New toilet fitted - low water level

1 Sep 2005
Reaction score
United Kingdom

Just had a new downstairs toilet fitted as part of an extension. The toilet flushes OK but just after it has flushed it makes a kind of popping noise, and the water level is low in the bottom of the pan.

We've lifted the outside manhole cover and the drain appears to be clear, however the builders may have let the odd bit of rubble drop down the drain that leads from the toilet. Could it be blocked or is it a venting problem ? And why the weird 'pop' ?

Also, I think, but can't be dead certain, that the water level in the existing toilet upstairs is also a little lower.
Sponsored Links
losing the trap seal sounds like a vent proplem to me
(i have been wrong in the past)
Hmmm - so how easy is that to fix ? The soil pipe runs through the roof space and then out of the building.
I have been wrong before and will be again, but I am sure you have a problem that could be either blockage or venting related. Have you spoken to the builder about the problem? If so what was their response? Was the work done under building control supervision?
Its hard to give detailed advise without seeing the drain/stack layout
Sponsored Links
I spoke with the builder yesterday - they checked the drain under the manhole and said they didn't think they had caused the problem !

The plumber is coming later in the week - I've mentioned it to him but he seemed a bit vague about what the problem was.

Yes all works to date have been inspected by the building inspector - but she hasn't been to sign them off yet.

The old soil stack was cast iron and we had it replaced by a plastic one. It only links into the upstairs toilet - it drops down into the roof space of the new extension (which is single storey) and then drops down again to the drain via the outside of the extension. Think it has two bends in it.

The downstairs toilet empties straight out of the building and into the drain. Doesn't go into the new soil pipe at all.
So it sounds like the upstairs is working fine. The downstairs one is not. By the sounds of it, the downstairs one shouldnt need venting so your problem is either a blockage or an installation fault on the drain. ie falling the wrong way!
Has it ever worked correctly?
If so for how long?
Also where the new wc joins the drain is there a new connection in the manhole or have they used an existing one which was in the area of your new extension. I'm wondering if they have connected to a trapped gully, which would create problems.
What do you mean by the drain falling the wrong way ?

From memory, I think they put in new pipework that linked into the existing drain - does that make sense ?

The pipe that comes up from the floor to meet the toilet is definitely new.

It was only installed on saturday - so has always been like this.
When i say falling the wrong way i mean just that :LOL:
The drain should be sloping downhill all the way from the wc to the man hole with a fall of about 1:40.
This is because of plumbings number one rule."water flows downhill"
If for any reason the underground drains have been installed flowing the wrong way ie uphill water will still flow but blockages will always occur.

Normally a builder would do the underground drains leaving a connection for the plumber to fix to. It is unlikely in my opinion to be a plumbing issue but a fault with the drains. Did the builder stick drain rods up the drain from the manhole ?.

It should be possiblefor the builder to remove the toilet and drop a suitable sized ball down the drain which if all is clear should roll all the way to the manhole( I have never had to try this myself but should in theory work,,( anybody else any views))

Talk to the builder again because there is obviously a problem that needs resolving.
Maybe the builder should contact a drain company with a camera to inspect the drain run and see where the problem lies.
You could be right about the drain. The drain under the manhole cover isn't very far down from the surface. The toilet is located about 2 feet away so I guess the drop from the toilet to the drain might not be very steep as it hasn't got far to go.

Not sure how we could resolve it - probably expensive to relocate the main drain lower down, and not easy to raise the toilet above ground level.

The plumber didn't fix the pan to the floor so it will have to be removed again anyway - I'll ask him to check for any possible blockages and pick his brains about the drain issue.

What would happen if it couldn't be resolved - is it still OK to use ?
Indeed it could be venting/blockage problems... To check the manhole and flush the wc....Blockage(partial probably) find the last manhole before the road/main there is an interceptor (trap) in there They are the devices of the devil :p . The 2 feet from the wc to the manhole is ok . that water will rush down there so fast it won`t touch the sides :eek:
Forgive me, but what is an interceptor (trap) and what does it do ?

From what I can see the drain goes down my drive to a water meter, there is no manhole cover between the one outside the toilet and the meter.

What is venting and how is it checked ?

Are you saying the blockage could be further down the drain as opposed to between the toilet and the first manhole ?
Forgive me, but what is an interceptor (trap) and what does it do ?
It`s like a wc trap in the pipe run just before the sewer. They always get blocked :evil:
From what I can see the drain goes down my drive to a water meter, there is no manhole cover between the one outside the toilet and the meter. ?
Meter position is coincidental ;)

What is venting and how is it checked ?
The stack must be vented open to air...I know you said it goes out thru` roof , but can you actually see it ...There is a tendency to fit Air Admittance Valves everywhere . Lifting the manhole (then flushing) vents the whole system pretty much so if things change then in the way the wc`s gotta venting problem

Are you saying the blockage could be further down the drain as opposed to between the toilet and the first manhole ?
Yes indeed . A camera survey will tell you exactly what`s going on in the pipe -all the way to the sewer.
The soil pipe is open to the air and yes I can see it. I've also fitted a cage thing on the top so birds, etc can't get into it. When we lifted the manhole and flushed the toilet nothing changed.

So from you've said I guess we have a blockage, or the drain isn't sloping enough from the toilet.

I might speak with the builders again as last week they capped off the former soil stack opening into the drain and also the roof down pipe entrance into the drain (it's been re-routed). When I asked how they would cap them off they said they would pour concrete down them !! Perhaps they have blocked something.
From reading this, does the new downstairs wc go straight to the manhole in this 2m length of pipe, with no vent. It may be that the flushed water 'plug' is just pulling the water seal out of the trap, making this pop noise, as it can't vent itself.

If you had a wash hand basin in the same room, the waste for that would connect to the 100mm soil pipe (behind theloo somewhere) and would have a anti syphon trap incorporated with the basin trap. This would have allowed air into the pipe following the water plug.

Does anyone know if you can put an air admittance valve in at the back of a wc? If so would that work!

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links