What the .... is that! Help needed.

[Marvin]

My neighbour has a bungalow with a feed and expansion (F&E) tank at the top of his loft.  It was completely cleaned out a few month ago (it had black gunge in it). About 2 months later the overflow started dripping. On inspecting the tank it was "mould growth" that stopped the ball valve from working.

 

I think this time it was white mould but I will check with him.

 

What is causing the mould? How can it be stopped?

 

Details:

 

Typical vented central heating system with radiators and oil boiler.

Hot water pump, 3 port valve and hot water tank (HWT) in loft!

Water treated with inhibitor

Standard size F&E tank.

Don't think the tank has a proper lid but it does have minimum insulation.

 

My guesses:

 

Pump pumping so hard that water running out of expansion pipe and going down feed pipe causing the tank to become a Radiator.

Inadequately vented loft causing a breeding ground for growth and suitable warmth for growth.

Feed pipe installed causing heat to rises into F&E tank

Port valve, and pump (and HWT which has no where else to go in the bungalow) shouldn't be in the loft. (I would never put a HWT in a loft!)

 

Has anyone come across this before, and if so what did you do?

 

 

 

  

[JohnMo]

Level in expansion tank would only drop if the central heating system is full of air.

 

Inadequate ventilation would be my guess, high humidity area around tank, has he checked the eaves vents are not blocked with insulation?

 

Feed pipe to F&E is supposed to be a vertical pipe, so will always transfer some heat.

 

You can install a cylinder in a loft, but heat losses in winter would be quite high and all wasted.

[Radian]

1 hour ago, JohnMo said:

Level in expansion tank would only drop if the central heating system is full of air.

 

 

It also increases the chances of pump-over as the change in pressure when the circulation pump starts and stops gets stored in the "springiness" of the trapped air and bounces back. This is particularly acute if the vent pipe is restricted (either by being too small a diameter, or if crud has lodged in the T-off from the system). I've had systems that would have intermittent pump-over that could be temporarily cured by bleeding.

 

The sludge that was in the tank indicates corrosion so frequent refilling has probably been happening for some time. I'd bleed the system, then close the isolation tap to the F&E cold supply and frequently monitor the level in the tank for a while. I'd also be looking around for leaks from pinholes, poor joints etc.

 

just remembered to add that if it has been pumping over, it makes a lovely lot of steam that condenses out all round the tank and provides perfect mould growth conditions. 

Edited August 4, 2022 by Radian