Fixing another Nest heat link - with easy tests

This appears to be a common fault with these units. If your heat link fails out of warranty it may be an easy fix to get it going again.

Here are some simple tests that can be done on an unpowered unit to check that the rest of the circuitry is still working.

Note that in the UK our boiler (furnace) control systems are usually 240V. In a way it might be better to adopt the American 24V system. Especially since electrical training standards have plummeted due to the activities of a financially motivated "voluntary electrical safety" company. All equipment associated with a heating system should be fed from just one circuit. Adding on other sections powered from other circuits can energise an isolated/disconnected system randomly via the control line.

These units have been educational. I really like the use of two different electrolytic capacitors in parallel. As well as being shorter than a single high value capacitor, if one is later found to be prone to drying out, the other will keep the system running. Likewise the use of two diodes in series means that if one fails short circuit the other will keep the system running. The small extra cost of these extra components is massively dwarfed by the cost of a warranty return/repair. Especially when the unit has to be replaced by a technician.

Note that the primary suspect component for failing is quite tricky to remove due to having a heatsink pad underneath. It is much easier to remove with two soldering irons being used as improvised heated tweezers. If you have doubts about desoldering and replacing the diode, then don't risk damaging the PCB. There are eBay sellers offering a fixed price repair service for these units.

I wonder what replacement component they are fitting in the warranty exchange units. The original is an S3100. The one I used is a B360B - which is rated 3A and 60V. I can't guarantee that this is a perfect replacement. A diode rated 3A or higher and 100V or higher is probably a better option. I've since found out that the original package is called a Powerdi.

Once I've got and tested some others in I'll add the stock numbers here.

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