Combi boiler shows zero pressure

[Marnhullman]

I have a Grant Combi 90 boiler and the pressure gauge keeps falling to zero when the boiler is not providing hot water - when it is heating then the pressure rises to normal. I used to open the valves and top it up but the pressure falls again very quickly so I gave up and it doesn't seem to make any difference to the performance of the boiler. However, the hot water pressure is quite low e.g. if the hot tap is opened in the kitchen then no hot water flows in the bathroom but this happens regardless of the pressure displayed on the gauge in the boiler so I suspect the heat exchanger is furred up.

Although the boiler seems to be working OK from zero pressure is this likely to be causing unseen trouble? As mentioned above I used to keep topping up the boiler so would this mean the inhibitor in the central heating system has become terribly diluted or this separate from the 'pressured' system?

Thanks

 

[oilman]

If the pressure rises from zero when heating, then the expansion vessel is probably not doing its job.

However, the hot water pressure is quite low e.g. if the hot tap is opened in the kitchen then no hot water flows in the bathroom but this happens regardless of the pressure displayed on the gauge in the boiler so I suspect the heat exchanger is furred up.

Tap pressure has nothing to do with the pressure gauge. How old is the boiler? It is more likely to be the blending valve causing the problem. These should be serviced annually if you don't have any form of water conditioner and you have hard water.

Although the boiler seems to be working OK from zero pressure is this likely to be causing unseen trouble?

Yes. These boilers are expensive, it would be worth contacting Grant who will give you the name of one of their recommended engineers. It will cost >£2000 to have it replaced, or <£200 to have it sorted. Which would you choose?

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[Marnhullman]

Oilman

Thanks for the advice will call a heating engineer

 

[JohnGuilbert]

HI Snowsride,

I had exactly the same problem. Starts off at 0 and goes up to 1.5 bars on mine now. I topped up until I realised that when heating was raised it shot up to 3.5 bar (tested at 9 bar). Was the problem OILMAN mentioned with the blending valve the problem?

Your help would save me a lot of money

Thanks.

John.

 

[swbjackson]

Faulty expansion vessel. Either needs recharging or swapping if diaphragm has gone. Look at the FAQs.

 

[JohnGuilbert]

Thanks Mike.

I have booked boiler in for a service and hopefully will get this replaced if faulty.

Thanks again.

 

[JohnGuilbert]

Faulty expansion vessel. Either needs recharging or swapping if diaphragm has gone. Look at the FAQs.

HI again,

We had the Engineer around to service the boiler and he called his plumber mate. I told him what you said and he said he thought that what you said was a possibility, but he ended up replacing the blow-out valve. This has not rectified the problem as we again are losing pressure as I originally described. Water is coming out of the blow-out pipe outside.

Oilman - am I being strung along here. My wife wasn't very convinced he knew what he was doing even though he is one of Grant's recommended Engineers.

I know you're not here and it's diffcult to make any decisions based on fact, but does it sound like to you the correct procedure has been followed. He said he will take it to the next step if it doesn't work....which we now know it doesn't.....more money.

Your help would be really appreciated.

John.

 

[swbjackson]

Pressurise the system to 1 bar. Turn the heating on and watch the pressure gauge. It should normally rise 1/4 to 1/2 bar but could rise up to 1 bar. If it rises further than that I would suspect a problem with the expansion vessel.

First check with the expansion vessel is to push the schraeder valve pin in and see what comes out. If it's water then the diaphragm is shot and it needs replacing. If it's air or nothing then it probably just needs re pressurising. To re pressurise the vessel you need to isolate the boiler from the system and open a drain off on the boiler (with a hose attached or into a bucket). You can then pump at the vessel to 1 bar using a foot pump or bicycle pump. Then test the valve to check if its leaking with some leak detection fluid or a bit of spittle will do the trick. If it's leaking it can be replaced with a schraeder valve available from most car accessory shops.

Then shut the drain off, open the isolation valves and check again. If you are still getting the problem then the likely cause is a blocked hose to the expansion vessel.

 

[JohnGuilbert]

Thanks for that.

I just checked the valve (like a bicycle pump valve) at the bottom of the expasion vessel by pushing the pin in. Black Water comes out - quite a bit.

I guess that means the diaphram needs replacing - is it just a case of going to the boiler shop and asking for a diaphram or does it come complete with valve?

Thanks.

John.

 

[swbjackson]

Unfortunately it's the whole vessel that needs replacing. Depending on layout etc. it may be possible to fit an external one.

 

[JohnGuilbert]

Thanks again Mike.

The expansion vessel is just a big red circular metal drum which looks easy detachable on my system (Grant Combi v90). I will give this a go next.

Thanks.

John.

 

[JohnGuilbert]

Hi Oilman,

I had another guy come around and look at the boiler - in not so many words he couldn't understand why they had replaced the pressure relief valve as water was already coming out.

Anyway - he came around and I told him that the pressure valve on the exapansion vessel leaks water. Not a gushing flow, but enough to make it difficult to pump. He told me "ahhh, you might get a bit of water come out but its not a problem, if I changed every expansion vessel that did that I'd have a lorry load.

What do you think - is this way of the mark what he said - I would of thought that if any water gets into the valve (which is for pumping) that's an issue - it's gotta come from somewhere hasn't it?

My next step is to rip up the carpet to access a trap door that could narrow the problem if there is a leak.

Can you give your advice on this?

Thanks.

John.