High flow heaters
No matter what shape the heater is, a high flow heater (typical name flow thru) utilizes a hi-watt density element (hwd) that requires a minimum 22.5gpm to keep it cool. A typical 2 speed spa pump provides this minimum flow at low speed. When flow is restricted below 22.5gpm, the element runs hotter than designed, & spa heater element life is shortened. Bath heaters are always high flow- hwd, as bath pumps always exceed 25gpm. Hwd heaters come in many shapes like flow thru, Tee, “L-shape & canister. You will not find hot tub parts catalogs separated into high flow & low flow, but you do need to know this info to be an expert.
Low flow heaters
Low flow heaters are designed for vertical mount operation. The exception is the 1” tube or Laing spa heaters we supply. Either design is made with a much longer, cool running low watt density element (lwd) These heaters & elements are made to operate on flows greater than 9+ gpm. It makes perfect sense that low flow heaters are paired with low flow circulations pumps like Laing & Grundfos spa pumps, which provide minimum 9+ gpm with a spa ozone injector. Low flow heaters contain more material & labor, this is why they are more money.
The enemy to all elements,
Besides corrosive water chemistry, slow flow, bubbles & air pockets reduce element life. Air that comes in contact with the element, allows the outer sheath to rapidly heat & promote a breakdown to the filament. 3” low flow heaters are mounted vertically, so the bubbles are constantly exiting without entrapment. 1” tube heaters like the Laing design, do not trap bubbles, so they can be bent into bizarre shapes, & even lay on the floor.
Spa pumps are designed to be flooded & should not gulp or be allowed to cavitate air for delivery through heaters. Additionally, placing an ozone injector before the heater, insures a more rapid element death & confused hot tub heater sensors.
Some repair techs either don’t know, or try to save money by placing a high flow heaters, into a low flow circulation plumbing system. It actually works for a while, then the element burns out, along with plastic parts, piping & wires. Be sure and confirm that your spa tech is using the correct heater for your spa.
Hope this helps.