Instantaneous water heaters corroding/leaking after a few months

[jacoscar]

I've been using an instantaneous water heater for the last 3 years; it was serving a downstairs toilet (far from the boiler) and it's a very convenient thing to have (especially in winter)

Galaxy GX1 Unvented Instant Water Heater 9.5kW

Order online at Screwfix.com. A space saving solution for providing instant hot water from the mains supply. Supplies 2 basins or 1 basin and 1 mixer shower. Half-power setting available. For use with manual showers only. For domestic use. FREE next day delivery available, free collection in 5...

www.screwfix.com

It has actually failed after 3 years, but reviews show that I've actually been lucky as that could happen much earlier
Mine started to make a strange noise like it was stuck on with no water moving through it; the water started to boil and I immediately switched it off (luckily the consumer unit is on the room next to it); removing the cover revealed a small leak and a lot of corroded bolts that seemed to have been sealing the pressure switch


As that model is hard to find, I was looking at this one;

Ariston Aures Slim Instantaneous Water Heater

• Front-facing dial and LED indication for ease of operation• Flow sensor system initiates instantaneous water heating when a tap is opened• 4-step dial to set the desired power level• Easy to maintain with frontal access to all internal components• Complete safety thanks to its double thermal...

www.toolstation.com

However, the reviews are also mixed, with most of them mentioning leaks starting within months or weeks from the first installation

Does anyone know why this happen and how to make it stop? Mine also had at least the first 20cm of neutral cable turning dark on the outside, while the line conductor was perfect; this made me think about something along the line of galvanic corrosion (as the neutral was permanentily connected, while the line would only be connected for the short time the heater is on)

 

[delmel]

you can replace with any equivalent water heater, but this may mean some change to pipe work, and poss cabling. From the comments of neutral cable changeing, this sounds likely to be overheating. This may be a loose terminal or undersized cabling. Check the cable size is suitable for the KW you are running. It isnt uncommon for Jo bloggs to change a water heater or shower unit without thought.

 

[jacoscar]

you can replace with any equivalent water heater, but this may mean some change to pipe work, and poss cabling. From the comments of neutral cable changeing, this sounds likely to be overheating. This may be a loose terminal or undersized cabling. Check the cable size is suitable for the KW you are running. It isnt uncommon for Jo bloggs to change a water heater or shower unit without thought.

It's unlikely to be due cabe sizing, the cable is 6mm2 in free air, the run is just a couple f meters, and the heater has never been on for ore than a few seconds; also I was keeping it at its low setting (which halfs the kW used); there is no sign of overheating at the neutral terminal
There was also a leak, obviously unrelated to the electrical collection

 

[Razor900]

Does it come with a dielectic coupling? If this isn’t fitted the unit will fail much quicker. A mini expansion vessel on the cold side is also needed to stop the pressure rising too high

 

[jacoscar]

Does it come with a dielectic coupling? If this isn’t fitted the unit will fail much quicker. A mini expansion vessel on the cold side is also needed to stop the pressure rising too high

No dielectric coupling but the Ariston connectors are hard plastic
Dielectric connectors won’t help if the metal coupling is inside the unit by the way
Expansion vessel? It’s an instantaneous water heater, it’s always at mains pressure (around 2 bar at my property)

 

[Razor900]

If they are prematurely failing it’s usually either electrolytic corrosion or excessive pressure.

Dielectric coupling will take care of the corrosion and an mini expansion vessel will take stop excessive pressure rise that’s usually caused by a non return valve in the water meter stopping expansion back into the main.

When the taps turned off the element is still hot and it doesn’t take much for the pressure in the pipe to rise significantly.

It’s the reason boiler manufacturers, (like Worcester), specify mini EV’s on combis

 

[jacoscar]

If they are prematurely failing it’s usually either electrolytic corrosion or excessive pressure.

Dielectric coupling will take care of the corrosion and an mini expansion vessel will take stop excessive pressure rise that’s usually caused by a non return valve in the water meter stopping expansion back into the main.

When the taps turned off the element is still hot and it doesn’t take much for the pressure in the pipe to rise significantly.

It’s the reason boiler manufacturers, (like Worcester), specify mini EV’s on combis

Even if there is a NRV at the meter, water still has the rest of the pipes in the entire house to expand. And by the same logic, any other hot water tap served by a combi boiler would have the same effect on the water heater, regardless of where it is.

 

[Razor900]

Oh, sorry I thought you was asking for advice. I didn't realise you was just trolling.

Perhaps you should write to WB and all the other manufacturers that specify one and show them where they are going wrong. You might get a medal...

 

[jacoscar]

Oh, sorry I thought you was asking for advice. I didn't realise you was just trolling.

Perhaps you should write to WB and all the other manufacturers that specify one and show them where they are going wrong. You might get a medal...

Despite the fact that it makes sense to have an expansion vessel in a heating circuit where all the water inside gets hot, I don’t see the point of having one on a cold mains feed where only the last section gets hot.

The instantaneous water heater manufacturers I’ve mentioned don’t specify an expansion vessel.