Brand new water heater: Starts hot, then water gets cold after few seconds
We just moved into a brand new condo which has its own water heater. We went to take a shower this morning and the first few seconds of the shower were very hot, then it went cold after 10 seconds. We tried again after half an hour, same thing. There was enough hot water for my husband to shave but it was impossible to take a decent shower. Each time we'd have to turn on the shower, we'd get the hot water for 10 seconds, then turn it off again, wait a minute then turn it on andget hot water again, then it goes cold and so on and so on to piece a normal shower together!
Any advice?