delayed tap!

[CXR100]

bath in a customers property, there is a 5-10second sometimes even more delay before water stars coming out of a 1/4 turn hot tap feed by a gravity cylinder, taps were new about 6 months ago , there is a blending valve befroe the tap , it is like there is air or something always in the pipe, the longer you leave it off the longer the delay , if i turn it off then straight on water comes out with no delay!!! any ideas???? thanks!

 

[PlumbBobBob]

tmv up the spout?

 

[AWheating]

could be the tmv is not suitable for gravity or supplys with different pressures?? most ive seen work up to a 1 bar diff between hot and cold supplys, high cold pressure and ive seen them overflow the domestic tanks