Expansion vessel position on unvented cylinder

[TheKeymeister]

Had an unvented cylinder fitted about 6 months back - there was a long winded thread on here about it as the fitter stuffed it up in numerous ways and didn't issue paperwork. Got the problems sorted by him and received a Gas safe cert for it in the end...

Not sure if this makes any difference, but on the 'multi function' valve that does the pressure relief and pressure reduction etc, there's a separate connector for the expansion vessel that is blanked off (second one down on the right), and the expansion vessel is actually teed into the cold feed from that valve to the cylinder (the 22mm tee going off to the left).

Does this make any difference to anything? When I turn the hot tap on for the first time, there's a big burst of pressure that calms down after a few seconds, could this be due to the expansion vessel being connected in the wrong place? Of course if it's perfectly fine I'll ignore it and carry on

 

[picasso]

Expansion vessel will be ok but where is the tundish ?

 

[Lower]

Does this make any difference to anything? When I turn the hot tap on for the first time, there's a big burst of pressure that calms down after a few seconds, could this be due to the expansion vessel being connected in the wrong place? Of course if it's perfectly fine I'll ignore it and carry on

That burst of pressure is exactly what I get when my megaflo bubble needs regenerating.

 

[TheKeymeister]

Expansion vessel will be ok but where is the tundish ?

Sorry that's an old photo from before the plumber came back and put everything right....there's a tundish on the 22mm pipe now, and it terminates at ground level now rather than pointing towards the top of the back door!

 

[Dan Robinson]

That burst of pressure is exactly what I get when my megaflo bubble needs regenerating.

No, you just don;t understand how these things work ,which is why you should stop trying to DIY things on the other thread.

 

[Lower]

No, you just don;t understand how these things work ,which is why you should stop trying to DIY things on the other thread.

Which i'm not doing.

 

[Dan Robinson]

Good

 

[Lower]

I get the brief burst of pressure from the hot tap when the hot water has been heating for a period without any tap being turned on. When the hot water is not being heated or if a tap has been recently turned on (and the hot water is still being heated) there is no brief burst of pressure. It only happens when the megaflo air bubble is depleted and when the relief valve is also passing small amounts of water into the tundish.

If i'm incorrect, why else does it happen?

 

[Dan Robinson]

With no bubble there's no burst. With full bubble the burst is much bigger. As bubble depletes burst gets shorter.

 

[Lower]

My megaflo doesn't do that. When the bubble has just been regenerated there is no burst when you turn on the tap. You only get the burst when the bubble is depleted.

 

[Mottie]

I get a short initial burst on mine if the hot tap has been off a while. I just assumed it’s the pressure build up as the stored water is heated and when you open the tap, as well as mains pressure water, you have the diaphragm in the expansion vessel releasing it’s stored pressure giving you that initial 'burst'.

 

[fezster]

With no bubble there's no burst. With full bubble the burst is much bigger. As bubble depletes burst gets shorter.

This intrigued me, because this is how I always believed it worked (and read many times on this forum).

But I thought it through logically, and arrived at the opposite conclusion - looking for validation on my thinking:

The expansion vessel (or air bubble) takes up the expansion as the water is heated. If you put a pressure gauge on your cold inlet, you'd see the pressure increasing as the water is heated if there was no expansion vessel.

With an expansion vessel, the pressure remains constant as the water is heated.

So I'd expect a "burst" of water when the air bubble is depleted as the pressure in the pipework has increased due to heating and the water has nowhere to go.

And vice versa, I'd expect no "burst" when the air bubble is working as the pressure has remained constant. However - I believe the air bubble/EV provides an accumulator type effect which results in a higher flow rate (the EV supplements the flow rate of your mains temporarily) which gives the same appearance of a burst of water, but because of a different reason.

Interested to hear if any of what I've said is right or not.

 

[John D v2.0]

As the air is squashed it stores energy and increases in pressure hence the burst.
If there's no air there's no where to store the energy, the excess water just increases pressure until it trips the prv and all the extra is lost down the drain.
Same reason you don't pressure test with air, it has a lot of stored energy. Water has almost none.
So your original thought Is correct and your new reasoning is not

 

[Dan Robinson]

This intrigued me, because this is how I always believed it worked (and read many times on this forum).

But I thought it through logically, and arrived at the opposite conclusion - looking for validation on my thinking:

The expansion vessel (or air bubble) takes up the expansion as the water is heated. If you put a pressure gauge on your cold inlet, you'd see the pressure increasing as the water is heated if there was no expansion vessel.

With an expansion vessel, the pressure remains constant as the water is heated.

So I'd expect a "burst" of water when the air bubble is depleted as the pressure in the pipework has increased due to heating and the water has nowhere to go.

And vice versa, I'd expect no "burst" when the air bubble is working as the pressure has remained constant. However - I believe the air bubble/EV provides an accumulator type effect which results in a higher flow rate (the EV supplements the flow rate of your mains temporarily) which gives the same appearance of a burst of water, but because of a different reason.

Interested to hear if any of what I've said is right or not.

Your pressure guage is before a non return valve. How will it see the pressure rise?

Explain how this works :

 

[fezster]

Your pressure guage is before a non return valve. How will it see the pressure rise?

Explain how this works :

Agreed, I don't dispute the reality. Was just interested in the why. I guess the expansion vessel increases/supplements the working pressure (pressure at a given flow rate) than just the mains can provide.