Question re- Semi-sealed system

Gobethewall

Hi, we got a new oil fired Grant oil boiler fitted 5 years ago. It has been running perfectly until recently when the hall rad needed to be bled on numerous occasions, and the system had gone very noisy and was obviously full of air. Boiler was serviced regularly once a year. We weren't told anything about pressures or re filling the system, so when I checked I couldn't find how the system was filled. On the last service the guy showed me how to fill the system and told that our system was a semi sealed system. It is filled from the attic tank and the filling pipe has 2 valves on it. One is a wheel type the other needs screw driver. We discovered that this pipe was closed off, the engineer thought that it should be open all the time, but because it was closed (we don't know who closed it) he decided to leave it closed. Now after 3 weeks the pressure is down again, so I opened it and refilled the system.

Sorry for long description, question is.. does anyone know if this should be left open or closed. I can find nothing on line regarding SEMI sealed systems.

Thanks in advance for any insight you may have

Wearb

It should normally be closed if the system is properly installed.

Some people leave them open to mask a leak, but constant fresh water being added will cause lots of damage to the system.

You should have a plumber out to have a look at it and advise. He might even advise to convert to a fully sealed system.

Gobethewall

Thanks for that Wearb

Any harm to ask? How often should you have re fill approx.?

Wearb

Short answer is as often as needed. The system needs to be full of water to work.

For most people, a couple of times a year, maybe a bit more often for the first couple of weeks after refilling (after having had work done), as air works its way out of the system.

Gobethewall

Wow! We went nearly 5 years without even knowing it could or should have to be filled or from where to fill it. So, possibly because this would have been the first time it was re filled it's quite possible it would have needed another fill fairly quickly as there would still have been some air in the system.

Wearb

It's not unusual to not need to top up. Some systems are clean and never need bleeding.

John.G

There are lots of semi sealed systems around me, when gas became available in my estate, some changed (from oil heating with OV systems) to gas fired boilers and for some reason or another (cheap & cheerful) the existing F&E tank was utilised, the vent was removed, a swing check non return valve was fitted to the cold feed and a expansion vessel fitted, they have worked quite well over the past 25 years or more, but I would think most with the cold feed (filling) left open, reason being that the max head (filling) is only ~ 0.45bar and the EV precharge pressure is usually left at 1.0 bar so as soon as any tiny leak appears or any micro bubbles collapse then the boiler pressure falls very fast to < 0.45bar, remember it will always fall to 0.45bar with a cold system. I hate leaving any filling/make up valve open, I keep the F&E tank (ballcock) filling valve shut on my OV system permanently and open it a few times a year as I will the hear the tank making up if any leaks. I have never heard as much a drop of water entering that tank in 50 years which is pretty amazing with a OV system.

If I had a semi sealed system, I would have the EV on the ground floor, I would pre charge it to 1.0 bar and then reduce the pressure again to 0.1 bar, this means that with a head of 0.45 bar you will then have ~ 2.5 litres water reserve in a typical 12 litre EV, you can then shut the filling (F&E ballcock) valve, if your pressure then falls below 0.45 bar it means that you have a leak but at least you will be aware of it pretty fast once that 2/2.5 litres reserve is used up.

Fully sealed might seem the preferred option but the semi sealed, if carried out as suggested above means that the maximum system pressure is < 1.0bar, it will only rise to 0.75/0.85bar with the system up to temperature.

Gobethewall

OK. JohnG Thanks for that.

Unfortunately most of your explanation is way over my head. I have no idea what an F&E tank is or EV. We certainly have nothing with a ballcock in the system. According to Grants literature the pressure should 0.5bar when not running, which it currently is.

The cold water just comes straight from the water tank in the attic. we had an overflow tank in the attic on our older system, but that's been removed. There is an expansion vessel in the boiler house ( OH! just realised that's an EV) at least I'm assuming the "red ball" thing is an EV.

John.G

Yes, EV is expansion vessel, "overflow" refers to F&E (feed&expansion tank), you are now supplying the cold feed water make up from your cold water storage tank (CWST) and that's why you have the double valve shut off, Presume and hope that the boiler pressure is increasing when the system gets hot, can you take a note of it with the two valves open and with them both shut, the boiler pressure should increase in both cases as there should be a non return valve in the line to make it "semi-sealed" and also to stop expanded contaminated water passing back into the CWST when the system heats up.

Gobethewall

With the 2 valves open the pressure rises to approx 0.8 and roughly similar when closed, this is with the system running. With system off gauge sits at 0.5. There is a non return valve on the feed.

John.G

That looks good, if you close the two valves now all you need to do is to monitor that 0.8bar hot pressure now and then but if falling to 0.45bar in a day or two with no venting going on would suggest getting your plumber to reset the EV pre charge pressure to 0.1bar if not already at that pressure and you will then get a good handle on leakage rate, if any, as explained above.

Gobethewall

Many thanks John.G