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Problem with new chimney liner?

Masons rebuilt my chimney and installed a TiMoly stainless liner. Previously the water heater(3") and the draft induced furnace(80% 3" with an expander 3" to 6" to fit the old exhaust) went into separate holes in the wall into the chimney. They now go into a Y connection attached to the liner and everything has been closed up with furnace cement. When the draft inducer blower starts and the furnace lights hot exhaust gas (the water heater is idle) comes down and out the water heater exhaust. Is this normal? I keep thinking CO. I know the system is sealed but it is best to know if it is OK. The water heater exhaust is now a 4"pipe that sits loosely on the water heater exhaust cap. If the water heater and furnace are both on maybe both will vent partially into the house? Thanks!

I think you have three options, It sounds as though you don't have enough draft for the volume of air the draft inducer is creating.
1. Try moving the draft inducer higher in the chimney column.
2.Install a bigger chimney liner all the way up.
3. Add more height to the chimney to create more draft.
Yes, you are correct, you are getting CO if the exhaust is coming down the water heater chimney, this problem needs to be corrected ASAP.
I think my first try would be to move the inducer further up the chimney, you are probably getting turbulence and that is what is causing the back draft.