Question regarding pipework size to an expansion vessel

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GasmanCam

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HI folks,

Does anyone know if there are any rules regarding what size pipework should be used to pipe up an expansion vessel that's working with an unvented cylinder.

I saw one yesterday that was installed with 15mm pipe to the combination valve, and I wasn't sure if this was acceptable or not. (The vessel was also connected into the wrong side of the check valve, which I'm going back to rectify)

Id appreciate anyones thoughts on this.

Thanks
 
15mm is fine, the rate of expansion isn't so great that pipework is going to cause a huge resistance. A lot of combi boilers have 3/8" hose with an internal diameter of about 4mm connecting the vessel to the system
 
Don't mean to suggest you're not but if you were qualified to work on unvented cylinders, then you really should know the answer to that.
 
Put it this way... Without "any" room for expansion, the force would "find a way" so, as long as the resistance of the comms tube is less than that of the PRV spring, there is balance (y)
 
Thanks Dilalio, what I specifically had in mind was would the smaller diameter pipe hinder the changeover of water within the vessel, which would matter more being secondary water, than if it was primary water.

Sounds like I was concerned about nothing though...
 
Good question! I always have piped them in 22mm copper. The vessels are 3/4” thread on any I install.
But the flexi hoses that are supplied with some brands are obviously very restrictive.
 
ReJect said:
Good question! I always have piped them in 22mm copper. The vessels are 3/4” thread on any I install.
But the flexi hoses that are supplied with some brands are obviously very restrictive.
Click to expand...

How much water flow do you think you're going to get to an expansion vessel??
 
aptsys said:
How much water flow do you think you're going to get to an expansion vessel??
Click to expand...

Not a lot and usually very slow heat up, therefore gradual expansion of water anyhow.
I am not saying anything wrong with using 15mm copper pipe for it. Flexies though in my opinion shouldn’t be supplied with unvented units, but I hate all flexies and see them as a weak point.
22mm is much stronger pipe which makes it less likely to get damaged when removing expansion vessel.
On domestic heating systems it is usually 15mm pipe used for to EVs
 
aptsys said:
How much water flow do you think you're going to get to an expansion vessel??
Click to expand...
With (say) 30 kW into the water, I estimate less than 20 litre/h expansion rate. That's only just after the boiler fires, when most of the heat is raising the water temperature. It drops off as the rads etc warm up and put out heat, so net heat into the water falls, down to zero when the whole system is at steady temperature.
 
Water expands just a few % over the full cylinder temperature range, so with a typical 120 litre vessel you're talking no more than a couple of litres.
 
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