COTHERM thermostat on a megaflo tank

[bexy]

My sister has a Heatrae Sadia megaflo hot water tank with two immersion heaters for her hot water. Heating is separate, by storage heaters (no gas supply to flats). She has had some problems, and an electrician has been called a couple of times. Last time, he told her how to reset the thermostat if it tripped. However, there are a couple of things I'd like advice on, please.

From my reading so far, I think that the lower immersion heater is the overnight timed one, and the upper is the boost one - is this always the case?

The thermostats are "COTHERM type: TSE T115". There is a little red plastic switch labelled SAFETY which the electrician told her to "pop out". However, using a small screwdriver to press this switch does not release it out again. Should it, in fact, be pushed in?

I ask both questions because I took the cover off the lower stat and the switch is level with the blue surround, whereas the top one (which the electrician adjusted last time) is depressed. However, the boost currently works and the main water heating does not.

Finally, if this happens again, is it most likely to be a dodgy stat, or will the whole element need replacing?

Sorry for the long post, thanks in advance for any advice.

 

[ericmark]

The only time a reset button should be used is when you have two forms of heating for example solid fuel. In a way you do have two forms of heating as you have two elements and since thermosyphon will cause water at top to also be heated from bottom element then top one can trip due to fault in bottom one. Since Megaflow is an un-vented system I would be worried about the trip and would want to replace the thermostat.

The boost is always top and storage bottom again because of thermosyphon.

Because the Megaflow is a special it may work at higher than boiling temperatures so thermostats need careful selection. Not really done much work on them most around here seem to use Glenhill as unlike Megaflow they don't need annual inspections as not pressurised.

 

[Taylortwocities]

I'm with Ericmark. The only time I have seen a CoTherm trip in an all-electric Megaflo is when the owner drained down the system and left the elements powered up

The red button should be depressed to just below level with the blue housing.

If you have a test meter, there are two test points on the stat (marked 1 and 2) you should get 230V between them with the power and the reset on and the thermostat turned up.
You'll only be able to do this on the lower element during the off peak times.

I'm not sure at what temp the safety trip on these goes off. I think the standard is 90ºC. Check the temperature setting on the stats and make sure its not set on maximum.

PS Eric. From April 2004, new and replacement Immersion Heater Thermostats need to incorporate an overheat safety cut-out. They are fiited as standard to comply with regs.

 

[TicklyT]

I would expect the lower thermostat to be set to a lower temperature than the upper one.

When the cylinder is heated, there will be a temperatue gradient from hottest at the top of the cylinder, cooling towards the bottom. Hot water rises, just like hot air.

If the lower thermostat is set too high, the thermosyphon could heat water at the top of the cylinder enough to trip the cutout in the upper heater before the water at the bottom of the cylinder gets hot enough to operate the lower heater's thermostat.

You could try setting the lower 'stat temperature about 10 degrees lower than the operating temperature the upper cutout operates at, then 20 degrees etc. etc. until you find a point where the upper cutout doesn't trip.