Please please help - hot water flow in combi system

[amc73]

Hi, I searched on previous posts but can't see what I am after, plus i am clueless!

I have a combi system and currently have a hot water problem. Essentially the boiler is working, the radiators are fine and no warning light are flashing. Its a Main Combi 30 HE.

When I run the hot tap, there is either no flow at all, or it runs normally for a few seconds and then the pipes start hammering, flow in in spurts, then it stops flowing completely.

If i run the cold tap, at mid flow strength, then the hot will run but very weak, but as I gradually turn the cold tap off, the hammerinf starts again and then flow stops.

This is affecting every tap in the house, excpet the hot tap in the kitchen which has good hot flow, which it tapped of the hot pipe about 1m below the boiler.

The cold supply is fine.

If this was my old gravity fed system i would suspect airlock and would blow the sytem out, but its a closed combi which is confusing me.

Can it be and airlock and if so how can i clear it??

Please help!

 

[Agile]

Its difficult for me to guess what is causing your problem.

You should start be ensuring the system pressure is about 1.5 and the filling loop is closed.

Tony

 

[amc73]

Thanks for the reply - pessure is just under 2 bar and the loop valve is closed off.

I briefly managed to get one tap working at 50% power but as soon as I opening it up more, the pipes knocked hard, water spluttered and it stopped again.

Is it possible to disconnect the hot water out pipe from the boiler in order to flush backwards though all hot taps without disruprting the internal boiler pressure?

 

[corgiman]

I would say that you have a partially closed isolation valve somewhere

it aint no air lock

 

[amc73]

I would say that you have a partially closed isolation valve somewhere

it aint no air lock

I'll have another look round at those. Thanks!

 

[amc73]

have checked all isolators and all operational and wide open.

I'm at a loss.

 

[corgiman]

Then it may be a partially blocked HEX

I cannot remember if these ones have a DHW HEX or not mebbe one of the others will know

If it is a DHW HEX I would replace it

 

[diverdee]

if you have hot good flow rate at one tap on a combi it is not the boiler.check rest of house hot pipe work for blockage.isolate the cold at boiler and back fill from a cold tap to a hot tap might disturb the blockage.of course might just move it elsewher too!!

 

[corgiman]

he says its affecting every tap in his OP Diver?

 

[diverdee]

nope corgiman he says he has good hot flow at kitchen tap!!

 

[diverdee]

check stopcock open fully may just be a lack of pressure !!

 

[amc73]

I've now got a list if things that happen.

1) Turn hot tap on in kitchen - all fine (hot water, normal flow)
2) Hot water not running at all upstairs taps or bath or shower.
3) Turn cold tap in upstairs sink on, then ease hot tap on and both flow. Turn cold off, knocking at hot stops. Turn cold on full and trickle of hot from hot tap
4) Dowstairs toilet sink mixer works on hot if you gingerly put it on and leave it about 50% open. Open fully pipes knock and water spurts in pulses with knocks then stops (like upstairs)
5) downstairs mixer type shower works if on tepid (i suppose same as hot open 50%.

So it seems that downstairs there is flow if gentle
Water comes through hot, boiler firing properly
Open up gflow fully and its game over
Upstairs hot is not happening
Cold flow in all taps fine.

The system has been in for just under 2 years. No changes made since installation

There are a few isolating valves in there but all new at the time *the quarter turn handle type. Both showers are the mixer valve type and have no return valves fitted.

Kitchen tap is teed straight of the hot water out pipe, that then feeds the rest of the house.

Alex

 

[corgiman]

nope corgiman he says he has good hot flow at kitchen tap!!

Missed that Dee I did

I would need to look at the job to see whats wrong OP sorry

 

[diverdee]

check filter on cold inlet on boiler could be starving its self then vacuming in upstairs pipes !!.hard one with out seeing it

 

[amc73]

Solved! There was an old stop cock on the hot water pipe, just after the T for the kitchen sink. This was well past its best. Put a new one ina problems solved!