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Uses two 7.4V batteries and Far InfraRed Ray Technology to warm your core. $390, www.HeatedWetsuits.com

From Thermalution:

The Thermalution Power Heated Undersuit’s purpose is to turn your ordinary wetsuit into a comfortable heated wetsuit. Designed to keep you warm while you are on/under the water for 1.5 to 3 hours, the product consists of a traditional short sleeve lycra modified to accommodate Thermalution’s waterproof battery system with non-metallic Heating Wire Technology and temperature controller. It must be worn under a wetsuit or drysuit and does not heat you up if worn on its own.

From TKB:

We were sent the 15m model which comes with the undersuit, two batteries, charger, and controller. It is good down to 75 feet and retails for $390.

Tom Reilly, Central California Coast: First impressions: This is a nice looking rash guard but has a lot of wires and connectors. I charged up the batteries and read the instructions. I wanted to see  how this thing worked. The fabric felt smooth and the seams were well done. The two sealed batteries sit in outside pockets that lay against your side or lower chest.

The batteries are completely waterproof and sealed. They have a screw on connector to keep the water out. The suit felt heavy when the batteries are connected.

I was SUP surfing in 58 F water and low 60 F air temp while using my 3/2 billibong wetsuit.

I felt that the undersuit was heavy at first but did not notice the weight of it when I had it on. The batteries seemed bulky but then I forgot about them over the course of the day. There is a sealed power switch that is connected by a cord that sticks out of the wetsuit collar. This powers it on/off and changes the temperature settings. Finding a place for the best spot to tuck it in while surfing would be dependent on the wetsuit style. I liked the color light that tells you what the temperature setting is. Green for low, yellow for medium, and red for high.  When the light is off , so is the heat.

I tested the suit at the car before heading into the water and WOW did it heat up FAST. I timed it to less than 10 seconds. I put it on the lowest setting and it felt very warm. I turned it off and then tried it in the water. I flushed my suit as suggested and then turned the suit back on. It felt like the sun was shinning on my back. The heat felt good but I would not go any higher than the low setting. As I was surfing, I turned it on and off over the course of an hour. I did not see the need to leave it on as the rash guard material added warmth to my suit. I did enjoy the second day wearing it since my wetsuit was cold and wet from the day before. This time when I turned it on while surfing it took the chill out even after some good wipeouts.

This would be a great addition for folks like me who get cold easily or when diving.

As for kiting, I usually kite in my dry suit so getting cold is not an issue. I would use it again in a wetsuit however. The placement of the batteries may get in the way of certain harnesses.

I would recommend this in very cold water. I would also like to see the batteries smaller, maybe half the size and the heat setting lowered to low and medium.

Marina Chang, Central California Coast: I get cold very easily and was eager to try this suit out as I basically wear a 5/4 year round. I tried the suit out on an unusually warm day in San Luis Obispo County but it was still cold enough that everybody was wearing at least their 4/3 suit and hoodies.

The air was around 60 and water around 51-53. The system is pretty intuitive but I read the directions thoroughly. Frankly, putting something electrical on my body in water freaked me out a bit.

I put the Undersuit on under a 4/3 knee-length Mystic wetsuit. I set the temperature on medium and in less than a minute(this baby heats up fast!), the suit was too warm, so I turned the temperature to the low setting.

The conditions were pretty gusty and the waves were 2 feet at best, so I basically cruised back and forth for about 1 hour and 20 minutes before coming in. I felt very comfortable the entire session, except I should have moved the battery packs up a bit as they were hitting the bottom of my harness. This was due to the top being a bit too big for me.

After getting out the water, I kept the Undersuit on under my wetsuit and took about 10-15 minutes to derig and get gear back to my car before changing into my street clothes. That night, I noticed I had two red marks on the left side of my back.

From the impression, it looked like it was caused by where the battery connectors meet. The marks did not hurt but they did last on my back for the next few days. I called Caleb from heatedwetsuits.com (also a kitesurfer) who said that they had had a few cases of this before but it was extremely rare and the red marks in other cases had not lasted more than a few hours at the most.  He then asked if I had flushed my wetsuit with water after putting it on and I told him no, but did fall into the water about 40 minutes after suiting up.

I am not deterred by this product and must also say that I have extremely sensitive skin that bruises very easily. I see great potential for this product and I might even be able to get away with a 3/2 suit in the winter on a medium setting, but next time I will for sure flush my suit with water right away.