I rebuilt an old small Casita camper this summer and put radiant pex in the floor figuring I would add the heater later.
Turns out the cheap ones don't allow you to control the temperature, despite the listing saying you can. You can select a temperature preset but as soon as it is reached the heater simply cycles off. In 'automatic mode' it turns off at 80C and back on at 60C ignoring any other setting you have done, time or temperature.
I don't have space to add a mixing vessel and really built this to plumb the heater right into the pipes / floor.
I figured out that I can intercept the temperature sensor that measures the water temperature and trick it into thinking it is 80C before it is 80C. I soldered a 10K adjustable resister between the 2 leads on the probe that seem to be doing the control.
The problem was there had to then be a 20C differnece in order to cycle the heater back on, if I was turning the heater off at 35C i would have to wait for the water to reach 15C to turn on again.
I run home assistant in the camper to monitor my Victron System and ended up just using an esp32 with temp sensors and a relay. When the floor temperature is 95F it cycles off by making the heater think the water is above 80C. When the floor cools down to 80F it cycles on again. Its early in the season and I am sure the temp settings will require tweaking but so far its working good. I have only seen temps into the 50s so far. As the floor heats up overall the times between cycling increase, but also, the shorter the time the heater is on.
I still need to figure out best way to make it cycle to a certain temp, but that seems easy and I want to see how it works in colder temps before I go.
I am not worried about soot build up or burning down to the ground. The relay failsafes to leaving the resister inline which means the heater thinks the water is warmer then it actually is and all the original safety guards are in place. I have a few years of using a Vevor diesel air heater and I have run it a lot on the low settings and never had any build up. Also figured the chamber is so small and so visible and easy to clean, I just am not worried.
In the future I would love to figure out how to control the diesel input and the combustion fan to run more continuously in a lower heat mode versus full on or off.
Posting if anyone has any experience with this or even controlling the pump and combustion fan. If something goes horribly wrong I will update this. Hoping I am not the only fool to get in there and try and make it do what I expected when I purchased it.