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Condensation on the inside of toilet lid

We have had property entirely replumbed and have experienced many problems with poor workmanship which we're getting resolved.
The bathroom gets quite hot as the hot water tank is situated in a cupboard behind the bathroom.
We have installed a new extractor fan that has a humidity detector and auto-operates.
On some occasions we've lifted the toilet lid to find a significant amount of moisture on the underside of the lid. But never on top of the lid or elsewhere in the bathroom.
This suggests to me that either the water inside the bowl is heating up and is evaporating. Or perhaps the cistern water (concealed Grohe) is getting hot and is flushing the bowl with hot water which is then evaporating.
It's definitely not splash back when flushing.
We also get some grey water feedback occasionally. When the toilet hasn't been used for a while, we return to find the water looking grey. Originally we felt that this was because the builder was disposing of materials (grout, etc) down the loo but it's being going on a while now.
Our builder is dismissing my concerns (always does) but we need to resolve this somehow. Any suggestions?

4 Answers from MyBuilder Plumbers

Best Answer

Hi Barbara, thanks for the question.

"The bathroom gets quite hot and the hot water tank is situated in a cupboard behind the bathroom." Make sure there is lagging on the tank and that it does not exceed recommended operating temperature... unfortunately you do not say how it is heated or type of tank.

You are concerned the water in your toilet tank is warm... try a thermometer in this cistern water and then water which has been run into a cup from the nearest cold tap. Obviously, should be the same.

Water can condensate on the inside of toilet lids but it may also be water that is splashing up... flush the wc with the lid off and have a look. Place a piece of card over the top and look at it after you flush... splash marks?

You do not elaborate on the gray water issue so I'm unable to say... however, "gray water" is a term for used water such as sink / bath water. If this is backfeeding into a toilet it can only mean that you have a main drain blockage.

Water can of course look a kind of gray due to light and reflections from the room or china pan... but it will be a clear, not cloudy / opake gray, allegidly.

Good luck... Bob.


Answered 27th Nov 2019

Hi your toilet should have a straight cold feed from the mains feed have you checked the water in the bowl to see if it is warm check the cistern connection and where it comes from.


Answered 27th Nov 2019

Hi Barbara
If your bathroom is very warm always you can get condensation on your toilet as the cistern is cold water and the u bend of toilet is cold water as the warm air hits the cold surface this could give a build up of water particles have you got a wooden toilet lid which normally stops this. But the grey colour water needs looked at asap which does suggest blockage or waste pipes fitted incorrect.


Answered 28th Nov 2019

Hi Barbara,

Your toilet is basically a big bowl of cold water in what sounds like a very warm room. There will always be condensation as a result.

Ensuring that the tank is correctly lagged / jacketed will help but may never fully eradicate the issue.

The “grey” water would indicate the presence of something foreign. Depending on how your system is fed it would be present from other outlets If from a cold water tank or mains. I suspect the builder is not being totally honest ref disposing of materials still and you may find that if it’s concrete / plaster they’ve been putting down there then some of it may not be clearing the U bend of the WC as it’s heavy / dense which is why it shows occasionally.

Hope it all gets better soon.


Answered 29th Nov 2019

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