DIY Doctor UK DIY Forums

Hi: I have a fairly mature microbore system. the upstairs radiators (especially the two larger ones) need regular bleeding. The header tank is positioned in a hotcupboard next to the main cistern and is quite low, only about four and a half feet above the radiator height, so I assume lack of pressure is causing the air problem. I do not want to elevate the header tank to the loft, but I have room to raise it maybe a foot and I thought I could maybe increase the size of the tank too.

Would this work, or is there an alternative answer?

Before you move the tank, you may want to examine the pipework layout as this, rather than the tank height, may be the route of your problem. Pay particular attention to where the feed from the tank joins the system and where the expansion pipe tees off the circuit.

If these points are not very close, the pressure in the circuit can cause either over pumping where water is ejected from the expansion pipe, or the opposite where air is sucked in. This is even more likely in a microbore system because of the reduced bore.

To prove the point, with the pump off, place a thumb lightly over the expansion pipe end. Have someone start the heating up and feel if a vacuum is formed for a second or two as the water starts to circulate.

For proper design, the expansion should be connected to the boiler flow at the highest point, and the cold feed no further than 6 inches away.

The height of the F&E tank above the system is unlikely to have anything to do with it.

"fairly mature" = sludge in system producing gases by chemical reaction and /or microleaks in pipework allowing air to be drawn in I would say. Gas/ Air settles at highest point - ie the rads which need bleeding. Time to think of a system clean methinks.

Alternatively you have other fundamental problems leading to water aeration. If the system is pumping over (simple test - is the water in the header tank hot) you will be continually cyclying aerated air leading to further corrosion leading to more gas. Causes / cures for pumping over; a) check there is no blockage where the feedpipe joins the flow - need to cut out and replace that section of pipe; b) pump operation speed too high - turn it down.

Another possibility is that there is a fundamental design problem. The correct sequence is boiler -> open vent -> cold feed -> pump. If its configured any other way you could be getting a condition known as see-sawing. That also leads to aeration but would have been there since day 1 and is the most unlikely IHMO.

Cheers

Thanks for the replies, guys - I am working through them. The first check revealed that a little water was making it out of the expansion pipe when the pump was switched on, so I guess that means the pump is too fast. NOw I need to get to that!

No water should ever, except in the event of a fault, flow out of the expansion pipe.

Slowing the pump down (there is usually a three position switch on the side) may stop the overflow, but may reduce the flow to the radiators significantly. Try it.

Remember, the height of water in the expansion pipe is the same as that in the tank. It may only have to be lifted by the pump a few inches before it overflows. You might try and raise the top curve on the expansion so the water has to be lifted higher.