No apparent leak but combi boiler loses pressure

[Marnhullman]

I have recently bought a property with a Grant 90 combi boiler which heated the rads OK but not the hot water. The previous owner of the house paid for a plumber to make some repairs which included replacing the diaphragm in the boiler and a couple of other things, including fitting a new pressure vessel, he claimed to have done but I'm not sure he actually did.

The cold supply has plenty of pressure and we have hot radiators and hot water although the hot water pressure is very low and it takes ages to fill the bath. I suspect the problem is associated with the fact that the system pressure keeps dropping and I have to keep re-pressurising it. Diagnosis is not helped by the fact that the house is frequently unoccupied but the heating system is left switched to constant with the room stat turned down to 5C to prevent freezing. When occupied - most weekends - it's necessary to repressurise the system and then I have to keep doing it although if I forget to check it doesn't actually seem to make any difference to the way the system works.

I have contacted Grants and they have suggested the low hot water pressure may be due to lime scale as the house is in a very hard water area and there is no scale prevention system fitted. Their solution was to replace a major pipework component in the boiler as they say it can't be descaled but I'm reluctant to spend money on a possibly doomed project.

Grateful for any clues as to why the boiler keeps losing pressure and why the hot water pressure is so low.

Finally, to repeat, I can't see any evidence of leaks.

Thanks

Mike C

 

[ollski]

I would imagine they have recommended you have the heat exhanger replaced due to scaling. If your area is hard water and you have no softener it's almost a certainty I have known combis scale up to unusable levels within 12 months!. This is why you have no hw pressure as the constant boiling of oxygenated water has scaled the pipe from the heat exchanger reducing the bore to a fraction of the original....sorry

The pressure loss will either be due to a leak (often evaporating off hot components before you can see it) or the system reaching high pressure which is the relieved through the pressure relief valve

 

[ChrisR]

Yep. ANd to clarify, the leak (which is causing the pressure drop) hass no effect whatever on how well the boiler works, until the pressure in the system drops below a certain level where the boiler will switch off. I've not heard of Grant gas boilers - perhaps it's oil but I imagine the principles are the same. I don't know of anything that can't be descaled, either!

Perhaps its time to consider a new boiler - later designs have heat exchangers which are far less prone to scaling up, and can be descaled routinely. There are of course external devices which can be used to lessen the problem.

Your system IS leaking. Maybe in the boiler somewhere, but you need to find it to be sure, or a replacement boiler will have the same problem. As above, most common is the pressure relief pipe.

 

[oilman]

ChrisR you are right, it is an oil boiler, and of course right again that the water side works much the same as gas boilers (but bear in mind I don't know about gas boilers).

Snowsride, if there is a system leak that can be cured, but first you need to establish whether the expansion vessel is ok. Please search the forum because there's loads of posts about it.

Once you have ensured the state of the expansion vessel (and you MUST do it first) you can attend to leaks if the pressure still drops. To do this reduce the pressure on the gauge to zero using the pressure relief valve. Then you will have to think this next bit out, you need to squirt 2 bottles of Radweld or similar in through the filling loop. If you can't sort this out, you can buy an expensive Fernox cartridge which you can squirt in through the radiator bleed valve.

Just make a check to see it's not the pressure relief valve thats leaking.

 

[Marnhullman]

Thanks to everyone who has replied - will follow up al suggestions.

Regards

Mike C