Roof vents, ductwork and exhaust air in-line drain

[Hastings]

I have got my 1st fix install kit - everything except the MVHR unit itself (Vent Axia Advance S) - from BPC. They have been great.

I have never installed MVHR before.

It will be sited in a cold roof loft, above a corridor/landing that separates two bedrooms. Supply and exhaust vents (supplied by BPC) in a new sarking boarded slate roof.

BPC provide a very good install guide pdf and almost everything about how to install seems clear except for some of the detail of how to connect the unit to roof vents.

There are 2 bits in the guide that puzzle me:

 

1/.

"Roof vents:

• In addition, roof vents must be installed at a higher level than the MVHR unit or condensation will form in the pipework.

• We recommend the installation of an in-line drain with waterless trap be fitted to the air extract roof vent to prevent water damage to the unit "

 

Under the "Wall Vents" section however, it is says just to add a small FALL in the duct run from MVHR unit to the outside. No drain needed.

 

I would like to understand why a wall vent needs no drain but a roof vent does. Why with roof vents the ducting goes upwards but with wall vents it goes down?

 

How is an in-line drain fitted to 150 pipe? Is it just a tee-ing in of, say a 32mm waterless trap, into the 150 pipe at the lowest point and nearest the MVHR unit?

 

2/.

"A short section of Insulated flexible ducting will be required to install roof vents..."

 

Is this in combination with rigid 150 PVC piping which can only do set angles? If yes, then does it matter where in the run it goes - before or after the rigid PVC section(s)?

 

 

If anyone knows of a photo of a standard loft to roof vents install, that would hopefully explain it all without a word!.

 

Thank you, in advance.

[Hastings]

I wonder if @Stones has a similar installation?

[Declan52]

I have a vent axia sentinel b and my vents out are connected by a straight run of insulated duct to a vent tile. It goes straight up and anytime I have removed the heat sink to clean it out, on my 5th year now, it's always been dry. 

Just beware I had a short run through my cold section of loft and even though it was an insulated duct I still had condensation form. Had to remove this section dry it out by leaving it infront of the stove and then refit. I then wrapped it in 150mm rockwool insulation and have had no issues since.

[Hastings]

@2Declan52, thank you for the reassurance. Insulation seems like a much better way to fix condensation if it happens, than draining it.

Edited October 24, 2020 by Hastings

[Hastings]

On 24/10/2020 at 18:31, Declan52 said:

I have a vent axia sentinel b and my vents out are connected by a straight run of insulated duct to a vent tile. It goes straight up and anytime I have removed the heat sink to clean it out, on my 5th year now, it's always been dry. 

Just beware I had a short run through my cold section of loft and even though it was an insulated duct I still had condensation form. Had to remove this section dry it out by leaving it infront of the stove and then refit. I then wrapped it in 150mm rockwool insulation and have had no issues since.

 

Was the condensation forming inside or outside the duct ?

[Declan52]

Inside. The warm air from the house must have cooled down enough to form condensation on the inside edge of the pipe. It started to drip from the ceiling vent that's how I noticed it. 

[Hastings]

11 minutes ago, Declan52 said:

Inside. The warm air from the house must have cooled down enough to form condensation on the inside edge of the pipe. It started to drip from the ceiling vent that's how I noticed it. 

 

Ah, so it was an individual room supply pipe, not a main pipe to/from outside atmosphere?

[Oz07]

I never got that advice with my bpc tile vent installation. I just got told continuous fall on duct back to unit. Surely if the unit has a condensate drain it will do the job there? My ducts are wrapped in the foil insulation, foil taped at joins of insulation and covered with loft insulation until they reach the tile vent..

@Declan52 how did the condensation make it's way into supply side ducting?

[Oz07]

This was mine. Intake was a bit long but needed 2m separation and wanted them on sane elevation. 

[Declan52]

18 minutes ago, Hastings said:

 

Ah, so it was an individual room supply pipe, not a main pipe to/from outside atmosphere?

Yes was from my utility room so of all the extracts this air would have been the warmest.

[Declan52]

17 minutes ago, Oz07 said:

I never got that advice with my bpc tile vent installation. I just got told continuous fall on duct back to unit. Surely if the unit has a condensate drain it will do the job there? My ducts are wrapped in the foil insulation, foil taped at joins of insulation and covered with loft insulation until they reach the tile vent..

@Declan52 how did the condensation make it's way into supply side ducting?

See above

[Oz07]

I though condensation in the supply side ducts was unlikely? I believe op is referring to the intake/exhaust pipes

[Declan52]

I only mentioned the issue I had with condensation as in the original post they stated that it was too be sited in a cold loft. Was more of a warning to be careful with the room extract ducts as these aren't usually insulated. Never had any issues with the insulated ducts leaving the unit heading to the tile vents.

[Hastings]

Thanks @Declan52. and @Oz07 for pics.

 

I think it's clear I don't need to worry about a drain for the main exhaust.

 

Good advice about insulating, (ie. you can't really have too much).