MHRV drain non return valve needed

[Colin Green]

I've recently had a Mechanical Heat Recovery Ventilation system fitted as part of a virtual rebuild of my house. It's great, apart from one thing. Occasionally it gives off sewer smells when it is on boost. Google has helpfully told me that it is because the condensate drain pipe is plumbed into the main foul stack. If the U bend in the condensate pipe is dry, it can draw air in from the stack.

Can I get a non-return valve for the plastic pipe used for the drain? It looks like about 20mm pipe. I've looked at B&Q etc and can't find what I need. Thanks.

 

[Malc]

You will probably need to use converts to change 22mm to 32mm and back so you can have a trap. I guess they might make a 22mm trap but getting one is another matter. Alternately why not just make the 22mm loop down and back to make a running trap yourself? 75mm should be enough. Or you could dump the condensate to the rain water pipe. BTW MHRV instruction will have said fit a trap if connecting to the soil stack.

FYI the best option is to go to rain water as the trap can dry out. For the rain water connection (i.e. into gutter or hopper) you don't need a trap of course as that's in free air. No trap to dry out. The condensate is on the outlet side so should always be positive pressure so it's odd you get the problem on boost. Are you sure it isn't the air blowing into the stack that is pushing the smells out?

 

[Colin Green]

The drain has 4 90° bends to form a trap, but as you say, it can dry out. It is connected to the stack as the stack is inside (with a thing on the top to only let air in). There isn't a convenient rainwater drain as it is more or less central in the loft conversion.

When it happens, the smell is around the inlet vents and not near the extract vents. But you're right, it's the warm moist air that is cooled and condenses. I hadn't thought of that. I guess the problem is that unlike a sink, there isn't always a flow of water to top up the trap.

 

[Malc]

You can get waterless traps. Screwfix have then for £21, and you'll also need 2 32mm to 22mm converters

Update:

Or maybe this would suit better