Hot water mystery - faulty boiler?

[pulck]

I have a shower running off bath mixer taps (i.e. not an electric shower) and for the last few weeks it has been driving me insane... hot water has been intermittent and water pressure is terrible. Plumbers can't figure out what is causing the problem.

What happens is this: when I turn on the hot tap I get hot water (albeit with poor pressure). But as soon as I turn on the cold tap, the water pressure drops even more and the boiler stops heating the water properly. Result: cold shower.

One plumber observed this: if you run the hot water tap anywhere in the house (e.g. downstairs kitchen) the boiler fires up just fine. But as you turn on the cold tap, the more you turn it on the less the boiler heats the hot water. You can literally hear the boiler decrease in volume as you increase the flow on the cold tap.

He checked water pressure coming into the house and it was 2.5 bar, which I believe is within the acceptable range. When he spoke to an engineer at Worcester, the engineer maintained that the problem was not with the boiler, as it has no pressure sensor.

Background info: my combi boiler (Worcester Greenstar Junior 24i) is upstairs and is 9 years old, but in good condition. It had a new heat exchanger and a new diverter valve last year. Central heating is working fine.

Some possible causes:

- Low water pressure into the house? (Or possible leak on ground floor?)
- Calcified/blocked hot water pipes? (NB/ It's a Victorian house)
- The boiler is not powerful enough for a 3-bedroom house? (Although why has worked fine so far?)

How can I test for these? Is there anything else I've missed?

I'd be very grateful for any thoughts or suggestions...!

 

[oldbuffer]

Could be a non-return valve failed, allowing cold water to pressurise the hot water circuit, resulting in the boiler deciding there is not enough hot flow to fire.

 

[Gasguru]

Possible dodgy mixer tap/shower allowing cold to pass to the hot pipework when not in use.

 

[terryplumb]

Are you saying that the problem is the same on all sinks/ basins and bath??
I.e turn on hot tap at kitchen sink , and boiler fires giving hot water non stop but if you then open any cold tap ,even on another basin or the bath ,the boiler then shuts down ??

 

[pulck]

Could be a non-return valve failed, allowing cold water to pressurise the hot water circuit, resulting in the boiler deciding there is not enough hot flow to fire.

Thanks - no-one has mentioned this before. Couple of questions:

How can I check if this has failed? Or do I simply get the valve changed and hope that's the problem?

Is the non-return valve definitely on the water heating? Some other sites suggest it is only on the central heating.

 

[pulck]

Possible dodgy mixer tap/shower allowing cold to pass to the hot pipework when not in use.

Thanks, although I don't quite understand how this works. If cold is passing into the hot water at the mixer tap, wouldn't the water come out at exactly the same temperature?

Also the problem occurs with other taps in the house. It's just much more pronounced with the shower mixer, presumably because that's where gravity causes pressure to be most critical (shower is upstairs and shower hose is raised above bath).

But if this is the issue, again can I test for this or do I just have to bite the bullet and just change the whole mixer?

 

[pulck]

Are you saying that the problem is the same on all sinks/ basins and bath??
I.e turn on hot tap at kitchen sink , and boiler fires giving hot water non stop but if you then open any cold tap ,even on another basin or the bath ,the boiler then shuts down ??

The problem is most pronounced with the shower presumably because of gravity.

I haven't specifically tested separate separate basins (will try that tomorrow) but it is true that if I open hot tap on any basin and then open cold on the same basin the boiler will stop sufficiently heating the hot water. As I said in the original post, you can hear the boiler shut down more and more as you open up the cold. By the time the cold is fully open, the boiler has practically shut down.

UPDATE: I've just checked and the problem persists even when I use hot and cold on separate basins. Does that help diagnose the problem?

 

[oldbutnotdead]

What flow rate are you getting on your downstairs cold tap (bucket, stopwatch, measuring jug, cold tap on full)- static pressure is only 1 part of the story

 

[pulck]

What flow rate are you getting on your downstairs cold tap (bucket, stopwatch, measuring jug, cold tap on full)- static pressure is only 1 part of the story

The plumber measured 2.5 bar on the inlet to the kitchen tap. Is that normal?

 

[oldbutnotdead]

Thats the static pressure (with no taps running). You could get that from a blocked lead supply pipe....but the flow from that supply would be pathetic, as would dynamic pressure.
How many litres per minute does the downstairs cold tap deliver?

 

[bernardgreen]

that the problem was not with the boiler, as it has no pressure sensor.

If the heat exchanger in the boiler is offereing resistance to the flow of water then the pressure at the input to the heat exchanger will affect the rate of water flowi through the boiler. Too little flow and the boiler will shut down. Opening a cold tap will reduce the pressure in the mains supply inside the house, hence the flow through the boiler will reduce.

The boiler is fired up by one of either a flow sensor ( if there is enough flow ) or a divertor valve ( if there is enough pressure ).

The boiler will ( should ) be shut down by an over temperature sensor if the water leaving the boiler is too hot. This can happen if the flow rate is too low for the heat being generated.

A low flow rate can be a result of the heat exchanger being partially blocked by lime scale.

 

[pulck]

Thats the static pressure (with no taps running). You could get that from a blocked lead supply pipe....but the flow from that supply would be pathetic, as would dynamic pressure.
How many litres per minute does the downstairs cold tap deliver?

I just ran 10L of cold water through the kitchen tap in 64 secs. I guess that's 9.375L/min. Does that sound normal?

(BTW I did the same for the shower upstairs - cold tap but without the shower head attached. That was 10L in 1min48secs = 5.55L/min.)

 

[pulck]

If the heat exchanger in the boiler is offereing resistance to the flow of water then the pressure at the input to the heat exchanger will affect the rate of water flowi through the boiler. Too little flow and the boiler will shut down. Opening a cold tap will reduce the pressure in the mains supply inside the house, hence the flow through the boiler will reduce.

The boiler is fired up by one of either a flow sensor ( if there is enough flow ) or a divertor valve ( if there is enough pressure ).

The boiler will ( should ) be shut down by an over temperature sensor if the water leaving the boiler is too hot. This can happen if the flow rate is too low for the heat being generated.

A low flow rate can be a result of the heat exchanger being partially blocked by lime scale.

Thanks, but I had the heat exchanger replaced last year, as I thought it might be scaled up. The old heat exchanger was in perfect condition but it was replaced nevertheless. So I think it's unlikely to be the heat exchanger.

I had the diverter valve replaced at the same time. But am I right in thinking that diverter valve problems would only be apparent if the hot tap was on at the same time as the central heating? That's not the issue I'm having.

But the flow sensor is not something that I've looked into. Does the flow sensor would gradually or is it an on/off sensor? Reason I ask is because the more I open the cold tap (whilst the hot is already open), the less the boiler is firing.

 

[oldbuffer]

For every sink / basin / bath / shower with a mixer tap:

1. Turn the hot on, and feel the hot pipe as close to the tap as possible.
2. Turn the cold on. If the hot pipe starts to feel cooler, then water is flowing back from the cold into the hot.

A mixer tap is one where the hot and cold are mixed internally, whether or not thermostatically controlled. A tap which feed cold through the outside of the spout, and hot through the inside (or vice versa) so that hot and cold only mix immediately after leaving the tap, is not a mixer tap for the purposes of the above test.

It is also possible that the bath / shower has a thermostatic mixing valve fitted. This is a valve, usually hidden from view (e.g. under bath), which blends cold water with the supplied hot to output hot water not exceeding a certain temperature (usually 38 or 40 degrees). They are in effect mixers as above. Often fitted in newly built or newly renovated properties, and those designed for the old and infirm.

 

[bernardgreen]

I had the diverter valve replaced at the same time.

If the boiler has a divertor valve then it is unlikely that it also has a flow sensor. The divertor valve diverts the water that normally circulates round the radiators and makes it pass through the secondary heat exchanger to heat the hot water to taps etc. . At the same time the divertor valve switches on the boiler. When drawing hot water to taps the hot water to radiators stops flowing.

edit to correct quoting