Intermittent hot water from taps

[plumb_know]

i was wondering if anyone has any ideas on a job i recently attended, i have just passed my plumbing apprentiship and attended a job where the customer was not getting hot water from her basin tap, she had british gas out to check her mains pressure and they said it was fine. she had a old combination boiler and when the basin hot tap was turned on it was not producing enough flow to activate the flow switch to ignite the boiler. i solved this issue by replacing the basin taps. but after i did this i checked the kitchen tap for hot water and it was only coming through luke warm but the boiler was igniting, the customer informed me that this has been happening on and off and that sometimes it gets hot and other times not. i was just wondering if anyone has any ideas why this might be happening?

cheers for any suggestions !

 

[Resolute]

When was the boiler last serviced? I'd guess it might be the heat exchanger being scaled up but I'm sure one of the boiler engineers will advise you better.

 

[plumb_know]

i thought it might be the boiler, but after replacing the basin tap it came out very hot so my understanding is that if it was the boiler all the taps would be luke warm not just kitchen tap?

 

[JCplumb]

Opening any of the hot taps whichever room they're in should activate the dhw, if not then a gas engineer is needed as the boiler needs attention.
I'm guessing you're not a gsr otherwise you'd have already fixed this problem. No offence intended but it sounds like it's time to pass the baton on this one mate.

 

[plumb_know]

yes i did advise the customer to get a heating engineer in, but that does not help me understand what was going on?? and opening the kitchen hot tap was activating the boiler as i could hear it firing up, i just posted this question to help my understanding so in the future i would have more knowledge of what may be happening.

 

[diamondgas]

yes i did advise the customer to get a heating engineer in, but that does not help me understand what was going on?? and opening the kitchen hot tap was activating the boiler as i could hear it firing up, i just posted this question to help my understanding so in the future i would have more knowledge of what may be happening.

In general terms the difference in temp at alternative outlets is due to the flow rate through the outlet! Greater the amount of water passing through the tap the lower the temp will be! For example say you have a boiler that'll guarantee 35[SUP]0[/SUP]C rise with 10lr/min flow. If the water is running at 7lt/min through the boiler it'll spend more time in the heatexchanger and would collect more than the guaranteed 35C, whereas if it was running at 12ltr/min it would spend less time in contact with the heat source and therefore be cooler! Does that make sense?

 

[ManchesterGas]

Sounds like a pinhole in the diaphragm to me, would need a GSR to sort it

 

[jtsplumbing]

You also did-not say how far-away the kitchen tap is from the boiler (are the bathroom taps closer to boiler) or what size boiler, why did she call BG to test water pressure ? is she on a BG service contract ?

 

[plumb_know]

cheers for the replies everyone, yeah diamond that does make sense and could well be the problem.
kitchen tap was about 1 metre away, dont know boiler size just big old combi boiler