Gravity hot water to fully pumped / sealed

eagleeye · Jan 3, 2020

One of our installers currently fitted a new heat only boiler to replace an old floor standing with gravity hot water. At the time he never fitted any valves or cylinder stat etc so I have been called back there to basically start again. Been there today to do it and found that he had the pump on the return. Anyway I drained system and made the necessary alterations to the pipework by the boiler and fitted a mid position valve and a wireless cylinder stat (as the run is impossible to get a cable between the two). So currently the feed from the F + E tank goes into the return of the cylinder and the vent on the flow. The boiler is in a tight cupboard low to the ground in an extension of the side of the house and the cylinder is on the 1st floor in the middle of the main part of the house so you can imagine how the pipework runs

Stu-B · Jan 3, 2020

Crikey that looks snug! The filter is gonna be a devil of a job to open up!

eagleeye · Jan 3, 2020

Stu-B said:
Crikey that looks snug! The filter is gonna be a devil of a job to open up!

Yeah very tight. The filter is tight but can be open now. I've attached a pic of what it looked like before. Our surveyor has really mucked up here to be honest. He should of quoted for a system boiler for a start.

SJB060685 · Jan 3, 2020

So this is an open vented system and not sealed? Pump can go on return on a vented but the cold feed should be piped to down into return shortly after the pump outlet, the system would be under negative pressure though. To be honest that looked a mess beforehand

eagleeye · Jan 3, 2020

SJB060685 said:
So this is an open vented system and not sealed? Pump can go on return on a vented but the cold feed should be piped to down into return shortly after the pump outlet, the system would be under negative pressure though. To be honest that looked a mess beforehand

yes, open vented at the moment. Problem is that I cant make any alterations to the feed & vent on the cylinder as its as tight as a .......... and there is no way i can get feed or vent down to pump / boiler position. Bloody nightmare of a job.

Do you think my idea will work ok ?

Chuck · Jan 3, 2020

Er, careful, this is a public forum... Might want to take it to the Plumbers' Arms?

ShaunCorbs · Jan 3, 2020

agree pump should be on the return as they tend to last longer due to lower temp

check valve on the feed from the f and e
and a 2 bar safety valve on the vent
expansion vessel

thats along as you have an okish head eg 3m+

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eagleeye · Jan 3, 2020

ShaunCorbs said:
agree pump should be on the return as they tend to last longer due to lower temp

check valve on the feed from the f and e
and a 2 bar safety valve on the vent
expansion vessel

thats along as you have an okish head eg 3m+

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So are you saying still use the F + E tank to fill the system and fit a check valve to the feed, 2 bar PRV on the vent as well as an expansion vessel or am i doing away with the f + e and filling via mains water ?

Also gonna have to leave the pump on the flow now as ive already changed it all now. Will that matter ?

Only place I can get vessel in is either above the boiler on the ground floor or in the place of the f + e tank.

SJB060685 · Jan 4, 2020

If you leave the vent as is then whenever HW is called for it will pump over, in time it will sludge up the system. Im assuming Shaun is suggesting seal the system and do away with f and e tank, which is what I would suggest. Preferably the expansion vessel will go on return where it’s not exposed to high temperatures, if the pump is on return the vessel should be close to suction inlet of pump to maintain a positive pressure through boiler and around the system, thus eliminating any air ingress.

ShaunCorbs · Jan 4, 2020

eagleeye said:
So are you saying still use the F + E tank to fill the system and fit a check valve to the feed, 2 bar PRV on the vent as well as an expansion vessel or am i doing away with the f + e and filling via mains water ?

Also gonna have to leave the pump on the flow now as ive already changed it all now. Will that matter ?

Only place I can get vessel in is either above the boiler on the ground floor or in the place of the f + e tank.

leave the f and e tank in so it will stay the same pressure

sealing could cause issues with leaking on joints etc if in a concrete floor

SJB060685 · Jan 4, 2020

ShaunCorbs said:
leave the f and e tank in so it will stay the same pressure

sealing could cause issues with leaking on joints etc if in a concrete floor

But the vent is on flow of HW, not only will this pump over when calling for HW but the vent is not open if valve is shut?

eagleeye · Jan 4, 2020

SJB060685 said:
If you leave the vent as is then whenever HW is called for it will pump over, in time it will sludge up the system. Im assuming Shaun is suggesting seal the system and do away with f and e tank, which is what I would suggest. Preferably the expansion vessel will go on return where it’s not exposed to high temperatures, if the pump is on return the vessel should be close to suction inlet of pump to maintain a positive pressure through boiler and around the system, thus eliminating any air ingress.

Thats what I thought. Think I will fit the vessel above the boiler on the return, remove the f + e tank and fit AAV's on what used to be the feed and vent. At least this way the coil can release trapped air
[automerge]1578097374[/automerge]

ShaunCorbs said:
leave the f and e tank in so it will stay the same pressure

sealing could cause issues with leaking on joints etc if in a concrete floor

[automerge]1578097473[/automerge]

ShaunCorbs said:
leave the f and e tank in so it will stay the same pressure

sealing could cause issues with leaking on joints etc if in a concrete floor

Would love to but cant see how im goona get over the airlock issue. This way I can fit AAV's on the old feed and vent pipe once i remove the tank

ShaunCorbs · Jan 4, 2020

SJB060685 said:
But the vent is on flow of HW, not only will this pump over when calling for HW but the vent is not open if valve is shut?

will have a 2 bar blow off on the end of it so no pumping over
[automerge]1578098152[/automerge]
Air lock issue ??

SJB060685 · Jan 4, 2020

Yes I see what you're suggesting Shaun. How many systems have you known to be configured like this, personally I wouldn't feel comfortable

ShaunCorbs · Jan 4, 2020

I’ve done one to date couldn’t seal due to pipework and not enough pressure

worked ok and no complaints so far been in 2 years

John.g · Jan 4, 2020

In case of any worries re pressurising the system (old pipework etc) it might be no harm adding that the E.vessel should have its pre pressure reduced to whatever the static head is and be as large as comfortably possible , that way the final pressure will still be very very low, probably < 1 bar with hot system.

SJB060685 · Jan 4, 2020

I agree with John to a degree. The expansion vessel pre charge should be set higher than the static head at that point. The system should be then be pressurised to roughly .2 bar higher. Yes a larger expansion vessel is not a problem but one you're tight for space and two theres no point spending additional money on a 50 litre if a 12 is enough. You need to calculate the vessel volume required using system volume etc. Like I said ideally you want the vessel on the return, tapped in just before pump inlet, this will give the neutral point at that tap in, negative pressure in short piece of pipe before pump and then positive pressure through boiler and rest of system back to neutral point, thus eliminating possibility of air ingress.

John.g · Jan 4, 2020

I wouldn't like to see the EV pre pressure < 0.4 bar in case it doesn't push the diaphragm up against the opposite end, and because you will always have "auto filling" via the F&E tank then there is no need for a EV water reserve so Pre & filling pressure in these systems can be the same IMO, assuming system volume of 100 litres and a mean boiler temperature of 75C and assuming a static head of 4M with a 12 litre EV then the maximum system pressure will still only be 0.76 bar when hot. If one "forgot" to reduce the pre pressure then 1.0 pre pressure will result in a final pressure of 1.52 bar and 1.5 pre pressure will result in a final pressure of 2.15 bar.
Bord Gais approved RGIs here have installed literally "tens" of these systems near me on converting from oil to gas with no problems whatsoever, they use(d) swing check NR valves (which have to be installed in the horizonal) rather than the poppet type.

SJB060685 · Jan 4, 2020

John where are you getting this info to calculate the vessel charge and system pressure when hot? It seems to be more in depth than what I have in front of me, can you supply a link?

John.g · Jan 4, 2020

I just work(ed) it out from first principles many years ago when I was involved in industrial pressure vessels, boilers etc, you (or anyone) can input your own numbers here.

SJB060685 · Jan 4, 2020

How do I change the numbers John to calculate?

ShaunCorbs · Jan 4, 2020

SJB060685 said:
How do I change the numbers John to calculate?

in excel I’m guessing well that’s where I would think it’s made

SJB060685 · Jan 4, 2020

Yes I've worked it out thanks Shaun. Still getting used to my new phone 🤦‍♂️

ShaunCorbs · Jan 4, 2020

SJB060685 said:
Yes I've worked it out thanks Shaun. Still getting used to my new phone 🤦‍♂️

same

got no home button now

John.g · Jan 4, 2020

That's it, it was in lotus 123 originally before excel was even heard of!!
I have just added another "box" to give a taste of a fairly typical (fully sealed) installation, its quite handy.

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