Pressure for expansion vessel tied into open system.

aria1

Bit of help needed pls.

I have an expansion vessel tied into an open system. The vessel is tied into the supply/return from the expansion tank with a non-return valve.
It was installed because the expansion tank was getting too much returns and near boiling off.

I was checking over the system today and the accumulator pre-charge was 0.5 bar. The label says it came from the factory with 1.5 bar pre-charge.
Should I top it up to 1.5 again?

I’ve been getting airlocked circ pumps of late, hence my checking the system over.

The expansion tank sits about 9’ above the boiler.
The expansion vessel is sitting about 6” below the expansion tank.

Hope there’s enough info there.

Thanks in advance.

John.G

aria1 wrote: »

Bit of help needed pls.

I have an expansion vessel tied into an open system. The vessel is tied into the supply/return from the expansion tank with a non-return valve.
It was installed because the expansion tank was getting too much returns and near boiling off.

I was checking over the system today and the accumulator pre-charge was 0.5 bar. The label says it came from the factory with 1.5 bar pre-charge.
Should I top it up to 1.5 again?

I’ve been getting airlocked circ pumps of late, hence my checking the system over.

The expansion tank sits about 9’ above the boiler.
The expansion vessel is sitting about 6” below the expansion tank.

Hope there’s enough info there.

Thanks in advance.

When did you get the system converted to this "semi-sealed" system and how long was it before you got circ pump air locking?.
The installer may have deliberately reduced the pre charge pressure to 0.5 bar because of the expansion vessel location. The pre charge pressure should be
lower than the filling pressure but your filling pressure (from the tank) is only ~ 0.05bar at the expansion vessel and 0.3 bar at the pump. Theoretically, if you increase the pre pressure to 1.5 bar and assuming that all the air has been released then as soon as the water starts getting hot the pressure at the expansion vessel should immediately rise to 1.5 bar (1.8 bar at pump) and depending on expansion vessel size and system contents go on to ~ 1.9 bar at expansion vessel and 2.2 bar at the boiler, it will then return to 0.05 bar and 0.3 bar when system cools down.
It may be worth doing this but make sure that all the air is vented at the NR valve first.

aria1

Thanks John.

The vessel was installed pretty much as soon as the house was occupied. The expansion tank nearly melted, I’m told.

This was about 11 yrs ago and the plumber is long gone I’m afraid.
Sounds like it should be ok at the 0.5 bar?

The house is unoccupied for months at a time but the air locking is a new problem.

John.G

aria1 wrote: »

Thanks John.

The vessel was installed pretty much as soon as the house was occupied. The expansion tank nearly melted, I’m told.

This was about 11 yrs ago and the plumber is long gone I’m afraid.
Sounds like it should be ok at the 0.5 bar?

The house is unoccupied for months at a time but the air locking is a new problem.

The system may be drawing in air at somewhere, in genera, IMO, if there is 1 to 1.5 bar in the system when cold then there is less chance of this happening but with your system the maximum cold head that you can have is 0.05/0.3 bar because its supplied from the feed & expansion tank.
I would try and get rid of the air first and then see how you get on, you could fit a automatic air vent on a T piece just below the NR valve as well but a fully sealed system with a filling loop from the mains may be the long term solution.
But to answer your question, yes, 0.5 bar should be OK but there is no harm in upping this to 1.0/1.5 bar. You could also drain down just enough water to remove the NR valve (or just its cover) and see if is moving freely.

Edit: I should really have stated this, the obvious, in my first post, ensure the Feed&Expansion Tank is "full" of water before doing anything!!, the ball cock can stick up due to little or no make up required normally.

aria1

Thanks again John.

The F/E tank was full, I actually then filled it to overflow level yesterday to check the overflow was clear.

I’ve pressed up the accumulator to 1.0 bar so I’ll see how I get on with that. I bled out the pumps on Thursday and haven't had any problem since, but as you’ve mentioned; I’ve no idea if the check valve is working.

John.G

aria1 wrote: »

Thanks again John.

The F/E tank was full, I actually then filled it to overflow level yesterday to check the overflow was clear.

I’ve pressed up the accumulator to 1.0 bar so I’ll see how I get on with that. I bled out the pumps on Thursday and haven't had any problem since, but as you’ve mentioned; I’ve no idea if the check valve is working.

Very good, if you bled out a lot of air on Thursday last then you may have heard the ballcock making up some water to the tank/system, this would prove that the check valve is opening If you have a pressure gauge anywhere in the (waterside) system then it should start rising as the boiler is firing, this would prove that the check valve is closing/closed and holding, if you havn't got a pressure gauge then if you take the expansion vessel pressure before and after firing the boiler you should also see a rise here.
Anyway see how it goes and keep us informed please.

aria1

No pressure guage, they just stuck in the vessel.

Great ideas for checking the NRV, all seems ok now but that’ll be next thing checked if I have problems again.

I’m away tomorrow for a few weeks but will update when I get back and see how the system is doing.

aria1

No problems since with airlocks.

After a few weeks away I went back up and checked the expansion vessel, it has held the pre-charge.

I then confirmed the NRV operation in one direction by draining water from a rad and confirming the expansion tank level dropped. I didn’t get an increase in expansion vessel pressure when I ran up the boiler though so not sure about the other direction.
I have the boiler stat turned down to about 50% and I was definitely not getting any heat into the expansion tank so I’m hoping it’s ok?

The house will be unoccupied now again till Feb. I gave all the rads and the pumps a good bleed down and got some more air out before leaving so hopefully it’ll all work if the weather turns cold and I fire it up remotely.

John.G

aria1 wrote: »

No problems since with airlocks.

After a few weeks away I went back up and checked the expansion vessel, it has held the pre-charge.

I then confirmed the NRV operation in one direction by draining water from a rad and confirming the expansion tank level dropped. I didn’t get an increase in expansion vessel pressure when I ran up the boiler though so not sure about the other direction.
I have the boiler stat turned down to about 50% and I was definitely not getting any heat into the expansion tank so I’m hoping it’s ok?

The house will be unoccupied now again till Feb. I gave all the rads and the pumps a good bleed down and got some more air out before leaving so hopefully it’ll all work if the weather turns cold and I fire it up remotely.

That's interesting, if you have a prepressure of 1.0 bar and a filling pressure of 0.5 Bar (gravity head) and IF the NRV is holding and IF there is no remaining air it the system then as soon as the the water starts heating up, the system pressure should immediately rise (because water is virtually uncompressible) to 1.0 Bar and then rise still further to 1.3/1.5 Bar or whatever. On the other hand if there is still air in the system then theoretically this will act as the expansion and the system pressure would again have to rise to 1.0 Bar before the expansion vessel starts to increase in pressure. One would think that there would still have to be a lot of entrained air in the water for this condition but I just don't know. The most likely explaination is that the NRV is passing/stuck open or the internals have been removed, in any of these scenarios, the system will just act a if there is no NRV fitted and will work away fine but you said originally that the reason the semi-sealed system was fitted was because of the pitching problem and the tank nearly melting so why isn't it happening now.
Now, your system may have been modified to have a combined vent and cold water make up from the F&E tank and the system was also made semi sealed, the combined vent & makeup is often used on its own to cure any pitching problems. What system have you got?.

aria1

Thanks again for coming back on this John.

I’m not sure I understand the question around the system I have. I think is a combined vent/feed as there is no goose-neck pipe going into the tank, just the supply from the ball-cock, the line from the bottom of the tank with the NRV, and the overflow.

I don’t think the NRV internals have been removed; also all the bits of cut pipe were just left there when the system was modified so I reckon they would have just chucked the guts of the NRV beside them.
I guess it could well be stuck open. I ran out of time to check that before leaving but I must do it next time.

John.G

aria1 wrote: »

Thanks again for coming back on this John.

I’m not sure I understand the question around the system I have. I think is a combined vent/feed as there is no goose-neck pipe going into the tank, just the supply from the ball-cock, the line from the bottom of the tank with the NRV, and the overflow.

I don’t think the NRV internals have been removed; also all the bits of cut pipe were just left there when the system was modified so I reckon they would have just chucked the guts of the NRV beside them.
I guess it could well be stuck open. I ran out of time to check that before leaving but I must do it next time.

You probably have the system shown in my schematic (center drawing) so that's more than likely the reason why you are not getting any pitching even with a non functional NRV, the system was probably converted from the one shown in the top drawing but may have been converted from the one shown in the bottom, if it has then you should see a 1/2" line from the feed tank which would then have been blanked off somewhere.

aria1

Definitely have the center one now. I think it was converted from the top one.

What’s stopping the pitching though, if the NRV is not functioning?

John.G

aria1 wrote: »

Definitely have the center one now. I think it was converted from the top one.

What’s stopping the pitching though, if the NRV is not functioning?

Pitching I think occurs when the circ pump pressure is too high (there are lots of posts on here through the years regarding this). It usually happens in systems that have a separate expansion and cold feed, this can then create a circulating flow, some plumbers on here have cured it by changing the cold feed from behind the circ pump to before it I think but the combined system like you have has been used in a lot of cases. I don't really know if there is any real difference between the top and middle system, mine is the top system from day one. I think either of these systems stop a circulating flow occuring.
I would have a look at the NRV sometime and if its definitely stuck open then I would either replace it with a new one or remove it altogether.
The combined expansion & feed is only allowed in systems fitted with a thermostat and a high limit stat such as in oil or gas fired boilers but should not be used with solid fuel because if the system started venting due to over temp then the cold feed is trying to come down the same line.....not a good idea.

aria1

Thanks John. I’ll definitely pull the NRV out next time and check it. I have an oil fired boiler so your explanation makes sense.