I run a small restaurant, we have operated it for ten years. I have recently been having some really offensive smells coming up under building. I had the plumbers in, we first emptied grease trap twice. Then got a jetter in to clean the pipes. We also, replaced a couple of sections that did not fall correctly. We have also, added an extra stink pipe. I myself, have checked all the in/out inspection ports, and the pipes are clear. However, for the last few days, since having all this work done.. the smells still comes back. And it comes back on the dot of 5.30pm. My plumber is at a total loss of what to do next. We have tried filling wastes, running taps, plus all of the above. Why does it only come back at 5.30pm. Obviously, something changes.. i.e... the air gets colder.. I know sewer gas is heavy.. I don't know of any other factor that changes at that time, except of course the kitchen is busier.. more water going down sink.. I don't know what to do next. It hasn't happened before, and we have been here for ten years.. and the lines are clean, and running correctly, the greasetrap is empty... thanks in advance for any help.