MVHR ducting through a passive slab foundation?

[Dreadnaught]

Hard hat on. It is possible to run MVHR ducting through a passive slab?

 

I am starting to think about ducting runs from my bungalow with vaulted ceilings. As a bungalow, there are no joists to take the runs. Passive slab not poured for another few months at least.

 

 

Edited February 12, 2019 by Dreadnaught

[Jeremy Harris]

There are a couple things to sort out, but I think it might be feasible, at least in part.

 

First thing would be whether or not the duct runs would adversely affect the structural integrity.  That really depends on the slab design, and ours at 100mm thick would be a non-starter, as the smallest ducting is 51mm deep I think.  A 200mm deep slab might take it OK, but I think an SE would need to say for sure, as you don't want the slab to crack along the lines of the duct runs.

 

Second issue concerns the heating impact on the MVHR ducts from the floor.  Fresh air feed ducts running in a warmed floor would be fine, it would just add a bit of heat to the air coming in.  Extract ducts would pose a problem, though, as you don't want them to be warmed up, as that would significantly reduce the MVHR efficiency.  Not sure how to address that, really, as I doubt that insulating the ducts within the slab would be very effective.

 

I've assumed that the ducts all run through the concrete, as you wouldn't really want them down inside the insulation layer.

 

 

[vivienz]

Could you build in some sort of false or suspended ceiling to take the MVHR runs and other services?  What sort of head height do you have and can you afford to lose any?

[mike2016]

I'd be worried about condensation in the ductwork intake contaminating the air coming into the house. Remember that German example where they buried the ductwork in the ground to get ground temperature warmth even when it was freezing outside but the ducts became contaminated and caused illness to the occupants. How are you going to inspect for this afterwards? I know you can insulate but it's a risk....

[Dreadnaught]

2 hours ago, vivienz said:

Could you build in some sort of false or suspended ceiling to take the MVHR runs and other services?  What sort of head height do you have and can you afford to lose any?

 

Very possibly yes. Ceiling height will will be in excess of 3m at the peak of the vault.

 

I am assuming that when I get that far my timber-frame designer and my M&E expert (now selected) can come up with ideas for where to put internal duct runs.

 

The MVHR in its plant room will be at the central corner of an L-shaped building so I fundamentally have two runs to think about. One will be easier than the other: the one to the bedrooms has a corridor which can have a false ceiling.

 

The tricky one is the other wing, getting a supply vent to the other end of a large open plan vaulted room, with exterior walls on both sides. The following might work. A low level duct run extending initially through a bathroom beneath a bath and then into the kitchen, under kitchen units and then finally continuing out into the open, underneath some built-in furniture might be one idea.

 

If it is important for the supply vent to be higher up, I might even then be able to run the supply duct up inside a shelved room divider made by a cabinet maker which I plan to have. Having said that, I have heard @jack mention that he has (some?) low level vents (I am not sure if they are supply or extract) and they work fine for him.

 

2 hours ago, mike2016 said:

I'd be worried about condensation in the ductwork intake contaminating the air coming into the house.

 

Good point. I do recall that German example. However I am thinking more about the internal ducting runs after the MVHR unit rather than the inlet and outlet to the outside. The outside connections fortunately should be straightforward as the MVHR unit will be on an exterior wall.

[Gone West]

@Dreadnaught There's a discussion here about MVHR duct routing where I have shown pictures of low level supply ducting. Is it possible to have a small flat ceiling at the top of the vaulted ceiling to conceal the ducting. I have the extract vents high and supply vents both high and low.

 

[jack]

10 hours ago, Dreadnaught said:

If it is important for the supply vent to be higher up, I might even then be able to run the supply duct up inside a shelved room divider made by a cabinet maker which I plan to have. Having said that, I have heard @jack mention that he has (some?) low level vents (I am not sure if they are supply or extract) and they work fine for him.

 

My floor-level vents are in the bedrooms, so they're outlets/supply.

 

I've heard it suggested that a higher supply position helps ensure that the fresh air being introduced diffuses better, but I'm really not that fussed. I don't notice any difference in air quality in these rooms compared to others with ceiling-mounted outlets.

[Jeremy Harris]

All the stuff I've read about MVHR air movement suggests that the primary process is diffusion, not forced air movement as such.  Diffusion will ensure that stuff like water vapour and CO₂ will move naturally from areas of higher concentration to areas of lower concentration, so it probably doesn't matter too much where fresh air vents are.  Extract vents are slightly different, as we usually want to ensure that water vapour is drawn out quickly, and water vapour is lighter than air, so will tend to rise to the top of rooms that have a high concentration of it (bathrooms, kitchens etc).  For that reason I think that extract terminals are best placed high up.

[jack]

50 minutes ago, JSHarris said:

All the stuff I've read about MVHR air movement suggests that the primary process is diffusion, not forced air movement as such. 

 

Yup, agreed, but I've definitely read at least once on BH that supply vents should be high up. 

[Jeremy Harris]

5 minutes ago, jack said:

 

Yup, agreed, but I've definitely read at least once on BH that supply vents should be high up. 

 

I think the main thing is to try and ensure that the path from the fresh air supply point to the stale air exit point in any room is as long as you can practically make it.  It's often the case that rooms with a fresh air supply will have a stale air exit out through a door that has a gap underneath it for this purpose, so there may be a slight benefit in having a high fresh air terminal in the ceiling diagonally opposite the door, but I'm not convinced that having it in the ceiling is an essential requirement. 

 

I tried to position all the terminals in the house so that air would have the longest path to travel across a room, but didn't achieve this on our bedrooms, just because fitting ceiling terminals in the vaulted ceilings would have been a lot of faff, plus it would have significantly increased the length of the duct runs.  In practice our low-level fresh air supplies in those rooms seem to work OK.