Originally Posted by dalto
I believe it needs time to heat the water. Those heaters are actually rated for a max throughput.
Honestly, I am not an expert in heating elements in flowing water. I am mostly going on information contained in the product manual.
I can't find a max flow rate in the manual that came with mine, nor the FAQ on line.
Heat transfer is this situation (all other things being equal) is directly proportional to the difference in temperature of the heating surface and the media.
There are two problems inherent in water moving too slow across the heater. The first is that if it moves too slowly, it can warm actually too much, and cause the heater to turn off before the tank is warm enough. This can also happen in an in-tank heater if there is too little water flow -- you get a warm area around the heater that turns it off, but the rest of the tank is colder.
That's unlikely with any decent amount of flow of course, but the second issue is always present -- the slower the water moves the higher the temperature of the water that is actually in the tube. This higher temperature means the heat transfer is less efficient; another way to think of it, the heater itself heats up with less water flowing through to cool it. This wasted heat is, well, wasted.
Provided you do not have a flow rate that causes air (or cavitation) to enter the lines, or damages your plumbing, or damages the tank on output, any heater is at its most efficient with higher flow across the heating surface. That's just basic thermodynamics.
All that said, I suspect the difference in efficiency at (say) 50gph (that you might use for a UV
kit) and 300gph (somewhat normal canister flow for this size) is extremely small.
But I would be curious to know where you saw a max flow rate?