Posted By: jockassairflow pro flexible ducting

It's not a flexible as you might think! Worth having a feel before you go down that route.

I would not use anything other than rigid ducting.

<blockquote><cite>Posted By: tony</cite>I would not use anything other than rigid ducting.</blockquote>

Although the airflow semi-rigid is easier for a diy install, and wont crush like normal flexi

I agree with Tony - don't use flexible ducting, it increases the specific fan power of the system amd costs much more to run.

Choose your MVHR system carefully - check that the model proposed has an EST Best Practice specific fan power (SFP) - heat exchange efficiency is what the manufacturers focus on but SFP is the really important one. Similar duty units can have the same HR efficiency but one unit can use 3 times the energy of the other. These are 24/7 operation so take care in the selection.

SAP software packages have access to the full database and can give an accurate assessment of the energy used (and saved).

The Itho models mentioned by Chippyclaus in his post above are amongst the best performers, but other brands such as Vortice and Xcell are also good. Don't settle for anything worse than 0.6 W/l/s. BTW, you could double the SFP by using flexible ducting - it's a complete false economy.

The DV71 is a dog however, not to be touched with a bargepole.

SFP of 3.03 for 3 wetrooms, 6 times worse than the best units available. Efficiency of 91% doesn't compensate for this miserable performance.

Controls are more important than SFP with regards retrofit, especially if fitting to help with allergies and Aq.

Go for unit with summer bypass, humidity sensors and the option for air quality sensors (which can be damn expesnive)
Also look into filter options. Which means you may wish to oversize the system to overcome increased resistance of better filtration.

Id rec VA kinetic all day long.

Thanks guys - .79 for k+3wetrooms so looks ok. Is there any guides for ducting out there other than what common sense would dictate?

Try:

https://www.google.co.uk/url?sa=t&source=web&cd=1&ved=0CDYQFjAA&url=http%3A%2F%2Fwww.b-espublications.co.uk%2FEcommerce%2FproductPDFs%2FDW154.pdf&ei=JQiPUOrJIsXG0QXY5YCICQ&usg=AFQjCNGFb4ASk7xqjz4-_YbhzUq-13TXjA

This is for commercial plastic ductwork. Use in conjunction with common sense.

Lindab have branches in various areas. Eventually I used them for all the ducts, grilles and air valves that I needed after the intial batch I got from a MVHR manufacturer had to be added to and the layout modified as the job proceeded. For my next project I went to them for everything except the MVHR unit and cooker hoods. If you have a local branch that is convenient compare their prices. I got a 25%+ discount when I ordered the bulk for the second job.
Second the suggestion from chippyclaus that a bungalow is a lot easier, even with no experience. I also have fitted 2 Itho cooker hoods directly to the MVHR systems; you may be aware that this has caused a lot of discussion on this forum in the past ( I note that you only joined GBF 9 hours ago).
Not sure how many 125mm spigots there are on your intended unit; several members have recommended oversizing the MVHR and the ducts to reduce noise and power consumption. Some units with the capacity you would need for your home have 150mm or even 200mm spigots.
For the 2 systems I have installed, I found that the most suitable location for the MVHR limited the choice of models. For instance, for a loft installation some need a surprising amount of headroom to remove the heat exchanger with the unit in situ.

RobinB

Yes it seems to be a smaller version of the DV72 which is EST compliant. I haven't studied the unit in detail but I note it is operating at its limit with 3 wet rooms and the poor SFP suggests an inefficient fan (possibly high speed) designed to meet the pressure drop requirements of a ducted system at minimum cost.

As a rule, the closer an MVHR system operates to its maximum capacity, the higher the SFP. I prefer to specify units which operate in the middle of their range as this gives a good balance between SFP and cost. They tend to run quieter, too.

Posted By: GaryBI agree with Tony - don't use flexible ducting, it increases the specific fan power of the system amd costs much more to run.

Once and for all, this ducting is semi-rigid not *flexible*. There is a significant difference.

Hi Borpin

<blockquote><cite>Posted By: borpin</cite><blockquote><cite>Posted By: GaryB</cite>The DV71 is a dog however, not to be touched with a bargepole.

SFP of 3.03 for 3 wetrooms, 6 times worse than the best units available. Efficiency of 91% doesn't compensate for this miserable performance.</blockquote>I investigated this unit and the SPF in SAP Q is misleading as it was (I believe) tested with and F3 filter which, as it is a finer filter, affects performance!

I have some significant reservations on these SPF figures.</blockquote>

An F3 filter has a pretty low resistance to airflow and low efficiency. I understood that all MVHR filters were F grade. I normally specify F7 (80-90% efficiency) for an office environment, F5 is 40-50% efficiency, so F3 is not impressive.

I checked out the Airflow ducting and yes it is semi-rigid with a smooth inner layer. This was not clear in the OP, just the term 'flexible' was used. The Airflow brochure does not quote the actual resistance factor but does state that in a complete installation the ducting is equivalent to a rigid system.

I suspect that the pressure drop for a given flowrate is slightly higher than rigid duct but the bends will be long radius compared to the short radius fittings used in a rigid system. None of the installers on the 172 MVHR systems we have designed have used semi-rigid duct or even offered it as an alternative, so there must be a cost penalty compared with rigid.

Hi Borpin

Thanks for the tip re quoting via html - never did understand that before!

Posted By: borpin
The Airflex Pro ducting wins for me as it is 75mm OD so I can get it round the back of my Oak Frame. I also think it will be significantly quicker to install and with long bends nice and quiet.

Reading up on it the other advantages are supposed to be -
- maintains integrity of airtightness (not relying on glued joints/duct tape)
- avoids sound transmission between rooms

What sort of cost increment are we talking about and are there any other disadvantages?

Posted By: Simon Still- avoids sound transmission between rooms

This is true but we have airflex pro ducting and we do get a little noise from the unit itself. Poorly laid out perhaps?