Morning,
Had reason to look at this and found the expansion pipe went down under the water in the tank - am I right in thinking this is wrong and it should end above the level of the water? What effect could this have on the sytem if this is wrong as I suspect?
TIA
It should be above the water level to create an air gap.
Someone will be along soon....
This pipe is also an open vent pipe, and should not be blocked in any way.
Why does it need an air gap?
Thanks for all the replies. Regarding the pipe being under the water
- the prat who did the original fitting made the pipe so long it nearly touched the bottom of the tank. I shall be giving a good look at the rest of his work. Thanks for the tips.
AFAIK when water is heated it expands & any excess overflows into the tank via the pipe. When the system cools the water contracts IYSWIM. If the pipe is below the surface, water could be sucked back into the system, which would cause the tank to overflow.
Yes.
Yes.
No it wouldn't, unless there's something I'm missing, in which case plase explain.
The tank obviously can't overflow as a result of sucking water out of it, but of course what could happen is, if it sucked out enough to cause the float to drop and to open the top-up valve, then too much new water would be added to the system. But this shouldn't happen because the float valve would already have been shut before the water heated up and expanded.
Therefore any water spewed into the tank (with or without an air gap) would raise the level, causing the float to apply even more force on the top-up valve to keep it shut.
If the same quantity of water is then sucked back out of the tank, it will cause the tank level to drop only to its previous level, which is not low enough to open the top-up valve.
Its only a theory :-)
Assuming the system is hot & the float is 'up' - the expansion pipe then falls into the tank. As the system cools, water would be sucked out, so the float drops & the tank fills back to the normal level. Then when the system heats up the water expands & returns to the tank - which is already full - so it overflows.
Can't see any other reason for an air gap?
You mean it's not very realistic?
No, as I've tried to explain, this should not happen, because the float won't drop far enough to open the filler valve, because the float will already have been 'up' when the system was cold. So when it heats up, the float rises even higher than 'up', if its arm is flexible enough, or if it isn't flexible, then it just pushes harder against the valve. Either way, when it cools and some water gets sucked out again, the float will drop or ease only to the level it was at before, and no actual filling takes place.
No, this shouldn't happen. When the tank is "full" (enough for the float to be up and the fill valve shut), there should be more than enough spare room in it to accommodate the increase in level you would expect when the system gets hot, without it getting anywhere near the overflow.
I can imagine another reason, but it doesn't seem very good either. It is that this "expansion" pipe actually has two purposes, only one of them being to squirt out expansion water (actually some of the expansion would be expected to come back up the feed pipe, would it not), the other is to allow air and miscellaneous gases exsolved from the water to escape, i.e. it is a vent pipe. If the vent pipe's opening is under water, any venting would cause bubbling. You don't really want it to do that, but if it does, it's not all that terrible.
There might be some confusion here on the actual pipes in question...
The "feed and expansion" tank will have a feed and expansion pipe into the bottom of the tank. This is where water is both fed into the system and also where expansion will force water out of the system and into the tank.
There will usually be a totally separate (although it can be merged with the F&E pipe lower in the system) vent pipe that should rise higher than the highest water level in the system before turning over such that it would discharge into the top of the tank should water ever flow up it. On a correctly working system water should not ever flow from this pipe. When it does, that is "pumping over".
There would be a risk if its end sat in the header tank of it becoming blocked (crud, limescale etc), and this is a no no from system safety point of view. Also if the end is immersed, then you could ultimately end up with a vent pipe full of water (held by the hydraulic lock), which would then circulate via convection.
Is it a tradiional heating system? Diagram please (with many clours for clarification) [g]
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