Controlling a 3kw immersion heater, variably with temp sensor

Hi,

I'm a beginner to electronics, but i have a good amount of experience in mains power so i'm safe on my execution.

I'm wanting to heat water to a certain temperature in a closed loop (the water is recirculating). My initial plan is to use an immersion heater and pump water through it, in the UK - 240v, 13Amp max, although i am open to suggestions.

I plan to have a temperature sensor on the output side of the immersion heater, and would like to control it to within a few degrees. I could simply use a heavy duty relay and switch it on and off, however i don't think this will provide the level of control needed, the water will either be too hot or too cold too quickly.

I'm not sure if using a triac would be better, and i can rapidly switch the immersion element on and off? I've been using this as a reference:

http://www.rotwang.co.uk/projects/triac.html

Any advice would be greatly appreciated.

Thanks,

Elliot

Use a solid state relay, very cheap on eBay. If the input is rated 3-32 V DC (or similar) it has a built in resistor and can be connected directly to an Arduino output pin.

Very low frequency PWM can be used to control the temperature.

Thanks for the response - i came to the same conclusion after reading up. I believe i need a zero crossing SSR, with opto coupling? Rated accordingly of course. Would that be adequate?

I think all SSRs have optocoupling. I doubt you need a zero crossing model, but it certainly would not hurt.

You will need a PID 3-term controller configuration for your application and for very little money you can get a system comprising t/c, ssr and 3-term controller all from the likes of e-bay

For example : http://www.ebay.co.uk/itm/220V-PID-REX-C100-Temperature-Controller-max-40A-SSR-K-Thermocouple-Sensor-/272012027513?hash=item3f552e2679:g:PZ4AAOSwT6pVoJQ1

You say the water is recirculating - what is the thermal mass in the vicinity of the heater? That needs to be large enough that the water temperature change rate is a lot slower than the thermal time constant of your temperature sensor. 10 litres or so is probably a sensible minimum (the size of the heater may already make this the case).

What is the maximum power needed to sustain things in the steady state? If its only a 100W or so, consider a smaller heating element!

use a SSR always optical, no need for zerocrossing as it is a resistor, control with time like a looptime of 1 minute, when faster it wil irritate in lights etc.

I think you’re overestimating the speed at which an electric element can heat water.

3000W is 10,000 BTU, and one BTU will raise the temperature of 1 lb of water 1oF in 1 hour. An example 5 gallons (20 liters) would be 40 lbs, so in one hour you could raise the water temp from 0 to 250oF (theoretically), or in one minute just 4oF (about 2oC). This speed of temperature change is not something that you need PWM or a triac or anything fancy for.

And of course like MarkT said you can just use a smaller element.

elliotp: Hi,

I'm wanting to heat water to a certain temperature in a closed loop (the water is recirculating). My initial plan is to use an immersion heater and pump water through it, in the UK - 240v, 13Amp max, although i am open to suggestions.

This is a fairly standard heating/ plumbing implementation in flats/ luxury apartments to ensure quick demand delivery of hot water. Modern circulating pumps in the eu are regulated and require energy saving methods to be employed. No need for arduino control . I'ts all built in.

Here is a sketch of the domestic heating system I recently built. It uses SSR to operate immersion heaters and conventional relays to operate pumps.

You might find some of it useful.

Temperature.ino (23.2 KB)