Rigid vs semi-rigid ducting for MVHR

[Great_scot_selfbuild]

Which is better in terms of MVHR performance - rigid or semi-rigid ducting? Anyone have experience of both types?

 

We have ample space to accommodate rigid ducting in our new-build and have visited a build nearby which has used rigid MVHR ducting throughout, but a lot of the quotes I'm receiving are proposing semi-rigid ducting (our plans were supplied, so they can see we have 425mm posi joists and can run pipes up alongside the 350mm timber I-joists to the loft space). My assumption is that the rigid ducting will provide a smoother airflow and therefore better performance of the system.

 

I'm busy reading up on some of the existing MVHR threads - some excellent resources. I'm weighing up whether I design my own system; some of the design costs (alone) are near £1k; is this typical?!

[Crofter]

Your intuition is correct. Smooth bore rigid ducting is preferable for best performance.

[JohnMo]

Either rigid or semi rigid can good or bad. It's all down to design. Semi rigid being way more tolerant of not so good design.

 

Rigid, needs good between room attenuation, semi rigid had this by default.

 

Airflow, disturbance really depends on how many bends or changes in diameter you have with rigid. Semi rigid is all generally smooth long radius bends.

 

System pressure drop, resulting fan speed, either can be designed with the same pressure loss - so exactly the same fan speed.

 

Air flow noise, 90mm semi rigid in most instances can be a single run from plenum to outlet/inlet, 70mm may need 2x runs. Rigid should be silent normally.

 

Ease of install, semi rigid wins every time. It's flexible, rigid isn't.

 

Cost to install semi rigid should be way cheaper, no inter room attenuation needed.

 

I had two designs rigid and semi rigid, both to be self installed. Went semi rigid, just so much easier to install - it's silent.

[Nick Laslett]

@Great_scot_selfbuild, like all self build questions there is no one right answer. For most of the members here, price seems to be the guiding factor. 
 

I like this page at Paul Heat Recovery that covers the ducting topic well. 
 

https://www.paulheatrecovery.co.uk/mvhr-and-mev-ducting-components/best-practice-mvhr-and-mev-ducting/

 

They recommend both rigid branch system and quality semi-rigid radial system.
 

I also like this website, Heat, Space & Light Ltd. 
 

https://www.heatspaceandlight.com/difference-between-branch-radial-duct-which-better/

 

They have lots of blog posts and pictures of various installs, there are good discussions in the comments on some posts. 
 

With a rigid branch system, there is potential for noise transfer between rooms, which might require additional attenuators. With a semi-rigid radial system you can send more than one duct to a plenum which can be useful. 
 

Not sure if rigid spiral ducting is smoother than semi-rigid, as one has seams and the other is seamless with a continuous unbroken duct. Rigid has lots of joints, which require the joints to be sealed. Lindab SAFE rigid ducting has double gaskets, but is more expensive than regular spiral ducting. 
 

For me, I went with Lindab’s semi rigid ducting at 90mm, but this is quite expensive. I was quite strongly influenced by Robin Clevett’s install video. 
 

 

[Mike]

2 hours ago, Great_scot_selfbuild said:

My assumption is that the rigid ducting will provide a smoother airflow and therefore better performance of the system.

Rigid ducting in the standard outside the domestic sector where larger diameters are required to move more air, and where installation is less impeded - there's normally no need to thread it between joists, for example. In regular domestic situations the pros of semi-flexible ducting predominate and the performance differences aren't going to be very significant.

 

Bigger factors are how the intake and exhaust ducts (to and from the MVHR unit from outside) are routed and insulated - keep them short and insulate them well - and the performance of the MVHR unit. The latter needs to take into account both the heat recovery rate (for which the standard way of measuring it is inadequate) and the electrical consumption of the fans (which will be running 24/7 for many years). The reliable resource for selecting those, based on independent real-life testing of both measures, is the PassivHaus database, where Efficiency Ratio = overall performance (heat recovery & electrical consumption).

 

2 hours ago, Great_scot_selfbuild said:

I'm weighing up whether I design my own system; some of the design costs (alone) are near £1k; is this typical?!

No idea, but there's plenty of information on here on DIY design.

[JohnMo]

4 hours ago, Great_scot_selfbuild said:

I'm weighing up whether I design my own system; some of the design costs (alone) are near £1k; is this typical

£1k to design is just bonkers. Once you get your head around it, the design is pretty easy really.

[Nickfromwales]

4 hours ago, Great_scot_selfbuild said:

I'm weighing up whether I design my own system; some of the design costs (alone) are near £1k; is this typical?!

This is average, but should be for design and on-site commissioning, so if just for design it’s time to look again.

[Nick Laslett]

On the topic of self design, self install, this forum gave me the confidence to DIY this part of my build. 
 

In no particular order, here are some threads that I found useful. 
 

 

 

 

 

 

In addition to the two Robin Clevett videos, I also found the detailed podcast and associated articles on HousePlanningHelp very instructive. Ben had a rigid system designed and installed for his build. There are some interesting design compromises that give you an idea how malleable a MVHR system can be. 
 

 

https://www.houseplanninghelp.com/hph202-designing-the-ventilation-heating-and-hot-and-cold-water-systems-for-bens-house-with-alan-clarke/

 

http://www.houseplanninghelp.com/wp-content/uploads/2018/01/Alan-Clarke-2.pdf

 

Link to the Good Practice Guide, MVHR for single dwellings (Passivhaus Trust) document  

http://passivhaustrust.org.uk/UserFiles/File/Technical Papers/2018 MVHR Good Practice Guide rev 1.2(1).pdf

 

Here are some specific notes I collected during my research: 
 

House planning help podcast notes

Active carbon filter for external intake, with log burners nearby. Turn unit to lower setting. Filter on kitchen extract. Boost when lot of people. Boost when cooking. Set fan speed 2 high enough to remove moisture when showering.

 

JHarris - BuildHub post

The issue regarding the number of extracts versus supplies is very important.  Extract rates will be higher per room very often, because building regs mandate the minimum extract level for kitchens, bathrooms, utility rooms and WCs, and these rates are significantly higher than the supply rates you would normally want in bedrooms, living rooms etc.  It's therefore best to have more supply vents than extract vents, to enable the system to be balanced more easily.  That way the flow rates through the supply terminals will be lower than that through the extract terminals, meaning noise will be lower.  A bit of noise on boost is normally acceptable in a bathroom or kitchen, but less so in a bedroom. Silencers must be fitted as close to the MVHR as possible, before the main ductwork. MVHR kitchen valve is located at least 600mm away from the cooker. 

 

HeatSpaceandLight - blog post comment

We put filters on each extract valve (where dust from the room will naturally accumulate) which can be taken out easily and cleaned/replaced to keep dust out of the ducts. We also put a grease filter on the kitchen valve to keep grease and dust out of the ducts. Finally, the unit has F7 and G4 filters which keep pollen, carbon dust and other nasties from entering the system/fans from outside, and these can be replaced and cleaned also.

 

Edited 13 hours ago by Nick Laslett

[Kelvin]

I used quality semi-rigid to the manifold which is really easy and quick to install. It’s also quite easy to damage if it catches on any sharp edges or random nails so some care is needed. We’d never have got rigid through the building. One mistake I made was not allowing quite enough room in the ceiling for the manifold, attenuation and pipe work to the MVHR unit. With a bit more thought I could have mostly hidden it. However it’s all in the plant room so not a big deal. The other issue we had is the MVHR unit is in the single storey part of the house so getting the ducting through into the other side of the house was a right pain. The design of the house made no allowance for this. 

[Great_scot_selfbuild]

2 hours ago, Nick Laslett said:

On the topic of self design, self install, this forum gave me the confidence to DIY this part of my build. 
 

In no particular order, here are some threads that I found useful. 
 

 

 

 

 

 

In addition to the two Robin Clevett videos, I also found the detailed podcast and associated articles on HousePlanningHelp very instructive. Ben had a rigid system designed and installed for his build. There are some interesting design compromises that give you an idea how malleable a MVHR system can be. 
 

 

https://www.houseplanninghelp.com/hph202-designing-the-ventilation-heating-and-hot-and-cold-water-systems-for-bens-house-with-alan-clarke/

 

http://www.houseplanninghelp.com/wp-content/uploads/2018/01/Alan-Clarke-2.pdf

 

Link to the Good Practice Guide, MVHR for single dwellings (Passivhaus Trust) document  

http://passivhaustrust.org.uk/UserFiles/File/Technical Papers/2018 MVHR Good Practice Guide rev 1.2(1).pdf

 

Here are some specific notes I collected during my research: 
 

House planning help podcast notes

Active carbon filter for external intake, with log burners nearby. Turn unit to lower setting. Filter on kitchen extract. Boost when lot of people. Boost when cooking. Set fan speed 2 high enough to remove moisture when showering.

 

JHarris - BuildHub post

The issue regarding the number of extracts versus supplies is very important.  Extract rates will be higher per room very often, because building regs mandate the minimum extract level for kitchens, bathrooms, utility rooms and WCs, and these rates are significantly higher than the supply rates you would normally want in bedrooms, living rooms etc.  It's therefore best to have more supply vents than extract vents, to enable the system to be balanced more easily.  That way the flow rates through the supply terminals will be lower than that through the extract terminals, meaning noise will be lower.  A bit of noise on boost is normally acceptable in a bathroom or kitchen, but less so in a bedroom. Silencers must be fitted as close to the MVHR as possible, before the main ductwork. MVHR kitchen valve is located at least 600mm away from the cooker. 

 

HeatSpaceandLight - blog post comment

We put filters on each extract valve (where dust from the room will naturally accumulate) which can be taken out easily and cleaned/replaced to keep dust out of the ducts. We also put a grease filter on the kitchen valve to keep grease and dust out of the ducts. Finally, the unit has F7 and G4 filters which keep pollen, carbon dust and other nasties from entering the system/fans from outside, and these can be replaced and cleaned also.

 

@Nick Laslett thanks so much for taking the time to put this collection of links and info together! I started on some volumetric calculations over the weekend and am coming round to the idea that this is something I could do myself (need to somehow free up the time!). I’ll spend some evenings this week going over this detail - TVM 😁

[Great_scot_selfbuild]

2 hours ago, Kelvin said:

I used quality semi-rigid to the manifold which is really easy and quick to install. It’s also quite easy to damage if it catches on any sharp edges or random nails so some care is needed. We’d never have got rigid through the building. One mistake I made was not allowing quite enough room in the ceiling for the manifold, attenuation and pipe work to the MVHR unit. With a bit more thought I could have mostly hidden it. However it’s all in the plant room so not a big deal. The other issue we had is the MVHR unit is in the single storey part of the house so getting the ducting through into the other side of the house was a right pain. The design of the house made no allowance for this. 

@Kelvin useful detail - thanks

[JohnMo]

Here is a simple flow requirement guide.

 

Passivhaus bedroom requirements are just about correct for our house, so good guide. Double bedrooms not (rarely) used treat as a 1.5 person bedroom.

 

If your house is large with only a few people, once building control is finished and house has dried out, you should look to set the flows as per passivhaus otherwise you will end over ventilating.

 

You don't need to wrapped in spreadsheets, you just end up going down never ending tunnel, keep things simple round numbers. Overall flow Inlet the same as outlet. Lounge flow more than a double bedroom - your awake so produce more co2, and water vapour and instead of two of you there maybe 4 or 5. We flow 70m³/h in to your lounge.

 

Set boost 10 to 15% than normal.

 

Edited 10 hours ago by JohnMo