Grant 90 Combi Mk2 Pressure Loss

[Rabby]

Hello

I've just moved into a new house with a Grant 90 Combi Mk2. Noticed there was some pressure loss over a day or two, maybe 0.2-0.4 bar, so topped up and bled radiators. No losses anywhere else in system, or at least that I can find. Still gradual loss continued.

When we started using the heating for an hour an evening, we were having massive pressure build up then losing pressure rapidly after timed heating shut off - complete loss every time to 0 bar.

After exploring various avenues, found out the Expansion Vessel was full of water, so replaced this. This controlled the pressure build up and now never goes above 2 bar. However, still same problem with pressure loss. So replaced the Grant 2.5 Bar Internal Pressure Relief Valve (MPCBS28). Old one was very grimy inside. But still get pressure loss to 0 bar - the outside pipe has a steady drip from it. Water from overflow looks clean, but lots of oil staining around area, think from flue pipe vent. Even without heating on, we lose pressure, entire 1-1.2 bar per day and overnight.

So, what should I do next? Is this new PRV a dud so should I replace it or is there something else causing an issue?

Many thanks in advance for your help.

Richard

 

[Agile]

You keep saying that "you" have replaced these things. Is that so?

Working on boilers is not just a case of replacing parts and walking away.

An EXV needs to be set to match the operating conditions and it needs to be sized to match the water content of the system.

Tony

 

[Rabby]

Hi Tony

Thanks for your prompt reply!

Yes, it is "me". Dad was a plumber and fully certified installation engineer and taught me quite a bit from my early teens about central heating systems and general plumbing. (But he's passed away so can't help now.) Also, with the move, we are very strapped for cash at the moment!

I have all the original specs from installation, including calcs on water content - previous owners supplied massive amount of original information (quotes, receipts, warranties, instruction manuals, etc) on just about everything!

I considered what was already present on the system and then did some checks to see if they were appropriate, including full reading of all instruction manuals and talked to several suppliers and Grant. Turned out all parts were appropriate so replaced like-for-like with proprietary Grant parts.

Having said all the above though, I'd never do "serious" work on the boiler and would get someone appropriately certified in, but I do want to make sure I can do the "easier" things and if it is a bit more serious won't be taken for a "ride" by any tradesman (as I said, very strapped for cash).

Regards

Richard

 

[Richardthe3rd]

Hi Tony

Thanks for your prompt reply!

Yes, it is "me". Dad was a plumber and fully certified installation engineer and taught me quite a bit from my early teens about central heating systems and general plumbing. (But he's passed away so can't help now.) Also, with the move, we are very strapped for cash at the moment!

I have all the original specs from installation, including calcs on water content - previous owners supplied massive amount of original information (quotes, receipts, warranties, instruction manuals, etc) on just about everything!

I considered what was already present on the system and then did some checks to see if they were appropriate, including full reading of all instruction manuals and talked to several suppliers and Grant. Turned out all parts were appropriate so replaced like-for-like with proprietary Grant parts.

Having said all the above though, I'd never do "serious" work on the boiler and would get someone appropriately certified in, but I do want to make sure I can do the "easier" things and if it is a bit more serious won't be taken for a "ride" by any tradesman (as I said, very strapped for cash).

Regards

Richard

Sorry about your Dad Richard, he sounds a great old fella & I'm glad he's passed on some of his skills. There's few around that can take a pragmatic approach to situations & I sure he's shared his outlook with you, so he's left you with a great legacy.

Perhaps your system is too big for the expansion vessel, which can be on the small side?? Sorry, I'm not a Grant fan, seen too many burst water jackets & their lack of interest in a fault of that nature, rsoles!!

It maybe worth checking your system water, if it's manky that can give you problems too......
HTH

 

[oilhead]

If you still have a drip from the pressure relief valve, then this needs attending to. The valve may be faulty, so needs a look at. Pressurise up to 2 bar, and release it through the prv. If you have any contamination holding the seat up, you may flush it away. If it still leaks, then replace it. Also, check for leaks around the DHW heat exchanger, and all auto air vents. If you have to isolate the boiler to change any parts, take the opportunity to remove the expansion vessel and check that the feed hose is not obstructed. If the new vessel is not taking water, it is the samre as having no, or a faulty vessel.

 

[Agile]

OK but if PRV dripping you could try clicking the knob!

But the EXV cannot be expected to be supplied at the correct air pressure for your system.

Some are supplied at about 2-3 bar and need to be reduced.

Others are supplied with hardly any air and need to be increased.

You have not said anything about setting the air pressure correctly when replacing it.

Tony

 

[Rabby]

Hi All

Many thanks for your replies - really is appreciated, especially so soon after posting! Been busy checking some of the things you've mentioned.

I got a 12L Expansion Vessel and this said it came pressurised at, and only should be pressurised to, 1 Bar. I checked it though before first installing as, well, I'm simply not trusting! But it was at 1 Bar. I've checked it a few times since, just in case, and it has remained at 1 Bar.

On the old PRV, I flushed it through several times by clicking it open in case, as mentioned above, dirt had clogged it open a little. I also checked to see what pressure it would actually operate at - according to my pressure gauge it opened at exactly 2.5 Bar as it says. Flushed several more times and still allowing depressurisation, so that is why I replaced it. When I looked inside, as I said, it was quite dirty.

Regarding the new PRV, I also thought that it may be a tad inaccurate - agin my untrusting self! - so I checked what pressure it released at. Again, bang-on 2.5 Bar according to my gauge and it reduces about 0.8-0.9 Bar before pressure release stops. I've flushed it through several times as well, but still doing the same thing. I've collected water outside at the overflow and it is very clean. Also collected water from radiators upstairs, and again all very clean.

I've contacted the shop (http://buyplumbingonline.co.uk) to see if they have many returns. Paul, who I spoke to, was most helpful and said they didn't, but would happily replace even in a week or two if still having the issue. He also contacted Grant on my behalf, and they suggested checking the flexi-hose to the Expansion Vessel.

So after peoples comments here on the flexi-hose to the Expansion Vessel, I investigated this. I had noticed before that this was crimped/kinked - think someone must have struggled or forced it in - but didn't think it would be a significant issue. I drained system, removed the Expansion Vessel and slowly repressurised system. Low and behold, only a slow drip of water came out of the flexi-hose, even at pressures just before PRV releases, so it is either blocked or kinked to such an extent to form a blockage. I didn't realise it was such a small diameter (only 5mm), so seems like it's a very delicate piece. I've a replacement on the way and will carefully fit this and reinstall the Expansion Vessel to see what happens.

All pressures referred to above are cold.

Still odd the PRV is dripping, even at 1.2 Bar, but will see if the new hose makes the difference.

Thanks all again for sound advice!

 

[Agile]

It seems odd that a new PRV is leaking like that. But it can take a few days to stop dripping.

The hose to the EXV is a common problem for blocking.

You are really lucky that you did not get yourself wet. It is very easy to do that and with very dirty water too.

Tony

 

[oilhead]

Forget replacing the hose without replacing the prv. If it is dripping now, it will continue to do so. For the paltry amount they are, change it NOW.