The vent pipe goes up and into the CH header tank.
The thing is, the pipe is actually in the water!
I don't have any problems with the CH (Touch Wood!) But is this OK?
In all the other systems I have seen, the pipe hovers just above the water - maybe it was done so no air can be introduced to the system?
If I follow the vent pipe down, it goes into the top of a little cylinder, about the size of a small tin of sweetcorn (not as big as a standard baked bean can, a bit smaller)
Two other pipes go into the side of this chamber - I take it the water enters to top side, ,and exits the bottom, the water spiralling around the chamber to try to get any air out? - Or am I completely wrong here!
With a header tank system, the vent pipe will also be full up to the level of the water in the header tank. So this idea of water spiralling around inside the cylinder thing won't, or shouldn't, happen.
Does the cylinder have a drain c*ck on the bottom of it ? Is the cylinder screwed on to a normal looking fitting ? Is the cylinder actually part of the heating circuit pipes ?
I'd be worried about it if my boiler wasn't intended for sealed pressurised operation. Less so, if it is. A boiler suitable for sealed pressurised operation has additional safety cutouts that prevent it producing steam. As well as sealed operation, such cutouts can be used to mitigate a whole range of design errors on vented systems, such as combined feed/vent pipes and the use of automatic bypasses.
RADS = Heating/HWC circuit. May be entirely blocked by controls (i.e. S-plan or all TRV)
ABV = automatic bypass valve BOILER = old boiler, not suitable for sealed/pressurised
It could not be used in the system above. This is because these systems require that a direct path for steam from the boiler flow to vent is provided, whilst at the same time to quench the boiler, a direct flow from the feed to the boiler return must be provided that doesn't conflict with the steam path. This is normally provided either by a fixed partly open bypass (or bypass radiator), or a Y-plan system with at least one TRV-less radiator.
If the heating circuit becomes blocked by the zone valves, or all TRVs closing, then during a boiler runaway malfunction, the water head may not provide enough pressure to open an automatic bypass valve, causing the boiler to run dry and cause damage/explosion. If the boiler is protected against this using a overheat (or water content) cutout (additional to normal thermostat), then this isn't so much of a problem, as venting isn't the primary safety mechanism.
I assume the two >> are the pump. Then this is fine, but the vent and feed pipes "must" be very close together. The feed must always be the first pipe towards the pump to suck in water rather than air. This system does not require double overheat protection.
There must never be "any" valve, pump, zone valve, etc between the open vent and the feed pipes and the boiler. That has always been the case. With high limit stats a one pipe system, as below, can be used. See boiler makers first though. Some like other don't other are indifferent.
Can you explain how quenching water gets to the boiler return? Fundamentally, the system as designed is a combined feed and vent arrangement when it comes to safety, as the automatic bypass valve and heating system may be closed. The feed/vent become combined. Therefore, boiler overheat protection is needed, or a manual bypass must be fitted.
You can't rely on the pump to open the ABV, as its failure is a likely contributary cause of the boiler overheat in the first place!
Via the return pipe from the rads, if the zone valves are open, which they should be. The water boils, cools and also water flows back in via the flow pipe. It is always the probability aspect. This system must be fully pumped. The chances of the pump packing in, the zone valves packing in and the boiler stat failing at the same time is super slim. About the same probability as both engines failing on a two engined jet.
I recall about 24 years ago British Gas reluctantly accepting this arrangement. The probability calculations convinced them. Think about a normal two valve setup and how it operates, they never used 3-way mid-position valves, which were starting to infiltrate (they were unreliable too; still are).
There would have to be a catastrophic failure in the zone valve to leave is stuck closed, the microswitch energising the pump (can't pump because the valve is stuck closed and boiler and the boiler stat failing too. Then if there is an auto bypass valve, if fitted, the pump would open this and create flow back to the boiler. The chances of boiler stat, zone valve, pump and auto bypass failing is super duper slim.
OK. You have a dodgy stat on the boiler. You have a system with all TRVs. The boiler starts to overheat, the radiators get boiling hot. The TRVs shut down. The boiler starts steaming, blocking the line between feed and vent. So far, this is exactly what you would expect upon stat failure.
You are now in the position that you are entirely dependent on the pump operating on a mixture of steam and water and expecting it to produce sufficient pressure to overcome the resistance of the automatic bypass valve. That is something that might not occur, especially if the ABV is set close to the pumps maximum head.
There is no need for the zone valves to pack in. An all TRV system without interlock, as many people seem to advocate would fail this way. If all the TRVs turn off, which is actually quite likely to happen when the radiators hold boiling water, the valves will close.
If you have zone valves, the interlock will likely cut the boiler power. However, there are failure modes of an old boiler that would cause even an S-Plan system to fail. Any internal boiler failure that causes the burner to lock on is likely to be dangerous.
You are now relying on the pump producing 0.4 bar (or whatever) to overcome the ABV, which may not happen with entrained steam. The pump may pack in because of the boiler temperature, too.
So, depending on the exact design, you only need 1 failure, or possibly 2 (1 of which could be latent and not discovered) to cause catastrophic failure.
i.e.
All TRV system:
Stat failure
S Plan system:
Internal boiler failure causing burner lock on.
Stat failure (and latent zone valve interlock defect)
The zone valve interlock failure could be latent, as if the microswitch or wiring failure defeats the interlock, the system will actually operate correctly, but will just be inefficient, pumping hot water through the ABV. Some people may not notice and fix the issue.
It is clear the regs heavily recokmend an interlock. It is foolish not to have one.
The pump will be pumping water down from the F&E tank. If you have ever observed a boiler boiling over you will see that little steam is produced. It is just boiling water with nowhere to go except the F&E tank, this will drop down the feed pipe with the pump grabbing it and sending it back to the boiler.
Luckily few boilers do not have a high limoit stat these days.
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