Extending warm air ducting/heating

[sgg123]

Hi all,

I have partitioned a room that had an existing warm air vent, and the new room doorway has meant removing that vent.

I now want to 'T' off the existing ducting so that I can a T joint coming up within the new stud partition for 2 vents, 1 serving each room. I would have liked to have vents in the floor but there is a joist obstructing access to the adjoining room.

As you can see from the images below, the existing ducting is 300 x 150mm and I want to T into the stud wall which is 90mm. I had thought about getting some new ducting fabricated for this but not sure if this is overkill (or would work)? I wondered also if there is some flexible hosing solution that would serve me better - but I don't want to lose too much heat. For info, the vents will be just above skirting board height.

P.S. The timber across the opening of the existing ducting is temporary to support a cover piece of timber floor.

Any ideas?

 

[stem]

The thing to avoid is to reduce the cross sectional area of the duct which would reduce airflow. This will be difficult narrowing it down to less than 90mm to fit inside the wall. Presently it's 45,000mm2 (300mm x 150mm) To get that at 80mm the duct would have to be 560mm wide.

I faced something similar when splitting a duct in a reception area when it was divided up into two rooms. I constructed a small box above the duct like below.

There was a vent at the back straight through into the new room behind.

As it was a timber construction, to prevent it getting damaged by the heat, I used a couple of old engine oil tins to fabricate a duct inside. Not very professional, but it worked, and was hidden from view by the timber cover.

Also to be considered is the return air flow. Obviously the same amount of air that is coming out of the duct into the room behind needs to be able to find its way back to the heating system's air inlet. Usually this is achieved by fitting a grille in the door or walls between the two rooms. I choose the door. If this isn't done then the air entering the new area will have to escape by other means and if it exits to the outside of the building warmed air will be being pushed outside.

 

[sgg123]

Hi stem, thank you for your detailed reply. The box idea looks great, but the constraint for me is that an internal door will open over that location in both rooms.

Over the last couple of weeks I went ahead and chatted to a ducting fabricator who suggested a cap for the duct that incorporates 2 spigots to serve 2 lengths of insulated flexi pipe (1 for each vent). I'll feed the flexi up through the stud base plate and along the stud wall and out into each room using the old vents I have (pictures below). I appreciate your point about reducing airflow, thanks for this. I sense I will just have to see how it goes. As for return airflow - the doors of these first floor bedrooms will be open most of the time, so I think this will suffice.

 

[stem]

Assuming the ducts in your picture are 90mm diameter then I reckon that equates to an outlet area of 6,362mm2 per duct [ 3.1415 x (45 x 45) ] or 12,724mm2 in total for both of them. If so, this would represent a 71% reduction of the current 45,000mm2. Or in other words (er.. numbers ) that's only 29% of the original size. Are you confident this will be sufficient?

If the door between the two areas was never going to be closed, then that's fine, but if it were; as well as heat escaping outside it would further restrict the airflow by increasing the resistance of the airflow.

There's a really good explanation here. It's from the USA, where ducted systems are used to heat most properties, and in their case cool them as well.

 

[sgg123]

Yes, I appreciate its not ideal - but I feel a little limited in what can be done. I don't want extra construction outside the existing stud wall, which is what would be required if 'continued' the existing ducting to new vent locations. The two first floor bedrooms open to a landing (see below), and 99% of the time the doors will be open/ajar. I feel I have 2 options:
1) Go with the proposed flexi option
2) Install radiators into the 2 rooms

We like the warm air system, the bedrooms won't require too much heating. I guess we will have to see how it goes.