Unvented water heater concern

nairb999 · 12 Jan 2009

I have a STATE gas fired water storage heater - SR8 50 NADS. It has worked fine for 11 years. However 12 months ago when the hot water was used in an upstairs basin the tap would splutter for a little while. This now happens in several other taps. I have read several notes on similar heaters on this forum and realise that a G3 registered engineer must work on the heater but I would like to gain some insight into what the issue may be.

Any thoughts would be graetly appreciated

simond · 13 Jan 2009

It may or may not be G3. This type of WH comes in unvented and vented versions, if you have a water storage tank the heater is open vented.

Spluttering taps is usually air in the system, caused often by overheating.

nairb999 · 13 Jan 2009

simond said:
It may or may not be G3. This type of WH comes in unvented and vented versions, if you have a water storage tank the heater is open vented.

Spluttering taps is usually air in the system, caused often by overheating.

Simond - thx for response. The system is unvented. Heat setting is 155 deg f which provides comfortable hot water ie not scalding.

nairb999 · 14 Jan 2009

This morning the spluttering from the downstairs tap was a lot worse. Initially iit cleared and then after the water flowed well for several seconds started spluttering again. So the problem appears to be getting worse.

I presume it can't be an air lock as it is a pressurised system and any air lock would be forced through the system.

The heater manual specifies the expansion vessel should be pressurided to 3.5 bar. This has not been touched in the 11 years the heater has been working. Could that be an issue ? ie either under or over pressure.

Ideas would be greatly received as the spluttering is at some force and can make a bit of a mess.

Onetap · 14 Jan 2009

There is dissolved oxygen in water which comes out of solution when the water is heated. With an unvented system this disperses up the open vent pipe; it can get sucked into the inlet of a shower pump and cause problems, hence the need for the Warix, Surrey or whatever flange to draw hot water away from the open vent and the air bubbles.

With an unvented system, there's no vent (No sh1t, Sherlock!) so the air comes out of the taps with the water. SFAIK there are no automatic air vents that are suitable for use with DHW. You may be getting more air now because the cold water is colder and can carry more dissolved oxygen (I think) or the supplier may be taking more water from a source with more dissolved oxygen (winter, high reservoir levels, so less borehole water used?). The temperature of the stored water is too hot, it should be 140 degF, 60 degC. Have you checked it with an accurate thermometer?

It could be is hydrogen from galvanic corrosion or electrolysis, but the volume of gas sounds far too much. Does the cylinder have a sacrificial anode and has it been replaced? Anodes were used on steel cylinders. When the sacrificial anode has been corroded, you then get a sacrificial steel cylinder. If there was an anode and it's gone, plan on replacing the cylinder before it splits.

Check whether the immersion heater has a leak to earth. Turn it off if it's also heated by a boiler and see if there's any change.

nairb999 · 14 Jan 2009

Onetap said:
There is dissolved oxygen in water which comes out of solution when the water is heated. With an unvented system this disperses up the open vent pipe; it can get sucked into the inlet of a shower pump and cause problems, hence the need for the Warix, Surrey or whatever flange to draw hot water away from the open vent and the air bubbles.

With an unvented system, there's no vent (No s**t, Sherlock!) so the air comes out of the taps with the water. SFAIK there are no automatic air vents that are suitable for use with DHW. You may be getting more air now because the cold water is colder and can carry more dissolved oxygen (I think) or the supplier may be taking more water from a source with more dissolved oxygen (winter, high reservoir levels, so less borehole water used?). The temperature of the stored water is too hot, it should be 140 degF, 60 degC. Have you checked it with an accurate thermometer?

It could be is hydrogen from galvanic corrosion or electrolysis, but the volume of gas sounds far too much. Does the cylinder have a sacrificial anode and has it been replaced? Anodes were used on steel cylinders. When the sacrificial anode has been corroded, you then get a sacrificial steel cylinder. If there was an anode and it's gone, plan on replacing the cylinder before it splits.

Check whether the immersion heater has a leak to earth. Turn it off if it's also heated by a boiler and see if there's any change.

Thanks Onetap. The water heater temperature is set so we can comfortably shower ie only a little cold water is required and a basin of hot water is hot but not piping hot if that makes sense. I did reduce the temperature setting as advised on this site but that did not make any difference other than my wife going mad because the shower was only luke warm. The unit does not have an immersion heater by the way.

I have run off a couple of gallons from the drain at the bottom of the boiler. The water came out with some force but steady and other than black for a fraction of a second crystal clear.

We are in a soft water area so I would be surprised if the Sacrificial anode would have depleted but agreed it does need checking. I think I will leave that till some warmer weather arrives.

I don't know whether this bit of information helps but the cold water pressure into the house is 1.5 bar and when a tap is turned on it quickly drops to 1.0 bar. The pressure rises to 1.75 bar when the water is heated.

Hopefully your oxygenated theory is the answer.

Onetap · 14 Jan 2009

nairb999 said:
I don't know whether this bit of information helps but the cold water pressure into the house is 1.5 bar and when a tap is turned on it quickly drops to 1.0 bar. The pressure rises to 1.75 bar when the water is heated.

Hopefully your oxygenated theory is the answer.

Having thought about it, there's probably too much air/gas. Is there an immersion heater, is it used? I think it may be a split immersion heater.

The pressure variations are normal but the expansion vessel needs servicing & possibly replacing after this time. So too does the whole heater, especially the pressure relief valves.

nairb999 · 14 Jan 2009

Onetap said:
nairb999 said:

I don't know whether this bit of information helps but the cold water pressure into the house is 1.5 bar and when a tap is turned on it quickly drops to 1.0 bar. The pressure rises to 1.75 bar when the water is heated.

Hopefully your oxygenated theory is the answer.

Click to expand...

Having thought about it, there's probably too much air/gas. Is there an immersion heater, is it used? I think it may be a split immersion heater.

The pressure variations are normal but the expansion vessel needs servicing & possibly replacing after this time. So too does the whole heater, especially the pressure relief valves.

Thanks again Onetap

We do not have an immersion heater. If the expansion vessel has lost pressure could that be a problem. Some more information we do not have any problems with the showers. The showers are approx 4 ft higher than upstairs sink taps.

Will take your advice on service

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Unvented water heater concern

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