C/H expansion tank is hot!

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Female friend by any chance?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
--
Tim Watts

This space intentionally left blank...
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In an earlier contribution to this discussion,

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.
--
Cheers,
Roger
______
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 5 Dec 2009 23:08:19 -0000, "Roger Mills"

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.
2. The pump maybe is set at too high a speed. I can reduce that and see what difference it makes.
3. It isn't fitted the wrong way round. It has been working OK for the past month or so.
4. 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.
5. 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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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).
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 6 Dec 2009 00:24:37 +0000 (UTC), snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

After further thoughts, please disregard the above comment!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
David J wrote:

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,
--
Cheers,

John.

/=================================================================\
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 06 Dec 2009 04:31:37 +0000, John Rumm

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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.
--
Dave Baker



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Dave, many thanks for this very comprehensive reply, which I have printed out. My thoughts now are that your Item 2 covers this problem.
#Whether the pump is in backwards we will establish later with the plumber. Now, after reading all the replies when folks realised the it was a gravity/pumped system, I'm not so sure again.
What I can do this morning is to restart the system, with the pump on the lowest speed setting, and see how it all performs. I can check what is going on in the header tank as well.
David
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In an earlier contribution to this discussion,

May I suggest that when you first turn the system on, you set it to HW-only mode and note whether or not the pump starts. If it's a fully pumped system, the pump *will* start on HW-only. If it's gravity HW and pumped CH, the pump won't run until you also turn the CH on. That should help to determine what sort of system it is.
--
Cheers,
Roger
______
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
--
Tim Watts

This space intentionally left blank...
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 06 Dec 2009 09:52:05 GMT, Harry Bloomfield wrote:

in

of a

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.
--
Cheers
Dave.




Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wibbled on Sunday 06 December 2009 09:52

That would be hard as the pipe to the tank is full even when cold. To do what you propose would require a magic ballcock that cut off when the water was several feet down the pipe below the base of the tank!
--
Tim Watts

This space intentionally left blank...
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
It happens that Tim W formulated :

No magic involved...
The boiler, rads, pipe and tank will of course all be full of cold water initially. As the system heats up there will be point at which the water in the pipe will hot below a certain level and cold above it. As the water warms and expands, the interface between the two will gradually rise up that pipe.
Thermo mixing will be a minimum in a narrow pipe, unless and until the hot water expands up to the tank, where it should quickly mix with the cold water in there and the tank will then cool. A warm header tank means something is wrong.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wibbled on Sunday 06 December 2009 22:01

OK - I see what you are saying - my bad. I though you were saying there would be no appreciable increase in the water level in the tank. You were actually saying the tank shouldn't get hot.
My apologies...
--
Tim Watts

This space intentionally left blank...
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tim W was thinking very hard :

No problem!
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.