Additional knob/valve inside shower wall

Hi,
First time posting in the forum - hope i get it right. I'm a plumbing novice and am not sure what the additional knob below my shower handle stem is. I've been trying to find out via the internet but it's difficult because i don't know what to search for. Most pictures of two-handle system (e.g. here http://www.diyadvice.com/diy/plumbing/faucets/tub-shower-stem-compression/) do not show the lower knob or valve.

I'm assuming it is, or was (my construction is original 1971) common as these tear-drop escutcheons are something i've seen a lot and I assume were intended to disguise this additional part. However that is as far as my deduction has taken me.

Has anyone seen this part? What does it do? I thank you in advance.

My guess is they are integral shutoffs so that you can work on it without having to turn off the entire house.

Well, an integral set of shutoffs would be that, a set, and there's only one!

If I had to guess (I'm not familiar with many different ones), it might be either the spool pressure balance valve, or it's a max temp adjustment.

jadnashua said:

Well, an integral set of shutoffs would be that, a set, and there's only one!

If I had to guess (I'm not familiar with many different ones), it might be either the spool pressure balance valve, or it's a max temp adjustment.

Click to expand...

The one your looking at is on the cold side. The second one would be on the hot side. It's a two valve shower valve.

John

jadnashua said:

Well, an integral set of shutoffs would be that, a set, and there's only one!

If I had to guess (I'm not familiar with many different ones), it might be either the spool pressure balance valve, or it's a max temp adjustment.

Click to expand...

I reject your reality and substitute my own.

Alas johnjh2o1 is quite right, i should have attached a photo of the entire set-up. Here it is:


Based on LLigetfa's suggestion, i found this: http://blog.********.com/2009/06/shower-valve-with-stops-what-does-that.html
Perhaps that is it. It is either stuck or requires a key, as it does not turn with a screw driver.

They usually snap the slotted head when you try to turn them. Usually completly useless after a few years.

hj said:

They usually snap the slotted head when you try to turn them. Usually completly useless after a few years.

Click to expand...

Oh no! Bad news. Thank you for letting me know. Do you know if, when that happens, it creates disaster (i.e. water spraying out)?

Also, any use spraying on some penetrating oil or something on it before I try it out?