I have a oil burner that heats the house and the water. My kitchen sink and bathroom sink both have hot water coming out but the tub is luke warm. Any suggestion on how to get it hot like the sinks would be helpfully..

I have a oil burner that heats the house and the water. my kitchen sink and bathroom sink both have hot water coming out but the tub is luke warm. any suggestion on how to get it hot like the sinks would be helpfully..
Not sur if this helps but I have a Temptrol Tub & Shower Unit
http://www.hardwarestore.com/pop-print/larger-image.aspx?prodNo=63855