Air locks in gravity fed mixer Showers

[Frankyb88]

I moved a pair of joined cold water storage tanks in a loft and removed a central heating header tank (pressurized it instead) to make way for a loft conversion. The tanks moved approx 5-6 meters away from previous position above cylinder and have moved further away from shower outlets. I raised the tanks approx a meter to compensate for the extra distance on pipe runs.

The pipework had to be re-routed and was therefore renewed. I used speedfit as flexibility is required for joiners laying beams and doing floors etc. Time was also an issue as heating and hot water needed to go back on as soon as possible for a large family with kids.
All pipework was done in 22mm except the 15mm rising main. The previous cold feed to the cylinder was 28mm. Part of this still remains in 28mm however, the bit i have extended i did in 22mm (maybe foolishly).

I filled the system and (after holding my hand over a mixer tap with the hot and cold side open) both heating and hold water worked as expected. However the two gravity fed mixer showers didn't! On one shower i could only get hot water and the other only cold. Realising there was no easy fix I tee'd off the cold mains and ran a temporary pipe to shower outlet to no avail. I then tee'd a cold main into the feeds above the showers. This gave me flow from the showers through the offending hot and cold sides. However when i reinstated the gravity feeds only one remained woking (and that with a lot of fiddling).

Only being 23 and i dont have extensive experience with conventional systems as i have had the luxury of mostly working on modern pressurized systems. I am running out of ideas and am reluctant to spend more time and money fitting a pump if there is a more fundamental reason for this issue?

 

[stevetheplumber]

a metre of xtra head should have overcome any problems do your pipes fall continously with no high spots above joists or walls etc?

 

[Frankyb88]

They run fairly level with some slight waves up and down over joists. At no point do they raise more than 3 or 4 inches and from the tank there is and initial drop of about 1 and a half meters.

 

[Best]

3 or 4 inches is a lot. Even 1 inch of a dip or rise will airlock a pipe on gravity supply. I would want the pipes falling slightly from the tanks or else dead level. That's one of the problems with plastic pipes. They would need clipped level.

 

[Frankyb88]

Ok thanks. Originally i clipped them level but when the joiner came he unclipped them to put his beams in. I can try and raise them at the beggining of the run and straighten them out and see if that works. Was also thinking of putting a couple of auto air vents along the run.

I have another question aswell. The customer wants a pumped shower in the loft conversion which due to the distance between the shower head and tank level i was going to use a negative pressure pump. However, if i fit this will that have any affect on the other showers that are not pumped?

 

[PlumbBobBob]

i would be inclined to pipe up all of the showers to the pump which will remove the chance of airlocks forming and give much more powerful showers.