Hot water goes cold unless it is trickling out

[uk-Pedro]

Hi,

I'm hoping someone has had this problem before and solved it and can help...

I've a combi boiler and when I switch on the hot water from any of the taps in the house everything seems to work fine. Water starts to flow and warm up but after a few seconds when the water is hot the flow rate drops off and if I don't open the tap more the combi will switch off. If I open the tap a bit more the hot water will continue. If I open the hot water tap fully then I only ever get cold water.

Anyone seen this sort of behaviour and know how to cure it?

Any help appreciated.
uk-Pedro

 

[learningplumin]

I can only, think that the domestic hot water heat exchanger, is blocked.
But don,t take my word.
Look at my username, thanks.

 

[Alanc]

I would suggest your HW divertor valve.

 

[learningplumin]

HI, Alanc, the diverter valve would not reduce flow??????
I assume.

 

[stevetheplumber]

Hi,

I'm hoping someone has had this problem before and solved it and can help...

I've a combi boiler and when I switch on the hot water from any of the taps in the house everything seems to work fine. Water starts to flow and warm up but after a few seconds when the water is hot the flow rate drops off and if I don't open the tap more the combi will switch off. If I open the tap a bit more the hot water will continue. If I open the hot water tap fully then I only ever get cold water.

Anyone seen this sort of behaviour and know how to cure it?

Any help appreciated.
uk-Pedro

help if you include some detail of the boiler make model etc

 

[Alanc]

HI, Alanc, the diverter valve would not reduce flow??????
I assume.

Well IMO it definently can affect the flow.

 

[stevetheplumber]

divertor valve will not affect flow of the dhw but will affect temperature

 

[Alanc]

If your diaphragm is pin holed then the switch can slowly close/open causing flow changes at outlet.

 

[stevetheplumber]

If your diaphragm is pin holed then the switch can slowly close/open causing flow changes at outlet.

if you say so but ive never seen that

Hi,

I'm hoping someone has had this problem before and solved it and can help...

I've a combi boiler and when I switch on the hot water from any of the taps in the house everything seems to work fine. Water starts to flow and warm up but after a few seconds when the water is hot the flow rate drops off and if I don't open the tap more the combi will switch off. If I open the tap a bit more the hot water will continue. If I open the hot water tap fully then I only ever get cold water.

Anyone seen this sort of behaviour and know how to cure it?

Any help appreciated.
uk-Pedro

i think its the taps that are dropping back and closing the flow down as your setting them to only slightly open
try opening the tap full and closing the main into the boiler to get temprature right

 

[payney1974]

Its got to be a blockage at the plate to plate heat exchanger or direct H/E depending on what sort of combi you have. Never heard of a diverter causing this though? Could be wrong, after all we are all learning!!

Just read your post again (must read more carefully) If the flow at your taps doesnt slow down then there could be other issues such as a diverter valve or thermistor. Could you let us know what model you have?

 

[rod A]

Hi, is this only happening with one tap or all, have you got the correct flow rate when the taps are fully open to suit the make and model of combi . ? Rod

Hi, i think steve is right it,s easy to get to deep to quickly, Rod.

 

[Beeline]

I think steve is right too. I seen this prob many times especially on old taps of large bore.
If the hot tap has a rising spindle this can stick when it heats up and effect flow drastically.
Best fix is get an engineer to fit a flow reducer to the cold feed, rated as per boilers spec. This will control the flow of water through the boiler, then any tap can be opened fully and only good DHW will come through - it will be a little slower but at least its hot!!
Changing the taps alone could improve the situation a little.

 

[stevetheplumber]

If your diaphragm is pin holed then the switch can slowly close/open causing flow changes at outlet.

id love to hear you explain how a divertor valve can alter the flow of dhw seeing as the dhw doesnt pass through it try it next time your infront of a boile take the water section of and push the divertor back and forth see if it affects the flow

 

[payney1974]

Steve, i think its all the taps that are affected so might not be the taps unless its a big coincidence.

 

[uk-Pedro]

Thanks for all the posts.

Sorry for not putting the combi details on but it is a Glow-worm Compact 100e.

Stevetheplumber: The taps are new as I replaced them hoping that would cure this issue... I'm opening the taps enough to get splashed by the water at the start but as soon as the water gets up to temperature then I only get a trickle unless I open the tap some more. What do you mean by "closing the main into the boiler"? Is there a tap I need to fine tune to get the right amount of water flow into the boiler?


Rod_A: This is happening to all hot taps - which were replaced in the hope of fixing this issue.

Beeline: I'd rather not restrict the water going into the boiler if I can avoid it but if that's my only option then I'll take it.

Thanks for all the suggestions so far.

Regards,
uk-Pedro

 

[charlie frederi]

Hi you may need to check if the right flow restricter is fitted in the mains to the combi as they can alter the flow rate a diverter valve will not affect the flow it will only affect the temp have you always had this problem

 

[uk-Pedro]

Always - since moving into this house 4 years ago.

 

[uk-Pedro]

Is the consensus to fit a flow restrictor on the boiler supply pipe?

Thanks,
uk-Pedro

 

[stevetheplumber]

Is the consensus to fit a flow restrictor on the boiler supply pipe?

Thanks,
uk-Pedro

just close the inlet valve till it delivers the flow it says in the manufacturers instuctions with the tap full open the check temperature against the spec if at the correct flow rate your not getting the correct temperature rise then you will know there something amiss with the boiler which will then need to be checked to see if its burning the correct amount of gas for that you will need a gas safe engineer unless you know how to rate an appliance from the gas meter it can be done with out opening the boiler so if you google it im sure you will find instructions and relevant info for your boiler this is the only way to test changing parts haphazardly will be exspensive

 

[learningplumin]

Hi all, really interesting post.
I am on the trail, that say the dhw heat exchanger was corroded, then on mains water pressure, no fault would be present.
If you call for heat, on a corroded exchanger, then the internal construction may compress, and reduce the flow and cause the over-heat stat too operate.
Just a theory!
THANKS.

 

[stevetheplumber]

Hi all, really interesting post.
I am on the trail, that say the dhw heat exchanger was corroded, then on mains water pressure, no fault would be present.
If you call for heat, on a corroded exchanger, then the internal construction may compress, and reduce the flow and cause the over-heat stat too operate.
Just a theory!
THANKS.

first rule of plumbing KISS KEEP IT SIMPLE STUPID nine times out of ten its going to be something simple the same faults and problems time after time so always do the basics before getting into more exotic faults and theorys

incase you take offence at kiss im not calling you stupid its just a good rule to remember

 

[learningplumin]

Thanx, steve good advice.
No offence takin.
Just thought, I would offer that !🙂

Damm, I cannot post a smiley!

 

[rod A]

hi, regards flow adjustment ,the older combi,s had an adjustment srcew on the body of the divertor valve this slowed the water down passing through the dhw h/ exchanger which did not affect the noise level, you might find that closing down a ball type isolating valve on the incoming main will make a [mains pressure noise though the boiler and system when hot water is drwn off ] ROD.

 

[uk-Pedro]

I've now run a test and think there may be a problem with the boiler.

According to my boiler manufacturer the flow rate I should be getting is below:

In the test I did, the temperature difference was 36.8C and it took 1:47 minutes (1.78) to fill a bucket with 6.4L of hot water. This seems to equate to a flow rate of 3.6L/min. At the temperature difference I measured I should have got around 13L/min but instead got 3.6L/min. This all assumes my calcs are correct...

Can anyone suggest which bit of the boiler to check? I live in a hard water area so this may be due to that but since I'm a newbie to boilers I'd appreciate your input.

Thanks again.
uk-Pedro

 

[rod A]

hi, back to the plate heat exchanger i think not alot else to look for. Rod