installation guides for the Nest show the output as being able to switch either 24 or 230 volts depending on the boiler's control voltage.
There is a difference of opinion between Nest and Baxi about where the 230 volt control supply for the boiler should come from
A speculation is that if the Nest is wired as per the Nest diagram ( Live from supply ) then bk1 on the boiler could be supplied with 230 V AC when bk2 is not providing the control supply. 230 V on bk1 via the Nest without 230 V on bk2 could result result in higher than expected current through the relay contacts in the Nest.
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Bernard, wiring the boiler like your example would make some of the circuitry within the boiler live when 2 pole isolator on the control panel in in the off position.
Furthermore, a lot of boilers are addressed to run or not via optoisolator, current for which is about 20ma at 2 volts input
nest wiring shown would almost certainly be used on a heat only boiler where motorised valves are employed. It is not the first time had to replace a pcb where a 240 volt demand was applied to a boiler that required voltage free switching. Nest and hive are fast becoming fashion controllers where the end users are self fitting not then confusion takes hold when looking at multiple terminals
Almost everyday there is a wiring query in plumbing section where an end user is seeking advice given free of charge by regulars like Stem and other professionals
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I suspect Baxis wiring diagram is more from a point of view of compliance with the Electricity at Work Act and safe working procedures when working on boilers.
Those regs require checking the absence of 240V between the L,N & E terminals. Clearly bodged wiring (say a boiler & controls fed from 2 voltage sources) would not be picked up by those tests,
so to mitigate that circumstance perhaps they insisit on the controls Live feed coming direct from the boiler terminal block.
Now of course many timeclocks feed their relay commons from the Live within the timeclock so would not be suitable
The whole situations a shambles but that's to be expected from the boiler industry.
Reading the forums a dead Nest is not that uncommon....looking at the teardowns there's a ridiculous amount of tech inside to turn your heating on (several ARM cores for starters).
Those regs require checking the absence of 240V between the L,N & E terminals. Clearly bodged wiring (say a boiler & controls fed from 2 voltage sources) would not be picked up by those tests,
so to mitigate that circumstance perhaps they insisit on the controls Live feed coming direct from the boiler terminal block.
Now of course many timeclocks feed their relay commons from the Live within the timeclock so would not be suitable
The whole situations a shambles but that's to be expected from the boiler industry.
Reading the forums a dead Nest is not that uncommon....looking at the teardowns there's a ridiculous amount of tech inside to turn your heating on (several ARM cores for starters).
I'll bow to the electronic experts here but I have experienced expensive board failures and questioned it with manufacturers and am not proposing this as an answer to the OP but... Surge protectors prevent over current mostly, fuses protect against overcurrent but voltage surges can be damaging too.
I had to replace very expensive GV solenoids on commercial boilers and relays on control pcbs following a power cut or maintenance in an area where supplies were switched to enable continuity of supply. Upon switching the voltage can be very high and cause damage IF the appliance is operating at the time.
A radio ham enthusiast told me after some expensive valves from a well known supplier suffered failures in Slough/Reading area they logged his incoming supply. On seven or seventeen times, I can't remember, there were surges in excess of 1,000volts in a year. Southern Electric denied it until shown the data.
A nursing home I worked for got compensation from Eastern Electricity due to the fact that the village supply could be switched from one end to the other during maintenance which would take out the lift motor (£1000 20 years ago) and the boiler solenoids if in operation. It happened several times till they notified him of the work beforehand so all could be powered down.
I had to replace very expensive GV solenoids on commercial boilers and relays on control pcbs following a power cut or maintenance in an area where supplies were switched to enable continuity of supply. Upon switching the voltage can be very high and cause damage IF the appliance is operating at the time.
A radio ham enthusiast told me after some expensive valves from a well known supplier suffered failures in Slough/Reading area they logged his incoming supply. On seven or seventeen times, I can't remember, there were surges in excess of 1,000volts in a year. Southern Electric denied it until shown the data.
A nursing home I worked for got compensation from Eastern Electricity due to the fact that the village supply could be switched from one end to the other during maintenance which would take out the lift motor (£1000 20 years ago) and the boiler solenoids if in operation. It happened several times till they notified him of the work beforehand so all could be powered down.
In the absence of 230 V on boiler terminal bk 2 it might be that the 230 V being fed onto terminal bk 1 from the Nest is supplying several parts of the boiler normally supplied from the boiler's own power system. This abnormal load might be enough to over stress the relay contacts in the Nest.
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