How hot should water in a CH header tank be?

Yes. Only in extreme circumstances should any hot water be going up the expansion pipe to the header tank. In normal use, there should be no hot water going into the header tank.

Could it be the pump running too fast, and pumping the water up the expansion pipe into the header tank? I've seen that before.

If not then you've got a problem, possibly the over-heating of the DHW.

I think you could have solved it for me!

Turned the pump up to full throttle a couple of weeks ago when the heating started kicking in because I thought one of the downstairs radiators had so me sludge in it, just to see if it would heat up normally. I then left the pump running at maximum speed and forgot to turn it down again - it usually runs at the mid-setting. Oops!

Let's see how the system behaves after it's settled down again...

Many thanks!

That doesn't help but pump over may not be a constant flow/dribble but just a spurt when the pump starts up.

There was a steady continuous trickle of very hot water going into the head er tank from the overflow pipe when I looked this evening, which I guess co uld happen if the pump is running flat out. My wife and I were both working from home today and it was a cold and damp day so we had the heating on fo r a prolonged period.

I'll take another look tomorrow after the heating has been running for an h our or two now that I've turned the pump setting down - hopefully the hot w ater trickle will have stopped.

A lid is fine (and normal) - just make sure its not airtight

Nope, its doing what it is supposed to do - absorb the expansion of the water as its heated.

Lid sounds good.

ok if its coming out of the vent pipe over the top of the tank, then that is pump over and is undesirable. It will also get the content of the tank very hot.

The other solution to pumpover is to convert it to a combined feed expansion and vent configuration - where the F&E from the base of the tank is teed into the vent pipe shortly after it leaves the main pipework run.

That seems rather too "definite" a statement IMHO. The whole purpose of a of a feed and expansion tank is to absorb the system primary water expansion. Hence how much hot water gets into it, is more a function of the volume of the system and then tank. Lots of rads, small header tank will tend to also mean fairly hot header tank.

Sure that is a possibility and far more undesirable than normal expansion of hot water into the tank. Not least because it causes far more condensation and also forces more oxygen into the water.

Is it possible that the pump has been installed the wrong way round? I always understood that the pump should pump away from the expansion pipe, i.e. it should be 'trying' to suck water down the expansion.

That was what was wrong with my in laws system - another professional install!

Peter

If you get the pump the other way round, it does not usually make it pump over (at start up anyway[1]), but it may draw air into the system - especially as it starts up. That in turn means lots of corrosion since lots of fresh oxygen is being introduced all the time. (and blockages from corrosion can cause pump over)

Its a common arrangement that normally puts the Feed and Expansion pipe on the suction side of the pump, so that it does not draw air into the system, and then the vent on the output side. This does however make it relatively easy to pump over if the vent does not pass high enough above the high water level in the header tank before returning to the header tank. (lots of plumbers install the vent just hooked over the top of the tank, which means there are only a few inches of dynamic pressure required to push water up and over the vent looping a couple of feet above it is better)

My favoured solution is to move the vent to the suction side, but then tee the F&E pipe into it well below the top of the vent, that way when the pump sucks on the vent you have two pipes full of water ready to feed into it - hopefully enough to keep the air out long enough to let the pressure equalise.

Looks like good news - after turning the pump down from 3 to 2 and running the heating for a couple of hours this morning I've been up and checked the header tank and it's now running cool with no hot water overflowing into it.

I've rigged up a small heater and a fan up there and left the loft hatch open, which will hopefully accelerate the drying out process.

Many thanks to everyone for their advice!

David.

Wasn`t` there a horrific accident few years ago involving hot water melting the header tank and dumping boiling contents through ceiling?

There was more than one case IIRC, caused by the immersion heater thermostat failing and the water continuing to heat, going up the expansion pipe to the cold water cistern and gradually heating up all the water in the cold water cistern until the plastic cistern failed (split, deformed? I don't know).

It's why all immersion heaters should have a over heat cutout as well.

So not the CH header tank, no.

That was the main cold water cistern and a hot water cylinder boiled by a faulty immersion IIRC. Dumped 40 gallons of scalding water onto an elderly couple sleeping directly under it.

There have been a few cases over the years.

A 30 year old woman in 2003

A baby in 2006:

Part of the problem seems to have been cold water cisterns that weren't properly supported as well.

Yup, either propped on joists or studwork, or often sat on a bit of loft grade chipboard flooring, which does not maintain its integrity when soaked.