CONSTANTLY smelly drains

We have just moved into a large house, and didn't notice a smell before we bought it. We are putting less water through the drains than previous owners though. Outside drains are running freely but show 'tidemark' evidence of earlier backing up. We propose to get these cleaned, but think the smell shouldn't get up past the traps. Are enzyme treatments and such a waste of money? House is 10 years old.

There is a constant sour drains smell in the main bathroom and adjacent airing cupboard (only heating pipes in here). There is a boxed section, presumably a Durgo valve, in the bathroom. Top of this is well higher than WC cistern. Can only think Durgo is leaking air, and that air is free to travel into space between the floor and ceiling and gets out in airing cupboard. The only visible bit of waste plumbing is the hand basin's, and it doesn't have an air admittance valve on the trap. Yes it could be dirty overflows, so I cleaned them, and why's it getting into the airing cupboard? And why is it constant?

This is not quite the full on sewer smell. I'd describe it more as a cross between sour wet dishcloths, accute BO, strong coffee grounds, and emulsion paint! UGH! Despite not drinking coffee, admitting to BO or putting paint down the sink, I get it (different sewer) in the kitchen sink occasionally and Mr Muscle foamer sees it off. But not this.

Would there be an anti rat device in a 10 year old house? Do you think something has died in there? No, I remember I forgot to check a mousetrap in another house once and found a dead mouse, poor thing. The smell was richer somehow. Dear God

All pipes and drains run freely, no gurgling or similar. Each behaves seperately, no backing up or bubbling elsewhere. No smell in downstairs loo below (which I presume shares the sewer pipe).
There are two other spurs from the house to the main sewer, and toilet/sinks off these do not smell either.

I have tried proprietory drain cleaners and unblockers.
No evidence of any leaks in ceilings below, but have not checked behind closed structures.

Grateful for your opinions!
Thanks

A leaking AAV would account for the smells - can you get to it to put a carrier bag over the top and seal with elastic bands?

Also, your presumption about smells not coming through water seals is wrong - if you have inadequate provision for soil venting then even with a correctly functioning AAV you'll get smells.

It's a common fallacy: AAVs do not obviate the need for a vent - they merely admit air, and don't relieve positive pressure in the sewer. If, for example, a nearby house has been altered (or built) then the balance of pressure in the shared sewer can change.

Also, there might be more than one problem. If the smell you're getting is 'rotten', rather than just 'farty', then then may still be a partial blockage underground (leading to the tidemark that you mentioned).

Some rudimentary checks would be sensible - lift a manhole cover and get someone to flush two sheets of loo paper while you watch to see if they emerge in a good flow of water.

Hi Softus, and Thanks.
I didn't know smells could get past a water seal - that's worth knowing!
We have had the drains cleaned and things improved a bit, but it is still there.

Pressure. There are 16 houses on the sewer, each with 3 or 4 toilets ( ridiculous I know!) and if they are like our house, there is at least one stack with a vent above the roof per house, so I hope there isn't pressure building up.

Not sure whether this next snippet of info was in my last post, but I do now notice that if I put bleach down the downstairs loo (below the smelly ones and on the same stack) I can immediately smell the chlorine in the airing cupboard and bathroom above, which sounds like freely moving air from the stack getting out, to me.

I will have to break into the boxed in section in the bathroom ( I do wish they didn't do that!)to reach the AAV, but I think it may have to come to that.

Thanks again for your advice
B