Intro
I personally really like having a quite low air temperature in the bedroom, but a cosy warm bed. In my teenage years I had a heated water bed which was awesome in this regard.
I tried to replicate this feature with “normal” beds before using heated blankets. This didn’t worked perfectly well and the amazon reviews of nearly all popular heat blankets show pictures of burn damage. The whole idea of putting a long heated wire in my bed and measure at just one point if the temperature is correct and not above the autoignition temperature of the surroundings does not let me sleep well at all. :)
Idea
So replicating the water heating known from my previous water bed should be a possible solution and only four general parts are needed.
- vessel
- water heater
- pump
- water to bed heat exchanger
Build
Heater, Pump and Vessel
My first idea was to take a small ~5 L wide-necked barrel and build in a 200 W - 500 W hater heater as well as a small heat resistent pump into it.
For the water heater and pump I looked through domestic and aquatic water heaters as well as aquatic and pc water cooling pumps. A friend who is just finishing his PHD in chemistry gave me a really good hint on how to build a prototype fast and easy, by just using an laboratory circulating thermostat for heater, pump and vessel. I bought a MDE CWB-02-20 circulating water bath on ebay for just 25,50€.
Fun fact: After I got the unit I noticed inventory labeling. Turns out it was used at the InstPharmToxBw (Institut für Pharmakologie und Toxikologie der Bundeswehr) which is the chemical warfare research institute of the German army. So I just hope they cleaned it properly before giving it away, the fact that the internal inventory number and instructions are still on …
Another downer for production use is the quite high minimal filling capacity. The CWB-02-20 needs to be filled with ~20 L of distilled water, this is a lot of thermal mass and buffers a lot of thermal energy before starting to transmit it to my mattress. For regular use I would prefer a system with less than 2 L of liquid, which is not giving much moisture to the surrounding air.
Cover
Sometimes you are lucky and things just fit. We have IKEA HOL storage tables next to our bed on both sides and the assembly fits perfectly in.
I just had to drill a hole for the power cable.
Heat Exchanger
For the heat exchanger I’m just going to use 10 m of 8 mm x 10 mm silicon tube. This is quite flexible, temperature resistant, does not smell and can transmit the heat to my box-spring bed topper. A better heat exchanger could be needed, but the silicon tube is also easily available and affordable.
A more perfect solution could be a flow through middle layer / blanked.
The tube is currently just pressed on, but I definitely need proper hose clamp before I run the system unattended.
Test
Unfortunately it became quite warm here after christmas, so I wasen’t able to do a real field test jet. I’m planing two simple tests, but for both I would like to lower the room temperature.
- Lay in bed, start the heating and note down when I start to notice.
- Use my remote BBQ thermometer to measure the temperature over time after I start the heater.
After I know how long it takes to head up the bed, I can automate operation.