Combi DHW pressure drop

[BWDuncan]

Hi all,

This is hopefully a simple question for you, but it seems interesting to me. Note: This is *not* a question about the pressure gauge on my boiler showing a low pressure, or my boiler refusing to start because the pressure has dropped.

I have a Worcester Bosch 24i. It's at least 5 years old, maybe up to 10, and was here when I bought the house. It has a thermostat for the central heating but no controls for the hot water.

When I open a hot tap, the boiler spins up, whirrs all its fans and what not, then fires up the burner. Eventually, hot water reaches the tap and everyone is happy. Soon, however, the pressure at the tap begins to drop. The water becomes cloudy as air bubbles come back out of solution in it (I presume). Eventually, if I don't open the tap enough or soon enough, the pressure drops below the point at which the boiler thinks a tap is open, and it shuts off the burner. I then have to open the tap further, wait for the boiler to fire up and repeat the process of waiting for hot water to come through. It's quite annoying.

For a year I have been assuming that the boiler is just old and a bit crap and an early example of combi technology. Having given it a bit more thought, I struggle to understand how anything other than the tap I am holding can reduce the pressure of the system...

Looking forward to any ideas on what could be going on or how I could fix it!

Thanks,
Bruce

 

[PEDANTICVINDICTIVEMAN]

It's not the boiler, it's the tap, replace the tap or the insides with a renovation kit, some modern taps seem to do this, don't know why.

 

[bengasman]

If opening the tap further solves the problem, just open the tap a bit further from the start.

 

[BWDuncan]

PEDANTICVINDICTIVEMAN,

All three taps in the house have the same problem. I have a disused hot water feed for a washing machine. I will test that.

bengasman,

I've tried that. I have to open it so far that I get soaked! Alternatively, the tap is open so much that the flow rate through the boiler is too high and the water doesn't get hot enough.

Bruce

 

[Agile]

In that case restrict the flow throught the boiler at the isolator at the boiler inlet.

You will then be able to open the taps fully and still only get the flow rate the boiler can manage.

The taps have a brass part called a jumper which expands when it gets hot and this change reduces the flow. Some are worse than others. I think that I know why.

Tony