DHW cylinders - internal versus external expansion pressure vessel

[redbarnman]

Anyone got any views on whether a DHW cylinder having an internal expansion 'bubble' e.g. TribuneXe is better or worse than having a cylinder with an external expansion tank e.g RM Prestel. It strikes me that if there is air in the new water entering the cylinder, it will rise up to the expansion 'bubble' area and you will need to vent some off occasionally?

Thanks

 

[joni os]

In practice, a cylinder with internal air bubble, such as Megaflo, needs recharging as the hot water absorbs air in bubble. In all cases, internal or external, regular service should be undertaken to maintain safety and performance.

 

[lame plumber]

internal one wont need the vessel replacing like an external one will

 

[townfanjon]

one very obviously fact is the internal expansion type are obviously higher, so you need to check heights of both types.

 

[Chalked]

In my experience, bubble top cylinders cause more problems. When the bubble diminishes, they then pop the tprv or the prv. These then get dirt and most of the time need replacing.
You do get the odd expansion vessel go on the remote type. But that's about it.

 

[SimonG]

My cylinder of choice is the RM with seperate expansion vessel.

 

[Ray Stafford]

I like separate components. My simple mind prefers to see complex systems broken down into component parts that can be easily understood, tested, and if necessary, replaced.

 

[redbarnman]

I think Ray Staafford has hit the nail on the head. Easy diagnosis of problems worthwhile and its a lot cheaper replacing an expansion vessel than spending hours trying to work out a tank problem. One final twist, we will run off a borehole water supply and the makers of the Tribune cylinder won't provide a warranty unless it's mains water!

 

[Best]

To your thoughts about if air enters the mains water coming into cylinder you would have to vent it off. That is not what would happen. If air enters an unvented cylinder that has air gap, the air will not get into the air gap, as it is separate. Air would just simply come out of hot taps when opened, same as with an unvented cylinder with external expansion vessel. (Air only can get into air gap when unit is drained basically)
I would prefer the external expansion unvented units, but note that they (as with air gap unvented) need servicing each year and the G3 qualified plumber needs to service the expansion vessel. If the vessel membrane burst then the mains water will enter the steel part of vessel and rot it, ultimately causing a mains water flood. So vessel needs replaced after a few years. Preferred location for unvented cylinders is at a low level floor and ideally close to boiler and not far from bathrooms and most outlets

 

[redbarnman]

Good point on rotting out of expansion tank. I see that you can get stainless steel ones for just over £100 so perhaps the extra up-front cost is worth it even if only to avoid having to mop up and dry everything out after a failure!

 

[Best]

Good point on rotting out of expansion tank. I see that you can get stainless steel ones for just over £100 so perhaps the extra up-front cost is worth it even if only to avoid having to mop up and dry everything out after a failure!

No doubt a stainless exp. vessel better. Or just make sure you get the unvented unit serviced properly every year and the expansion vessel should then be checked, including if its membrane is intact and replaced if faulty or old.
Trouble with having the extra expense of a stainless steel vessel is once the membrane goes inside it you will then have the bigger cost of identical vessel. The membrane can be replaced in some of them, but doesn't seem to be done.