Hot water mystery - faulty boiler?

bernardgreen said:
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
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OK, thanks. So I think I can disregard both flow sensor and diverter valve.
 
oldbuffer said:
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.
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I have only 2 mixer taps: the one in the downstairs kitchen and the one on the upstairs bath/shower.

Unfortunately, I cannot perform this test on the bath/shower mixer without causing damage to paintwork and grouting. Of course, I'd be willing to do this if I've eliminated all the easier to ascertain possibilities.

The kitchen mixer is attached to the hot and cold inlets by braided hoses (approx 20 inches long). When I turn on the hot tap fully (no cold) its hose does not heat up significantly. I cannot feel it cooling down at all when I turn on the cold tap, but I'm guessing the hose would not change significantly in temperature whatever is running through it. Further down, where the hose joins the copper pipes, the hot tap's pipe does get very hot with just the hot tap on. It does not cool down at all when I turn on the cold tap.

Given that I can replicate the boiler-not-firing-up-properly issue with whichever taps I use (it's just more pronounced with the shower), can I rule out that the mixers themselves being the cause of the problem?
 
The boiler incorporates a flow turbine to detect hot water tap operation...they are very reliable component.

Considering your claimed static water pressure and very low flow rates from the cold kitchen tap and shower I would be checking stopcocks (inside and out) are fully open...there is clearly an issue there.
You are verging on the minimum flowrate stated in your water companies contract so it might be worth calling them out if the boundary stopcock is suspect.
It's quite possible that with such low flowrates the boiler flow sensor will not trigger a demand when the cold tap/shower is in operation.
 
Gasguru said:
The boiler incorporates a flow turbine to detect hot water tap operation...they are very reliable component.

Considering your claimed static water pressure and very low flow rates from the cold kitchen tap and shower I would be checking stopcocks (inside and out) are fully open...there is clearly an issue there.
You are verging on the minimum flowrate stated in your water companies contract so it might be worth calling them out if the boundary stopcock is suspect.
It's quite possible that with such low flowrates the boiler flow sensor will not trigger a demand when the cold tap/shower is in operation.
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Thanks. I have actually called Thames Water already and asked them to come and check my flow rate. The main stopcock is definitely fully open.

Would you happen to know what the water company's obligation is regarding flow rate? By my rough test today, I'm getting 9.375L/min through the kitchen tap. Is that within the acceptable range?
 
You'll have to look at their website but Thames water are hopeless...and their minimum may even have dropped to 8 litres/min.
 
EDIT 10 litres/minute is poor, 5l/min at the bath is pathetic
Alas there's no easy way for you to check dynamic pressure (ie pressure in the main when cold tap is wide open) but i expect it would be b*gger-all. If you're on a lead supply from street to house that would be a prime suspect for furring up internally or being partially crushed by incidental works nearby ( new fence posts? Garden decking? Virgin/bt digging the pavement up). Look at sorting that before spanking cash on boilers or mixer taps
Also worth asking your neighbours if they're having any problems with water supply
And check your internal stop tap is wide open- if it is accessible it might be worth stripping it down to check the internals are working properly (switch the street stoptap off first!)
 
oldbutnotdead said:
EDIT 10 litres/minute is poor, 5l/min at the bath is pathetic
Alas there's no easy way for you to check dynamic pressure (ie pressure in the main when cold tap is wide open) but i expect it would be b*gger-all. If you're on a lead supply from street to house that would be a prime suspect for furring up internally or being partially crushed by incidental works nearby ( new fence posts? Garden decking? Virgin/bt digging the pavement up). Look at sorting that before spanking cash on boilers or mixer taps
Also worth asking your neighbours if they're having any problems with water supply
And check your internal stop tap is wide open- if it is accessible it might be worth stripping it down to check the internals are working properly (switch the street stoptap off first!)
Click to expand...

Thanks. I'll start by checking with the neighbours. I have never identified an internal stop tap but again I'll check with the neighbours (we live in a row of identical Victorian terraced houses).
 
pulck said:
....... I have never identified an internal stop tap but again I'll check with the neighbours (we live in a row of identical Victorian terraced houses).
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I'd be surprised if there wasn't one, usually under the kitchen sink (or where the Victorians put the sink, if you have a fitted kitchen you may have to look in the service void, some fitters have been known to bury such things, good ones will have put a cutout in the back of the unit. Remember to take any drawers out- you might be surprised what you find). When you find the tap you should be able to determine what sort of supply pipe you have (the bit coming into the house)- probably lead (grey,soft, when scratched it'll show bright metal), might be copper or iron or black plastic or blue plastic.
 
oldbutnotdead said:
I'd be surprised if there wasn't one, usually under the kitchen sink (or where the Victorians put the sink, if you have a fitted kitchen you may have to look in the service void, some fitters have been known to bury such things, good ones will have put a cutout in the back of the unit. Remember to take any drawers out- you might be surprised what you find). When you find the tap you should be able to determine what sort of supply pipe you have (the bit coming into the house)- probably lead (grey,soft, when scratched it'll show bright metal), might be copper or iron or black plastic or blue plastic.
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I've looked everywhere but can't find it. It may have been removed by previous tenants or concealed behind the fitted kitchen.

However, I got Thames Water to come out today. They measured 8L/min flow on my kitchen tap and 25L/min at their main stopcock. (They also measured 15L/min on my garden hose tap.)

They said there's no sign of a leak so the pressure discrepancy between 25L/min and 8L/min must be because of a faulty stopcock in my front garden (until they pointed it out, I hadn't realised there was one!). As it's on my property, they wouldn't fix it for me. But they told me to try closing and opening the front garden stopcock a few times to see if that would help. Apparently there's a rubber valve inside that tap which sometimes drops down and blocks the flow.

Anyway, I did find the tap - after considerable digging - and turned it off and back on and it made no difference. However, the tap was very seized up and in turning the tap I believe I have caused a minor leak - when I closed the tap water starting dribbling out of the top of it. So now I've got to look for someone to replace the whole tap.

The saga continues....
 
Joy joy. Given you've got a stoptap in the garden there may well not be one in the house.
Did you see/did they tell you if the supply pipe was modern blue plastic or something else? If it is modern blue then replacement is fairly easy (gets more difficult the deeper the tap is, especially if it is in an enclosure)
 
And if it's brass you can normally just change the headgear with no need to replace the part connected to the pipework.
 
oldbutnotdead said:
Joy joy. Given you've got a stoptap in the garden there may well not be one in the house.
Did you see/did they tell you if the supply pipe was modern blue plastic or something else? If it is modern blue then replacement is fairly easy (gets more difficult the deeper the tap is, especially if it is in an enclosure)
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They didn't tell me anything about the supply pipe, but the stopcock in my property looked like it hadn't been opened in decades The hole was filled with soil and rubbish. Took me a good 30 mins to get down to the tap.

It's about 2 feet down and in quite a tight enclosure, so will be tricky to work in. I didn't really take note of what material it was made from, but I'd happily just get the whole tap replaced by someone who knows what they're doing if it's going to resolve once and for all this wretched low pressure.

Incidentally, the pressure inside has been worse than ever since Thames Water visited. Is that further evidence that there's something dislodged which is blocking the pipe between the Thames stopcock and my kitchen?
 
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