Fill and expansion tank getting hot

[rcflyers]

I have a traditional central heating system that heats the radiators and also a hot water cylinder.
I went in the loft recently and noticed quite a bit of condensation.
I switched the central heating on and noticed that the vent pipe into the feed/expansion tank (the end of it is below the water level) is as hot as the radiators and the water in the feed/expansion tank is steaming hence the condensation.
It seems to be plumbed correctly looking at the diagrams I have found online.
The vent and feed pipes are tee'd next to each other on the pipe between the boiler outlet and circulating pump inlet.
Should the vent pipe be below the water level?
Any other ideas what the issue might be please?
Thanks.

 

[stem]

Heat can rise from the pipework below and warm the water in the feed/expansion cistern, and if that is what you are describing it's fairly common, especially if the pipe rises vertically up to the outlet of the cistern directly from the hot flow pipe. If that is the case, provided that the pipe is well insulated and the cistern insulated and sealed so that moisture doesn't escape into the loft it doesn't normally present a problem.

Sometimes a small loop is included in the pipe (shown inside the black circle below) to reduce the likelihood of heat rising.

However, if water is being pumped into the cistern via the vent pipe then that's a different matter. But from what you have said this doesn't appear to be the case.

 

[fixitflav]

I have a traditional central heating system that heats the radiators and also a hot water cylinder.
I went in the loft recently and noticed quite a bit of condensation.
I switched the central heating on and noticed that the vent pipe into the feed/expansion tank (the end of it is below the water level) is as hot as the radiators and the water in the feed/expansion tank is steaming hence the condensation.
It seems to be plumbed correctly looking at the diagrams I have found online.
The vent and feed pipes are tee'd next to each other on the pipe between the boiler outlet and circulating pump inlet.
Should the vent pipe be below the water level?
Any other ideas what the issue might be please?
Thanks.

Restriction in the pipe between the vent and feed, caused by build-up of sludge.
Edit - the end of the vent pipe should not be submerged

 

[Dereekoo]

Restriction in the pipe between the vent and feed, caused by build-up of sludge.
Edit - the end of the vent pipe should not be submerged

But is not over pumping so is not an issue?

 

[fixitflav]

But is not over pumping so is not an issue?

Where does it say it is not overpumping? With the end of the vent pipe submerged, it wouldn't be easy to tell.

 

[rcflyers]

Thanks, I have now cut the vent pipe so it is no longer submerged.
Will test and report back soon....

 

[fixitflav]

Thanks, I have now cut the vent pipe so it is no longer submerged.
Will test and report back soon....

The top of the vent pipe inverted U should be minimum 450mm above the tank water level

 

[rcflyers]

Cut the expansion pipe so it is about 2" above the water level and it's now working fine with no hot water ending up in the tank. Thanks again.

 

[fixitflav]

Cut the expansion pipe so it is about 2" above the water level and it's now working fine with no hot water ending up in the tank. Thanks again.

That's good.
If the problem had been happening for some time it's worth making sure your inhibitor is up to strength, as it will have been pumping aerated water round the system. Bright wire nail test needed!

 

[rcflyers]

Bright wire nail test?

 

[Dereekoo]

Cut the expansion pipe so it is about 2" above the water level and it's now working fine with no hot water ending up in the tank. Thanks again.

Where does it say it is not overpumping? With the end of the vent pipe submerged, it wouldn't be easy to tell.

Same place as it says it was over pumping I guess ,but cutting the pipe has stopped it from doing so for some strange reason so all is good?

 

[fixitflav]

Same place as it says it was over pumping I guess ,but cutting the pipe has stopped it from doing so for some strange reason so all is good?

Not with you. The only reference to overpumping I can see is in #2, "However, if water is being pumped into the cistern via the vent pipe...."

 

[fixitflav]

Bright wire nail test?

It's described in #6 of attached thread, to save me typing it out, credits to Mottie! Personally I use 3-4 nails, but it's not critical.

Inhibitor

 

[fixitflav]

My guess as to what happened, with the vent pipe submerged, is the air in the loop disappeared, dissolved in the water or bubbled out. Then you've lost the 450mm "protection" and some of the flow can easily go via the F/E tank. Similar to when rad return pipes are not all commoned before the cylinder return joins, causing rads to heat when HW called.

 

[rcflyers]

If the feed pipe from the loft tank was say 22 or 28mm what is the point of the expansion/vent pipe?
The feed pipe would both top up the water in the system when required and also allow for the hot water to "expand" back up into the loft tank.