Switching from Gravity to fully pumped - open vent position?

[AndreGH]

Hi, Our gravity HW tank split and without a long delay the only one we could get was a fully pumped tank. Fairly easy replumb to get the tank in, however on all diagrams I've looked at, the fully pumped system has the open vent coming off between the boiler and the pump, whereas in our current gravity install, the open vent comes off the connector at the top of the coil.

Are there any specific requirements of where to put the open vent when moving to fully pumped? The pump is very close to the boiler so will be a bit awkward to "T" in to the pipe between boiler and pump. I've attached a before / after - is the vent pipe new position okay, or do I even need to move it there?

 

[SJB060685]

To comply with current building regulations the system should be fully pumped, although there are still many systems like yours out there. If you’re going to make the system fully pumped (which I advise you do for a number of reasons) then you’re going to need to reroute the cold feed and vent, you will also need either two 2 port valves or a 3 port, diverter or mid position. The system will need to be rewired as well. I would advise on having someone else carry out the work.

 

[John.g]

The recommended way (below) is Boiler, Vent, cold feed, Pump. (VCP). this sometimes also gives problems but if you tee the cold feed directly into the vent right at the header tank, preferably also in 22mm pipe then a almost 100% guarantee of no problems. (See mine, 50 years).

 

[ShaunCorbs]

Easier to low pressure seal it eg 0.5 bar as they always pump over as the feed and vent are in the wrong position

 

[EvilDrPorkChop]

Or leave it Gravity.

Replaced many old gravity cylinders with one's that are meant to be only fully pumped with no issues in the past.

 

[SJB060685]

Or leave it Gravity.

Replaced many old gravity cylinders with one's that are meant to be only fully pumped with no issues in the past.

If leaving gravity the system should at least be wired as a C plan. That way there is no constant cycling at the boiler during a HW on period at the programmer and cylinder at temperature

 

[EvilDrPorkChop]

If leaving gravity the system should at least be wired as a C plan. That way there is no constant cycling at the boiler during a HW on period at the programmer and cylinder at temperature

Why? It would just be like previous on the boiler stat.

In agreement that, yes it would be better and very much more energy efficient. But there's many systems like this, and I would only see the need in upgrading controls etc. if replacing the boiler. But there's no requirement when replacing the cylinder in my eyes.

 

[SJB060685]

That’s exactly why the C plan valve was designed. To prevent boiler cycling and save fuel. If someone doesn’t want that then that’s on them but a no brainier in my eyes.

 

[John.g]

There was a post or two on here about someone who replaced the gravity cylinder with one that allegedly would work likewise but couldn't produce a drop of hot water seemingly because it had a corrugated coil.

 

[EvilDrPorkChop]

There was a post or two on here about someone who replaced the gravity cylinder with one that allegedly would work likewise but couldn't produce a drop of hot water seemingly because it had a corrugated coil.

Yeah a corrugated coiled cylinder wouldn't work.

 

[SJB060685]

Yeah a corrugated coiled cylinder wouldn't work.

Can you explain why a cylinder like mentioned wouldn’t work on gravity?

 

[EvilDrPorkChop]

Maybe @John.g can explain it better, but I always believed it was because of the resistance it caused along with air getting trapped in the corrugations made them a lot more prone to air-locking.

 

[John.g]

I think that's as good a reason(s) as anything, I might be able to find that long thread and post the link.

 

[SJB060685]

I think that's as good a reason(s) as anything, I might be able to find that long thread and post the link.

It was a long thread and I can remember very little of it.

 

[ShaunCorbs]

Yes due to the internal resistance of the coil manufacturer don’t recommend it for gravity

 

[John.g]

Link

How can I cure my gravity circulation problem? | Central Heating Forum | Plumbers Forums

Find How can I cure my gravity circulation problem? Advice and Help. How-to How can I cure my gravity circulation problem? in the Central Heating Forum advice boards on Plumbers Talk | Plumbing Advice | Plumbing Forum - Professional and DIY Plumbers and Bathroom Fitters Forum. How can I cure my...

www.plumbersforums.net

 

[AndreGH]

Thank you for all the feedback, we bit the bullet and moved the vent and feed, wasn't the easiest fit as it was so tight but we got there and all is working perfectly now. Also had hot water inside half hour of it being turned on as opposed to the several hours with the gravity system lol

 

[John.g]

Where did you move the vent and feed to? (which one of the schematics?)

 

[AndreGH]

Where did you move the vent and feed to? (which one of the schematics?)

As per John G's reply.

 

[John.g]

That's either the VCP method or the combined cold feed and vent like my own.
To put the advantage(s) of the fully pumped vs the gravity system in perspective and assuming the HW cylinder is ~ 2.5M above the boiler then the circulating head is ~ 0.023M with the gravity method vs ~ 2/2.5M with the fully pumped method so easy to see the reason for the the vastly improved cylinder warm up time.