Hot Water Cylinder Stat Wiring Question

[2BobSparky]

Came across a Heatrae Sadia hot water cylinder in my travels and was intrigued as to why the single phase supply was wired through two stats before going to the element terminal.

Enquired with a colleague who said one stat is used for temperature cut out and the other for temperature regulation. That's fine I thought, but both stats have a manual temperature dial on them, which seems wrong. Surely the intention is to wire a dedicated temp cutout as well as the adjustable temp control stat, not two temp control stats.

I was advised to set one at maximum temp for the cutout and the other set at the desired tap point temp, either one or the other, it shouldn't matter.

So my question is, if I've got one stat set at say 80 degrees and the other set at 40-50 degrees, how is the desired 40-50 degrees regonised and not instead the 80 degrees?!

 

[Deleted member 9648]

Well the 40-50 degrees stat will cut out before the temp gets to 80 degrees....therefore that stat will always operate first. If it fails then the higher rated stat will cut out before the water boils.

 

[HandySparks]

Are the two stats wired in series? Where are the two stats located? A photo would be good.

 

[2BobSparky]

Are the two stats wired in series? Where are the two stats located?

Yes, in series and they're located on the under side of the cylinder alongside the element connections.

 

[Richard Burns]

Whichever stat operates first will stop the circuit if the 40-50 does not work then the 80 would operate.
Not ideal really but would work.

 

[2BobSparky]

That's what I'm thinking, it can't exactly be an ideal setup for this kind of installation.

Would it be better than to install a dedicated temp cut out stat in series with the manually controllable dial temp stat?

In retrospect, after checking other Heatrae Sadia cylinders in the same building they're all setup the same way, line in through manual dial temp stat through to the next stat, which has the reset button on it.

 

[Specialist]

Probably the reason that they both have a setting dial is that they use the overheat cutouts in several different applications, so it's cheaper for the Manufacturer if they make 1 adjustable stat rather than several different pre-set stats.

 

[2BobSparky]

Okay, I see what you're saying, but these stats didn't come with the cylinders from the factory, they're two different brands, the regulator stat is Cotherm and the one being used as a fail safe is Sunvic inserted in a Heatrae cylinder.

So it seems as though this was the route taken by the original installer and as a few have already stated, it's not exactly the correct way to do things, although it does work.

Hmm...

 

[Specialist]

Ok: Just because the stats are of different manufacture does'nt mean they are'nt original, Manufacturers often source components from different maufacturers. If for example Co-Therm did'nt make a resetable stat of the type Heatrae needed then they'd source them elsewhere. Domestic appliance manufacturers are notorious for doing this.

 

[ukiri]

Take another look at the stats, one of them may also have fixed temp trip at 88 degrees with a reset in a small hole on the top.

 

[One_of_them]

bet it looked like this



if so, what wirepuller said is about the norm

 

[mhar]

Just a thought, are they mains pressure or fed from a header tank? If they are at mains pressure then the consequences of overheating are severe, it effectively becomes a pressurised hot water bomb waiting to go off.
I have a pressurised hot water system at home which is the best thing I have ever spent money on. It is (water) mains driven but we had to regulate our incoming water pressure to 3.2 bar. As well as having an expansion vessel and a mechanical blow-off valve we have two motorised valves. The first is a three port one which is connected to a cylinder stat and is used (as normal) to control thee water from the boiler to the tank or the cylinder. The second is a two port one, has it's own cylinder stat and it's sole purpose is as a back up safety device if all others were to fail, to stop the cylinder heating any more and further pressurising it
In a header tank fed installation there is pressure relief through the feed or vent pipes, in a mains fed pressurised system there is no such relief therefore one of your stats could well be for day to day temperature control, the other being to stop the tank overpressurising and exploding in the event of a failure of the first stat. I see no need to have either one dedicated to just one function. In fact being able to set them at different temperatures would be the equivalent of our discrimination, if the water suddenly got a lot hotter it would (should / may) alert the customer to the fact that something odd is happening ie a stat failure. This would not be the case if you had a preset stat set to 65deg and an adjustable stat also set to 65deg as you would not know which one was working

 

[2BobSparky]

bet it looked like this

View attachment 9936

if so, what wirepuller said is about the norm

Yes, more or less like this.