Hot water recirculation system thermal activation control

Hello guys, in this lock down times I am looking for some project I did not finish.

Here is one where I need someones experience and help as whatever I found so far did not work as well as I thought it would.

A short explanation. Hot water tank heater. Water temp is at around 50 C. The farthest faucet (happens to be kitchen) is about 15 m away (50 ft). I am using a thermal switch to activate the small circulation pump. However, the switch has a very "short fuse", meaning its reset temp is just below set temp. Hence, I am looking for some advice what to use to control it a bit better.

The ideal situation, based on my test would be to have a thermo switch that would have set temperature at 42-45 C and reset at about 38-40 (say about 5 degrees below set temp).

To make it more complex - I started thinking of adding one more switch, but at lower temp. This one would open at 40, but close at 48-50. Call it cold water safety switch. Something like the heater is not heating and water is getting colder.

So, any ideas which pieces (just two thermo switches) to use? I got some cheap KSD-9700 switch. Works "ok", but resets too quickly. I do not want to keep buying them only to find out another one is a piece of junk.

The who system is made of PEX tubing (plastic), but the switch is sitting on a copper Tee, so heat transfer is not a problem.

by PLP
April 05, 2020

Sounds to me like a perfect application for an Arduino UNO or equivalent, a couple of thermistors and an "SSR" or two. The SSR means solid state relay; it is controllable, off/on by the UNO output, controls up to 40 Amps ac at 240 volts (I've used it at this level) and gives optical isolation between input and output. It would be a DIY job including software but would not be difficult with some study and planning

by Foxx
April 07, 2020

OK, I see. Yes, I guess that would be the best option giving the most control. However, despite being comp save (I used to build/troubleshoot computers for living a few years ago), software part was limited to installation and some DOS/Command prompt... so programming Arduino is like a next step for me.

Any guidance?

Sounds like a great challenge, though.

by PLP
April 07, 2020

Don't be spooked by the Arduino programming-it's a dumbed down version of C++ and I've found the Arduino Reference + tutorials to be excellent. My own programming experience was limited to basic many years ago and quite a bit of PLC programming which is like no other. I have a masters degree in electrical engineering but this was done when the only computer we had was a slide rule. If I can be of any help say so.

by Foxx
April 07, 2020

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