Can I vent a hot water cylinder to a different header tank?

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Due to lack of space the header tank for a hot water cylinder is about 5m away, but the header tank for the boiler circuit is directly above it. It's a gravity feed from a solid fuel backboiler.
Is there any reason not to run the vent pipe from the hot water cylinder to the boiler header tank instead of it's own? To save an awkward pipe run and because it seems safer to me to have a short vertical vent not a long nearly horizontal one.
The top of the boiler header tank is slightly above the top of the hot water header tank.
 
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The only problem I can think of is that it could cause the overflow to run when there's no fault.
 
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eeerrr... that is the point of an overflow.
Isn't the point of an overflow to run when there IS a fault?

But my question is does it matter? Will enough water come out of the vent pipe to cause am overflow?
I would have thought only if the boiler is being continually overheated, and then a warning from the overflow pipe might be a good idea.
Is there any other reason not to do it that I haven't thought of?
 
I'm sure I've read somewhere, that if corrosion inhibitor is added to the heating F&E tank it will eventually become distributed around the heating circuit as the water expands and contracts, thus raising & lowering the level of the water in the tank, caused by the system heating up & then cooling down.

If so, would the reverse not also be true? in that the fresh water introduced in to the F&E tank will be absorbed thus diluting the corrosion inhibitor.
 
The vent pipe from the cylinder should be dry unless there's a fault.

The feed and expansion pipe one assumes comes from the cold water storage tank.
 
When water is heated it expands. 120 litres of water in a cylinder could increase by approx 2 to 3% (if my memory back to my school days serves me correctly) when heated from 5 degrees (cold mains temperature) to 60 degrees, increasing its volume by 2.5 to 3.5 litres which goes up the vent pipe and into the F&E tank. OK, if the temperature differential is less, then the expansion will be less, but it will still increase nonetheless.
 
When water is heated it expands. 120 litres of water in a cylinder could increase by approx 2% when heated from 5 degrees (cold mains temperature) to 60 degrees, increasing its volume by 2.5 litres which goes up the vent pipe and into the F&E tank. If the temperature differential is less, then the expansion will be less, but it will still increase nonetheless.

Don't be silly stem it does nothing of the sort.

When the cylinder is heated the expanded water goes up the cold feed into the storage tank
 
OK, my mistake :oops: I wasn't thinking clearly. But the purpose of the post was to show that the level of water in the F&E rises and falls, which it does via the cold feed as you rightly point out.

Stem.
 
Correct and unless there's a fault the heating water and domestic water can't mix.
 
If so, would the reverse not also be true? in that the fresh water introduced in to the F&E tank will be absorbed thus diluting the corrosion inhibitor.

I'm just basing this on logic, not experience - but I would have said no, as long as it's only very small amounts, because it will only replace the water lost by evaporation, which would otherwise be replaced by the ball valve opening.
 

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