Combi hot water pressure drops

[rupert]

I've got a problem will my hot water pressure coming from my combi boiler. From cold I turn on the hot tap and the water comes out at good pressure and heats up still with good pressure. However, when the water is fully heated the pressure drops reducing the flow to a tickle, this is particulary noticeable in the (thermostatic mixer) shower where the flow stops and I have to shake the hose to get the flow to resume, all be it at a very low pressure.

Any ideas?

Thanks
Rupert

 

[COG]

Just a gues but does it shut down when you put the tap on full from the start , if it stays on I would ges it was your tap jumper etc expanding and shutting down the tap , especially if you can turn the tap on more as its shutting down,its quite common.

 

[corgiman]

maybe the hot water calorifier is scaling up. the intial burst of pressure your getting is due to the fact that the presuure has time to build up but after you release this the true flow rate is shown, its this you should be concerned about not the pressure.

What make is it as Vaillant is the main one with this type of problem

Call an engineer out to descale or replace it

 

[rupert]

If have the tap open full from the start the pressure drops a little but it does not shutdown. This would point to the tap jumper then - thx

What about the thermostatic shower? Why does the pressure drop right off when it gets up to heat? The other thing it does is that the pressure returns after 3 or 4 minutes but it also goes cold when the pressure returns, then quite quickly the pressure drops and the heat returns.

Does this make sense?

The boiler is a Potterton Puma 80E.

 

[COG]

Not very familaer with your boiler but your best to ignor your shower problems til you sort out the tap flow first,if the pressure and therefore the water temp is going up and down the shower thermostat (and anti scald device if fitted) is bound to be confused, do you have decent mains water pressure and flow ?

 

[rupert]

The mains pressure and flow are ok. The hot water pressure and flow in the kitchen are ok. The hot water pressure and flow in the bathroom sink are medium ok, however the hot water pressure and flow in the bath is poor - both the bathroom sink, bath and shower are on the same spur.

 

[ChrisR]

MEASURE the hot water flows in litres per minute & report back. ANd get a new shower hose!

Thermostatic valves can do odd things with combis. The valve has too much hot so it reduces its volume. So the hot gets hotter then the boiler cuts the gas down and the hot cools down, and then the cylcle repeats. Scale in the heat echanger, or a scaled up shower valve, can do it.

 

[rupert]

I measured the flow with interesting results:

Kitchen 7.5 l/min
Bath 8.5 l/min
Bathroom Sink 8.5 l/min
Shower around 8 l/min

I took the shower head off the shower hose and the water stayed hot for 3 or 4 minutes before going cold - the longest ever! The pressure seemed more constant as well.

What does this mean?

What sort of shower hose (and shower head) should I get? will a shorter hose, so it doesn't hang down, help?