Thanks, will check out a PID controller.

Some updates:

Considering a water-tank instead of the liner. A bit less construction, and more sturdy. Cut off the top, and perhaps paint the inside with a boat top gel coat to get a nicer color.

Found a local Nopro 2000 litre flat tank. 244x192x67 cm - perfect for my plans, i think. Even has a hatch on top so I can "try it on" before I buy. :-D

Still not decided what to do about the ceiling to prevent drip and provide a nice interior. Maybe I'll make a polycarbonate plate (green house type) pyramid, good isolation and a steep (60 percent) smooth surface. Will be able to put colored leds on the outside. Hrm..

Update 2:

Nope after closer consideraton, no water tank. :-) Will do the self-build with liner, so that I can use theavailable space better.

Np. If you have a PID controller on the loop, I don't see why it wouldn't be able to control with the same level of accuracy. It might take a bit longer to reach program stasis, given the cause/effect delay of the more complex setup, but one it is reached, PIDs do a great job of maintaining a consistent temp. If you don't have a PID controller on the loop, you may need to limit the floor heating temp to max out at "half the temp differential" (meaning if the temperature fluctuates 4 degrees, than you will want to cap at about 32 degrees so that it never goes above 34 degrees) and then use a waterbed heater & PID to maintain the difference.

One concern I have with this setup is that you will create a dependancy on a fluctuating variable. PID controllers do well to maintain static temperatures of the target within a relatively stable environmental temparature. It learns how long it takes to heat up and how long it takes to cool down and adjusts the thresholds for activating and deactivating to maintain the set temperature accordingly. When the temperature of the environment, that the target is located in fluctuates, it can mess with the "learned" programming. Honestly, I think I'm over-anazyling, but still something to consider if the fluctuations are large.

Thank you for the feedback. I will have the floatroom base on a separate pex circuit. Our system is way over-dimensioned.:-) 7 free circuits, only use 1 so far. Its a 30year old house which we refurbish, and floor heating is so far only in the hallway.:-)

the system may supply (individually controlled) up to 50degree celsius hot water, so should be no problem. Only thing is a accuracy. I dont know how specific i will be able to control it.

plan to start with a simple intex pool for a year, before rebuilding the room. Test if heating, ventilation and tech equipment is sufficient, and finetune everything. Our floor heating manifold is right next door, i can pull a pex pipe over, and make an isolated pool base without making it permanent.

may consider other types of ceiling to get a steeper angle, than barrel ceiling. Might be able to get a cool cathedral look perhaps? the room is only 230cm high, so there are limitations to what i can do, and still maintain some headroom.;-)

All power is on gfci circuits already. Will make sure to ground everything. Dont want to electrocute anyone.;-) thanks for the advice!

You got me thinking that the future may have houses built with float rooms. This is my first exposure to air to water heating. Amazing. I will assume use of this relating to heating a float room will be through PEX or other floor heating. This may work to heat the tank depending on a few things. You will need to acheive 34-34.5 degrees (celsius) which is much warmer than the rest of your flooring, so unless you can isolate the room and independantly control the heating, you will need another method for the rest. Given you will have a liner, the waterbed heater is a perfect solution.

The passive ventilation in the ceiling should suffice. A human only needs about 8 cfm for adequate air supply, so as long as the verticle differential between where the hot air leaves and the cool air enters is great enough, that will work. It should suffice for humidity in the room too.

Filtration seems fine. Make it accessible because they do require maintenance. Be sure that your filter element lid is higher than the solution surface so that cleaning/changing filter doesn't make a mess.

Size seems good and barrel ceilings do tend to work well if the material is slick. It generally takes an hour for condensation to form so simply wiping down before entering will give you a drip free float for a good period.

Outside of these things, do consider grounding and bonding. keep all electronics on GFCIs, ground your solution, and do your best to bond the area around the room. Electrical charge of the floor and the solution can be different depending on weather, buiding, etc. Its not dangerous, but feeling an electric current while stepping in can ruin a relaxing environment.

Good luck and post pics.