First of all, Welcome to BC, Ziggy!
And....where are your from?
Now that we have those details out of the way, we might need a little more information to diagnose for you.
Does the temperature change at all when your select a lower temperature, or does it just pump out heat?
You say it has not worked "properly" since the water pump change, how so? and did you disconnect the battery when you changed your water pump?
It sounds like it could be a cracked gear in the airmix door actuator. Many here have had the gear crack after a disconnect of the battery. We don't know exactly why, but it may have something to do with the actuator cycling through on start-up and the gear ends up cracking somehow.
If you enter in "airmix" in the Search function at the top of the page, you will get quite a few topics for the airmix actuator issue. To try to see if your actuator is moving when changing temperature, you need to remove the glovebox and look through a small hole in the dash support to see if the actuator rod moves when changing temperatures.
This pic is looking through the hole, behind the glovebox that shows the airmix door actuator rod in place in the white connector that is attached to the airmix door to determine how much hot/cold air. There is small black plastic box that you can't see that drives that rod. While looking through the hole, adjust your temperature on the contol panel and observe what the rod does. It should move all the way to the passenger side for full heat, and all the way to the driver'* side for full cold. If it binds, pops, or clicks, and doesn't move or moves erratically, then the airmix actuator gear is broken or has failed