Air in CH system

[Nick]

Conventional fully pumped, vented, heating system. Storage, F+E tank & DHW cylinder in loft.

24 rads. All seems to be in good order and working correctly.

Last July I drained the system to replace the failing circulating pump. Refilled the system, inc. inhibitor, and bled all rads also the two bleed valves in loft (1xCH, 1xDHW).

System working fine except that there is constant air in the CH. I would have expected this for a short while in a fairly large system, but not 6 months.

Trapped air congregates fortunately in one rad only but can also be heard in the return to boiler.

The offending rad is on a long drop (about 4m). No other rads are affected. I'm surprised that air is being forced down a 4m drop as there are other rads in the same circuit, both above and below, which are air free.

The F+E tank is ok, ballvalve works correctly and there is about 4" of water above the outlet to system. No sign of overpumping and have tried the pump on different settings.

(Thinks: changed pump. Could air be leaking via the central screw in pump?)

Any ideas please?

Nick.

[newshound]

Water leak under ground floor. Being refilled with fresh water + air from header tank.

24 rads is a fairly big system, would need about three tubes of inhibitor.

Wouldn't expect in-leakage at pump.

Collection on low rad is unusual but I suppose pump delta P could be responsible? I had a system once where you had to turn the pump off before bleeding otherwise it would just suck air in from the top rad.

[cynic]

If the pump is in the return leg I suspect a higher performance pump - probably drawing air in because of greater suction than the old one.

[Steven Langdale]

Are you sure it's air?

Next time you bleed it, do it gently and try and light the air as it comes out. If it burns (or sometimes pops) it's hydrogen and means you don't have enough inhibitor.

Steven

[Man at B&Q]

I had Hydrogen. Drained, flushed, refilled with plenty of inhibitor. Now after a month I have air, like the OP.

Biggest worry is it might be the pipes under the concrete ground floor, but there's no sign of damp from them.

MBQ

[Roger Mills]

That sounds unlikely, BUT since the problem seems to be related to the pump change (unless it's just a coincidence), I'm wondering whether the new pump is installed the same way round as the old one. If water is circulating in the opposite direction, funny things might happen.

My first thoughts were too high a pump speed (or a more powerful pump) causing pump-over in the F&E tank - but you seem to have eliminated that possibility.

Others have suggested a leak somewhere where you don't see it - with the lost water being made up by introducing fresh oxygenated water. You can test for this by tying up the ball valve in the F%E tank and seeing whether the level goes down when no new water can get in. [Allowing for the normal cold/hot level difference, of course].

Incidentally, how much is the level below the overflow? If the tank overflows when hot, then takes in fresh water when it cools . .

[Nick]

Thanks very much. I have checked every inch of pipework and there are no visible signs of leakage. On refilling with fresh water I would expect some air in the system but not after bleeding regularly for 6 months. I always shut the system down and let it settle for a while before bleeding. Also set the motorised valves to manual open, just in case.

[Nick]

Your new (more powerful) pump is drawing air down the expansion pipe because the pipe layout is wrong. If you look at your pipework layout and sketch it out you will see what is happening. You will see that the cold feed and expansion pipe are on opposite sides of the pump, they need to be on the same side. ( In effect).. You can confirm this by putting your thumb over the expansion pipe outlet (over the tank) while some one turns the pump on. You will feel the suction. The converse of this is when the system "pumps over" ie water comes out of the expansion pie into the tank. Again wrong pipe layout. Unfortunately the only cure is to turn the pump speed down or sort out the pipe work. (By moving the cold feed or expansion pipe so they are both on the "boiler side" of the pump)

That's odd because the system worked perfectly from the time it was installed until the pump change. The new Danfoss pump is exactly the same as the original, not more powerful. Although the original pump may have lost efficiency over the years.

I have also tried the pump on all available speeds (for a period of a week per speed). This has absolutely no effect.

[Nick]

Thanks, I have already tried this. There was about 100mm of water above the system inlet, I have increased this to about 150mm. I might try shutting off the water supply to the F+E tank and watching it closely to see if there is any level change over a few days.

[Nick]

Thanks, The new pump is a direct replacement for the old and is definitely pumping in the same direction. The level is about 100mm below the overflow and there has never been any overflow. I will see what happens when the water supply to F+E is turned off.