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We have been in our house for 5 - 6months now and never had a problem with the water draining out of the kitchen sink...until now!

Under the sink we have the usual connections where the washing machine outlet joins the sink catchment pipe (U-bend) where it connects to another connection, connecting to the pipe running through the wall into the outside drain.
Recently the water in the sink takes longer to drain, or if we run the water from the tap into the sink it will drain up to a certain point, whereby it will backflow into the sink again.
We have also found that when the waching machine reaches certain cycles and discards some of it's water, that it will backflow into the kitchen sink, which leaves a nasty PONG in the air. It does eventually flow away.
We have also disconnected the catchment pipe (if this is the correct term) and poured a cleansing chemical (acid) down the shute which dissolves any muck in the pipes. This still does not solve the problem.
At times, there is also a waft that comes from the outside drain into the house, but we can't check it out, as the conservatory is built onto the dining room and kitchen, thus the outside drain is under the conservatory floor with the pipes boxed off.
Any ideas on a solution or what the problem might be? Could it be a gravity problem, connection pipe set-up or muck in the pipes?
Please help!

id go for a blockage as the main cause
try and disconnect all the connections under the sink and clean them out
try finding a external manhole cover and see what the water flow is like
you may need to get some drain rods and rodd it
if possible try sticking a hose pipe down the pipe and blasting water thru

[quote="bobplum"]id go for a blockage as the main cause
try and disconnect all the connections under the sink and clean them out
try finding a external manhole cover and see what the water flow is like
you may need to get some drain rods and rodd it
if possible try sticking a hose pipe down the pipe and blasting water thru[/quote]
Thanks for the info bobplumb.
I tried it all last night. Removed all the connections, checked for blockages etc. Unfortunately I could not check for the manhole, as the conservatory floor is still over it, for the moment.
We did also do a load of washing last night, and the washing machine water was backflowing into the sink; this was until I decided to twist the outflow hosepipe from the washing machine around, so that the big loop was above the outflow. This immediately solved that problem. It seems that the water used to flow out upto a certain point & due to gravity, it might have flowed back into the washing machine hosepipe, pushing water back up the kitchen sink when it became too full.
When I then tried flushing water throught the sink outlet, it flowed well up to a certain point, but slowed down again, taking it's time to empty. I will tackel that problem this weekend.