What exactly does a header tank do?

cannugec5

Please keep this simple!
Our overflow pipe is overflowing so Hubby ( 76 and not very agile) went into the loft to investigate. 

The float in the header tank was underwater but not on the bottom of the tank. He raised it manually and it stopped the incoming water. 

However it stopped it below water level. 

Also the overflow pipe was leaking in the attic (as well as going outside) and the fibreglass insulation is wet. 

Plumber/ heating guy can’t come for a few days. 

In the meantime Hubby has taped round the leak and the water is no longer flowing. 

But! I can’t get my head round what this tank actually does. I’ve read that it tops up the central heating to allow for expansion. Does that literally mean when radiators get hot, hot water back tracks up into that tank? And conversely when they are cold it  goes back down the pipes and fills them up again? 

We are soon (agreed to proceed but awaiting dates from heating engineer) changing our entire oil fired system to an ASHP. Will the header tank still play a part in this ? In which case it needs a permanent fix rather than a botch, or would we likely have a completely new and different header tank system anyway? 

Our current upstairs radiators always gurgle prolifically and have cold tops despite hubby bleeding them regularly. To me that means air is getting in the system. Could that also be connected with this header tank issue? 

Section62

cannugec5 said:

But! I can’t get my head round what this tank actually does. I’ve read that it tops up the central heating to allow for expansion. Does that literally mean when radiators get hot, hot water back tracks up into that tank? And conversely when they are cold it  goes back down the pipes and fills them up again? 

Yes, although ideally the water which moves up towards the "Feed and expansion" (F&E) tank will be cold rather than hot.  The actual amount of water involved is relatively small - the total amount of water in a typical central heating system is only a few tens of litres, and the expansion of that volume when heated from (say) 20C to (say) 60C is not that great.

It would be a different story if the water starts to boil - in which case the first sign is likely to be steam coming from the "vent" pipe (the one which usually curves up over the top of the F&E tank with an open end discharging into the tank).

If the float valve fails and you get a constant flow into the F&E tank then the water in the system will have more air in it than normal, so this isn't ideal.  But a more likely cause of the problem with the radiators is probably corrosion causing a build-up of gas.

If you are having a major upgrade of the whole system I wouldn't spend too much time and effort fixing this issue.  The F&E tank will probably go, and you can ask the installer to give the whole system a good flush.  There's also a possibility they will need to upgrade/replace radiators to work more efficiently with the ASHP so there is little point spending much money on the old system until you have a better idea of what will be kept and what will be scrapped.

ThisIsWeird

Hi Cannie.

You've pretty much nailed it.

Yes, when water is heated, it expands. So, when your CH comes on, the largish volume of water in the rads and pipes will expand by a fair amount (a few litres?) and that will need to go somewhere if it isn't going to burst open a rad And when the system cools down, this water returns.

The correct water level in the tank, then, should be a few inches below the overflow outlet. When the system expands, the level will rise, but should still stop short of overflowing - 'overflowing' is a sign of summat gone wrong.

The ballcock is there to top up the tank when needed, say after a few months when some water will have naturally evaporated, or when a radiator has been bled. The ballcock does not (should not) be operating during normal use.

Your new ASHP system will almost certainly be getting rid of all that 'old-school' stuff, and you will have a 'sealed' system (which will have its own way of coping with expanding water).

The float in your tank went below water level? Almost certainly the 'ball' has a leak, and had filled with water. Assuming it's plastic - is it bright orange? - then it's simple to screw on a replacement. I would suggest you do do this, as you will want it able to top up if needed.

The air in your rads - classic cold tops - will likely be caused by a separate issue, most likely something like a build up of sludge in the pipes, or just a poorly-designed pipe layout. This can cause a negative pressure drop behind the circulating pump when it fires up, and can be enough to 'suck' in a gulp of air via the expansion pipe (the one that Hubby sees arching over the F&E tank, and pointing down in to it). Since your system is being replaced, I wouldn't worry about this - it'll soon (hopefully) be history. I suspect - but obviously don't know - that your system will have significant sludge in it, as air being drawn in will compound any existing corrosion going on. If you are keeping the existing rads with your ASHP, your system will likely require a very good clean :-)

A few splashes of water on your insulation should soon evap away, so I wouldn't worry.

If you want to lower the water level in the F&E tank, then you can do this by bleeding a rad - it'll take a while to get a litre out, tho'! - or scooping water out and in to a bucket (could be awkward up there?), or by making a funnel to stick in the overflow outlet in the tank, and cupping the water out that way, or via a drain !!!!!! if there's one fitted somewhere on the lowest pipe in your system - drain off 2 or three litres. 

jennifernil

Is this definitely a tank for the central heating rather than for the hot water?

GDB2222

jennifernil said:

Is this definitely a tank for the central heating rather than for the hot water?

How big is it?

cannugec5

Thanks for all your replies and suggestions. Our heating engineer got back to us that a friend in the same business might be able to help today. 

He’s been and sorted the problem. 

The float was actually not the issue. The pressure was too high in the CH system causing a large flow into said tank every time the heating kicked in. Too much water was overwhelming the float and causing the overflow. 

Simultaneously the overflow pipe was leaking indoors . Enough water that it is now showing through the ceiling of the garage and we have a puddle on the floor! We have a bedroom above that garage but because of the design of the house we are lucky it is unaffected. We have what is known locally as a one and a half storey house. The header tank is in the attic above the bedroom but the leaky overflow feeds through the eaves at the edge of the bedroom so we are very lucky. 

Today the guy has reduced the CH pressure and also replaced the leaky pipe. 

He showed me the pipe after he removed it. It is a plastic pipe with what looks like a brown tarry fabric wrapped round it. He suggested that was a previous fix. We’ve had the house over 20 years and it wasn’t done in our time…

cannugec5

Oh, should add Hubby has been round and bled all the radiators again now! 

ThisIsWeird

Thanks for the update.

I wonder if, by 'pressure too high' he meant the circulating pump was set at too high a speed?

Anyhoo, very pleased it's fixed.