Unvented Heating system pressure increase


Postby saintadam » Mon Nov 17, 2008 4:19 pm

I have an unvented heating system installed (in house when bought it) and it has developed a strange problem. The hot water tank has the necessary expansion tank installed and is fed by the mains. The mains also feeds the boiler circuit that runs around the radiators via a shutoff valve that feeds into another expansion tank. The problem is that the pressure in the expansion tank on the boiler circuit (not the hotwater tank) is creeping up to its three bar limit then continually leaking out the 'relief' pipework, even with the sustem off.

I don' think it is the valve that feeds the tank as you can hear water coming in via that if it is opened. My quesiton is, is it possible that the hot water in the tank is somehow finding its way into the boiler circuit water and pressurinsing the expansion tank?

:cry:
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Postby plumbbob » Mon Nov 17, 2008 9:35 pm

That is a possibility, but I would first head straight for the filling loop for the heating system. Often these valves fail to shut off, and allow mains water to seep past eventually over filling/pressurising the system. It's a common fault.
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Postby Steve the gas » Tue Nov 18, 2008 7:20 am

Yep disconnect filling loop when not in use
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Postby saintadam » Tue Nov 18, 2008 7:29 am

[quote="plumbbob"]That is a possibility, but I would first head straight for the filling loop for the heating system. Often these valves fail to shut off, and allow mains water to seep past eventually over filling/pressurising the system. It's a common fault.[/quote]

Plumbbob, Thanks for the info. That was going to be my first test today. The fill system is controlled via serviceman type valve so I will have a look at that and hope that is the issue rather than the internal tank pipework.

A quick question, what would happen if the "Anode" in the tank was turned off for an extended period of time? Is this an anti corossion device?

If it is not the fill valve, then can I presume it will be a new tank as the internal pipework cannot be changed?

Thanks again for your help.
saintadam
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Postby saintadam » Tue Nov 18, 2008 9:15 am

[quote="Steve the gas"]Yep disconnect filling loop when not in use[/quote]

Steve, Thanks, but the filling loop is isolated via a srviceman type valve. I am going to unpressurise the system and check out this valve and hope that is the problem.

Thanks for the reply.
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Postby saintadam » Tue Nov 18, 2008 1:40 pm

Well, it looks like my worst fears have been realised. I disconnected the mains supply to the bioler circuit, turned back on the mains feed to the hot water tank, and the pressure in the boiler circuit started creeping up.

Looks like the internal boiler circuit circuit has been breached in the tank. Can I pressume this means a new tank? It is an Ariston STT 300 so I am looking at around £600 ( :shock: ) to replace.

I suppose if I get a direct replacement, there is a chance the plumbing should fit reasonably easily!!

Thanks for your continued help and advice.
saintadam
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Joined: Mon Nov 17, 2008 4:13 pm


Postby plumbbob » Tue Nov 18, 2008 10:25 pm

Maybe you have found the cause of the problem with the anode. These sacrificial anodes are installed to prevent internal corrosion, and should be examined every so often (year or so) and replaced if necessary.

Personally, I would not change the tank without proof that the coil was faulty. You could disconnect it from the heating and see if water trickles from the coil connection. Maybe even cap off the heating tails so the CH can be run whilst you source a replacement and heat the hot water with the immersion.
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Postby saintadam » Fri Nov 21, 2008 5:48 pm

[quote="plumbbob"]Maybe you have found the cause of the problem with the anode. These sacrificial anodes are installed to prevent internal corrosion, and should be examined every so often (year or so) and replaced if necessary.

Personally, I would not change the tank without proof that the coil was faulty. You could disconnect it from the heating and see if water trickles from the coil connection. Maybe even cap off the heating tails so the CH can be run whilst you source a replacement and heat the hot water with the immersion.[/quote]

Plumbbob,

Looking into this further, the cheaper tanks (like this one) only have around 5 or ten years guarantee and come with either a sacrificial anode or an electronic anode system. This tank is around 9 years old, and I know that the anode(s) have not been checked for at least the last 2 years or us living here as I did not know what was needed. I also suspect that the last people hadn't changed it since it was put in!

I have proven to my own satisfaction that the internal coil is faulty. Turned off boiler, left hot tank refill open (normal operation), removed cold fill to boiler system and watched the pressure in the boiler system increase from 1 bar to 3 bar when the pressure relief kicks in.

I have found that by closing the cold water supply to the tank, it finally levels out at around 1.5 bar with the boiler system so I can run the CH. I also tried to heat the water in the tank with the immersion, but it looks like this is FUBAR as well!! Have ordered a new tank and am having it fitted next week hopefully. Luckily we have a couple of electric whoers in the house as well as the ones run from the hot tank, so we don't smell too much!

Thanks for your help and advice again.
saintadam
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Joined: Mon Nov 17, 2008 4:13 pm


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