Just reading up on MHRV systems for my new build and have come across the one site touting the benefits of radial ducting vs branch ducting.

Below is some of the reason why they recommend radial ducting over branch ducting - sure some of it is sales speak but does anyone have any option on if this is valid. do you have to buy the ducting from the MHRV unit manufacturer, or is there good inexpensive ducting that will fit most units?

•A ‘branch’ ducting system with flexi, metal or PVC pipe is also prone to acoustic issues, either a whistling from the supply/extract valves nearest to the fan motors or 'cross talk' travelling form one room to another.

By far the best kind of ducting in a domesitic or small commercial property is the new semi-rigid ‘radial’ ducting. The benefits radial ducting include:

•Flexible enough to thread through joists, between stud walls and other tight spaces (ideal for retrofit).
•Eliminates the risk of cross talk between rooms as every room has its own dedicated air pipe connected to a sound-proof manifold. The manifold also eliminates any motor noise from the MVHR fans.
•Can be cleaned in the future as it has a smooth internal bore and there are no sharp bends. While advisable at the moment, this feature is likely to become a requirement under building regulations.
•Incredibly low air resistance meaning the MVHR fans have less work to do, making the unit quieter, more energy efficient and (in theory) extending the life of the motors.
•Far easier and quicker to install as it clips together with no gluing & taping. One continuous length runs from (near) the unit to the room. The pipe can be cut neatly with a blunt knife rather than awkwardly with a saw. The install time is normally a lot less than that of rigid PVC ducting. It’s ideal for DIYers or builders new to MVHR work. The risk of boxing in a leaky pipe is almost nil.
•Partially insulated to reduce heat loss through the ducting. It can also be lagged with mineral wool without the condensation risks associated with PVC ducting.
•Extremely durable with a higher tensile strength than any other.

Radial/semi-rigid wins for me every time, UNLESS you are designing/building a larger than normal dwelling, where you can 'lose' 200mm dia rigid metal/plastic ducts?
The 75mm dia semi-rigid lends itself to threading through metal web joists, or through I-joists, and up/down through walls.
Having tried both, it is a no-brainer.
The extra capital cost is outweighed by the ease of install, and cleaning if required.
Cheers

I think the gig with Rapid is that you have to buy 2 manifold units as well.

So from the MHRV unit you have a large dia to the manifold, then the radial runs off it to the rooms.

I looked at Airflows and they have prices on there websire of £305 (not sure if includeds VAT or not) for 50m for their Airflew Pro

but I found 50m of Lindabs semi rigid ducting for £120 inc VAT.

That's a huge difference!! So wondering if you can use whatever ducting you want with a MHRV unit? Or does if have to be used with an approved duct for SAP Q

Have fitted a radial system in my new build, vertict still out on whether it was a good choice. The 75mm pipes went in easily, all whole pieces from the manifolds which was nice. But with hindsight I should have been more stringent over fan choice, static pressures and air velocities.

Suppliers don't mention the 0.3l/s per sqm meter Part F requirement, and max fan flow does not tell you want the fan will do with long duct runs. At regs flow I have a noise problem, probably due to air speed, although the noise seems to be from the terminal valves rather than the ducts or fan. Calc the air speed in each duct if going radial, an quiz the supplier about back pressures.

Posted By: jfbwhat are back pressures Greenfish?

Probably using the wrong term.... I mean be sure to get some evidence that the fan will be able to deliver the required flows down the lengths of ducting needed, what gets quoted is often the max possible flow that fan can give with no ducting to impede it or the max pressure (when flow stops). Five different suppliers offered me various ~115l/s max flow units (max pressure ~450Pa) for my 217sqm house, and listed the whole dwelling ventilation rate based on numbers of bedrooms (Table 5.1b of part F), in my case 25l/s. I looked at the specs and thought the fans on offer were more than adequate for the job, thus I would be running well within their capability (25 out of a possible 115) and enjoy a quiet system. I picked one on that basis, but wish I had asked more questions, I am using more of the capacity than I expected.

Reality, the regs require 0.3 x 217 = 65l/s air flow, and I have no idea what resistance the radial ducting presents to the fan thus what pressure it is running at, but to deliver 65l/s it is cranked up quite a way and the system is unacceptably noisy.

Also - do you know what maximum speed you are getting in the ducts? Isn't there a regs max of 2 or 2.5m/s?

I don't think there is a speed limit in the regs (someone tell me if there is), but I have read elsewhere that sound could become a problem above 2.5m/s. Having just commissioned the system I know what flow rates I have at each supply/extract terminus, I can calculate the speeds from that and the duct dimensions, unfortunately almost all of them exceed 2.5m/s. Should I be surprised at the noise!

If the reg required area based flow is reasonable (see my other thread) I have to wonder if air speed is a flaw in the radial/semi-rigid approach. Let's do some quick calcs.

My 75mm semi-rigid ducting is 63mm internally, thus a cross-sectional area of 0.00312sqm. If air was flowing in the ducting at 2.5m/s then the air flow rate would be 7.8l/s. That is less than the minimum continuous extract rate for bathrooms (8l/s) and kitchens (13l/s) given in table 5.1a, so either the air is faster and potentially noisy or even a tiny bathroom needs more than one duct serving it.

I guess that having 2 ducts for every 26sqm of floor (extracting and delivering 0.3l/s per sqm at less than 2.5m/s) doesn't sound too onerous. In my case to achieve 65l/s balanced flow at a speed of 2.5m/s or less I would need 9 ducts supplying and 9 ducts extracting - that is a lots of terminals in each room and lengths of ducting! Supply is not so bad, as it is I have 8 distributing the air to the rooms, but locating 9 extracts between the kitchen and 2 bathrooms would have been difficult, I have only 4. None of the suppliers mentioned having more extracts, so maybe they don't consider air speed in their designs?

I would love to know from other with radial/semi-rigid systems what flow rates/speeds they have.

Posted By: slipstreamhow many ducts are you using?

Hi Slip, sounds like you have a similar size ventilation problem to mine. I have 8 supply and 4 extract terminals with only one duct each, thus 12 duct runs compared to your potential 31! As along as I keep the flow rates proportional to the number of occupants (less then half the Part F area based requirement) noise is not an issue, but I fear not enough care went into the design.

We went with a Polypipe system, but 4 other manufacturers quoted the same number of terminals and ducting (as far as I could tell, most liked to give a price eventually but keep design detail to a minimum). I don't think that any considered air velocity or noise, or they could have used it as an effective selling point. It seems that your first supplier has a better grasp of the issues than any I had contact with. Good luck!

Slip - who was your first supplier you contacted ? i might give them a call

also - can anyone tell me any actual differences between Lindab's and airflow's semi rigid duct other than price?!

You could use normal duct from the kitchen and bathrooms, then the flexible to feed air into all the rooms.

DJH - what incompatibilities have you found in semi-rigid? I'd been assuming they were all based around the same 3rd party 75mm ducting (in various colours) so could source the duct itself from the cheapest source whatever my manifolds