Hot pulling air through open vent.

[samuelmaccarthy84]

Hi, I'm new to the forum and just some background on myself. I have 4 years experience as a plumber but haven't worked in the trade in 15 years. I would say my overall knowledge is not great as I did most of the 4 years working on rinse and repeat estates of houses. I was also quite young and uninterested!

I'm having some trouble with the hot on an open vented system (I have little to no experience in these) and I'm hoping to get some input please.

The attached diagram is the current pipework layout. In diagram 1, you can see there are offsets and bends as well as the pipes are running parallel to the rafters. If you were to look in at the pipework as the arrow indicates you would see what is shown in diagrams 2 and 3. In diagram 2, the hot to taps runs horizontally across the wall before dropping down below the floor to feed the bathrooms. In diagram 3 the vent runs across the attic (rising continuously) to terminate in the cold water tank.

I'm having an issue with the 3/4" hot feed to the bath, in that it is pulling air in through the open vent. The 1/2" hots are working fine.

My main question here is, must the vent and hot to taps run vertically up and down for this system to work? i.e. is my problem being caused by the fact that the pipes are running parallel to the rafters and also the horizontal run on the hot to taps?

Thanks in advance, and apologies for the long post.

 

[snowhead]

If your drawing in air from the vent line then you must either be running out of cold water into the cylinder or there's a restriction, maybe scale in the hot elbow above the cylinder or the junction with the vent to taps line.

I'd guess that if you ran all the 1/2" taps at the same time you'd probably draw air through the highest 1/2" tap.

However my first guess of a fault would be the cold shut off to the cylinder part closed / sheared off part closed.
What size pipe is it for the cold feed to cylinder?

 

[samuelmaccarthy84]

Hi snowhead, thanks for the reply. I should have mentioned in the post that this is a brand new system, so the first fix has just been filled and tested. It is possible that some insulation or wood shavings have made their way into the tank and lodged in the cold feed to the cylinder so I will drain and flush the pipes to check. The cold feed to the cylinder is 3/4".

 

[John.g]

Even though taking the HW & vent at the same level isn't the best, it looks like the cold water feed to the HW cylinder can't keep up with the bath HW draw off for whatever reason, blockages etc, what type of isolation valve is on the cold feed to the HW cylinder, is it a gate valve (which is full bore) or a lever valve which may not be full bore?. If a gate valve. ensure ~ 3 full turns from shut to fully open.
As above, obviously check that the CWST isn't running low or out of water.

 

[samuelmaccarthy84]

Thanks John. It's a gate valve, fully open and there plenty of water in the CWS. There was a lid on the tank from day one also, so while there could be debris in there I think it's unlikely. I'll inspect the gate valve and flush the cold feed pipework today to rule that out. Thanks again.

 

[John.g]

If the push comes to a shove and you don't find anything amiss then one of those fancy dip tubes like a surrey flange (or homemade dip tube) would probably cure it.

 

[samuelmaccarthy84]

Thanks John. The cylinder is SS and the connections are compression. I had a salamander s flange bought (for a shower pump) but with the compression connections, the "dip tube" of the s flange barely goes a few mm down past the top of the tank. Still though,it might fix this new issue, hadn't thought to try.I'll put some photos of the pipe work in here today to see if it helps.

 

[samuelmaccarthy84]

Hi again, just attaching some pictures for context. Also, I think you are both correct and it is a feed issue. When I choke down the isolation valve I have on the bath to a little under fully open it stops pulling air. I cannot find any blockage or issue with the gate valve on the cold supply so I'm going try John's suggestion of a dip tube. I have an s flange and I will need to modify the dip tube part of it to work with this type of cylinder (no male or female connectors so dip tube needs to fit through 1" copper with space around it for air).

Thanks very much for the advice.

 

[John.g]

Are the bath Hot & cold taps 22mm??, old fashioned type that give almost unrestriced flow?. I have those, 3/4"ID around here, (51 year old) but never get any aeration, the HW take off is similar to yours but the horizontal run is only about 0.3M.
The CWST feed to the HW cylinder is separate to the feed to the bathroom/bath. They both flow 13.6LPM. (gravity)

 

[samuelmaccarthy84]

Apologies, I keep talking about the bath but it's not actually installed yet. What I mean is I have bit of pipe connected temporarily from the bath pipe into the water pipe so I can test. See attached. When I open this iso valve fully is when I get air being pulled in. Like your system I have a dedicated feed from the CWST to the cylinder hot feed and the hot taps each.

 

[John.g]

A open ended pipe (even with a ball valve) will give a far greater flow than even a 22mm tap, run it into any container for even 15 secs, then measure this with a 1L milk bottle or whatever, X 4 to get the flowrate in LPM, then throttle it untl no air drawn in and repeat the measurement.

 

[samuelmaccarthy84]

Just ran that test there John. Fully open it's 20 LPM, closed down enough to stop drawing in air it goes to 12LPM. Do you think that's enough of a flow rate?

 

[John.g]

I think that should be quite acceptable Samuel, I have seen flowrates of 10LPM mentioned with taps that are even designed for very low heads.

 

[samuelmaccarthy84]

Thanks very much for all the advice John, it's much appreciated as it can be very hard to find good info on open vented systems.