MVHR Duct Design

[Triassic]

@JSHarris   Thanks for the feedback, just a couple of questions..

 

The 0.3 l/s per m² - is this the internal floor area of the rooms being subject to ventilation or the whole house floor area?

 

Could you clarify where the 177 figure comes from?

 

 

 

Edited February 14, 2019 by Triassic

[Jeremy Harris]

45 minutes ago, Triassic said:

Thanks for the feedback, just a couple of questions..

 

The 0.3 l/s per m² - is this the internal floor area of the rooms being subject to ventilation or the whole house floor area?

 

Could you clarify where the 177 figure comes from?

 

The floor area figure used is the internal floor area of all floors in the house added together.  I thought you'd posted earlier that your floor area was 177m², hence the use of 177 in the sum, but I may have got that wrong.

 

Edited to add:

 

Just realised I have got the floor area wrong.  I saw 177m³ and misread it as 177m².  If the internal floor area is 310m² then the total supply (or extract) rate needs to be 0.3 x 310 = 93l/s

Edited February 14, 2019 by JSHarris
Realised I'd got the area wrong...

[Triassic]

Ah right..

 

The total internal floor area of the house (every room, over all three floors = 388m2

 

So 388 x 0.3 = 116 l/s

 

So having reworked the calculations to us 75mm pipe throughout and use more pipes to reduce the velocity, the total flow rate / hour= 75.

 

If I understanding this correctly I need to extract an additional 41l/s  to get the rate up to 116 l/s?

Edited February 14, 2019 by Triassic

[Jeremy Harris]

You'll probably find that figure dominates the others given in building regs, as it usually does.  It will also over-ventilate the house by a fair bit, as 177m³ at the typical PHPP ACH of 0.3 would mean a whole house ventilation rate of around 15l/s, and the ~0.4 ACH figure we run ours at would give you around 20l/s.

[Triassic]

So having reworked the figures and based on a total floor area of 388m2 x 0.3 ACH the whole house ventilation rate is 116 l/s .

 

It's starting to look like a very large MVHR system with lots of pipes!

 

[Jeremy Harris]

Looks about right to me.  It's a shame that the building regs mandated figures are so high, especially in your case where the disparity between the floor area-mandated rate and the sensible ventilation rate is so great.  I think you could probably rely on having the MVHR running at a higher than normal level in order to achieve BR compliance, through, then wind it down to something more sensible as others have done.  This might mean you could accept duct velocities of around 2.5m/s in order to achieve the BR rates, secure in the knowledge that in reality you'd be running at a lot lower flow rate pretty much all the time.

[ProDave]

I have not measured or ballanced ours yet.  But I am running it at the slowest fan speed which makes it totally silent.  Anecdotally, it is working  fine. the air in the rooms always appears fresh and free of smells, so I see no need to increase it.  If I find I do need to increase it, it will only be briefly for compliance,  then it will go back down again.

[mvincentd]

I've a kitchen/living/dining area in the 170m3 ballpark....3x 125mm inlets and 2x 125mm outlets which expand into 150mm diameter ducting back to unit.  I'm on a branched system with rigid duct so not sure if this is a helpful comparison or not...see attached.

 

[Triassic]

@mvincentd thanks for sharing, it’s good to see an alternative solution.

[Nickfromwales]

55 minutes ago, Triassic said:

@mvincentd thanks for sharing, it’s good to see an alternative solution.

FWIW, I would avoid the larger bore series run systems ( personally ) after seeing how ducts and soil pipes would become quite problematic at junctions / crossovers ( eg a crossover would be near impossible ).

Doubling up to 2x75mm ( 82mm O/D iirc ? ) to single inlet - extract point / locations are seems excellent insurance against audibility at high / boost rate. To better clarify, on the current project we have put a single outlet either side of the large living area with a single duct run to each outlet, where the alternative could be to run 2x ducts to one 'double-barrel' outlet ( all outlets were supplied with the ability to connect 2x ducts to a single outlet, with one open and one with a factory fitted, removable blanking disc which you remove and dispose of to connect 2x ducts to 1x outlet ). Putting multiple outlets with individual radial runs to each is favourable if that's is feasible, as there is a better spread of lower airflow over the whole space vs one double-barrelled outlet trying to do the lot at one point / location within a space. 

One person I spoke to was advised by a reputable MVHR supplier to beef up their posi-joists and go for a large bore series system. Naff results, difficult to balance ( as reducing at the last plenum slightly increses flow to the plenums upstream and vice versa ) and an uplift in cost for the posi's of over £4K !!!

Plus then you have the constraints of rigid ductwork, so when your posi's are in, you cannot traverse the house perpendicular to the joists without introducing lots of cuts and joints. No ta.

 

[Triassic]

When planning the MVHR system I’ve visulised the duct runs and checked the architects drawings to make sure what I had in mind was doable. What I’d not considered was how the timber frame is constructed, it has loads more, and chunkier, timber than the previous house, lots of structural timber elements etc, all making it more difficult to run ductwork.

[Jeremy Harris]

I just did a rough design initially, to get the terminals more or less where they needed to be, and to estimate the amount of ducting needed, then ran the semi-rigid 75mm/63mm duct along the best routes I could manage, trying to keep the runs as short as practicable and also trying to avoid too many sharp bends.  It's hard to predict in advance the exact alignment of the openings in the posi-joists, and the angles you can run ducting across a room when aiming to get a near-diagonal run.

[Triassic]

I think part of my problem is the orientation of the posi- joists, especially around the landing area, I’ve ended up with a few solid timber trimmers in the way!

[Gone West]

2 hours ago, Triassic said:

I think part of my problem is the orientation of the posi- joists, especially around the landing area, I’ve ended up with a few solid timber trimmers in the way!

I had a strongback in the way of one of the ducts so I cut out a section of the strongback and replaced it with pieces of timber between the top and bottom cords of the joists.

[mvincentd]

12 hours ago, Nickfromwales said:

FWIW, I would avoid the larger bore series run systems

I wouldn't disagree...it's a pain.

However on a point to point system I would have had to get 12 runs of flexible ducting through steelwork, 4 times...the last of which is only a 2 metre long steel.

 

[ProDave]

7 minutes ago, mvincentd said:

I wouldn't disagree...it's a pain.

However on a point to point system I would have had to get 12 runs of flexible ducting through steelwork, 4 times...the last of which is only a 2 metre long steel.

 

Can you relocate the plenum boxes to the other side of the steel so only 2 large pipes have to pass through that?

[Nickfromwales]

9 minutes ago, mvincentd said:

I wouldn't disagree...it's a pain.

However on a point to point system I would have had to get 12 runs of flexible ducting through steelwork, 4 times...the last of which is only a 2 metre long steel.

 

I mitigated against that by being involved from early on in the build eg before it was even started. A few penetrations needed to be made in the main steels, so that was all pre-planned on annotated CAD drawings to / from the TF supplier. Other key penetrations made at that time were for small bore plumbing, and a few letterboxes for the main drag of cables. Worked a treat with just a little fine-tuning here and there.

[Pete]

We had issues with our build and I have had to lower the ceiling in the corridor to run all the ducting down. There was no other way of doing it as the roof above is the roof terrace which splits the house down the middle. We lost 100mm but it is has not made a big difference.

[mvincentd]

1 hour ago, ProDave said:

Can you relocate the plenum boxes to the other side of the steel so only 2 large pipes have to pass through that?

It is what it is now, but I don't think so....I have 122mm wide posijoists at 400mm centres either side of the steels and they're not aligned webs due to the house being curved.

heyho!

[le-cerveau]

I had a similar problem, 472m² floor area, which gives an extract rate of 131.37 l/s or 472.92 m³/h.  I have 2 x MVHR units both running well below max as this reduces noise and increases heat exchange efficiency.  My system was designed (by myself) at 495 m³/h to comply with building regs but I run it at about 316 m³/h and that more than copes with the house and is probably still quite high.  My system auto boosts back up to the higher rate or even high 570m³/h based on the RH sensors in the extract ducts and these work well detecting shower usage.

 

Layout I used the Ubbink system with 180mm rigid ducting between MVHR units and the outside/plenum boxes and 90/75mm (Outside/Inside) semi rigid elswhere.  The kitchen has two extract terminals each with two ducts, everywhere else I used single ducts to terminals, though wet rooms have two extract terminals and bedrooms two supply terminals.  This was to keep velocities down.

 

The whole system is silent though you can hear the MVHR units in the plant room when at the Building Regs speed, but at the normal running speed nothing.

[Triassic]

All the quotes I have specify 2 MVHR units, having looked on line, the majority of units are for homes up to 150m2 floor area. As I have a large plant room and wondered, is there anyone manufacturing a large capacity single unit ?

Edited February 17, 2019 by Triassic

[dpmiller]

what have you been quoted? Vent Axia's Kinetic Highflow will do 350m2.

 

have a look here:

 

https://www.bpcventilation.com/heat-recovery/full-diy

[Pete]

3 minutes ago, dpmiller said:

what have you been quoted? Vent Axia's Kinetic Highflow will do 350m2.

 

have a look here:

 

https://www.bpcventilation.com/heat-recovery/full-diy

That is what BPC recommended for us.

[le-cerveau]

19 minutes ago, Triassic said:

All the quotes I have specify 2 MVHR units, having looked on line, the majority of units are for homes up to 150m2 floor area. As I have a large plant room and wondered, is there anyone manufacturing a large capacity single unit ?

I originally looked at a single large unit and they are available, the issue is the 250mm connections, going to the outside with that size and then splitting it into 2 and then into your plenums, you will need 2 plenums each for supply and extract for a house that size, I have a pair for downstairs and a pair for upstairs.

 

5 minutes ago, dpmiller said:

what have you been quoted? Vent Axia's Kinetic Highflow will do 350m2.

 

have a look here:

 

https://www.bpcventilation.com/heat-recovery/full-diy

The issue at running these units at these speeds is noise and efficiency.  At full tilt they do make noise that will then be transmitted through the ducts to some extent, and if you look at the details they are far more efficient (heat exchange) at around 50-60%.  PassiveHaus specifications don't allow them to run at those speeds, a PH certified unit has a lower PH capacity that it's absolute capacity.  My Brink Renovent 400's run at 140 and 175 m³/h respectively, they are capable of 400 m³/h but as you go above 200m³/h they start to become audible.

[Triassic]

18 minutes ago, dpmiller said:

what have you been quoted? Vent Axia's Kinetic Highflow will do 350m2.

 

have a look here:

 

https://www.bpcventilation.com/heat-recovery/full-diy

Looking back some old quotes (for around 2 years ago, our old house took longer to sell, hence the delay) BPC had quoted for a system based on one of these. I recently asked them to update their quote and they came back with a quote specifying two smaller Vent Axia units, not sure why?

 

I’ll  take a look at the high flow unit, as it’s the sort of thing I had in mind.