Hot water problem with combi boiler - any ideas?

[sammydavis]

We have a Worcester Bosch Greenstar 25si (mk iv) combi boiler which has developed a problem - I think!

Central heating aspect is fine, boiler pressure at recommended level and so on.

Last week, we noticed that, when having a shower, instead of a constant flow of even-temperature water, the shower was producing hot water, then cooling slightly, then getting hot again, then cooling slightly - constantly over the period of the shower.

The 'boiler lit' light stays on when water is running - although I think the 'burning' sound is switching off - so I think it's switching itself on and off.

There are no flashing 'FAULT' lights or indicators on the boiler - all seems fine.

Our shower is just a wall mounted mixer tap over a bath - nothing fancy, one control lever, pull out for on/off and turn left or right for hot/cold -cost about £20 and has worked fine for last year.

I've tried to test the water temperature variation in the bath and kitchen sink and I 'think' it's doing the same at these taps.

I've turned the water temperature UP and DOWN at the boiler and it makes no discernible difference to the problem - but does make water hotter and colder.

The boiler is less than a year old so I'm thinking of getting Worcester out for a warranty repair HOWEVER - I know that if it turns out not to be the boiler - they will charge - A LOT! - so I'm trying to eliminate all other possibilities before calling them.

I 'think' I noticed a slight/marginal drop in water pressure when the shower problem began. I've been running cold water to check - the pressure still seems high but maybe slightly lower than it was before the problem began - not lower enough for me to remark upon it independently of the shower problem - it's probably gone from 'blast your face off' high pressure to 'splashes on the front of your shirt if you're not careful' - so probably a fair bit of a drop.

After this long tale of woe - would the drop in water pressure cause the boiler to switch on and off during a water heating cycle (if so, can this be remedied?) OR is it more likely to be an internal/mechanical problem with the boiler?

The weather here has been colder but not freezing and didn't cause the boiler any problems last year (colder) so I don't think it's that.

I've googled this and there are other people out there with the same problem - but I can't see where anybody has found a solution - nobody posts the solutions - buggers!

I wonder if the amount of water flowing through the boiler is insufficient to keep the flow switch on.

turn on the bath hot tap (only) and

• run it into a bucket to see how many litres per minute it delivers
• see if the water is continually hot

then turn on a cold tap, perhaps at the kitchen, and measure the flow and heat at the bath hot tap again.

you say the boiler is fairly new. Was it installed on an old system with old radiators? Was the system powerflushed? Was corrosion inhibitor added? Was a system filter fitted?

How old is the house, and how old are the radiators?

Exactly the same problem here! Though Vaillant boiler.
Having trouble finding "a man" as suspect the job is too small. Vaillant helpline is useless.
Will follow with interest.

Thanks for the replies.

I haven't measured the flow/quantity of water from the taps yet (it's observably lower to the eye and feel of the pressure on the hand) but I have checked the temperature issue and the temperature of the water is varying from every tap as well as the shower.

The boiler is about 10/11 months old fitted to an old system of pipes and radiators which was powerflushed when boiler fitted.

It says on the benchmark checklist that it was flushed with SM-4 and inhibitor SM-1 was used.

Is a system filter one of those extra wide pipe attachments? Don't have one of those. House is 1930's but radiators are probably about 20 years old/ish.

Could it really be clogging up already - inside a year? We have limescaley water but not awful - I've lived with worse without boiler trouble.

Lots of internet stuff goes on about a drop in water pressure/flow causing the boiler to stop burning OR some sort of failure with a flow sensor.

If it's a drop in water flow - can that be fixed? Although I can't imagine the water board would be best pleased if I phoned up and asked for a bit extra at number 42!

I can imagine that wouldn't be covered under a warranty but could a plumber adjust the boiler settings to compensate for reduced flow?

OR, could it be a boiler sensor/burner problem which would be covered by warranty?

I suspect it would be a lot cheaper to get a local plumber out to do the diagnostics but that might not be covered/reclaimable under the warranty if it turns out to be a boiler fault.

But if I get the warranty people out, and it turns out to be low water flow and not covered, then I'd have to pay their high call out rate PLUS the cost perhaps of then getting another local plumber to do the fixing

Any thoughts on best further courses of action?

the temperature will vary with the amount of water passing through the boiler, so will be different at different taps.

When you are measuring litres per minute, also measure it at cold taps to see if it is markedly different.

During winter, when the incoming water is colder, the supply of hot water at the taps is bound to be cooler and/or slower, because there is a limit to the output of the boiler.

Well I don't know anything about boilers but for what its worth, mine did exactly as you describe and it was the heat exchanger that needed changing...and has done roughly every year since I've had it...luckily I have boiler breakdown cover but it is a pain in the butt.

Try changing your showerhead to a larger free flowing item with bigger jets, this usually cures the problem. Scale can also block the jets in a showerhead reducing the flow so I doubt if there is a problem with your boiler at all.

I gather from the website that it can deliver
Domestic hot water flow rate: 10.2 litres per minute at 35 degree temperature rise (provided adequate mains water pressure is available)

so it will be important to know how much hot water it delivers, and compare this with the cold water flow.

Thanks again all.

I've been busy with the measuring jug (unfortunately no bucket to hand) and the alarm clock with a second hand so I finally have some flow figures.

Those figures are interesting PigletJohn given my results.

The water main enters the house at the bathroom and the boiler is in the bathroom.

Hot flow at bath = 7 ltrs per min approx

Cold flow at bath = 12-14 ltrs per min approx

The cold measure was hard to do accurately with a 1 ltr measruing jug as it filled so fast and every time I emtied it probably another half a litre went down the plug hole - suffice the say, the cold flow is fast.

The hot flow was (I think) cooler generally than usual but varied from very hot to hand hot and then very hot again coinciding with a change in the sound of the boiler (even though the 'lit' indicator remained on throughout).
The force of the hot water was variable too.

The kitchen is further away from the bathroom and boiler only by about 10 feet.

The taps style is different - a mixer with a very narrow (and annoying) jet effect which forces you to turn the tap down or it ricochets back in your face!

Hot flow was slightly less at 6 - 7 ltrs per min.

Cold flow was slightly more 12 litres per min but hard to be accurate with all the splashing.

What does this mean for the boiler do you think? From PigletJohn's figures it seems it should be able to cope but is producing less than it should.

Central heating is working fine - no complaints there.

Have also removed and descaled showerhead - unfortunately, no difference noted.

Without knowing the flow rates when new, hard to be sure, but the boiler, or its piping or valves, appear to be restricting fliw. Are you in a very hard water area?

I think I would ask a trusted local heating engineer, probably the original installer, to take a look first. You may get some improvement for changing any ball-o-fix valves (look it up) for full-bore valves, especially if there are several. Braided tap hoses are somewhat restrictive unless you fit a larger size.

It might be the heat exchanger though getting clogged.

Thanks PigletJohn - the original installer didn't complete the flowrate box on the benchmark checklist so no idea on that I'm afraid - can only go by own sense that it seems to have fallen.

I understand where you're coming from in terms of getting a local plumber to check it out but I'm on a tight budget and that would be a charge whether he can fix it or not or whether he then recommends getting the manufacturer to examine it.

Do you think (on the basis of probabilities) that the items you describe would be covered under a standard boiler warranty - the valves, heat exchanger and so on?

Ps. yes, we do live in a hard water area - is that a get-out for the manufacturer, do you think?

I still think it sounds like a flow restriction. Having a local person round to diagnose would be my first suggestion. It might be limescale in the heat exchanger, they can be chemically cleaned or replaced, not very cheaply. I don't know if your boiler has an aluminium heat exchanger, or a stainless steel one which stands up better to acid cleaners. Some people thing a magnet/conditioner on the incoming pipe prevents scaling, I don't believe it. A water softener does prevent limescale but costs some hundreds of pounds. It would be possible to take the hot outlet pipe off your boiler and measure the flow through that to see if it is the boiler or the house pipework causing it.

You should be able to ask your installer first, he has to take some responsibility for it working properly.

If you are lucky, and the problem is (partly) due to cheap restrictive valves on the pipes like this they can be fairly easily replaced with full-bore ones like this Pegler. Note that it is fatter in the body to allow more water through.

I would still try a local person first to diagnose the problem and advise after seeing it.

Look up your boiler warranty to see if it is one year or five.

p.s. the valves supplied by the boiler manufacturer, which are usually in a row underneath it, are pretty sure to be full-bore ones. The others are more likely to be found in the supply pipe, or in the bathroom or under the sink. Metak braided flexible tab hoses are also restrictive, unless you buy the next larger size and fit adapting olives.

The cheaper ones are very popular for DIY plumbing as they make it easy to cut off the water when changing a tap washer or something, but I have found they sometimes start to leak for no apparent reason.

Thanks again PigletJohn - I take your point on the installer responsibility. My feelings (and I suspect the installer's too) would be that the fault didn't develop until the boiler was in and operating for 10 months.

I'd love to get him back for an opinion but I know he will charge me for that because he will take the view that this is a problem unrelated to installation - a new issue unrelated to any previous work he has done.

I won't be able to prove otherwise so getting him round is going to cost me his call-out fee and I can't help feeling - to what end? If he can diagnose the problem, he'll say 'you need a new heat exchanger - it's all scaled up' - it's not reasonble for me to then say 'well, 50% of that is down to you using the wrong valves' because I can't prove that.

I think if I got him out under those pretences, he would say you owe me for the call-out, and you either pay me to fix it or you get Worcester out to fix it and we're no longer mates any more because you accused me of doing a bad job - everyone's more sensitive these days

I don't want to get someone out for a free estimate - diagnose the problem and then not use them because part failure is covered under the warranty. I would just be taking the pee then.

Much as the problem may well be part installation, part hard water and part product failure - for me the customer, to resolve it without going from pillar to post - I think I'm better off starting with Worcester - however, if they then start sucking on their teeth and saying - it's down to you water' - well, that won't be very amusing!

Is your gut feeling that it's not something likely to be covered under a standard warranty (2 years phew!)?

We had this problem and it was all caused by a leak in a pipe in the back street.

Go to your tap in the kitchen and listen. (don't turn any taps on while listening)
If you can hear a quiet background noise like running water, the chances are there is a leak somewhere.

If it is a leak in the back street (if you have one) then it is up to the water company to fix it but you would have to get them onto the problem.

The drop in water pressure would cause the boiler - heat- to not stay on long enough while you shower (we had this happen)

Best bet is get a plumber round to investigate.

Good luck and let us know how you get on.

because the water flow from the cold taps is quite good, and better than from the hot, I don't think it's due to the water supply.

It might possibly have been higher a year ago, which would contribute, but is not a probable cause on this one.

It will do no harm to check that all your stop-cocks and isolating valves are fully on.

You can ask a local person around for a diagnosis and recommendation, tell him you will pay him for his time before deciding what else to do, that's fair.

Cheers - like PigletJohn, I don't think a leak is causing the problem after all the cold water flow is minimum 12 ltrs a min and most the boiler could heat is 10 ltrs a min - so even if there is a leak, the boiler is still being fed more than enough water.

It's the output that's the problem.

For some reason, in the last week/10 days the boiler has stopped heating the same quantity of water AND the burners are audibly switching on/off hwere they didn't two weeks ago.

Something is amiss with the boiler. Something: a valve/a sensor/ a gauge - something is telling the burners to cut out where they didn't cut out before. Something is restricting the amount of hot water and the constancy of hot water leaving the boiler.

What is that something?

In the end, I paid more money for a Worcester because they're supposed to be better. Worcester give warranties because they beleive their boilers are better. I've paid for that warranty, for that reliability already through the higher price.

When there's a problem with the boiler - would I not be being daft if I get someone other than Worcester to fix it?

I've been lucky enough to be able to ask you for your advice and I've searched the internet - all in the hope I've diagnosing the problem myself - I'm trying to let Worcester off the hook - after all I don't want to pay if it's not a Worcester problem - but in the end, if it is a Worcester problem - I've already paid for the warranty with the higher purchase price - I think I'd be daft to pay another plumber to do what Worcester have warrantied when I bought it .... but I guess there's only one way to find out!

If you think this is defintely not a warranty issue - tell me now - as I'm going to call in Worcester - I'll advise of the outcome and how much I get fleeced for

We had this problem with our Ferroli boiler. British Gas changed the control panel (looked like a circuit board) and it's been fine since.

don't know

but the two symptoms are related. The amount of water going through the boiler turns the burners on, and if the water flow is low, it turns the burners down or off.

It might be interesting to know if behaviour changes when CH is turned off.

Thanks all - Worcester on the case.

Water problem has been the same with central heating on or off.

Logically, as a layman, I would have thought if there's a broad 'heating'/'burning' problem then the cental heating would also be affected. For example, not getting as hot and so on.

But this hasn't been the case. CH is perfect.

The problem is exclusively hot water and specifically the burners / ignition and output reacting differently for hot water demand than they did a week ago.

Even if my water mains pressure has been reduced - it's still entering the boiler at a higher pressure that the boiler can feasibly produce hot water - so I'm optimistic water pressure is not the issue.

If the heat exchanger was 'scaled up' or gunged in any way the surely the heating efficiency might also affect the CH too - but the CH is fine.

I don't know - I'm flummoxed - I just hope there isn't a big bill on the horizon - call out is included in warranty so that's really good - and parts and labour as long as it's specifically a boiler issue.

Not many people can be hoping they have a boiler problem

It sounds just like hours. Intermittent and then got gradually worse. Was covered under our British Gas contract, but no big parts so can't have cost them too much.

Hope you get it sorted OP.

A solution to this at last! After many Worcester Bosch visits we have solved the issue. Still not entirely sure why it has happened or whether it actally is a boiler 'problem'.

But... the solution was to reduce the flow of water into the boiler at the mains!!!

Don't know why something that had worked fine for a year suddenly changed. Worcester couldn't say. They rigged up all their laptops to the boiler and said it was fine - nothing wrong - totally able to handle the water flow etc etc.

But it was only when the flow was reduced at the mains that the problem stopped.