IPhone keeps using data instead of home Internet?

I have an iPhone 4. When I'm at home, my phone randomly starts using data (3G) instead of my home Internet connection. Because of this, I'm really close to passing my monthly data limit. Is there anyway that I can prevent my phone from going to data at home, something in the settings maybe?

Turn on WiFi and turn off mobile data. I do that with my droid and it feeds off my WiFi and remains disconnected from my phones internet network- idk much about Iphones but you might like a droid have a switch in your data settings. Try that - if it turns off all data on your phone you might have an older model or a phone that feeds off of the mobile data when the WiFi disconnects and their may not be a button for it. You can always speak to your provider- they have trained techs who know about your phone so they can sell it too you in the first place

I don't have the iPhone. However sometimes my phone does this as well. It might just be because your wifi connection is low, or because you're too far away from your wifi monitor. If all the sudden you notice that your phone is using 3G instead of your home wifi you can try going into your settings and making sure that its connected to YOUR wifi. If that doesn't work well then I don't know.

Make sure it always shows the wifi symbol instead of 3G whenever you're using it. Make sure you're not streaming on 3G.

Turn data roaming off, since there isn't a need to use up extra data while you're away from wifi.
Settings>General>Cellular

You could also turn off 3G while you're at home, but remember to turn it back on. Same with Cellular Data, if necessary. Remember to turn those back on when you leave. That's all I really have to suggest. Make sure your internet is stable, since outages or resets could switch it back to 3G and it may not automatically change back to wifi. Your iPhone should automatically connect to known networks though, so I'm not exactly sure what the problem is. Sorry if this wasn't any help, but this is all I can think of.

Go on settings and turn off mobile data or roaming, or 3g lol