Longer immersion

I?m guessing there?s a reason you want a low wattage heater. Are you using solar power & batteries?

Without a cheap electric source gas would still be the cheapest way to heat your water.

Tim

Google gives me these people

Domestic Immersion Heater - Backer Electric

The link isn't all there but you can google them directly

It would be simpler to just use a high-power diode in series with the element. That would effectively cut the power in half.

Yes I contacted them and they would do it as a special if I order a batch, I am still waiting for a final response by email.

Yes I wondered about that but with any semiconductor other than a mosfet it means dropping about half a Volt at 13 A for a half cycle so that would only be 7W I suppose.

Yes the battery will only supply 2.5W and the standing load seems to be about 300W.

Boat immersion heaters can be low power

This one example:

It may not be 36" but 27" should get you a hot bath full.

What about getting a dual element and putting the elements in series?

Wouldn't that switch off as soon as the shorter element thermostat was satisfied? If not then wouldn't the water at the top of the tank start boiling?

I think the tank is about 100 litres so that would be 50 litres of hot water and probably adequate but I really wanted 2kW to get it hot in under an hour.

You don't wire up the thermostat associated with the short element.

Why would it be any different from using a longer element with the associated longer thermostat rod.

There may well be 2kW versions, that was the first I found.

A 27 inch immersion heater will heat 3/4 that of a 36 inch version. If quantity is important then turn up the wick on the thermostat for more hot water, which you'll then mix with sold anyway.

Another possibility is an under cabinet in-line water heater. They only store 10 litres but are generally rated at 2.2kW. I'm sure after the 10 litres are exhausted you can slow down the flow for your desired bath temperature.

If you want o modulate power, then I see no reason why your suggestion of a dimer won't work. Not many work with 3kW loads:

A regular diode drops 1 - 2v at high current, sometimes more. It's amazing how many people think they drop 0.65v at max rated i. A diode is far simpler than a thyristor & all the associated circuitry. A self oscillating bimetallic relay is another option. A segment timer would probably also do, but beware of ratings.

One of my favoured options is a capacitor dropper.

Yes but I bet the inverter couldn't come with the bad power factor and I wonder even about supplying a half wave via a diode even given your 2V voltage drop.

On 01/03/2022 16:51, Fredxx wrot

Yes there's that, I would worry a bit about scalding hot water though.

I actually have a 3kW variable controller in the form of a Vellerman kit but obviously prefer a simple resistive element, especially as the inverter specification mentions a power factor of 0.8

That can be learned by seeing performance via the inverter web based graphic.

Yes, I've only discovered that you are using an inverter.

What's the inverter, and it's rating?

Assuming it's an accurate representation of power. Another way is an auto-transformer, or use a transformer to reduce voltage. If it's a modified sine wave then all bets are off for a dimmer style controller, and any transformer is going to sing.

What is sourcing the inverter? Are you telling us that you're using 2kW from a set of batteries for an hour? How many times will your batteries do this? Are you using Lithium?

Is this a boat application, or residential?

More information would be helpful.

Yes but I bet the inverter couldn't cope with the bad power factor and I wonder even about supplying a half wave via a diode even given your 2V voltage drop.

I'm not sure but think because water is a bad conductor, hot water is stratified at the top of the tank, but the bottom thermostat is not satisfied so both elements continue heating, the water at the top boils before the bottom thermostat is satisfied