We've been trying to identify the cause of what smells electrical in a wall near the dining room in our old house built in 1911. The smell has been there for a long time, at least 6 years. The house has been renovated by others over the years and we had the knob & tube wiring replaced about 3 years ago.

We've had 2 electricians and the fire department come to test the wiring and (dining) light switch where the smell seems strongest, but so far, none can find a cause: the wiring tests okay, and a heat camera didn't reveal any issues, but the smell persists regardless of whether the power is on or we're using the switch. One of the firemen thought it smelled more like a dead mouse, but that smell is different and usually disappears after 6 weeks.

Sometimes the smell seems stronger when the light is in use. But not always.

Even over Christmas when we were away and the light switch was off, there was still a noticeable odor from the switch/ wall.

Any suggestions for determining the smell i.e. if its electrical or its source?

Would you describe that odor as a "fishy" smell? Is the switch a dimmer switch?

If there is a label on the switch, remove it and clean the glue off of the switch.

Sometimes the glue from a label will heat up and release an odor. I've never heard of it happening on a switch though, all of the problems I've seen were on light fixtures with labels.

Harold: good question. Perhaps "fishy" sometimes, though more towards the burning plastic/ electrical side of the spectrum. The switch was originally a dimmer, then replaced with a new dimmer over this summer, until we replaced it with a regular rocker type early in the fall at the suggestion of the electrician.

Don: not sure if there's a label - will peek tonight.

A "fishy" odor has been reported when plastic (usually plastic light defusers) is burned or over heated. Dimmer switches, especially the older type use resistors. Resistors give off heat.

What size and kind of box is the switch in? If it is the original metal box it is probably pretty small. How many wires are in the switch box? There is a limit to the number of wires that can be in a box. That limit is there because electrical current on a wire gives off heat.

Suggest removing the switch cover plate (that would allow heat to escape) and see if odor dissipates. If box is original small box, suggest replacing with larger one.

It's a typical double box i.e. two light switches: one on the left, for the dining room light, the other for the stair light (which can also be switched on/off from the top of the stairs.)Attachment 45556

I'll have to look at the number of wires tonight.

The switch plate has been off for months now, but the smell is still evident.

Can't see your link.
You said the knob and tube wiring was replaced, Is the switch box metal or plastic? Do you see the end of the cable sheathing in the box that would indicate Romex wiring? If the box plastic there should be a small mounting screw at the top right and bottom left, indicating that it is an "old work" box which could be replaced.
Unlikely to be a heat build up in the box if the cover is not in place.

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Please see attached jpg.
Attachment 45562

Box is metal. Will reply shortly re: wiring/ cable sheathing.

I was able to see both attachments but curious if you have inspected your attic or have a crawl space or basement? Are their heat registers nearby? Or a fireplace?

We have no attic (flat roof) but have looked in the basement. We haven't smelled the smell there.

We have no fireplace but a forced air supply duct at floor level about 4' (1.2m) from the switch in question. The heated air or duct doesn't have the same smell.

hkstroud: Still have to extract the switch boxes to check the wiring and age of box.