Combi only briefly heating water - passed between plumber and boiler engineer...

[MattB83]

(me again trying to make progress with various problems in in-laws house).

Worcester combi boiler only a couple q of years old, putting out loads of heat to warm the house but hot water only comes on briefly when taps are first switched on and goes off never to return. Boiler engineer said def not boiler and suggested it was mixer taps - although the issue is with ALL hot taps, not just mixer so wasn't sure exactly what this meant - and we need plumber. Plumber came out and said nothing wrong at all.

What I have noticed during my stay is that the boiler is losing pressure and has to be topped up every couple of days, I have spotted a radiator dripping badly which we've got someone coming out to fix later in the week. But even when boiler is up to pressure the problem persists, could the leak still be causing the water temp issue?

Any thoughts appreciated as not sure where to go from here.

 

[CBW]

The reference to the mixer tap could be its passing. A way to test this is isolate the cold inlet to the boiler and run the hot tap, if you still get water then that’s what is passing. However, I don’t think this is the issue here. Is the boiler under warranty? if yes then call Worcester out.

 

[MattB83]

Thanks - yes under warranty and they've been out but said boiler is not causing the issue.

 

[CBW]

Thanks - yes under warranty and they've been out but said boiler is not causing the issue.

If it’s definitely not the boiler, then I’d be looking at the mixer(s).

 

[MattB83]

If it’s definitely not the boiler, then I’d be looking at the mixer(s).

This seems to be the problem, the plumber spent a lot of time and said he couldn't identify any issues so we're not sure what to.do next. Another plumber - potentially someone willing to look at boiler + everything else?

 

[CBW]

This seems to be the problem, the plumber spent a lot of time and said he couldn't identify any issues so we're not sure what to.do next. Another plumber - potentially someone willing to look at boiler + everything else?

Is the mixer test something you could do, rather than await a plumber/gas engineer?

 

[bernardgreen]

If it’s definitely not the boiler, then I’d be looking at the mixer(s).

Something to discuss with the trades people

One reason a combi boiler does not continue to produce hot water is an inadequate flow of water to the hot taps.

Either
(a) flow rate is too low to keep the boiler's flow switch activated ( might involve the boiler's diverter valve )

or

(b) flow is too low to absorb the heat from the boiler and the water temperature becomes too hot and the boiler's safety ( overheat ) sensor activates and shuts the boiler down.

Have you tried opening several hot water taps at the same time ? If the boiler stays fired up with several hot taps open then a low flow rate is likely to be the problem.

 

[Johntheo5]

The reference to the mixer tap could be its passing. A way to test this is isolate the cold inlet to the boiler and run the hot tap, if you still get water then that’s what is passing. However, I don’t think this is the issue here. Is the boiler under warranty? if yes then call Worcester out.

Thats the first easily carried out test, if no water coming from the hot tap restore the cold mains to the combi then open one or more hot taps only to give a combined flow of exactly 5LPM and set the DHW temperature to 60C, this should be no problem for any combi boiler as it only requires 18kw to heat the water from the now mains temp of 8C to 60C, if this test doesn't work out then hard to argue against a boiler fault?. If the test is OK reduce the flowrate to exactly 3LPM and repeat, boiler output required 10.9kw.

 

[MattB83]

Thank you all. Really appreciate your time. I'll try all these things and take it from there.

 

[gas112]

if you have a shower off the combi they are usual culprits for passing .

 

[MattB83]

Thats the first easily carried out test, if no water coming from the hot tap restore the cold mains to the combi then open one or more hot taps only to give a combined flow of exactly 5LPM and set the DHW temperature to 60C, this should be no problem for any combi boiler as it only requires 18kw to heat the water from the now mains temp of 8C to 60C, if this test doesn't work out then hard to argue against a boiler fault?. If the test is OK reduce the flowrate to exactly 3LPM and repeat, boiler output required 10.9kw.

Thank you. Either the 5 or 3LPM test results in the same, the boiler initially kicks in and the water gets really hot, then after about 30 secs it stops heating the water.

 

[gas112]

Thank you. Either the 5 or 3LPM test results in the same, the boiler initially kicks in and the water gets really hot, then after about 30 secs it stops heating the water.

So the actual boiler stops firing up for hot water or does it carry on firing but the water isnt hot ?

 

[Johntheo5]

Thank you. Either the 5 or 3LPM test results in the same, the boiler initially kicks in and the water gets really hot, then after about 30 secs it stops heating the water.

Did you carry out the first test as per post #2?

Don't know your boiler output but some/most boilers have a flow restrictor installed to give a max 35C temperature rise so suggest opening all hot taps fully then monitor if the boiler continue firing, then measure the total flow rate(s) again.
If you multipy the boiler output in kwX0.41 and compare this with your measured flowrate then if both ~ the same then the max DHW temp will be 43C, if measured flowrate > the calculated flowrate (kwx0.41) then the DHW temperature will be lower.
For example if your boiler output is say 30kw then you could expect ~ at least 12.3LPM (30x0.41) actual flow.

You might repeat this test with the DHW setpoint temperature control (if fitted) turned down to its minimum.

 

[MattB83]

sorry for drip-feed but have been in touch with the previous plumber and asked if he could send me more info re: what checks he'd done - he e-mailed a report this morning:

Assessment/Investigation of warm water only at all outlets on a combi system. Customer previously advised by boiler engineer that taps were passing. Upon assessment, found no indications of this. In my opinion, this is a boiler issue and testing at all outlets indicate this as chances of all taps being broken is massively high.

 

[Johntheo5]

The previous plumber might be implying that the boiler kept firing because "of warm water only at all outlets on a combi system" which might be at variance with your post# 11.

One sure way testing for a boiler fault is to remove the combi DHW outlet, make up a bit of pipe with a lever or gate valve and attach it to the DHW outlet and put in the end in a large bucket or whatever, open the isolating valve to give a reasonable flowrate of initially say 3 to 5 LPM and see what the boiler does, then try it at various flowrates based on the calculation above, there can't be any arguments if this is carried out?.