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Rick Ridley
Rick Ridley, UK Heating Engineer
Category: UK Plumbing
Satisfied Customers: 2708
Experience:  30 Years on the tools, now retired, Ex CORGI
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Water pressure below 1 bar on Greenstar 30Si Combi Mk III.How

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Water pressure below 1 bar on Greenstar 30Si Combi Mk III.

How to raise pressure to operating level?

Thanks for help.

J Mack
1Understand how they work. Combination boilers work on what’s known as sealed central heating systems. Meaning they are completely free of Tanks, and natural air supply. This is essential not least because if a mains water holding component within that boiler became defective and thus enters the Heating side of the boiler there would be potential for all sorts of problems with floods in the loft area and much more. There are many advantages of sealed central heating systems. So we need to manually regulate the amount of water actually circulating your radiators. And this is achieved through a filling loop, usually located close to the combination boiler.2Identify the filling loop, the pressure gauge and the boiler.3Practice switching the boiler controls to off before attempting to top up the pressure.4Make sure the pressure gauge can be monitored whilst accessing the filling loop. The biggest mistake that can be made is over pressurizing the boiler; this is usually caused by not monitoring the gauge as you increase the pressure. If this is not achievable then ask for a friend or family member to assist by looking at the gauge as you take care of the filling loop.The ideal pressure for any sealed system is 1.5bar (displayed on the pressure gauge).The filling loop will have a silver flexible tube with two handles on each side of it. When the handles are in the off position they will be across the flow of the pipe. You will need to slowly turn them so that they straighten up in line with the pipe.By now you should hear the water flowing through into the boiler.5Keep an eye on the pressure gauge as it slowly creeps up to the green area or the 1.5 bar marker. Anywhere between 1 bar and 2.2 bar is fine.6Turn the handles back into the original position.A
Customer: replied 3 years ago.

That solution worked onmy previous non-combination boiler.


Can't locate filling loop on present one. I have discovered a white plastic key which has the message 'remove after filling' but can't see where it fits. There is a black tap by one of the copper pipes but turning this has no result.


The white key seems to fit inside a short black plastic tube beside this tap but it doesn't engage with anything. I haven't pushed it hard inside the tube.


More thoughts?


BTW the red alarm light is flashing - presumably because pressure is low.

Not sure about that one. I'll opt out and another expert will help shortly.
Customer: replied 3 years ago.

OK. It's late here so Friday will do. The boiler seems to be working despite the red light.

Hi my name is Brian,
I have been in the Plumbing & Heating trade for approximately 15 years,
lets see if i can help. Push the white key in fully twist it half a turn so it wont come back out then turn your black tap on, I think there is only one just have another look under there if it doesnt fill when you turn it on. Get to a pressure of 1 to 1.5 with the boiler cold. turn tap off and remove key. Hopefully job done..
Customer: replied 3 years ago.
Tried that. It seemed to be the logical way to raise the pressure. Only problem: it didn't work. All I got was a dribble of water from the black tap (the only one). No sound if rushing water and the indicator didn't move from the half bar level.


sounds like the valve or keyway has got stuck, I will not be able to help you with that. so will Opt out for you

Hi, I’m a moderator for this topic. It seems the professional has left this conversation. This happens occasionally, and it's usually because the professional thinks that someone else might be a better match for your question. I've been working hard to find a new professional to assist you right away, but sometimes finding the right professional can take a little longer than expected.
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Hi I am Rick, UK Plumber, and I specialize on Boiler problems, OK there are two possible problems here, 1, the key system was not fully fitted and there is a separate filling loop at another place usually near the boiler but can be any place that the cold main runs near the heating pipes. 2 the key is not going in correctly, often this is due to the O rings being dry, so apply a smear of petroleum jelly or similar to the key and insert it as far as it will go before twisting clockwise fully then open the square plastic valve it may be white or black, you should hear water flowing, if not then there is a blockage that will need to be manually cleared, after draining down, Rick
Customer: replied 3 years ago.
That's more like it! Previous helpers didn't mention the white block-style tap, only the black one.

Water pressure now restored to 1.5 bar. Alarm light still flashing but I suppose this will shut off when the next boiler on cycle starts. I'll keep a beady eye on it.

Many thanks.
if the pressure light does not go out but boiler works then there is a problem with the board or pressure switch, some times they are just slow to readjust, Rick
Customer: replied 3 years ago.

The boiler has gone through three cycles OK and the red alarm light is still flashing.


Should I wait a while longer? There are override buttons by the alarm light but I presume they apply to turning the boiler on or off outsice the programmed times. Anyway, prodding them hasn't made any difference.


Does this mean I will have to call in a physical plumber?



Mmmm not sure here, the boiler I assume is running OK? can you tell me exactly which light is on? and the GC number of the boiler it should be 47-311-85 for natural gas or 89 for LPG, I ask as there is no red light on the mentioned boilers, Rick
Customer: replied 3 years ago.
The red light is on a plastic box to the side of the boiler. It's market 'Draytpn Digistat SCR Wireless System' and contains an 'on' green light, the flashing red Alarm light and override buttons labelled 1 and 2 beside a 'Receive' mark.

Any thoughts?

Ah all is clear, this is a radio room stat, nothing to do with the boiler or it's pressure, you have a problem with the digistat probably the remote units batteries are going, look on the remote unit, if you see a small batery symble then change the batteries 2 x AA do it quickly and it should preserve your programs, this is realy another question, Rick

Rick Ridley, UK Heating Engineer
Category: UK Plumbing
Satisfied Customers: 2708
Experience: 30 Years on the tools, now retired, Ex CORGI
Rick Ridley and other UK Plumbing Specialists are ready to help you
Customer: replied 3 years ago.
Haven't found any battery symbol or way of opening the remote unit but I'll keep trying.

In any case, you answered the pressure question, for which I am grateful.
Customer: replied 3 years ago.

Rated Excellent. Pressure etc. all . Many thanks

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