I am so frustrated! I do not think my bathtubs were installed correctly. I run hot water into the tub and by the time I sit in it, the water has become bone-cold (especially on chilly nights). We have a large water heater, and the water that comes out of the tap is H-O-T. The shower/tub combo is on a slab foundation, and I don't think it was insulated correctly. Does anyone have any suggestions?

Here are some possible problems that need to be considered:

1)-Your water heater (if electric) has a heating element that has failed. Symptoms of a failed heating element would include hot water intiially coming out of the tap quite hot and quickly going cold.

2)Mal-adjusted temperature adjustment valve for the shower/bath combo. Modern shower/bath diverters are required by plumbing code to be temperature and pressure sensitive. What this means, among other things, is that the hot-cold handlemust be able to be adjusted to control the temperature. If there are temperature differences by raising or lowering water heater temps, or if the water heater has failed producing only mildly heated water, the temperature balancing valve in the shower will also nee dto be adjusted.

Bottom line is, if you can't figure this out for yourself, you need to hire a plumbing professional who can do it for you.