What is this leaking valve near the central heating pump?

[Benjy]

Hey all, I'm relatively new to DIY pluming and I've found this forum super helpful so far This is my first time posting.

I've got a leaking isolating valve on one of the pipes running I think back to the boiler from just after the central heating pump. I'm pretty sure I've read that this specific kind of valve ought not to be used on a central heating system - and if they are, are prone to leaking? So rather than me replacing it like-for-like, I'm trying to understand what the purpose of it being there is, and what I should replace it with!

I'll do my best to describe where the valve in question is. In between the central heating pump and the mid-position valve, the 22mm section of pipe tees off towards an expansion vessel, also in 22mm. In turn, the pipe to the expansion vessel tees off in 15mm, and this is where the leaky isolating valve is. I think that pipe then goes back towards the boiler, as it is joined by the return from the hot water tank. Is this a bypass valve? In which case, could I replace it with an auto bypass valve like this one?

Any pointers on what to replace it with and also just clarity on what the valve is there for would be greatly appreciated!

 

[steadyon]

1. Its a gate valve, and perfectly OK for use in central heating. Virtually every pump has one of these either side to enable the pump to be changed without draining the system.
2. Gate valves are not in some ways the best valve in the world. They rely on a metal to metal seal of a wedge shaped "plug" being forced into a conical seating to stop the flow. As soon as something gets between the mating faces or they wear, the seal is less than perfect. They also have a tendency to bind, such that the spindle snaps internally when they are forced, making it impossible to open a closed valve, or close an open one. When opening, always open fully, then close back 1/2 a turn. Help to stop them sticking.
3. However, they are the valves almost invariably used for pumps.
4. If the leak is from the point where the spindle enters the body of the valve, then the most likely culprit is the seal around that spindle. The seal is held in place and compressed by the gland nut, which is the small brass hexagonal nut to the left of the body (not spindle) of the valve in your picture. Two possibilities for a cure:
4a. Just tighten the gland nut up. Turn clockwise facing the end of the spindle. Don't tighten it so much that the spindle can't be turned at all, but making it stiff to turn (with a handle or small spanner on its squared end) is OK.
4b. Re-pack the gland nut. As below:

To fix a leaking gland nut on a gate valve.

• Close the valve completely. Turn clockwise looking down on the handle.
• Undo the gland nut.
• If possible, remove the gland nut. If not possible, carry out 6 and 7 below as far as you can.
• Clean any limescale from the shaft of the valve as far as possible. Use a limescale remover like Kilrock if possible. For any remainder, use fine sandpaper (120 grit or greater) or a fine file.
• Clean the shaft as much as possible, and make sure there is no roughness to it. A polish with grade 00 wire wool or Brasso is good.
• Cut a length of PTFE tape. Start with about 10 inches / 250 mm. Twist it into a “string”
• Wrap the string around the spindle, clockwise looking down from the handle end.
• Put the gland nut back on the spindle if removed.
• Push the PTFE down into the body of the valve with something like a thin flat bladed screwdriver.
• Screw the gland nut back into the body of the valve.
• Tighten the gland nut so that the spindle can just be turned by hand (see above).
• Open the valve and check for leaks.
• For a slight weep, try tightening the gland nut a little more.
• For a more serious weep, repeat as above and use more PTFE.

5. If the leak is elsewhere:
5a. If may be from the compression fitting above the valve. Try nipping it up a little tighter.
5b. Otherwise, change the valve. This involves draining the system down to a water level just below the lower valve (if changing that as well), removing the pump, removing the valve(s) fitting new valves, reinstalling the pump and refilling the system. It is a DIYable job, but the pump nuts are often very tight, and can require a very large spanner to undo. It can be easier to remove the valves and pump together and use a vice to hold the pump nuts. If you go this far I'd renew both valves, and the pump sealing washers at the same time. Use decent quality valves, NOT the quarter turn screwdriver slot type, which often seize up in my experience.

 

[BN9Pete]

1. Its a gate valve

I think the arrow in the photo is pointing to the isolation valve ( cheap ballofix )
Looks like it’s being used on the bypass

 

[Benjy]

Thanks for the replies. Yes sorry my picture wasn't very clear - it's the smaller valve just behind the pump in the picture. I'm now pretty sure it's being used as a bypass valve in a half open position.

I'm thinking of replacing it with a Danfoss AVDO15 auto bypass valve. It's quite pricey, as it wasn't easy to find an auto bypass valve for 15mm . But am I going along the right lines? Boiler spec doesn't say it needs to be a 22mm bypass, just that a bypass is needed.

steadyon, those instructions are still super helpful as I have this problem too with one of the gate valves leaking from the gland.