I have no hot water in my house .My gas hot water heater goes on but only runs for like 30 seconds and stops

Is your pilot light on? Read these articles before you start changing stuff.

Gas Hot Water Heater Troubleshooting | Gas Water Heaters | Water Heaters | Plumbing | Repair Topics (http://www.home-repair-central.com/gas-hot-water-heater-troubleshooting.html)

https://www.thespruce.com/water-heater-problem-no-hot-water-4103882

Make sure you watch this SERIES of videos. Be cautious. Gas is dangerous.

https://www.bing.com/videos/search?q=gas+hot+water+heater+troubleshooting&qpvt=gas+hot+water+heater+troubleshooting&view=detail&mid=2B18509FB17D9F4174F52B18509FB17D9F4174F5&FORM=VRDGAR

Hi guys

Call the manufacturer of the water heater and connect with the technical support guys. Give tech. support the model number and the serial number of the water heater and describe your problem... they will tell you what to do next.

If this heater is 15 year old or less it sounds like the vents to the heater are clogged and the closed combustion chamber is starved for air. If the heater is older than 15 years then you should have an open combustion chamber water heater and there are number of different things that could cause the problem.

Call tech, support and get straight down to answers on the problem.

Good luck!

Mark