No hot water in tank coil but radiators good?

[gec]

Can anyone advise me? I'm pulling my hair out with this!
I've got an indirect hot water cylinder coming from an Oil boiler via a 3 way port valve. I've got hot water to the tank coil input but very cold at output of coil from tank. Suspecting an air lock in the coil I've refilled the system loads of times - from tank above, from mains into the boiler drain point etc........

The radiator system coming off the other port on the valve is behaving correctly.
The 3 port valve has been changed (no Joy).
Initially the hot water system was only being heated (very very hot) when the radiator programme was selected. I found that this was happening via flow through an old back boiler in the fire place. I have capped off this back boiler pipework at the tank and into the radiator pipes.
I have never had hot water at the cylinder tank when it is selected at the programmer.
Any advice welcome!

 

[Nige F]

Oil boiler and (redundant) back boiler(solid fuel ?) interconnected and supposed to work in tandem...prior to you disconnecting the back boiler ..These can be linked, and I installed just the one system about 20 years ago But I`ve also found that when systems don`t work as they should it`s quite often incorrect piping .....If anyone here can diagnose without some pics or a diagram ..they`re a better man than me ...and they`ve got corgi, oftec ,partP, hetec ..........

 

[gec]

I've had another visit under the floor and as you suggest Nige, it may well be a piping layout problem.
It looks to me now, like the return pipes aren't installed correctly.
The only return to the boiler is coming from the Radiator circuit. The return connection from water system seems to be via the radiator system joining some yards further down the pipe.
Should I remove this and repipe the return directly from the water system to a connector joining the radiator return just before it arrives back at the Boiler?
Therefore the water and radiator systems will be almost independent apart from the pump line up to the 3 way port and the last few inches before it goes back into the boiler?

 

[meldrew's_mate]

...I've had another visit under the floor and as you suggest Nige, it may well be a piping layout problem...
...Should I remove this and repipe the return directly from the water system to a connector joining the radiator return just before it arrives back at the Boiler?...

Absolutely, and you are right to have the HW return connected as the last 'tee' before the boiler return connection, this helps to avoid reverse circulation in summer.

 

[chrishutt]

There's no point in altering the return pipework as long as you have the HW return joining the main return after all the heating return pipes have joined together.

 

[Expertboy]

Went out to one last week. Old house new side extension, 3 extension rads where getting hot with the hot water, seems the builder tee'd into the flow and return after the last tee.

Lets just say he had some explaining to do to the customer.

 

[Softus]

I've got hot water to the tank coil input but very cold at output of coil from tank.

How are determining that hot water is reaching the coil input? For example, conduction in the copper pipework will carry heat from the MZV further than the static water inside the pipe could.

Suspecting an air lock in the coil I've refilled the system loads of times - from tank above, from mains into the boiler drain point etc........

Is there a vent on the primary inlet? Using (or fitting) one would solve an air lock problem.

I have never had hot water at the cylinder tank when it is selected at the programmer.

Does the MZV operate, and the pump run, and the boiler light, when you select DHW only? You say that the MZV is new, but that doesn't mean that it's working.

Is there a cylinder thermostat? If so, have you tested that?

 

[gec]

Thanks for your assistance folks,
The Valve does operate, the pump does run and the boiler lights when water only is selected. The boiler heats up it's internal tank till extremely hot then stops. Basically the hot water is not circulating away from the boiler tank.
As you say "softus" the heat at the cylinder talk coil is probably due to conduction.
I am taking it that the port valve is good seeing as I have the same symptoms with the original one.

I have not tested the cylinder thermostat - but because the pump & boiler run for a few minutes before stopping and the water is not moving out of the boiler, I am taking it that the stat is not the problem. I am working on the premise that it is a flow/pipe/air lock type of problem.
There is not a vent on the primary inlet (I think). Where would one be placed (remember I'm a diy'er).

Is it possible that I'm not filling the system in the correct manner?
I'm draining it with a hose from the drain at the bottom of the Oil boiler with the filler pipe from the loft tank turned off with a wheel valve up just above the cylinder.
To refill I'm just closing the drain and opening the wheel - then bleeding the Raiators if required.
I only have radiators downstairs and the cylinder is upstairs. The radiator cct is operating perfectly well every time!

 

[meldrew's_mate]

...I only have radiators downstairs and the cylinder is upstairs. The radiator cct is operating perfectly well every time!...

Sorry if I've not read any previous posts, and am repeating what others may have said. If you have no air bleed at the highest point then you should add one at the top coil connection at the cylinder (Obviously air collects at the highest point)
In the meantime put a towel at the base of the cylinder and slowly undo the top primary pipe connection either at the coil or where it joins the 3 port valve (port B), but no more than 1 turn of the nut!!. Bleed air until water starts to come out, then retighten. Re-test and report.

 

[gec]

I have gone right back to square one.
To prove there is no air lock I have reconnected the back boiler pipework and the whole system heats up lovely (this is with the programmer set only to radiators) the radiators come on good and there is hot water in the cylinder.
To re-iterate, my hot water system will not heat up when selected. I am trying to remove the unused back bolier tank and pipework because it's costing me a bomb to heat it all up just to get hot water.
The only way I can get hot water is with the old back boiler plumbed in and the program set to radiators.