Recently my water heater's thermostat failed. It's just a simple mechanical one, with a piece of metal that "pops" and connects or disconnects the heating element.

I would guess that what made it fail was arcing in the contacts. How would I calculate a capacitor (or RC snubber) for this application?

The heating element is 1500W 220V.

I remember I used to have a small space heater with a similar thermostat and arcs were pretty spectacular whenever it switched on or off.

  • \$\begingroup\$ This question appears to be off-topic because it is about appliance repair, there is no design element and very likely very little electrical as this is fundamentally a mechanical switch. \$\endgroup\$ Jul 11 '14 at 17:11
  • \$\begingroup\$ The DIY forum would likely be a better place for this question. \$\endgroup\$ Jul 11 '14 at 17:12
  • 3
    \$\begingroup\$ @placeholder actually the design question is very specific. How to calculate a capacitor or RC snubber for this application. It happens to be aimed for a water heater, but it applies to any sort of mechanical relay arcing. Even the question itself doesn't mention the water heater in particular. \$\endgroup\$
    – hjf
    Jul 11 '14 at 18:00

My first point is that you might find that the mechanical thermostat is cheaper than the capacitor to achieve the same reliability. My 2nd point is that adding an extra component is potentially halving the MTTF (mean time to fail) of the system. It could be worse than that - the capacitor could, in certain circumstances make the problem worse (3rd point).

With a resistive load (the heating element), there will be an initial surge of current as the contact closes because the resistive element is cold. If you put a snubber capacitor across the contact then this snubber will have been charged from the previous opening of the contact and now, when the contact re-closes, it will see the normal peak associated with the element plus a discharge from the capacitor. In other words you' might make the thing fail quicker than before.

It's unlikely that there is much inductance in the element so the contact opening isn't going to create much of a spark so I'd say the bigger issue is when the contact closes and a snubber capacitor won't help.

Advise - try and understand the mode of failure then conjure up a decent fix

  • \$\begingroup\$ Actually the replacement thermostat was $80 USD... and I was also surprised that it failed so early (the water heater is about 2 years old. I had other units last 15 years without any issue). I thought it must have been just bad luck. When i asked the seller "Do these fail often?" he said "well, I don't know but i can tell you we sell these every day". \$\endgroup\$
    – hjf
    Jul 11 '14 at 16:56
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    \$\begingroup\$ @hjf - it looks like you need to buy thermostats elsewhere! \$\endgroup\$
    – Andy aka
    Jul 11 '14 at 17:01
  • \$\begingroup\$ Hhaha yes! But the worst part is, the seller assured me these are the "original" Rheem replacement parts. It doesn't say Rheem anywhere on it but it's exactly the same that my heater had. \$\endgroup\$
    – hjf
    Jul 11 '14 at 17:03
  • \$\begingroup\$ @hjf: Counterfeit parts are rife these days... \$\endgroup\$
    – EM Fields
    Jul 11 '14 at 18:29
  • \$\begingroup\$ @EMFields Oh I know. I often use power transistors. I know a thing or two about counterfeit parts :P \$\endgroup\$
    – hjf
    Jul 11 '14 at 21:51

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