Bath water gets colder the longer it runs

[ChiHomeOwner]

I just had a new bathtub installed, including a new stack for the shower. The problem I'm having is that when I turn on the water, it starts off hot, but gets colder very quickly, until there is no hot water running out of the faucet. This does not happen with any of the other faucets in the house, and they still run hot after the bathwater is running ice cold, so it's not a problem with the water heater.

I've got a contractor working in another part of the house, and he says it's because when the stack was installed, it wasn't installed with an air chamber and that affects the pressure of the hot water. Is this correct? Is there another reason that might be fixed without pulling down the shower wall to install air chambers on the pipes?

[jimbo]

That air chamber crap is a lot of hooey. It is possible that the tub filler is a pressure balanced type of valve, and for some reason it is shifting to all cold. More details would be required, and probably some on-site Dick Tracy work ( like possilby checking performance with the balance spool removed)

[jadnashua]

If the supply lines were installed backwards or the cartridge was removed during installation then replaced backwards (on some types), you could be getting cold when it should be hot. Can you make it hot by turning it towards the cold side rather than all the way towards hot? Also, the high temperature limit adjustment could be way too cold and you just can't turn it towards hot as far as you need to. The incoming cold water that's in the pipes has warmed up some, but once you purge that, since the cold is going to be colder, the mix will cool off, too.

[ChiHomeOwner]

Thanks for the feedback, all. The hot water does come out on the "hot" setting, it just diminishes within a minute or two of being turned on.

The faucet is a Price Pfister Avalon (http://www.pricepfister.com/bath/product_catalog.aspx?room=Bath&category=Bath%20-%20Tub%20and%20Shower&collection=Avalon).

If I leave the tub off for an hour or so, the hot water comes back, but again, only for a minute.