Why would water be coming out of my shower when I turn on the bathroom sink?

Question

The other day we finally installed a sink faucet and down pipe connecting the sink to the drain in the floor of our bathroom. Up until now we have been using the tub to wash our hands. We have been working on finishing the interior of a new construction here in Argentina for over a year so we were excited to finally have a civilized bathroom to use.

To our horror when we used the sink faucet for the first time, water simultaneously came out of the tub faucet. The shower has 3 knobs, left hot, right cold, and middle selects shower or tub. Water comes out of both the shower and tub facet depending on the selection while the sink facet is on.

I would like to add that we have a tank (1000 liters? enough for one home) on the roof to provide the water pressure and that the tank may not have been at full capacity/pressure at the time, since we don't have the permanent pump hooked up yet. I don't want to over react to the contractor if it just that we had low pressure, but based on other things I have witnessed I am a bit concerned that I will need to remove all our furniture and fixtures for them to make a correction.

Please tell me it has something to do with low pressure.

Answer 1

Any single handle/single spout faucet in effect connects the cold and hot water lines. That's part of your answer. You'll have to do more experimenting with the tub controls and both hot and cold water supplies.

Low pressure itself is not the cause, but low pressure could lead to flow reversal when a tap is open.