I have a 5'-diameter whirlpool bath. I recently installed a 1500W in-line heater but it doesn't keep the tub warm - in fact it has no discernible impact on temperature at all. There's voltage to the heater and it appears to be working. I removed it and plugged it in dry for a few seconds and verified that the element gets hot. I've surveyed the ads and the largest heater advertised is 2000W, which isn't that much bigger than I already have. Is the heater just too small for this size tub? Would a 2 KW heater make enough difference to justify the cost of a new 220V circuit? Any other suggestions would be welcome. Thanks.

Is there a way that you can slow down the flow of the water going through the heater? Maybe you could install a flow control valve and this would give your heater a chance to heat the water hotter as it passes thorough.

Please let me know if this helps.