Gravity Circulation not working

[verdigrey]

Hi Guys ,
Had a call from a customer whos oil boiler is not heating the cylinder properly and i dont know why so i would be glad of any advice .
Oil boiler with gravity 28mm primary to cyl and pumped to radiators . The boiler is working fine and rads work ok if needed . At the moment they just want to heat the cylinder , but allthough the flow to cyl is really hot the return from cyl is only just warm at cyl and cold down near the boiler . The water is getting warm but not hot . The F & E tank is full of water and i bled one rad to see if there was perhaps a blockage in the 15mm feed , but the rad bled quite fast , but the water was a bit black / grey . The expansion pipe is 22mm as usual .
The only thing that i could think is perhaps the 28mm return pipe is partilly blocked with sludge as it enters the bottom of the boiler !!!
Any thoughts please .

 

[Mike Jackson]

Sludged primaries or coil would be my guess.

 

[Best]

When you let water out of system, was the ballvalve on f & e tank then filling in? If it wasn't & cylinder primary flow is the highest point of system, then feed blocked.

If not that then blockage maybe at coil connections or tees. Try tapping any of these to see if flow improves. Will need drained & sorted. Just because the primary return is cold does not mean it is blocked, - the flow of water may be very slight, making the flow pipe very hot, but loosing the heat into the cylinder. So can be either pipe, probably at cylinder. That's assuming the primaries rise & fall haven't been altered - like someone putting a heavy weight on top of flow in a roofspace, as I recently witnessed!

 

[jase158]

Was there a 2 port valve for central heating? is it working properly
Dont know how much you know, but was thermostat turned up? (customers love to turn them down this time of year)

 

[secret squirrel]

I'd go with the blockage, I think that is the only logical solution.

 

[tamz]

The only thing blocking it is air. The expansion tee should be the highest point and it falls back from there. Split the fitting connecting to the coil or pull the stat on the boiler and let the steam move it.

 

[secret squirrel]

The only thing blocking it is air. The expansion tee should be the highest point and it falls back from there. Split the fitting connecting to the coil or pull the stat on the boiler and let the steam move it.

I'm way to much of a girl to split the fitting connecting to the coil, I'm having kittens thinking about it. LIke the idea of the stat on the boiler though, never thought of that.

 

[tamz]

I'm way to much of a girl to split the fitting connecting to the coil, I'm having kittens thinking about it. LIke the idea of the stat on the boiler though, never thought of that.

And here is me thinking you were a man of adventure :lol:

 

[Leo21]

I've only ever known this to be blocked cold feed or air. Drain some water and wait for ball valve to fill. Other tan that pull the stat. Never hard of a blockage in 28mm. But as my wife tells me I wrong on regular basis.

 

[diamondgas]

28mm are unusual to collect air enough to cause poor circulation unless you get long straight runs. However air or sludge will be your culprit! Pulling the stat is great if its in the flow, not so successful if the airlock is in the return! If you've never pulled a stat before then pack some spare underware!!!

 

[Leo21]

Your right usually the gavitys only get aired up because of a blocked cold feed. Good opportunity to fully pump it surely.

 

[tamz]

Your right usually the gavitys only get aired up because of a blocked cold feed. Good opportunity to fully pump it surely.

They get aired up because they are usually piped wrong. Most gravity feed hw systems, especially bbu's, are conversions from solid fuel. Solid fuels would never air lock as they have no stats so they would boil and push the air out.
This is how they are.
or a combination of the faults.

I learned this doing a 1000 solid fuel to bbu swaps where 950 of them air locked which either ment pulling the stat (once it went, it went for years) or regrading the pipes if i had time.

 

[Best]

That was my point about the rise & fall of the primaries, - critical they don't fall the wrong way & they need well supported. No way is it air if they are properly fitted, but few will be. An 1" of a dip in the middle of the flow & oil boiler won't circulate & boiler stat stays mostly off.

 

[verdigrey]

Thanks for all your help guys .
Jase there is no motorised valve , it is a non cndensing boiler so when it gives up the ghost the cylinder will have to be changed to pumped .
I am too much of a wimp to try the steam method . Thanks Tamz , I will probably try splitting it where it gos into the coil and see what comes out . It is a really old cylinder and I have allready warned them that they may have to have a new one .
If I finish up putting a new cyl in does anyone know if one of these cheaper stainless ones with 22mm coil connections will work on Gravity ok ???
Thanks

 

[Best]

Just try letting the boiler come near to boiling point, - shouldn't be a problem.
If nobody can tell you if those s.s. cylinders do gravity, I suggest always ringing the manufacturers for technical advice before buying one. I remember Range copper cylinders said some of their coils didn't suit gravity as too flat, so you had to specifically ask them for a gravity cylinder which probably had the coil pulled open more. Few plumbers probably knew that, & just bought standard cyl.

 

[Leo21]

The other not so obvious thing to check is that the return into the boiler isn't blocked, that's where all the crap collects. Does the heating work?

 

[buckley plumb]

Just try letting the boiler come near to boiling point, - shouldn't be a problem.
If nobody can tell you if those s.s. cylinders do gravity, I suggest always ringing the manufacturers for technical advice before buying one. I remember Range copper cylinders said some of their coils didn't suit gravity as too flat, so you had to specifically ask them for a gravity cylinder which probably had the coil pulled open more. Few plumbers probably knew that, & just bought standard cyl.

no good for gravity they have tracpipe coils which could sag
ant

 

[Best]

Thanks for that info Buckley Plumb. I checked on Internet & saw that Gledhill make them o/v cylinders for solid fuel gravity, so assume others do them also. Not something I have ever used.

 

[Roger Welton]

I had a cylinder from telford specifically for gravity primaries, it came with a 28mm coil (not sure on any other internal differances though)

 

[buckley plumb]

Thanks for that info Buckley Plumb. I checked on Internet & saw that Gledhill make them o/v cylinders for solid fuel gravity, so assume others do them also. Not something I have ever used.

pts do their boss cylinder and that has 28mm coil as standard and compression fittings as standard. on offer at moment

ant

 

[verdigrey]

Morning Guys ,
Thanks Leo , the heating does work ok . Thanks Best , Roger and Buckley , I will check out that PTS cylinder , cheers

 

[WHPES]

My advice would be to give the system an overhaul, flush if necessary and convert to fully pumped S plan or Y plan.

 

[jtsplumbing]

Is their any sort of stat fitted to Gravity side ?? or other control ??

 

[verdigrey]

Hi Whpes , yes prob does need overhaul , if I cant get the gravity working i will recomend it . Hi JTS , no there is not any control on the gravity side at all , I will get on with the job next week and report back what happens . Cheers