CH Header Tank Overflow - or Not?

[TheScullster]

Hi all

As I was out-and-about, I thought it worthwhile to do a circuit of the house and see if anything looked awry.

The old ticker sank when I saw a long icicle hanging from one of the overflows from the loft. So I went up in the loft to investigate and discovered that the icicle was hanging from the CH header tank overflow, rather than the main water store tank(s). Lifting the (somewhat improvised) lid I found that the level in the tank was fine and that the float valve appeared to be sealing securely.

As this tank runs fairly warm, I reach the (hopeful) conclusion that the water escape/icicle is down to condensation in the overflow pipe (approx

1.5m long) rather than a failure.

Anyone else with a similar issue in these conditions? [In east yorkshire, where snow and ice are prevailing currently]

Phil

[Ian Jackson]

Doesn't the CH header tank act as an expansion tank for when the heating cycles from off (coldish) to on (hot)? This causes the water to expand and contract and, as it expands, some warm water finds its way back into the header tank.

I'm pretty sure that my header tank does get somewhat warm, and, in frosty weather, wisps of 'steam' can sometimes be seen outside, coming from the overflow pipe (protruding, just under the eaves of the roof). This happens mainly soon after the CH has fired up after having been off overnight. As the top of the tank is covered (but not tightly sealed), any hot water in it will tend to produce 'steam' at the overflow. However, I don't think it has ever produced icicles - possibly because, in the cold weather, I usually leave the CH on overnight, turned down a few degrees.

[Dave Liquorice]

Coefficient of expansion for water 0.00021 per C. (ish)...

20 to 70C = 50C so that is a 0.0105 volume increase. 1 cu m of water is 1,000,000 cu cm = 10,500 cu cm expansion or 10.5l... Now I doubt 'yer average 3 bed semi heating primary has a cu m of water in it, maybe 1/4 of a cu m but we are still looking at 2.5l (2,500 cu cm) of expansion. Rather more than a "few cc".

I know when I put in a new expansion tank here it needed more than the normal 10 gallon one. Think I put in 25 gallon and the level in that varies about a foot hot to cold. This is a big system though, there are 12 gallons of water in the boiler alone...

[Ian Jackson]

Of course, there's also simple convection. Some of the hot water from 'down below' is bound to find its way up to the header tank.

[Ronald Raygun]

Perhaps instead of a few cc he meant a few cm. If the footprint of the tank is (say) 40cm x 30cm, that's 1200 sq cm, so adding 2500 cu cm to that will raise the level in the tank by only about 2cm.

[Dave Liquorice]

Feed pipes are normally 15mm, and should be connected to the bottom of the boiler. In the 3 bed semi with tank in loft and boiler on ground floor that is quite a length of relatively narrow bore pipe to get convection in.

10m of 15mm tube holds about 1.4l of water, quite possiby less than half the expansion of the water of the primary.

[dennis]

I don't know what you fitted as a system but mine has

[Ian Jackson]

The pipe may indeed be narrow but, with the CH on 24/24, some of the hot water must eventually find its way 'up top'.

[Andy Champ]

It isn't hot by the time it's spent a couple of hours working its way up a 15mm pipe full of cold.

Andy