Air in system with negative head pump

[Ben Beardmore]

Some years ago we had a loft conversion done. The loft had and still has two large ( 80 gallon ?) header tanks - one for the the HW and one for the cold water piping. They were repositioned during the conversion.
Rather tha go for a complete change to a closed heating system, we decided to stay with the open vented system and use a negaive head pump to supply hot water to the loft. So far so good.

Some time after the loft conversion when we drained the system ( to replace radiators) we found that we had poor water flow in both the cold and hot water on the ground and first floors. I guessed that this was due to an air lock because the tank was full and only a dribble was coming out. I fixed this by using a lose at mains pressure to force water into HW and CW taps on the first floor. The problem then did not affect the supply to the pump.

Fast forward, 15 years and we had to drain down again. My back-fill worked again on the HW and CW pipes but has failed on the HW pump to the loft. The layout is as per the attached diagram.

The back filling has not worked as indicated that no water comes out at the bleed valve at the top of the loop. Even opening the bleed valve whilst back filling still does not produce water.

Should I look at getting a plumber to create a connection (controlled by a gate valve) on either side of the loop whereby I can connect a mains pressure hose when required?
Or any other suggestions.?

 

[Johntheo5]

Is there a vent pipe from the cylinder top anf if so, does a T off this supply HW elsewhere?.
Have you removed and cleaned out the air vent, is it a AAV?.

 

[Ben Beardmore]

Can't see an AAV,. The air bleed valves are simple screw threaded type with a knurled end for opening and closing.
I attach an annotated pic which is better than any description. It's quite a complicated bit of plumbing in a small space. But here goes:-

At the top of the cylinder there is a single off take which looks like the supply to basins and sink.
CW supply to the cylinder is at the bottom on the LHS
There is a twin Stuart Turner pump that supplies only the shower on the FF This is located immediately behind the wall that is behind the cylinder. It gets its water from an (Essex ? valve on the LHS of the cylinder
The Techflow single end pump only supplies HW to the loft bathroom.
The air bleed on the LHS for the ST pump works OK and lets water come out.
The air bleed on the RHS for the Techflow pump does not release air or water.

For the last 10 years or so, the air bleed on the RHS would release air and water.

One question could be to insert a one way connection that would allow me to force mains pressure water into the supply loop for the loft pump.
Something like 2nd pic?

What do you reckon?

 

[Johntheo5]

I would start at finding the reason as to why the vent for the Techflow pump isn't venting air/water. Its lower than the bottom of the CWSC by 400mm??.

 

[Ben Beardmore]

I think its related to the way the builders (in 2005) routed the pipes coming from the 85 gallon CW storage tank. There has been intermittent problems with air locks in the with the hot and cold water taps and we have cured this by connecting a hose at mains pressure and forcing water up via the taps. This clears the airlocks - until such time as we have to drain the system to do such things as replacing radiators.

Last week we had to drain the system down so that the plumber could remove a radiator. The trick with the hose fixed the air lock in the hot and cold taps but not the one for the Techflow pump. I am inclined to the view that the impeller is restricting the back fill from the hose which was back filling from a tap in the loft ie the back fill was being applied to the delivery side of the pump . It hay be that the impeller is somehow restricting/blocking the flow getting to the intake side where the air bleed valve is.
I cant think of any other reason for not venting air/water at the valve.

Q1 -Could it be that there is air at the top of the HW cylinder?

Q2 Can you think of a reason why a Tee connection with a hose connector for both the Techflow intake and one for the cylinder intake would not work??

Because of the loft conversion, the floor is boarded over and carpeted so it would be a major issue to expose the piping.

 

[Johntheo5]

If you are getting water from whatever the cylinder top pipe is feeding then unlikely there's air in the cylinder top but could still be air in the pipe supplying the Techflow pump. The Techflow pump impeller will not stop reverse flow but there may be a NRV installed in the pump. The suggested Tee connection may not work for this reason, you may be able to enable the hose to feed into the Techflow pump vent.

 

[Ben Beardmore]

I will research the NRV on the pump but they went out of business a long time ago. Meanwhile, rather than directly in via the Techflow vent, would it be similarly OK anywhere on the the intake side?

 

[dilalio]

If you've drained the CH system for radiator work, this would have no impact on the domestic hot and cold water?

 

[Johntheo5]

I will research the NRV on the pump but they went out of business a long time ago. Meanwhile, rather than directly in via the Techflow vent, would it be similarly OK anywhere on the the intake side?

Whatever it takes!.
Have you proven the vent clear by removing the cap and sticking a bit of wire down into the pipe?.

 

[dilalio]

Can't see an AAV,. The air bleed valves are simple screw threaded type with a knurled end for opening and closing.
I attach an annotated pic which is better than any description. It's quite a complicated bit of plumbing in a small space. But here goes:-

At the top of the cylinder there is a single off take which looks like the supply to basins and sink.
CW supply to the cylinder is at the bottom on the LHS
There is a twin Stuart Turner pump that supplies only the shower on the FF This is located immediately behind the wall that is behind the cylinder. It gets its water from an (Essex ? valve on the LHS of the cylinder
The Techflow single end pump only supplies HW to the loft bathroom.
The air bleed on the LHS for the ST pump works OK and lets water come out.
The air bleed on the RHS for the Techflow pump does not release air or water.

For the last 10 years or so, the air bleed on the RHS would release air and water.

One question could be to insert a one way connection that would allow me to force mains pressure water into the supply loop for the loft pump.
Something like 2nd pic?

What do you reckon?

Looking at your pumps x 2 photo,
There appears to be an AAV for the right hand pump. Your back filling may have jammed this shut.
Loosen the nut that holds it onto the pipe or gently ease it out of the speedfit fitting and see if you get a release of air... Don't let it come all the way out though as you'll get wet.

 

[Ben Beardmore]

I have marked with arrows the two air bleed locations. Presumably the upper one on the left is an AAV . I pushed a 2mm stiff wire down it and there was no difference.
I tried to squeeze in the collar on its Speedfit coupling but could not budge it. Can you suggest a device I can use to put the squeeze on it without breaking it?

 

[dilalio]

Speedfit... Rotate the plastic nut ccw till it's loose then push the collet down and gently slide the copper up... If you pull too hard it will come out and you'll have to put it back in again whilst getting wet.

Pin down the aav will do nothing as there's a float there.