CH system has no vent on primary

[Keith Refson - real email addr]

I have posted before about the wonderful Servowarm gas CH system I inherited with the "new" house, but now as I am preparing to upgrade the controls and pipework to a more modern configuration I have just noticed a new twist.

There is apparently no vent pipe on the primary!

In brief the system was installed by servowarm about 12 years ago consisting of an Elite 50 boiler feeding a fully pumped system and an indirect HW cylinder. The system appears to be a prehistoric version of a Y-plan except that the diverter valve is a manual one and for the complete absence of any thermostats (other than the boiler stat). The primary circuit is fed from a conventionally set-up expansion tank/cistern arrangement in the loft which is hooked in just before the diverter valve splits the feed from the boiler.

I believe the usual recommendation is for a 22mm or larger vent pipe to discharge over the expansion tank, but there is no sign of this at all. This leaves just the 15mm feed pipe to the cistern which is far from straight.

There is a vent pipe on the *secondary* circuit discharges over the larger cold -water cistern, but this is of course not connected to the primary.

The only possible safety feature I can see on the primary is a device fitted to a Tee at the highest point in the primary circuit which I can not identify. This is a vertical brass cylinder about 5cm high and 4cm diameter with a screw-on brass lid with an offset plastic knurled nut just under 1cm diameter with a screw head in the middle. There's a single 15mm pipe entry at the bottom connected to the primary circuit.

Can anyone help me identify this device? I'm rather hoping it's some sort of pressure relief valve, but if not it seems that the system is potentially dangerous in its current state.

Since I'm redoing some of the pipework anyway, it would actually be a relatively straightforward matter to install a vent if that's necessary. I'd very much appreciate a comment from one of the well-informed regulars on this system and whether adding a vent is essential or not.

Keith Refson

[Keith Refson - real email addr]

I think I have now identified the device, and it's just an air separator. See

scroll down to "Automatic air vents".

am I correct in believing that this company installed a system which is against recommendations and dangerous as used?

Keith Refson

[Christian McArdle]

I'd convert to sealed pressurised operation if the boiler, cylinder and pipework can take it.

Christian.

[Ed Sirett]

This is standard fair for Servowarm. You may also find that the header tank is a manual top up device i.e. not with an auto top-up float valve. You just have to keep aware of when the sytsem is getting low . 8-(

The boiler may be able to take a sealed primary system if the manufactuers say so and there is at least one manually (= non-automatic) overheat cutout. They usually interrupt the thermocouple circuit on this age of boiler.

However there are so many other difficiencies in the control and efficiency of the system it might be cheaper to decide do a major overhaul (likely involving a new boiler).

-- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at

Gas fitting FAQ CH FAQ

[Keith Refson - real email addr]

But is it actually dangerous?

There is another cutout which appeas as you describe. (It tripped once and needed resetting manually before i could relight the pilot.)

I'm actually in the middle of a medium-level overhaul including the replacement of the manual diverter and conversion to a proper electronically controlled S-plan system, adding some thermostatic radiator valves and a pressure bypass valve. So if I am going to add a vent, now would be a good time!

The trouble with this is that I have spotted the other defect in the system which is that the expansion tank feed is on the wrong side of the pump, and a likely culprit for the air ingress problems I have been seeing.

If I were to move the expansion pipe and add the vent to the boiler output these would be on opposite sides of the pump, and I'd be asking for pump-over. I could put the vent and the expansion pipe close together on the boiler output but would not then cure the air ingress.

The pump is integral to the boiler housing and well separated from the area I am working on by the cylinder, so it would be a larger operation than I am happy with to move the pump at this stage.

Funnily enough the boiler itself appears to be performing well and I don't think it needs changing at this stage. I was rather hoping to upgrade the rest of the system so a drop-in replacement would be sensible when it becomes necessary.

Keith Refson

[Christian McArdle]

If you are rejigging the system so much, you really should convert to pressurised operation rather than faff around with vent pipes, tanks, feeds, pumping over and airlocks. You will need to check the boiler is suitable, but the presence of the manually reset cutout is a good sign. It will be far easier to convert to pressurised operation than to debug a poor vented design. To convert, a competent DIYer should manage it in a hour or two, if you exclude removing all the old detritus, tanks etc. Don't forget to add an additive filling point. Far less hassle than faffing with radiator injectors that squirt inhibitor over the carpet.

Christian.

[Keith Refson - real email addr]

Is it allowed for a DIY job? I thought this was forbidden, or is that only for a pressurised secondary circuit?

Keith Refson

I'm not familiar with the design of pressurised primary circuits. Is there some information you could point me at?

Keith Refson

[Christian McArdle]

The primary circuit is fine for DIY. It is having large volume tanks at pressure that call for special safety measures.

A boiler primary circuit just needs an expansion vessel, 3 bar pressure relief valve, detachable filling loop and pressure gauge.

You can buy a kit that contains all the required parts which may even have instructions. Fundamentally, you just plumb them into the system anywhere that can't be totally isolated off from the boiler. The pressure relief valve needs to be near an external wall. The pressure gauge needs to be next to the filling loop. The filling loop needs a mains water supply. The expansion vessel can go anywhere. Often these requirements can be fulfilled in a single location, next to the boiler.

Look in the FAQ for futher info.

Christian.

[Ed Sirett]

Well you could start with the SealedCH FAQ below and see what else you need to know.