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 dont 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.