Grant Combi 90 losing pressure

[mrbriggs]

Hi all,

new member here, so be gentle!!!

OK, we have the above model combi in a 5 year old house. It has been working very well until recently. It is losing pressure I'd say every few days. I know what you will all say, it must be a leak! But I have looked everywhere on all the rads, and can't find a leak. If just the hot water is used, the pressure stays constant, as soon as the heating is used, the pressure raises to around 2 bar while in use then returns back to 1 (I think this is normal) but gradually over the next few days, it drops to 0.

Any ideas?

Thanks

 

[clf-gas]

Why have you altered your post

Pressure relief valve

 

[mrbriggs]

I didn't change my post?!

OK, pressure relief valve. I heard it could be that especially as I did once fill it above 3 bar and it leaked out of the prv. I read the FAQ, and once water has leaked thru there, it can stay open due to dirt/sludge/whatever!

Where can I get one of these from? Is it straight forward to fit? Do I need anything on the joints when reconnecting the new prv? (like locktight, etc?)

I havn't done much work on boilers, but I am quite handy with a spanner!!

Thanks all.

 

[oilman]

I didn't change my post?!

OK, pressure relief valve. I heard it could be that especially as I did once fill it above 3 bar and it leaked out of the prv. I read the FAQ, and once water has leaked thru there, it can stay open due to dirt/sludge/whatever!

This is a myth. I always reduce the pressure at EVERY combi service using the PRV before checking the expansion vessel air pressure, as specified in BSS7074 part 1. It is a rare occasion when the valve leaks afterwards when it was not leaking before.

When was the expansion vessel air pressure checked, and what was the pressure?

The FAQs should be changed, as they are misleading, and they discourage people from operating the valve to check it is NOT stuck. How do you know a safety device will work if you don't test it?

 

[mrbriggs]

OK, thanks. I may just turn it to see if there is any dirt in it keeping it open? Also, I will have to work out how to get into the thing as it doesn't appear to have removable side panels so access must be via the top (then the side panels can be removed) The boiler is in our utility room and there is a worktop on top of it going over the boiler and washing machine. It may be a pain to remove all that!

The expansion vessel was checked today by myself using a foot pump. It was reading a little below 1 bar so I topped it up to 1bar. I also noticed that the expansion vessel MOVES! Is it supposed to be fixed in place?

Cheers for your help so far.

Lee.

 

[oilman]

The exp vess does move, did you check the air pressure with zero on the water pressure gauge? How did you measure the pressure?

You will HAVE to get the worktop out as access is through the top.

 

[Agile]

Its a little different for a heating engineer to lower teh pressure as he will have spares and be able to easily fit them.

A DIYer will not and I would always advise a DIYer to use a rad bleed screw to reduce the pressure.

Tony

 

[simond]

Why are we going down the exp vessel route? if the pressure doesn't go above 2 bar (and the gauge is working ) the PRV isn't going to lift until >3bar.

What about a simple balloon over the PRV relief? Are we not barking up the wrong tree here? Clearly the exp vessel is not in its first flush of youth and 5yrs seems to be typical for a Grant exp at the moment, but....if the OP is correct about the pressure then its not the expansion vessel.

 

[mrbriggs]

Oilman, no, I don't think I did check the pressure with the water pressure reading 0! OOOPS! I will check that again tonight as the boiler will be reading 0 by the time I get home tonight! I used a standard car foot pump to check the pressure. Is that ok? Also, the worktop is actually in 2 halves (joint in the middle) so someone was actaully thinking when they installed it!!! It should be easy to get to it tonight when home from work.

Simond, I tried the balloon over the PRV last night and it did fill with water but I'm not sure if the boiler raised to over 3 bar when the heating was on. (the pressure raises when heating is on, which is right isn't it?) If it did, then the PRV is probably working ok and there is a different problem with the boiler causing the pressure to raise over 3bar! (just a thought!)

Another thing, I think it may be due for a flush of the system as it's 5 years old (comments on this?), plus after all that topping up the system, it's probably nicely clogged up. How do I go about doing this? I read the FAQ, but how do I get the flush into the system? Through a radiator? How do I get it in there? I'm presuming through a bleed valve, but do I take the valve all the way out? Also, what do people recommend for the flush? Finally, I'll have to get inhibitor. Is there something I can use for leaks just in case there is a small leak on one of the rad pipes, like a radweld or something? Will this mix with the inhibitor or is there something all in one that does it all? What is the safest thing to do?

Sorry to sound useless, but I've never done this before and like I say, I like to have a go at these kind of things.

Thanks people! Great help so far.

 

[oilman]

Why are we going down the exp vessel route? if the pressure doesn't go above 2 bar (and the gauge is working ) the PRV isn't going to lift until >3bar.

What about a simple balloon over the PRV relief? Are we not barking up the wrong tree here? Clearly the exp vessel is not in its first flush of youth and 5yrs seems to be typical for a Grant exp at the moment, but....if the OP is correct about the pressure then its not the expansion vessel.

What "route"??? It needs checking so it's being CHECKED!!!

mrbriggs, it's better to use a pressure gauge, the gauges on foot pumps arent the best measuring tool.

Look up flushing, there will be loads of opinions, and the topic will go on forever.

Radweld will fix small leaks, though now watch as all the "don't ever touch the PRV" brigade vent their spleens about that one.

 

[mrbriggs]

OK, when I get home, I will check the expansion vessel for leaking water via the valve and top up as necessary then top the water pressure up and give that a go. If that all works, I still think I will need to flush and put inhibitor back in after.

Any other ideas?

cheers

 

[mrbriggs]

One other thing, can you not use the PRV to release the pressure off?

For example, when i go home tonight, if the gauge is reading above 0 (I fully expect it to have lost all it's pressure by then, but hey!) I will need to release the water pressure so I can check the expansion vessel pressure. Wouldn't an easy way to release this be to just turn the PRV tap?

 

[mrbriggs]

While i'm on the matter of fixing the boiler, ever since we moved in, we notice that the hot water doesn't stay hot for long. I.E. when running a bath, it comes out scorching for say 3-5 mins, then becomes luke warm. This is no good for a bath if you are like my missus and like it hot (and a hot bath )

Could this be related to the other problem we have with the boiler?

 

[JPH/01]

As oilman has allready said useing the p-r-v to drop the pressure is a good idea, as this checks to make sure it will open, this is the method I use at a service.
Aim for 0.8 to 1.0 air pressure at 0.0 system presure, the other weak point on the grant is the cheapo auto air vent, it is worth looking at this when you have the top off the boiler.
A combi will be slow to run a full bath of hot water, try only opening the bath tap half way, that way the store will not be depleated so fast and the burner will have a better chance at keeping up.

 

[mrbriggs]

OK thanks JPH, what am I looking out for on the auto air vent? What does it do and how can I test it? Where abouts is it on the grant 90?