C/H expansion tank is hot!

[David J]

Visiting a friend today, I noticed that a lot of air was circulating with the water through the radiators. I found a key and bled the downstairs rads. Upstairs the rads were lukewarm, and one was full of air. I then heard an overflow happening from the loft tanks.

I climbed into the loft to investigate, and found that the DHW header was OK. However the C/H header was full of dirty water which was quite hot and overflowing the tank. The ballcock was half submerged.

I immediately turned off the pump, and rang the plumber who had installed a new pump in October. I assume that he checked the system before he left the property. He cannot visit until next week sometime, but said there must be a 'blockage' causing the overflow of C/H water into the expansion tank. He told me to isolate all the upstairs rads, and just run on the downstairs rads.

I have no idea how the upstairs & downstairs piping runs are configured, but found the situation so scary that I have left the pump off for now. This is a 1950s ex-council house with a back boiler gas fired C/H system, possibly added after the original build.

Any ideas what might have happened here?

David

[...]

Female friend by any chance?

[Tim W]

David J wibbled on Saturday 05 December 2009 21:59

I might be missing something but it's quite normal for the CH water to flow back into the expansion tank - that's why it's called and expansion tank ;->

Now, if there was lots of air trapped, then air expands more than water when it gets hot, so assuming some of the air got hot, it may have caused the expansion tank to actually overflow to the drain/outside overflow.

Under normal conditions, the water level will rise and fall within the tank. When it rises, it may do so with normally hot (not boiling) water. The ballcock should be set (arm bent usually) to fill the tank enough to cover the outlet to the CH. When the system heats, the water level will rise (typically a few litres) but will not overflow if the ballcock has been set right.

Did the plumber drain and refill the system when he changed the pump? Usually this is not necessary if the pump isolator valves are working, but either they weren't or he wanted to clean out and refill the system, perhaps with fresh inhibitor. If so, then even if the system is bled, it may need rebleeding later as dissolve air comes out of the new water.

On the face of it, it really doesn't sound that bad. If the water is rising and falling in the tank, it is not about to blow up. If the overflow is working then there shouldn't be any danger in running the system.

Can you continue to bleed the air out of all the rads and run normally?

I would bet that after the last of the air is gone that the tank will no longer overflow, but that the water level will rise and fall withing the confines of the tank. It is perfectly normal for the ball to be fairly submerged as it should be set low anyway as I mentioned above.

On an aside, if the water is dirty looking, it sounds like there *may* be a lack of inhibitor which should prevent iron oxide formation from the rads. Having said that, CH water doesn't always look too clean even with chemicals...

HTH

Tim

[Roger Mills]

Has this problem only started since the pump was changed, or has it always been like it?

*Why* was the pump changed?

Sounds to me like it's either pumping over (is water flowing out of the vent pipe back into the tank?) or it's sucking air through the vent pipe. If it's a new problem, either the pump is set at too high a speed, or it's fitted the wrong way round - and pumping in the wrong direction.

[David J]

Hi thanks for prompt answers here.

Lots of questions from Tim & Roger. And yes it is a lady!

1. Pump was changed recently because it was knackered and was making knocking noise. (It had been doing so slightly for the past 2 heating seasons, but my advice to get it changed was ignored!)

While I was away on holiday it got seriously loud, so a local plumber was called in. As I was not around I have no idea if he totally drained the system. He probably did because the old pump had corroded isolating valves which have now been renewed.

He certainly did not add Fernox or any inhibitor.

1. The pump maybe is set at too high a speed. I can reduce that and see what difference it makes.
2. It isn't fitted the wrong way round. It has been working OK for the past month or so.
3. I cannot believe that the header tank (that is the expansion tank

-right?) should ever be warm. The one in my own house is always cold, and at a steady level. The hot water must have come from the overhead pipe which is the expansion line. And then the surplus overflowed via the overflow pipe to outdoors.

1. Just before I left the house I also noticed another fault. The hall (the only one) thermostat is NOT controlling the pump. It clicks at the ambient temp, but the pump is not inhibited when the temp setting is reduced.

I am now wondering if this C/H system is simply over-heating and is relying on the programmable timer to not loose more water from the system.

I did successfully bleed one upstairs rad with the usual key. Masses of air in there. But 2 others need a large screwdriver to effect the bleeding. When did those radiators stop being used?

David

[Andrew Gabriel]

How do you know?

Could end up with insufficient flow throug hthe boiler.

The clue is in the name _expansion_ tank. As the water in the heating system is heated, it expands back up into the _expansion_ tank, and yes, that will be warm, or it wouldn't have expanded.

Pump is probably controlled by the boiler, and it will likely leave it running for up to a couple of minutes after it stopped firing.

Overflowing the expansion tank could be because there's insufficient clearance between the fill level and the overflow to allow the water to expand without overflowing, or it could be because there's a leak between the hot water system and the central heating system, which is usually in the coil in the hot water cylinder. If the level doesn't drop back to normal and stay there when the heating system is cold, then it's the latter (holed coil in hot water cylinder).

[David J]

I don't know about the maths and the volume of water in the system that has to expand when it is hot - but there is a length of vertical expansion pipework before the header tank. Surely the water expansion will mostly take place in there?

About the idea of a hole in the coil. Would this not make the domestic hot water as dirty as the radiator water?

David

[David J]

After further thoughts, please disregard the above comment!

[John Rumm]

No, because the main header tank is higher than the CH header usually - so the water in the cylinder is at slightly higher pressure than that in the coil. So fresh water enters the heating circuit, rather than the other way round,

[David J]

OK John, thanks for that info.

Going to revisit the problem this morning and look again with a 'cold' start before I switch the pump on again. I'll take a infrared thermometer along to see how the radiators heat up.

I'll see what the pump speed is currently set at, and I'll look into the C/H expansion tank and see how that has settled down overnight.

I'll use a big screwdriver and bleed the two odd radiators upstairs. I'm pretty sure they are full of air.

I'll set the hall thermostat at min, and leave it there initially. The C/H should not start at that setting, I know. If it does then the wiring to the new programmer need attention, I would think.

I'll report back how it all works out.....

David

[Tim W]

David J wibbled on Sunday 06 December 2009 01:13

The maths is easy - there are net expansion calculators on the web, or use the expansion coefficient of water (this varies by temp, but it's good enough to stab an average value in and see what the expansion is between say

15C and 80C. The harder bit is estimating the volume of the rads and pipe.

Nowhere near. That pipe will be "full" to the same level as water in the tank. You'll get a good "couple" (+/-) of litres expansion out of a typical system depending on volume of system.

[Harry Bloomfield]

David J brought next idea :

You are correct, it should certainly not be warm. Warm can cause moist air in the loft space helping to rot roof timbers. The tank is just there to allow some expansion, to keep the system topped up and to provide a steady pressure or head of water in the system.

I would suggest trying bleeding the system with the pump turned off, so that there is no chance of the pump causing negative pressure and sucking yet more air in rather than it being bled out. Once the majority of the air is bled out, then watch what is happening in the tank with pump running - first with system cold, then with it heating up.

[Harry Bloomfield]

Tim W formulated the question :

Much of which I would expect to be accommodated by the pipework on its way upto the tank and once the system has heated up, there should be no more expansion - hence little heat rising into the tank.

[Dave Liquorice]

No, think about it. The water in the vent pipe is connected directly to the feed pipe via the system. Water finds it's level, so the water level in the vent pipe is the same as that in the tank.

When the system starts heating up the water expands and that increase in volume is accomodated by water moving up the feed pipe into the tank. Raising the level in the tank and thus in the vent pipe but that is only a inch or so of pipe so not a significant volume compared to raising the level in the tank by and inch or so.

Correct. At worst the tank should only get a little warm. Hot is bad so is any flow from the vent pipe, even a regular drip or a short dribble when the pump starts.

[Roger Mills]

It's also worth checking that it *is* air which is in the radiators. It there's no inhibitor in the system, it could be hydrogen caused by corrosion. If what comes out of the bleed hole burns with a blue flame when you stick a match to it, it's hydrogen!

Is there a lot of black sludge in the bottom of the F&E tank? This would be another indicator of corrosion. If hydrogen *is* being produced, and is collecting in the radiators, it will be displacing the water - and could account for the rise in level in the F&E tank.

[Graham.]

If your system is fully pumped, and the DHW cylinder was still calling for heat, the stat would have merely closed the motorised valve on the CH side leaving the pump running to supply the DHW.

[cynic]

Back boiler - probably gravity hot water and pumped central heating. At a guess there could be a hole in the primary coil of the hot water cylinder. Is the hot tap water supply tank ball float valve continuously filling? The continuous introduction of fresh water with dissolved air in it will cause corrosion as well as the air coming out of solution when the temperature rises and being noisily pumped round the pipework.

[Dave Baker]

Hi David,

There are several possibilities for this problem and it's easy to work out which is the cause. I've suffered from all of them on my own system at one time or another.

1) Holed coil in the immersion tank. With the heating and hot water switched off this problem will still manifest as the water levels in both of the loft tanks trying to equalise. The level in the large cold water supply tank is usually higher than that in the small heating system supply tank and often the CW overflow goes into the HS supply tank and thence to the outside via that tank's overflow. If the HS tank is overflowing with no obvious inflow from the ballcock or the CW overflow pipe then a holed coil is present. Only clean water should enter the HS tank though.

2) Pumping over. If the boiler pump is at too high a setting or fitted the wrong way round, or indeed if the pipework has been badly planned so the vent pipes are on the pressure side of the pump, then the immersion tank or boiler vent pipes can pump over into the HS supply tank. However this should not cause the tank to overflow because no additional water should be entering the system. The overpumped water just finds its way back into the circuit again via the tank and keeps circulating. However the HS tank will of course get hot.

3) Ballcock set too high. The HS tank needs some room for expansion as the water in the system heats up. Usually you set it at about half the height of the tank and the ballcock will end up somewhat submerged. If the level is too high the tank starts to overflow as the water expands into it however it should then stop overflowing as equilibrium is achieved and the water in the tank should not get hot.

To those who suggested that expansion might cause the vent pipe to overflow into the HS tank, that I'm afraid is just silly. The water in the vent pipe and HS tank will always be at the same level because they're both open to atmosphere i.e at the absolute most this will be the level of the HS tank overflow pipe. Only if pumping over is occuring can water come out of the vent pipe.

So the most likely cause of both an overflow plus hot water in the HS tank is pumping over which will obviously only manifest with the boiler running. By seeing what happens with the boiler both off and on you should be able to determine the cause.

FWIW when my own system started pumping over and it couldn't be cured by reducing the pump setting I just capped the immersion tank vent pipe off with an old radiator valve. Expansion, how my system was arranged at least, could occur just as happily back up into the HS tank via its outlet pipe and the vent pipe was actually redundant. I fact I can't envisage a system which actually really needs a vent pipe although I'm sure that will lead to a storm of protest.

Finally when my immersion tank sprang a hole in the coil and while I was still in shock at the cost of a new one a lorry turned up with a brand new tank on it which someone else had ordered and misdelivered it to my house (or at least they left it with a neighbour because I was out). I never did find out who it was really meant for but was grateful to the plumbing gods for such a timely intervention.

[David J]

Thanks everyone for all the follow-up suggestions. However this morning's checks have revealed something unexpected.

I re-checked the new pump fitted in October. It is a DAB EVO VA55/130 with 3 settings. This one was set to 3, the 70W setting (the largest). Too high, I thought.

This pump is available for about £25, I note, so it's not the best on the market.

I then re-checked the fitting direction of the arrows on the casing, and I'm pretty convinced that is (as Roger suggested yesterday) fitted the WRONG WAY around.

The arrows are facing downwards. That is the direction of the boiler. Upwards is the direction of the hot water cylinder.

As I understand it, this will put pressure on the pipe to the boiler (and to the expansion pipe at the C/H header tank) and suction on the pipe to the cylinder. Please tell me if I am wrong here.

I decided to leave the pump switched off. I bled the 2 other upstairs rads & both were full of air.

This wrong pump configuration will surely explain:

1. excessive air in the upstairs C/H radiators.
2. some hot water being returned up the expansion pipe, and occasional overflowing of the header tank via the pipe provided. And the water in the header tank being dirty.
3. The C/H system 'working', but only just.

I will discuss all this with the plumber when/if he shows up tomorrow. The electrics & the thermostat are something else. His mate, an electrician, connected that up.

David

[Roger Mills]

Is it a fully pumped system, or is it - as someone has suggested - a gravity HW and pumped CH system? If the latter, it's not unusual to have the pump in the return leg.

How many water pipes connect to the boiler? Where are the vent and fill pipes connected into the circulatory pipework? If you could draw a diagram of the system and upload it somewhere, with a link posted here, it would be a great help.