Advice needed re air in pipes

[Fedup]

Hi all, I hope someone can help. We have just had our boiler replaced and now we can't get the hot water in the bath tap to stay hot. It runs hot then cold, then hot then cold. When you first open the hot tap this causes an extremely loud banging noise in the pipes. This only seems to happen with the bath tap, the others appear to be ok.

It has been suggested that this may be due to air in the pipe, and I've read the technique for rectifying this, involving connecting the hot and cold kitchen taps together to force the air back up the system to the water tank. However, we have no water tank as it's a combi boiler, so assume this would be a bad idea.

Is there another way of doing this, and does it sound like the problem is being caused by air or something else? Unfortunately the guys who did this job turned out to be cowboys so I really don't want to get them back to sort this if I can help it. Thanks.

 

[sonray ltd]

how longs it been doing it since boiler fitted ?
and no you can't use the cold to push back the hot as they are both mains pressure.

hot/cold is the boiler staying alight while the bath tap is going hot/cold ?
or does the boiler cut in/out ?

 

[Fedup]

Boiler was fitted on Thursday and it's been doing it ever since. Not sure whether boiler is staying alight as it's in the loft but I can check that. However I assume it's staying alight because it's only the bath tap I have trouble with. If I turn on the hot basin tap it stays hot and no banging noise.

 

[sonray ltd]

does it still run hot/cold if you only have the bath tap 1/4 open instead of fully open ?

 

[Fedup]

Yes it does. The banging only occurs when the tap is about 1/4 open.

is the boiler staying alight while the bath tap is going hot/cold ?
or does the boiler cut in/out ?

Have just checked this and the flame is going off and on every 30 seconds or so, corresponding with the water going hot and cold. What does this indicate?

 

[Dal5band]

maybe the tap has a loose washer - bouncing up & down at a certain setting or it could be a loose pipe causing water hammer. Either way, the water hammer causes a shock wave in the water, causing the boiler to go on and off (same as turning a tap on and off). Check the tap, and all stop cocks in the system, and any loose pipes.

 

[traineedrip]

out of interest is it unusual to get airlocks in hot/cold pipe work using combi boilers?

 

[stevetheplumber]

virtually impossible to get airlocks in mains fed services to be honest you have to realy do a bad job to get an airlock in the heating with a combi
Thing of the past airlocks on any modern system

 

[traineedrip]

thanks steve. thought as much but nice when its confirmed

 

[danielp]

did he convert it to a combi boiler from tanks? , the pressure will cause water hammer from the ordinary

 

[jobsrus]

I am presuming that your combi boiler boiler is shutting down because the flow from the bath tap is being resricted,the plumber may have small bore flexi to the hot side.
The bang is the pressure being relieved when the tap is turn on which starts the boiler but then when the pressure has been relieved the flow drops thus boiler shut down.
Have a look at the pipe to the hot or is it a mixer that would be a possible problem

 

[migoplumber]

what setting is the temperature control of hot water set at??

ive been to a couple lately, where the temperature is maxed out,
full gas going into unit, not enough cold water flow through it so the water kettles
knocks off the gas for 20 seconds or so, boiler cools down, then fires up again

hot and cold flow through the "hot tap"

simple solution is to set the temperature lower, which is effect higher because it will be constant

good luck
shaun