I successfully made mine with that free Heyco sample
but it has little effect for what I wanted. I made an inline heater for cold water line for my autowater changer.
I want to do the same. Can't use hot from the tap because it goes through a water softener. From the spigot it is about 63F. I figure with a 3x400w heater, I can heat 30 gallons of water about 10F over 45 minutes (30g is how much I need when doing changes in all my tanks).
Even though my flow rate is slow, and I used larger 2" pipe, it still makes only a minor difference (with a 400W heater mind you).
I don't think the pipe diameter matters. Keep in mind that the heat has to go somewhere. As long as the heater stays on, and you don't have stagnant area that result in heat escaping through the PVC, the heat will transfer to the water. In fact a smaller diameter probably would be better so you always get a good flow of water over the heater. With a big pipe, water could be somewhat stagnant and cause the heater to turn off.
Even though my flow rate is slow, and I The only thing that would concern me for regular aquarium use, at least with the Theo heater (not sure about others) is the minimum water level for the heater seems to be right at the end of the glass. So inside one of the cordgrip fittings the actual water line is almost 2" away from minimum which might effects its temperature control. Apparently this isnt a problem for most, or someone would have mentioned it.
Good question. I have 7 heaters in various tanks and storage. All have the min water line just below the temp control. Maybe this is a problem and is why yours wasn't working well.
It would great if anyone who has this working would mention which heater they are using. I want to use the Haydor Theo, since using 3 400w Theos is the cheapest way for me to get 1200w. I see some are using Jagers, but a 250w Jager is as much as a 400w Theo, and I'd need to deal with 5 heaters instead of 3.