Pressure Problems with Vokera linea 24/28

[colinnic]

I have a problem with the pressure in my central heating system. The user instructions state that the system should be pressurised to between 1 and 1.5 bar when the system is cold. I have recently drained a radiator and needed to restore the pressure back to between 1 and 1.5 bar.

The pressure now rises above the maximum reading on the gauge (4 bar) when the system is switched on for a long period (usually in the morning). The user manual states that it should not exceed 3 bar. Once the system is cold, again the gauge reads just under 1 bar but doesn’t seem to drop any further.

1. Is there a problem with the pressure exceeding 4 bar ?

2. Is there a problem with the system returning to just under 1 bar when the system is cold?

Any advice would be appreciated.

 

[oilman]

There seem to be 2 problems, one is bad, the other very bad. The first problem is it sounds like the expansion vessel is faulty. Search the forum for posts about this in October. The air pressure should be set first, and unless your boiler won't work with less than 1 bar, 1 bar is too much.

The other problem is the pressure release valve is operating too high. Get this looked at, because if it is a problem, it could get worse with disasterous consequences.

 

[masona]

Am I righty saying combi can be dangerous than non-combi boiler ?

Reading Colinnic post regarding 4 bar pressure that is equal 58 psi ! ( 1 bar = 14.50psi ) At what pressure does the pressure relief valve operate at ? If the pressure relief valve failed what would you say the next thing would happen ? 4 bar seems a lot to me in a sealed system.

 

[ChrisR]

The pressure valve should certainly open at 3 bar. If it fails to open, there is the possibility that the system would burst. If the temperature is below 100 that's undesirable - squirting very hot water.
If both the normal boiler thermostat AND the high-limit thermostat failed, the temperature could rise well above 100, with a large pressure increase. Then when the system burst, all the water in the system would instantaneously vapourise, increasing in volume by over 1600 times. It would make the news.

 

[oilman]

Some unvented cylinder valves operate at around 4 bar, but I don't know of any on a sealed system that operate at other than 3 bar.

1.5 bar sounds too high, and since a two storey house with the highest system piont about less than 16 feet above the lowest point the air pressure needs only to be 0.5 bar. Anything greater than this increases the stress unnecessarily on the expansion vessel. Next the system pressure is set 0.2 bar above the air pressure when cold. Then it should not get much above 1 to 1.2 bar when hot. If it does, get a bigger vessel.

I don't know that combies are more dangerous than non-combies, the problem is caused by having an internal expansion vessel that's too small for the system, then it fails then you have no expansion space.

If the pressure relief valve failed, like stuck, it would depend how stuck. Modern ones are designed not to stick. If it was totally stuck, pressure would increase until something went. This is likely to be the expansion vessel rupturing. As the water would be hot, as in effin, I wouldn't like to be around. You just have to hope you don't have two coincidental failures, the other being the thermostat. Water boils higher than 100 deg C under pressure, if it' higher and the system ruptures you get a steam explosion. Very un-pleasant

 

[masona]

That's frightening,have any of you known it to happens ?

It certainly put me off buying one

 

[oilman]

Don't be put off for that reason, just make sure the expansion vessel is big enough and regularly checked.

What I am put off by is the ridiculous dribble from each of two taps when they are both opened together.

If you can have an oil boiler, have one. How many problems do you see on the forum for gas boilers, and how many for oil boilers?

 

[masona]

If you can have an oil boiler, have one. How many problems do you see on the forum for gas boilers, and how many for oil boilers?

And guess what,I work at the oil refinery and before you say anything,NO I cannot get cheap oil

 

[ChrisR]

The fuel is irrelevant, the dangers are the same for sealed systems. And the "dribble" from the taps is due to a poor mains water supply.
One reason why I just installed a 35mm copper water main coming off 50mm mdpe

 

[masona]

I would've thought oil would be better as you wouldn't have scaled problems or do oil coke up in the vessel as well ?? and also oil properly lubricate the system better ??

 

[oilman]

. And the "dribble" from the taps is due to a poor mains water supply.

I knew that would prompt a response, ChrisR, the last one I did was on a monster farm supply, with enough coming out of the cold tap to splash all over the kitchen floor at full tilt.

The problem is "how much heat can raise the water temperature from about 5 deg (cold water) to 55 deg (tap output) at what flow rate?" and so flow may be ok in the summer but not in winter.

I would've thought oil would be better as you wouldn't have scaled problems or do oil coke up in the vessel as well ?? and also oil properly lubricate the system better ??

Oil boilers have a much bigger heat exchanger than gas boilers, so scaling is less of a problem. It also mahes them heavy, 150lb for an oil wall mount. Oil burns with a lumnous flame and so a lot of the heat is transmitted by radiation, which is why oil combustion is so efficient compared to gas. Both burn fairly cleanly if set up properly.

Diesel/gas oil/35 sec oil whatever has good lubricating properties, but kerosene makes a good degreaser, so provides no lubrication whatever.

 

[colinnic]

Sorry it took a long time to respond to all the advice I received.

Just to let anybody that's interested the problem turned out to be the pressure valve at the top of the expansion vessel. After several plumbers told me the expansion vessel need to be replaced (none of them wanted to do the work i.e. didn't turn up again). Finally I got a British Gas engineer in and he said that on these systems sometimes it just a faulty pressure valve (the one that looks like it belongs on a car tyre). He replaced it with on charge and that fixed it.

Since then I’m on my third Safety valve and I think the pressure valve or expansion vessel has gone again.