Intermittent loss of combi hot water

[djavery]

Twice now we have lost hot water from our combi boiler. The boiler is firing and the hot water outlet pipe is getting hot, but the tap water is just tepid.

British Gas visited, but the engineer was stumped and thought it might be a mixer tap gone faulty. I isolated all cold supplies to the taps and the problem persisted.

A few hours, after the BG engineer had gone, the fault disappeared!

Today, around 2 weeks later, the fault is back again!

I know many posts on here say the diverter valve is the cause, but the BG engineer said the pipe was getting hot and he didn't think that was the problem.

Anyone had similar problems?

 

[Agile]

Your problem is unique to your boiler/system.

From your description I cannot guess what your problem is. But I expect it to be external to the boiler.

If the BG engineer could not identify the problem. What was his next move to be?

Do you have BG cover?

Tony

 

[djavery]

Yes, I have BG cover for the boiler only.

The BG engineer was going to change the diverter valve, but when he came back and the hot water was working, he said to call BG if it happened again and he went away again.

 

[bengasman]

Turn boiler off, and let the whole system cool down.
Turn ch off, and boiler back on. Run dhw and check if the ch pipes on the boiler get hot too.

 

[Agile]

You have not mentioned the boiler model!

Is it a Vaillant 824 or similar?

Tony

 

[djavery]

It's a Worcester 28CDi.

I have narrowed it down to the possibility that a tap we have in the kitchen might be the problem. It's the type with a hose and a lever operated spray function. The mixer control was left in the mid point and on, so I am suspecting that the cold was feeding the hot side, but not sure why. I pushed the control off and we had hot water back!

I am going to install a check valve on the hot side and hopefully that will be an end to it.

We shall see.........................

 

[DeviledAvocado]

Bear in mind that, with a combi system, the hot and cold will be at the SAME pressure. Where are you thinking of fitting this check valve - on the supply pipe to the hot side? To prevent cold water going down the hot pipe? It won't, so is not needed.

Surely the kitchen sink isn't the only hot tap you have in the house?! What happens with the bathroom hot tap when the kitchen 'hot' tap is only luke-warm? Is the bathroom tap delivering 'hot' water? If 'yes', then it's your kitchen tap that's at fault. If 'no' then I guess it's the boiler...

 

[djavery]

Bear in mind that, with a combi system, the hot and cold will be at the SAME pressure. Where are you thinking of fitting this check valve - on the supply pipe to the hot side? To prevent cold water going down the hot pipe? It won't, so is not needed.

Yep, that's exactly what I wanted to do.

Surely the kitchen sink isn't the only hot tap you have in the house?! What happens with the bathroom hot tap when the kitchen 'hot' tap is only luke-warm? Is the bathroom tap delivering 'hot' water? If 'yes', then it's your kitchen tap that's at fault. If 'no' then I guess it's the boiler...

When the fault is present all taps in the house are only delivering luke warm water. The upstairs ones get very hot for 5s or so and then luke warm again and continue to only deliver tepid water. It makes me think that the boiler has heated the water in some of the pipe work (and isn't being diluted by cold) and this comes out when an upstairs tap is run initially.

I'm at a loss of what to do now then. The more I think about it, it must be this mixer tap as the fault only happens on the weekend when the missus is cooking and using this tap. This is logical is it? The cold water could be going back down the hot side if the mixer is left in the ON position?

This is the mixer in question - http://bit.ly/eMdwuI - the lever at the bottom controls the mix of hot and cold and needs to be pulled forward for either of the taps to work, but they have their own levers too and they need to operated as well to get water out. The mixer was left on and at the mid point, but the taps were not being used.

 

[DeviledAvocado]

Hi dj, re my first comment in your post above, the cold mains water supply at your tap CANNOT force the hot water down its pipe because the hot water is ALSO at mains pressure.

The mains pipe comes into the house and splits - one goes to the boiler, is heated and feeds the 'hot' taps. The other goes straight to the cold taps. They are both effectively joined together just before they split before the boiler - at that point they are at exactly the same pressure. At the actual taps, you'll typically find that the hot has less 'flow' as it's been passed through a matrix in the boiler, but the actual 'pressure' it's being delivered at will be the SAME as the cold. Ergo, the cold CANNOT push the hot back down the pipe.

However, whether the mixer tap in the kitchen is somehow 'diluting' the hot water pipework with cold water by somehow allowing cold to be added to the hot pipe when taps are being run in the house is a different matter.

Ah! That's what you were suggesting, isn't it? Oops - sorry

Hmm, that seems unlikely to me, but that's not to say it can't happen - that would be one 'ell of a dodgy tap. But, er, your idea of fitting a check valve on the hot should cure the effect if that's what it is.

In which case ignore all my comments above and go for it!

Interesting one - look forward to hearing how it goes. Fingers crossed - if this cures it, you've worked out things better than a BG service fellow. And, er, a few others...

 

[djavery]

Hi dj, re my first comment in your post above, the cold mains water supply at your tap CANNOT force the hot water down its pipe because the hot water is ALSO at mains pressure.

The mains pipe comes into the house and splits - one goes to the boiler, is heated and feeds the 'hot' taps. The other goes straight to the cold taps. They are both effectively joined together just before they split before the boiler - at that point they are at exactly the same pressure. At the actual taps, you'll typically find that the hot has less 'flow' as it's been passed through a matrix in the boiler, but the actual 'pressure' it's being delivered at will be the SAME as the cold. Ergo, the cold CANNOT push the hot back down the pipe.

However, whether the mixer tap in the kitchen is somehow 'diluting' the hot water pipework with cold water by somehow allowing cold to be added to the hot pipe when taps are being run in the house is a different matter.

Ah! That's what you were suggesting, isn't it? Oops - sorry

Hmm, that seems unlikely to me, but that's not to say it can't happen - that would be one 'ell of a dodgy tap. But, er, your idea of fitting a check valve on the hot should cure the effect if that's what it is.

In which case ignore all my comments above and go for it!

Interesting one - look forward to hearing how it goes. Fingers crossed - if this cures it, you've worked out things better than a BG service fellow. And, er, a few others...

For £1.70 for a check valve I'm going to have a go and see what happens. I can always take it back out.

This tap is effectively connecting the cold to the hot and vice versa when the lever is open and at the mid point. Really funny design, but there you go!

Many thanks for your advice anyway

 

[djavery]

Well - installed said non-return valve today and hey presto, problem has gone. Doesn't matter now if mixer control is left on or off. Really pleased

 

[DeviledAvocado]

Wow - fast operator, and first class result!

Coooooooool