Cold Water Mystery

[lockett]

Hi
I have a Atag ic Economiser Plus 27, which now and again stops supplying hot water. Everything works fine then after a week or two weeks cold water from the hot taps. If you leave it for a while, half an hour, or first time it happened a day, the hot water is back. Since the boiler is only a few month old called Atag they diagnosed it as a faulty flow sensor they contacted the installer a new sensor was installed, engineer said he could not see a problem with the sensor (no missing vanes)but would change it anyway. This appeared to fix the problem, unfortunately not. Engineer retuned diagnosed it as mixer tap or shower bar passing. As when he isolated the DHW at the boiler (don't quite know how he did this, as the service manual shows the DHW is the second output from the left, which on my boiler does not have an isolation valve. The isolation valve on my boiler is for central heating, maybe this is common to both DHW and central heating) the hot tap continued to flow for a short while. After feeling the pipework on the shower he suspected that it was the shower mixer that was passing cold to hot. Had a new shower fitted, after a couple of days the problem is back again.
Any suggestion very much appreciated

 

[CBW]

Could be another mixer passing, do you have any others?

The isolation valve for the hot water is on the cold feed (cold in hot out).

 

[lockett]

Yes I have two other mixer taps, if I isolate them then turn the feed to the boiler off I should not have any water through the hot water taps. Thank you for your help now I know which isolation valve to close, that will be a good test to try next.

 

[lockett]

I have tried the following, turned supply off to the boiler, turned the cold water off to the mixer taps, turned on the hot tap, no flow, turned on the cold to the first mixer, no flow from the hot tap, turned on the cold feed to the second mixer no flow to the hot tap. If the mixer tap was passing when you opened the cold water feed to the mixers if it was passing you should get water coming from the hot tap?

 

[lockett]

Could it be the water held in the boiler that allows water to flow from the hot tap briefly when the cold water supply is isolated?

 

[lockett]

What position should the mixer tap be in to minimise bypass, fully to hot position, or fully to the cold position.

 

[CBW]

If you’ve isolated both taps at the cold and no water passes then not necessarily a mixer then. Had any other plumbing work carried out recently? When the water is cold, does the boiler still recognise the hot water flowing, eg fires up or tap symbol on boiler?

 

[lockett]

Haven't had any new pluming work done. When the problem first happened the Atag tech asked the same question, there was no square around the tap simple, he suggested flow sensor which was changed, did not fix the problem.

 

[Johntheo5]

I have tried the following, turned supply off to the boiler, turned the cold water off to the mixer taps, turned on the hot tap, no flow, turned on the cold to the first mixer, no flow from the hot tap, turned on the cold feed to the second mixer no flow to the hot tap. If the mixer tap was passing when you opened the cold water feed to the mixers if it was passing you should get water coming from the hot tap?

Repeat the above with mixers turned to fully hot (with boier cold supply isolated), no water should ( then, also) flow from the the hot tap?. because the cold mixer port should be fully closed off with hot port fully open to the hot supply, mixers should have non return valves if the mixing does not take place in the spout.

 

[lockett]

Will this give me any more information than doing what I posted, isolating the mains supply to the mixer taps?
Could it be the water held in the boiler that allows water to flow from the hot tap briefly when the cold water supply to the boiler is isolated?

 

[CBW]

Will this give me any more information than doing what I posted, isolating the mains supply to the mixer taps?
Could it be the water held in the boiler that allows water to flow from the hot tap briefly when the cold water supply to the boiler is isolated?

If you’ve isolated the cold taps, then this doesn’t prove mixing as then no water is allowed to pass over giving a false fault finding excercise.

 

[Johntheo5]

Will this give me any more information than doing what I posted, isolating the mains supply to the mixer taps?
Could it be the water held in the boiler that allows water to flow from the hot tap briefly when the cold water supply to the boiler is isolated?

Should have said boiler mains supply only isolated.

 

[lockett]

I have just followed your suggestions, mixers turned to fully hot with boiler cold supply isolated. Turn on the hot tap, water flows from the tap for 30 seconds then stops.

This model of boiler has a pre-heat hot water function (which is not activated) could the water that is stored for this function allow water to flow from the hot tap for 30 seconds before it stops.

 

[Johntheo5]

I don,t think water is stored for the preheat function which just keeps the plate heat exchanger hot I think, if the boiler is higher than the taps it may be just the water in the pipework draining down, there doesn't seem to be any xover from the cold though.
You could isolate the cold water to the mixers, reopen the boiler cold water, turn the boiler DHW temperature control to its minimum, leave mixers to max temp setting and open a mixer outlet, the boiler should then fire up and continue to supply HW at the reduced temperature, if t doesn't then a boiler problem as long as the "Hw" flow is > say 5 LPM, or open both mixers to ensure boiler keeps firing.

Since your problem is intermittent then if it does continue firing with the cold water isolated you could just set the desired HW temperature on the boiler and let the cold water off for a prolonged test, maybe open the shower mixer cold water (if not all ready open) while having a shower for safety reasons even though the mixer thermostatss will thermally shut off the HW in a few seconds if the HW temperature rises by a few degrees.