Electrical Boiler Element Problem

TDS and the resulting scaling are one possible aspect to look for. Another is the possibility of chlorides or dissolved oxygen in the feed water, which could lead to corrosion (pitting type). Depending on what you find, some countermeasures might include preheating/deaerating the feed water, chemical dosing, or selecting another sheath material for the heating elements.

It could be just a simple problem of inadequate water flow over the elements themselves. Without a proper volume and even flow over all of the element surfaces it will create hot spots and steam bubbles that will allow for the elements outer copper or stainless steel jacket to melt or develop pin holing problems or most often ruptures from over expansion of the filler material between the internal element coils and the outer jacket.

I have seen this a number of times where an old or incorrectly designed system cannot produce adequate flow rates in the actual heating unit. A bigger pump or a check over of the system to find restricted or shut off heating circuits and lines is needed.

The system may be set at a temperature thats too high for the fluid due to inadequate heat transfer at lower working temperatures to the area or application its producing heat for and its right on the edge of flashing to steam when in contact with the elements.

It could be possible that the elements you are using are too high of output for the application as well. Using a lower wattage element may solve the problems if they can still deliver the needed heat capacity that the system uses. Just because it has a 22 KW capacity doesn't mean that it is needing the full 22 Kw of output. It may very well be cycling on and off fairly often and only needs to deliver a lower average power to what ever it is producing heat for. The actual load may only need or be capable of transferring a 15 kw heat input.

Also there is the real chance your just getting crappy low quality replacement elements. Just because they may come from an authorized dealer doesn't mean they are not really just low grade junk with a premium price tag attached to them!

Hi, The boilers are used to heat the water used in shower area. The set temp. is 50 Deg C only. The boilers have sanicube inside (coil type pipe) thru which the heated water used flows. The boiler is also filled with water in which the element is immersed. The supply water is hence heated thru this filled water. I have also tried using the lower capacity elements. Now I am using only 18 KW element and they also blow out after some period.

If I understand this correctly, your boiler is a tank filled with water that stays in it all the time. The water for the showers is separate; it goes through a coil of pipe that is immersed in the boiler water.

If this is correct, then the suggestion to check the chemistry of the boiler water and look for sediment or scale buildup over the heater elements is definitely a good one. Another possibility is electrolytic corrosion if the boiler metal is different from the heater shell. If the heater element is touching the coiled pipe, the reaction could be taking place at that point. (A quick check of this could be made by measuring the voltage between the heater case and the boiler or coiled pipe, especially at operating temperature. A non-zero DC voltage implies electrolysis is occurring, even if is on the order of tenths of a volt.)

Is the failed spot on the heater element in the same place every time? That would be a valuable clue.

you have understood it correctly. It is the way boiler internal setup.

No, there are no sediments in the boiler and scaling is also very less. I have my RO plant connected before the boiler to supply water as the hardness of the raw water is very high. The water used inside boiler and for showers, or I should say, any water in use is thru RO plant and the TDS is in the range of less than 100.

Boiler element is not touching with anything except water inside the boiler. The failed spots are hardly visible hence cant say they are in the same place or not.

It appears that Tcmtech is on the right track, and the convection currents in the water around the heating element are inadequate to prevent local burnouts.

Is the coil a single helix spiraled around the heater? If so, make sure the cold water enters at the top of the helix and the heated water is taken from the bottom. This will accelerate convection in the heat exchange water that stays in the boiler and will also heat the shower water more quickly.

If that isn't the fix, two alternatives for solving the problem are to shorten the thermostat cycle or go to some sort of solid-state electronic drive so you get the same average power but lower peak heater temperatures, or install a propeller in the boiler to stir the heat exchange water (but don't stir so rapidly you get cavitation around the heater!).

OOPS! Those ideas may not eliminate the problem if nobody's taking a shower, because no cold water is coming in to generate adequate convection. So, perhaps the solution is to install a dual thermostat and a flow switch. If there is no water flow through the coil, the lower temperature thermostat is used. When flow is detected, the switch changes the connection to the higher temperature thermostat. The lower temperature would be set just high enough to keep the people in the shower from getting impatient waiting for the really hot water. Note: if this is a demand-type water heater, there should already be a flow switch somewhere in the internal plumbing. Perhaps it is not turning the heater off or to a low-temperature setting as it's supposed to do. If the water is going through in the wrong direction, the flow switch might not work at all.

Of course, I assume you've already verified the existing thermostat is working to spec and checked the heater element leads up to the boiler to make sure there is no odd problem such as insulation breakdown at a pinch point or crossing of a sharp metal edge.

1). Faulty Thermostat.... constantly on.

2). Boiler with little or no water

3). Scaling

4). Incorrect supply voltage

5). Incorrect heater element

too think of a few..

Several good suggestions regarding static water in boiler.

I suggest an alternate solution that will also improve efficiency.

Install an external water heating heat exchanger to replace the current internal coil heater- plate and frame would be ideal with hard water because of ease to clean periodically and high speed, turbulent flow that will reduce scaling anyway. If plate and frame are not available quickly, run shower water through the internal tubes of a shell and tube to allow easy cleaning of scale with a wire brush.

Put boiler on timer matched to about 30 minutes before water loads will be occurring and install a small circulator pump for boiler water through Heat Exchanger that will run continuously whenever boiler is energized.

That will eliminate the risk of localized hot spots and any risk of steaming around the elements and will produce higher heat transfer efficiency to incoming hot water.

Another added-value solution is to install a hot water storage tank between HXU and showers with circulating pump between HXU and storage tank to even out the flow of hot water and allow reduced power requirements for the boiler since total water flow for showers will be spread out over entire day. If this happens, eliminate the timer and use thermostat in storage tank to activate boiler and pump.