Customer service

The following is an actual telephone conversation I had today with a customer.  She had called us because she had no hot water.  We replaced a faulty immersion heater.

Stupid woman: Hello, this is Mrs X.  You sent a plumber to my property this morning to change an immersion heater.
Me: Hello, yes.  How can I help?
SW: Well I'm a bit worried about it.
Me: Oh dear.  What seems to be the problem?
SW: Well my old one made a sort of whistling noise when it was heating the water, you know?  But the new one isn't making any noise at all.  It seems to be dead.
Me: Right.  So you still have no hot water?
SW: Oh the water's nice and hot, but the immersion heater isn't making any noise.
Me: Well if your water's heating up that would suggest that the new immersion heater is working.
SW: But it isn't making any noise.  I'm just worried that it's not working properly.  Could you come back and check it?
Me: If your water's heating up then it must be working.  I don't think you need to worry.
SW: Really?  Even though I can't hear it?
Me: Yes.  The old one probably made a funny noise because it was scaled up.
SW: So you don't think there's a problem?
Me: No, I'm sure everything is fine.
SW: Okay.  It's just that it seems to be dead.  I'm a bit worried.
Me: It can't be dead if your water's getting hot.
SW: Oh, I see.
Me: Okay?
SW: So I don't need to worry then?
Me: No.
SW: It's just that the old one used to make a noise when it was heating the water.
Me: It was probably scaled up.
SW: But the new one isn't making any noise at all.  It seems to be dead.
Me: And yet your water's getting hot.
SW: Yes.
Me: So it can't be dead.
SW: Right.
Me: Okay?
SW: So what you're saying is that I don't need to worry?
Me: That's right.
SW: Even though it doesn't seem to be doing anything?
Me: Your water is heating up.  It must.  Be.  Working.
SW: Yes, I see.  I'm just a bit worried, that's all.
Me: Try not to worry.  Everything seems to be fine.
SW: The old one used to make a noise though.
Me: Probably scale.
SW: This new one doesn't make any noise at all.  I've held my ear to the cylinder and I can't hear anything.
Me: Right.
SW: I just don't think it's working.
Me: *sigh*
SW: Hello?
Me: Your water is getting hot.  That seems to suggest, quite strongly in fact, that the immersion heater - the thing that heats the water - is working.  If it wasn't, the water wouldn't be getting hot.
SW: Ahh.
Me: You see?
SW: So what you're saying is, the new heater is working even though I can't hear it working.
Me: Exactly.
SW: Right.  It's just that the old one used to make a noise...

This is the abridged version.  The full conversation, during which I considered ending my life, continued along these lines for ten minutes.