electric hot water controller - Thermar?

Has anybody made a temperature controller for a Thermar electric hot water demand heater? Or have any suggestions? The Thermar company was bought out many years ago and the products discontinued. The Thermar is a tankless 220VAC in-line hot water heater that operates only when you open the hot water tap. The control circuitry has died on it. It used a special IC no longer available.

When the Thermar worked it was very good and I have 2 of them, with burned out controllers. Temperature was monitored by a thermistor and the heating element was controlled a triac (which kept burning out). I replaced the triac with a SSR. The Thermar uses a reed switch & magnet to detect when the water turns on.

Most of the heater works, except the controller. I can heat the water manually by operating a push button switch to turn the heating element on and off via the SSR and a small 6V battery. For my wife to have a shower I press the button approximately 2 seconds on, 2 seconds off while listening to the water begin to "sizzle" and press less often to avoid getting it too hot - which pops the thermal cutout. I use a wood stove to heat the water in winter.

I propose to monitor the thermistor input and control the heating element by the SSR. I could use the output from the reed switch as a go/no go logic switch. Has anyone tried something similar? This seems like something the Arduino should be able to handle quite well. A good project.

I don't need to know what temperature the water is beyond too cold, too hot.

A 'bare bones' Arduino would be good for this application. You can use the thermistor as part of a voltage divider (like in the LDR example) to get a temperature reading. You could connect the power supply through the water-flow switch: if the water isn't running you don't need to check the temperature. :)

Hi, Interesting project...

So, I think I understand this:

  1. Reed switch / magnet senses water is flowing. (Does this cycle on-off like a rotating magnet, or stay on??)
  2. Thermistor senses output water temperature
  3. SSR is cycled to control water temperature.

So the Arduino has a pretty simple job:

  • IF Water is flowing enable the temperature control (Otherwise make sure no heating happens!)
  • IF Temperature control is enabled, monitor temperature with thermistor, cycle SSR appropriately.

(This implies you have a way to set the desired temperature. Can be a simple external Potentiometer that you later calibrate.) Or "His" and "Her" buttons? (You have to decide what to do if "Both" :) )

As in that saying from the 60's "Save Water .. Shower With a Friend".

Later you can control your wood stove...

Let us know how it goes....

Certainly sounds very plausible as an Arduino project, and well worth it to avoid being the shower wallah every morning. The control mechanism is simple enough though, that one of the electronics experts (paging Crossroads, Mr. Crossroads to the project guidance forum) could likely suggest a discrete component circuit that would do the job without the need for Arduino.

wildbill: The control mechanism is simple enough though, that one of the electronics experts (paging Crossroads, Mr. Crossroads to the project guidance forum) could likely suggest a discrete component circuit that would do the job without the need for Arduino.

Ah, but where's the fun in that? :)

Ah, but where's the fun in that?

Exactly, the fun is in figuring out how to do it with Arduino with class. Also buttons. Also flashing LEDS.

I do believe Arduino can solve most problems. I tried to nominate Arduino to be the official arbitration element in the Middle East situation. It could do a lot better than the present players. Right now, people are getting shot and killed North, South(35 yesterday), East and West of where I am. So It's not an idle speculation.

But the kids in school here are not allowed to discuss "the situation" at all. If they did they would have to mention that country that starts with "I", which is not even on the maps here.

I'm so glad we have freedom to talk here. Even OT.

2 weeks and I'm Outta Saudi Arabia on a short hop to Cairo. Shortly after that I'm headed to the Atlantic, New York, Vermont.


I can't wait to get to my Log Cabin in the woods, turn on my scope and plug in an Arduino.

Please... be very careful with the 220Vac!!!

If you have a flow sensor, you'll probably have to measure the flow so you can control the temperature based on flow as well... higher flow, more power to heat the water up. I'm just guessing there is some sort of power control going to the heating element instead of controlling the temperature by cutting circulation.

Some very interesting suggestions. Thank you.

Regarding the reed switch. It provides no power control mechanism beyond water flowing/water not flowing. Water enters from the bottom and lifts the encapsulated magnet in the water line which then closes the reed switch. The temperature is regulated simply by reading the thermistor. In any event, we have gravity-fed spring water, and realtively low water pressure (15 psi), so for a shower the water is "full on".

I like the idea of connecting the Adruino power through the reed switch. This seems simpler than some logic control. I just wasn't sure how much start-up time would be required before the Arduino was in action. Not much time I guess in the overall scheme of things.

So far no responses from anyone who has done something similar.

Insta hot water heaters come in two basic versions. One monitors discharge temperature and controls it The lessor version chcks if flow is over x then goes full power. Temperature changes with volume.

The first needs to know the flow rate. The second only that flow is above x

You can easily measure and control the discharge temperature with an arduino