We tried one of the hot water heaters, but we did not try the other one. When we were to try the other one, we realized the plumber forgot to connect it to the cold water. There was no water flowing to the hot water heater at all. Since the drywall was already done, and we hoped that the walls did not need to be opened to connect the hot water heater to water.
The plumber came and fixed it without having to open the wall. Luckily our plumbing is accessible from under the house. When the plumber left, we decided to try the hot water heater and we noticed that the hot water heater turned on even when we flushed the toilet or used cold water in the sink. After some testing and troubleshooting, we realized the plumber connected the water for the entire house to go through the hot water heater. This means that we had hot water in the toilet as well as the cold water valves leading to the sinks, etc. Now there was no cold water in the apartment at all!
We called in the plumber and his boss who both came out to investigate the situation. After a day of work, they finally diagnosed and fixed the problem. We now had both hot and cold water!
We started using the sink and toilet as normal. We could now stay in the house while we worked on it, as we had electricity, a sink, a toilet, hot and cold water, and heating and air conditioning. After a few days in the house, we had another plumbing issue. The main drain to the house backed up. We realized this when the drain water started coming out of the unfinished drain in the downstairs bath. We called the plumber back, and again he fixed the problem.
So far, we haven't had any other plumbing issues. We really hope that we don't need to call the plumber for a while.