F and E vent pipe overflowing when Hot water on.

[Bristol plumber]

Hi, I just put an indirect cylinder in for someone, simple one in one out..or so I thought.

When I fire up heating and hot water all is well, when hot water only is selected, the water from the boiler spills over into to f and E tank in the loft.

I've tried turning the pump down but it still happens.

It's a potterton condensing boiler and seems to be a bit patchwork system where there are two pumps, one above the boiler with a zone valve for the hot water, and the same underneath for the heating.

It doesn't seem to be the expansion of system water getting hot as if I turn the actual boiler off, but keep the controller etc.. on the pump and zone valve run/open and cold water flows over the top.

The vent is located on the flow just before it goes into the coil.

I'm stumped I must say..any ideas?

Thanks

 

[CBW]

Any backflow issue? I take it this issue wasn’t there when the old cylinder was in?

 

[Bristol plumber]

I dont think so, there is a check valve on the flow and is going the right way.

To describe the system (HW side) a little more. I suspect there was an old floor standing boiler, ideal Mexico or such, in the garage that was probably one of those that used convection, then pump only fired for heating. The garage has been converted since and now has a wall hung boiler.

The pump is on the return above the boiler with the zone valve.

The boiler fill comes down from loft and vanishes under the floor. The flow and return come up from underneath, the flow went into the top of the coil with the vent carrying on up and into loft (the check valve is as it comes up through floor), and return comes out bottom of coil and under floor, then to heat leak rad next in bathroom next to cylinder and back to boiler, the rad vented fine so I don't think it's an air lock.

The part I don't get it when heating is on too it all works perfectly both heating and hot water.

The original cylinder had a leak which is why I was called out. The system or parts of it are old enough that it was 3/4" hot and cylinder fill and 1" flow and returns...

 

[Hugh Jaleak]

Suspect from the description, it was originally installed as Gravity HW/Pumped CH, with a floorstanding lump of a boiler. This has since been replaced with a low water content wall hung Boiler unsuitable for Gravity circuits, so a pump was fitted in the cylinder return to overcome this. How old was the cylinder you replaced? I have to wonder if it was the original, designed for Gravity Primaries, and the new cylinder being for pumped system is providing a greater resistance through the coil, hence the water is taking the path of least resistance, and going up the vent.

It may not overpump when heating is on as well and some of the flow is being absorbed by the heating side, so the system just about copes. Had similar experiences before, recall as an Apprentice we went to look at an issue with the cylinder not getting hot, Gaffer thought it was blocked Primaries, then decided it was sludge in the boiler. To overcome it he tried to convert the system to fully pumped, with the net result it kept overpumping. Connecting the cold feed into the vent was suggested, but I cant remember if that solved the issue now.

First suggestion at the moment would be to create a Neutral Point where the vent and cold feed join the system, but if the cylinder is the issue, I'm not convinced that would help. Failing that, either swap the cylinder for one suitable for Gravity circulation, (if these are still available), or possibly look at converting the system to Fully Pumped, using a Y or S Plan setup.

No easy solution I dont think, sadly.

 

[Bristol plumber]

Suspect from the description, it was originally installed as Gravity HW/Pumped CH, with a floorstanding lump of a boiler. This has since been replaced with a low water content wall hung Boiler unsuitable for Gravity circuits, so a pump was fitted in the cylinder return to overcome this. How old was the cylinder you replaced? I have to wonder if it was the original, designed for Gravity Primaries, and the new cylinder being for pumped system is providing a greater resistance through the coil, hence the water is taking the path of least resistance, and going up the vent.

It may not overpump when heating is on as well and some of the flow is being absorbed by the heating side, so the system just about copes. Had similar experiences before, recall as an Apprentice we went to look at an issue with the cylinder not getting hot, Gaffer thought it was blocked Primaries, then decided it was sludge in the boiler. To overcome it he tried to convert the system to fully pumped, with the net result it kept overpumping. Connecting the cold feed into the vent was suggested, but I cant remember if that solved the issue now.

First suggestion at the moment would be to create a Neutral Point where the vent and cold feed join the system, but if the cylinder is the issue, I'm not convinced that would help. Failing that, either swap the cylinder for one suitable for Gravity circulation, (if these are still available), or possibly look at converting the system to Fully Pumped, using a Y or S Plan setup.

No easy solution I dont think, sadly.

Solved the problem in the end.

I had a look at the old cylinder last night and you are right, strange coil inside so must be one for gravity. I do believe it was the original one.

Going back with a fresh mind today I noticed the vent pipe was a little low, the top only 25-30 cms above water line in f and E so I raised it up to the 45 cms and all worked fine after.

 

[Nige F]

Solved the problem in the end.

I had a look at the old cylinder last night and you are right, strange coil inside so must be one for gravity. I do believe it was the original one.

.

Strange coil ? like a large can with top and bottom open > it's an annulus .Classic old skool heat exchanger for gravity primaries

 

[Bristol plumber]

Strange coil ? like a large can with top and bottom open > it's an annulus .Classic old skool heat exchanger for gravity primaries

That's the one..here's a pic.

Learn something new everyday!