https://social.technet.microsoft.com/Forums/en-US/7819a3cc-2b07-4656-aa75-872e400e2629/sharepoint-search-need-to-search-only-list-items?forum=sharepointsearch

1. In search center in results page, click Edit page.
2. In search results web part, click dropdown in the upper right corner -> click Edit Web Part.
3. Click Change query -> in Property filter field -> choose contentclass, Contains, Manual value, type STS_ListItem_Announcements -> click Add property filter.
4. Repeat Step 3, type contentclass of list items you want to search excluding document, such as STS_ListItem_Comments, STS_ListItem_GenericList, shown as below figure -> ok

For generic list add

"STS_List_GenericList": // Custom List

https://social.technet.microsoft.com/Forums/en-US/7819a3cc-2b07-4656-aa75-872e400e2629/sharepoint-search-need-to-search-only-list-items?forum=sharepointsearch

1. In search center in results page, click Edit page.
2. In search results web part, click dropdown in the upper right corner -> click Edit Web Part.
3. Click Change query -> in Property filter field -> choose contentclass, Contains, Manual value, type STS_ListItem_Announcements -> click Add property filter.
4. Repeat Step 3, type contentclass of list items you want to search excluding document, such as STS_ListItem_Comments, STS_ListItem_GenericList,   -> ok

For generic list add

"STS_List_GenericList": // Custom List

If it's a custom list template you can specify the type in a similar manner. For example, for template type 10001 you could do STS_ListItem_1001.