My nephew has a hot water tank with two immersion heating elements.

tank with two immersion heaters

He states that the bottom heating element no longer works, and says he tested it by swapping over the wiring from the top element (which is working, and is controlled by a 'boost' button) to the bottom- he waited for two hours and the water was unheated.

However, we are not sure if it is the heating element that does not work, or the thermostat which controls it.

ts220 thermostat

The TS220 thermostat appears to be wired up with access ports at the side:


Can we test the thermostat by measuring the resistance across the two side ports? (ie the ones connected by the brown and white-sheathed wires)

My understanding is that a zero/very low resistance would indicate a short (and so it should be permanently on in that case, which seems unlikely); and an 'infinite'/unmeasurably high resistance would indicate it was stuck 'open' (and therefore permanently off).

Is this a practical approach? I welcome other suggestions if there are better ways to do it!

  • 1
    Suggestions for post improvement are welcome, as always. I also explicitly welcome frame-challenge answers that suggest better ways of testing, especially if they suggest ways of testing the heating element without removing it for inspection- which would involve draining the tank and physically removing it and would be great to avoid if practicable!
    – bertieb
    Commented Feb 5 at 15:28

3 Answers 3


Your understanding is fairly accurate where as zero ohms would indicate a short but low ohms would indicate a load, not a short. After turning the power off,you could test the thermostat by removing the brown wire and the white wire from the thermostat and replace them with the leads of your ohm meter. Turn the orange dial all the way to the right and look for zero ohms. Turn it to the left and look for infinity ohms. Make sure that little reset button hasn't tripped. Keep in mind that just because you get some reading doing this, doesn't mean the thermostat is good. There could be a problem with the heat sensor that triggers the switch. You can remove the thermostat and probe and stick the probe in hot water and then turn the orange dial and look for ohms.

As far as retesting the element, connect the brown wire to the white wire, bypassing the thermostat, and turn on the power and see if the water heats up.

  • I went round today and measured- there's a very obvious 'click' as the thermostat latches and completes the circuit , dropping the resistance versus the infinity reading in the other direction... unfortunately, this points to the element being faulty which is a less easy change, particularly as the drain valve doesn't seem to work 0_o
    – bertieb
    Commented Feb 6 at 18:11
  • @bertieb But does the Click happen when the thermostat is heated? did you check the element?
    – JACK
    Commented Feb 6 at 18:14
  • unfortunately not yet- because of space (washing machine in front of tank) and time (nephew heading out) constraints I wasn't able to perform a more thorough assessment- I am reasonably satisfied that the thermostat was responding appropriately, but you are right that for completeness I should do both the hot water test and bypass the thermostat on the element
    – bertieb
    Commented Feb 6 at 19:06

Simply measure the heater resistance, *with all power off. You can figure out what it should be from Ohm's law, but I'd guess it'd be in the range of ~10 to 20 Ω. As an example 4 kW heater, at 220 VAC, would have resistance of ~12 Ω at operating temperature, and might be a bit lower at room temp.

If the heater is open ("infinite" resistance), it's dead. If it's not open, the issue might be in the thermostat, or the wiring to it.


If this unit is like a conventional domestic hot-water cylinder, then its often rigged so either element can run but not both at the same time.

The incoming cold water lays at the bottom and the hot floats to the top. With no mixer, there's a definite "boundary". This allows you to keep drawing hot water from the top of the tank and not have a steadily-decreasing temperature over time.

(My understanding here is subject to improvements - feel free to make edits)

  • If the tank is full of cold water, the upper element turns on by thermostat. This heats the next-water to be used.
  • Once the water in the top of the tank is at temperature, only then does the lower element turn on heating water further down the tank.
  • And once the lower element hits the cutoff temp it stops heating and the thermostat will cycle as needed. The upper element only turns on when the ready-use water starts cooling.

The two elements have the same power rating which equals the max-current draw on the label, and therefore both elements cannot run together.

So by swapping things around, you're not proving much because the element and temp sensor are at different heights.

A current clamp-meter on one leg of the input wire can tell you when an element is heating.

Re-visit So this heater is basically two separate elements, each with its own isolated thermostat.

My testing would be to disconnect both from supply power, and test resistance across the Brown and Blue terminals. Should be between 10 and 30 ohms for a fullsize domestic heater. This heater is physically smaller, so it might be a little higher resistance, up to 50 ohms. If it tests below 5 ohms, or infinite/megaohms of resistance, then the element is dead and needs replacing.

If the element isn't too expensive, replace both along with any anode. Saves doing it again next month.

Locating an Element to suit this heater might be a challenge.

enter image description here

You need to match pretty closely on the insertion depth, else it could be too close to the far wall. The thread specs need to be exact else it will leak, and wattage needs to be similar.

IMO this is a good excuse to buy a clamp meter - for someone with a practical bent like yourself, its another tool in the problem-solving arsenal. You don't need a Fluke, something like this $90 goldstar would be serviceable for decades. https://cdlnz.com/GSM-850 (no connection)

  • 1
    Hey Criggie, this is a good explanation of a hot water tank arrangement works and I have upvoted it on that basis since it would help other querents, but I don't think it's the case here- the two heating elements are independently supplied. I don't have a clamp meter and I assume a non-contact tester wouldn't suffice... I do have a sacrificial lamp that I don't use, could I wire that up in series to the lower heating element (bypassing the thermostat) as an indicator?
    – bertieb
    Commented Feb 9 at 9:51
  • @bertieb +1 for making me google "querent" I've revisited this answer to show what I'd do.
    – Criggie
    Commented Feb 9 at 20:10

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