MVHR - Intake and exhaust ducts lenghts

[NickK]

Hi,

 

I have read on a few sites that to achieve the best possible efficiency, it is best to keep the intake and exhaust duct length below 1.5m. Depending on the location of the unit, it may not always be possible so I was just curious...how much impact it would really have, if the length was over this 1.5m recommendation.

 

Thank you

Edited September 2, 2020 by NickK

[tonyshouse]

Low impact, up to 3m after that you should increase the size of the duct to the next size up, 200 to 250mm for the whole length 

[MJNewton]

I'd focus more on the straightness of runs rather than the length, and making sure they are well insulated.

[Nick Laslett]

This seems like a bit of a contradiction, if the minimum recommended distance between Intake and Exhaust is 3m, how do you keep the distance to 1.5m. Unless fitted in the roof, one pipe will need two 90 degree turns to exit building and be far enough away from other pipe. 

[joe90]

Mine are about 2m apart on the North outside wall fed by about 1.5m flexible pipes and insulated.

Edited November 15, 2020 by joe90

[Gone West]

1 hour ago, Nick Laslett said:

Unless fitted in the roof, one pipe will need two 90 degree turns to exit building and be far enough away from other pipe. 

 

IIRC our outside vents are 3.5m apart so our pipework is 5m for one vent and 1.5m for the other. I used 45 degree bends to reduce restriction of air flow.

[Nickfromwales]

1 hour ago, Nick Laslett said:

This seems like a bit of a contradiction, if the minimum recommended distance between Intake and Exhaust is 3m, how do you keep the distance to 1.5m. Unless fitted in the roof, one pipe will need two 90 degree turns to exit building and be far enough away from other pipe. 

Correct. That’s not a problem at all. 

[Nickfromwales]

23 minutes ago, PeterStarck said:

 

IIRC our outside vents are 3.5m apart so our pipework is 5m for one vent and 1.5m for the other. I used 45 degree bends to reduce restriction of air flow.

Most decent 90o bends are 2x 45’s back to back in one piece. 

[Gone West]

28 minutes ago, Nickfromwales said:

Most decent 90o bends are 2x 45’s back to back in one piece.

I prefer the ones that are 4 x 22.5's myself.

[Nick Laslett]

3 hours ago, PeterStarck said:

 

IIRC our outside vents are 3.5m apart so our pipework is 5m for one vent and 1.5m for the other. I used 45 degree bends to reduce restriction of air flow.

Thank you Peter. Is the 1.5m pipe the intake?

[Gone West]

15 minutes ago, Nick Laslett said:

Thank you Peter. Is the 1.5m pipe the intake?

No it's the exhaust. I thought because the exhaust is more likely to have condensation in it I would want it to drain quickly. The ducting slopes down to the outside vents and I've used 45 degree bends spaced apart.

[joe90]

Initially I did not insulate my two outside connections and it was the intake that formed condensation on the outside. Cold incoming  air in a warm roof. I insulated both to make sure.

[NickK]

Hello,

 

From what I’m researched the reason to keep the lengths short was mainly to maximise summer bypass but then it depends on each projects specifics.

 

 

[PeterW]

If the intake and the unit itself are properly insulated then it doesn’t matter too much how long the intake / extracts are. It’s more important to keep them large diameter and straight as possible. 

[Levo]

I have done some calcs on this and given the other drops (pressure) in the system, it appears acceptable to have a few meters of pipe on that side.

The following is for my nominal flow of 132m3 and ~5m of pipe.

I allowed for 4 bends (1-2 to form a U after leaving the unit upwards, 3 to turn horizontal on the floor and 4 to hit the cowling).

Tony is right though, if the runs get longer you may need to go up a step on diameter.

By comparison, a 15m 75mm double run with one bend along the way to a bathroom extracting 45m3/h will have around 40Pa drop.

This is excluding the pressure drops in the pipe, mufflers, bends and the distribution box.

P.S. These are all my own calculations.

Edited November 15, 2020 by Levo