Open Central Heating system with an expansion vessel.

Shoog

Hi all,

Maybe someone can help me out here with a bit of advice. I have been renovating a three bed semi from around 2010. The oil fired central heating system has been out of action for around 7months as I have changed a few radiators and the immersion tank. Got to the point where I am comissioning the system again today. Have managed to get the upstairs radiators all working but no joy on the downstairs. I am going to try rebalancing tomorrow but I have noticed something that troubles me. Its an open system, or so I think, but it only has a 15mm feed line with a stop valve on the line from the header tank and no visable expansion line back to the feed tank, however what it does have is an expansion vessel in the hot press. I have never seen this and it seems like a bit of a bodge to me.

There is no obvious point to pressurize the system from mains water pressure and I am very reluctant to attempt to run the system with the feed line valve closed (though there is a blow off valve on top of the boiler itself). What possible benefit could this arrangement have if its not possible to purge air with pressurized water ?

It all seems a terrible hodge podge of a system with all downsides and no benefits. Chances of getting a plumber to come and look before Xmas seems remote.

Pete67

It's a fairly common arrangement, I expect it was done that way by house builders to avoid call backs due to loss of pressure in a fully sealed system. In the event of any small leaks the system will be topped up automatically from the cold water cistern in the attic, and in theory the check valve prevents back flow from the heating system to the roof tank. In reality it's not a great idea for several reasons. It can mask a system leak, and the constant topping up which will lead to corroded radiators. A leaking check valve will allow the heating system to contaminate the cold water cistern, assuming that a separate feed tank is not installed, which is often the case, and a sticking check valve will prevent the system from filling at all as usually there is very little head pressure available to open it.

A fully sealed system filled from the mains water supply via a detachable filling loop and with an appropriately sized expansion vessel is better in almost every respect, other than you need to keep an eye on the system pressure and top it up manually once or twice a year.

Sealed systems are common, by definition they do not have an open feed pipe, expansion is taken care of by the expansion vessel, and if it fails, by the pressure relief valve within or close to the boiler. In your case before filling the heating system check that the bladder in the expansion vessel is pressurised to around 2 bar and is not leaking, if so it should be safe to use but I would consider converting it to a fully sealed system when you can. It would not be a major job. You may find that the pressure drops off more often that it should, in that case you will need to find and fix any leaks.

10-10-20

https://www.boards.ie/discussion/comment/121512469#Comment_121512469

That's exactly what I did three months ago. Then found one weep at a rad which just needed a quick tighten. Wasn't a difficult job, just needed some forethought and fittings.

Shoog

I would say that this whole arrangement has given issues previously as there are signs of extensive modifications at some point in the past. A neighbour also said it took considerable effort to get her system fully working.

I suppose this has something to do with the difficulty of successfully using qualpex without generating air traps all over the place.

10-10-20

https://www.boards.ie/discussion/comment/121512997#Comment_121512997

Qualpex doesn't generate any more issues than copper unless it's the ancient pre-barrier stuff which had other issues, or was installed incorrectly. Mine is qualpex throughout (2002) and I have no issues.

dathi

https://www.boards.ie/discussion/comment/121513049#Comment_121513049

have to agree pipe is just pipe it just a tube that holds water whether the heating system works or not is down to the competency or not of the person who installed it

Shoog

Got things working today, sealed off the upstairs loop and opened a pipe in the bottom leg to ensure all air had an easy path out. Pump jumped from a variable 20w draw to a steady 32w draw which indicated a larger part of the network had come online. Rebalanced everything and its a good un.

John.G

https://www.boards.ie/discussion/2058327486/open-central-heating-system-with-an-expansion-vessel

You probably have whats known as a semi sealed system where the cold feed has a non return valve so that it becomes a pressurized system every time it heats up, very common around here (50 year old systems) where, when gas became available, some people then changed to gas fired boilers, most then also converted to these semi sealed systems. Even if the valve is closed (with no NRV) then no danger since you have a pressure relief valve on the boiler.

youtheman

https://www.boards.ie/discussion/comment/121515391#Comment_121515391

Exactly. This is how I ended up with this arrangement. My house was built in 1991. Now on my third boiler. Original system was 'open' with header tank in the attic and pressure relief was via a vent back to the header tank. When I went for the first 'upgrade' the boiler itself was designed to be 'sealed'. But the plumber didn't want to risk pressurizing an old system. So the 'sealed' system was converted to a 'semi sealed' system by keeping the header tank for top up, fitting a NRV to the pipe from the header tank and removing the vent pipe back to the tank. I assume the low pressure switch also had to be 'frigged' to allow the system to run on low pressure (just the hydrostatic head from the header tank, not the main water pressure you would get form a sealed system).

Shoog

https://www.boards.ie/discussion/comment/121513173#Comment_121513173

In an open system with pipes ran in floor voids with no attention to ensuring secured straight runs - it's very easy to see how a raised bump in the pipe run would be a place where air could accumulate and cause an airlock.

In the old days when everything was copper a plumber paid great attention to ensuring correct falls on all pipe runs and inbuilt bleed points for air at unavoidable high points.

All of this maybe less critical in sealed and semi sealed pressurised systems but it would still be best practice not to just stuff long lengths of qualpex into inaccessible voids and hope for the best. I personally hate concealed pipe runs for this and other reasons.

Shoog

In our system they didn't put a NRV on the down line (or any sort of bleed nipples anywhere), its a stopcock. Going to have to remedy that.

John.G

Would suggest you use a swing check valve with a rubber seat installed horizontally, the spring loaded type may cause lack of water in the upstairs rads. Then install a isolating gate valve on the feed&expansion cistern supply to the ballcock and close it after refilling, by doing that you can monitor any leakages, is there a pressure gauge anywhere?.