Ventilation: DIY MVHR design and calculations

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- CommentAuthorMarkyP
- CommentTimeFeb 15th 2017
following my other thread (getting started with MVHR) and lots of reading, I am embarking on designing my own system and the associated calculations. I've had little luck getting this done for free so will have a go myself. I'd like to use this thread to capture that journey as I think others might find it useful to see the process worked through end to end, especially the various calculations which I've found hard to get my head around despite reading numerous threads and manufacturer documents. I'm hoping that with help and advice from the many experts here, I can work this out. I apologise in advance for what may be stupid questions, I want to avoid making any assumptions.

So to get started with the first questions. I've read the part F tables with both minimum rates and boost rates for wet rooms.The total internal floor area is 258.5m2 which gives me a background (based on 0.3l/s) rate of 77l/s. This I know is considered high and advice is to after signoff experiment with lower system speeds but will work to this to size the system. My first question is how to use this overall requirement to work out air flow through a given duct. How do you divvy up the 77 l/s across my 5 extracts and 7 supplies?

next question is do I need to look for a unit which can cater for background 77l/s plus boost extract rates from wet rooms which is another 45l/s? I know I will also need to allow for pressure loss but, as a starting point, is worst case ventilation rate the system will need to deliver 125l/s? (77 + 45l/s)

I will be working on calculations spredsheet which can be viewed by anyone and which I will update as I go.

https://docs.google.com/spreadsheets/d/1NkEv0bN9S3EUHkfL0H3XfZp7HWId4t69-8Ynhz4xFmw/edit#gid=724798357
- CommentAuthorgravelld
- CommentTimeFeb 15th 2017
Great idea to work on an openly visible sheet!
- CommentAuthorDarylP
- CommentTimeFeb 15th 2017
@MarykyP, aim for a unit that is overspecced ....! then you can always turn it down, whereas if you have to turn up a smaller unit, you will be more likely to get noise issues?
- CommentAuthordjh
- CommentTimeFeb 15th 2017
The 77 l/s is the SUPPLY rate. Divide it up according to the suppy room volumes. The extract rate is the same as the supply rate, so first add up the individual extract rates to check they're not greater than the supply rate. If they're not then scale them up proportionally so the total matches the supply rate. (If they are greater, then scale the supply up instead.)

Then tweak all the figures so they are practical for whatever ducting system you are using and check all the flow rates are low enough to be quiet. Make sure the areas without vents (e.g. halls) are on a transit path for air and if not add vents and redistribute the flows.

You've only listed four extract rooms but I see five in your overall list of rooms. I also don't see a plant room anywhere listed - do you have one?
- CommentAuthorEd Davies
- CommentTimeFeb 15th 2017 edited
Have you seen Wookey's ventilation spreadsheet? Goes into all the resistances for straight and angled bits of the duct and what have you.

Isn't the flow required the greater of the sum of the supply rooms and of the extract rooms, rather than the total of both?

Edit to add: page 2 of this thread: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13405 though the whole thread's worth a skim.
- CommentAuthorMarkyP
- CommentTimeFeb 15th 2017 edited
DJH - no plant room. MVHR will sit in an eaves cupboarding on a landing. Ideally positioned to access long eaves runs for ducting. this is a room in the room/extension and loft conversion. The only plant room option I had was what will be a utility room but it's narrow with a small exterior wall area and already an oil fired boiler flue going through the wall so want MVHR intake well away from that. On the rooms point, I updated the sheet to make clearer - first table is generic regs guidance per room type. Later table is actual rooms. I will add volume data and then divvy up supply proportionally. When you say add up extract rates, not sure I follow. What numbers would I be summing here - the boost rates or do I need to calculate background extract proportionally by extract room volume - basically sharing out the 77 l/s across the 5 extracts?

Ed- yes, I have a copy of Wookey's sheet. And thanks for the discussion reference. More questions will no doubt follow!

thanks both.
- CommentAuthordjh
- CommentTimeFeb 15th 2017
Posted By: Ed DaviesIsn't the flow required the greater of the sum of the supply rooms and of the extract rooms, rather than the total of both?

Doesn't quite work like that . See Table 5.1a and 5.1b of Part F, especially the footnotes, and then 5.2d. The minimum whole dwelling supply ventilation rate is based on the total area, not just the supply area. I have no idea why.

MarkyP, step 2 of 5.2d. Add up the individual room rates for â€˜minimum high rateâ€™ from Table 5.1a. Then just multiply each individual rate by whatever factor is required to make the total equal the rate determined in step 1. They will all get changed again by the practicalities of ducting. You just need to make sure you exceed the minimums in the regs.
- CommentAuthorMarkyP
- CommentTimeFeb 15th 2017
ah, got it. So my minimum high extract rates sum to 45 l/s, I'll just scale those up by an adjustment factor until they match the 77 l/s supply.

the supply room volumes - bit of a fiddle as many are room in roof with odd shapes. Any reason to not just proportion out the supply based on supply room floor area?
- CommentAuthorringi
- CommentTimeFeb 15th 2017
Posted By: djhMake sure the areas without vents (e.g. halls) are on a transit path for air

In most cases this is not important, as gases do defuse about and mostly sort themselves out.

Posted By: MarkyPah, got it. So my minimum high extract rates sum to 45 l/s, I'll just scale those up by an adjustment factor until they match the 77 l/s supply.

Do don't need to sale them up, you can just increase any one or more of them so as to get them to match the required supply. This can be done so as to make best use of duct sizes and/or based on your wishes, e.g. if you are going to dry lots of washing in the utility you may give it a higher extract, but leave a shower rooms that will get little use with the part F minimal requirement.

I think to much fess is made about getting all the airflows correct between rooms etc, and provided they are reasonable the building will just sort itself out. (Assuming you donâ€™t have close fitting auto closing doors on your rooms!)
- CommentAuthorRobL
- CommentTimeFeb 15th 2017
I originally had an MVHR extract in our drying room, and found it took days to dry stuff. I swapped it round to an inlet, and it works much better, generally overnight is fine to wear clothes, and we usually leave 24 hours to fold clothes away.
The MVHR boosts if CO2 is high, and controls the humidity to 50%. With 4 people + washing + cooking, it's easy to keep humidity to 50%, but CO2 can still get to 1000ppm. So I think the extra humidity is handy with MVHR - it allows higher fan speeds (so keeping CO2 down) without drying out the house excessively, which isn't great for the skin. We live in Cambridge, if relevent.
- CommentAuthorringi
- CommentTimeFeb 15th 2017
Posted By: RobLI originally had an MVHR extract in our drying room, and found it took days to dry stuff.

Was your drying room cooler then the rest of the home?
- CommentAuthorbarney
- CommentTimeFeb 15th 2017
but CO2 can still get to 1000ppm

By any metric that's still representative of good air quality in an occupied space

Regards

Barney
- CommentAuthorjoe90
- CommentTimeFeb 15th 2017
<blockquote><cite>Posted By: ringi</cite><blockquote><cite>Posted By: RobL</cite>I originally had an MVHR extract in our drying room, and found it took days to dry stuff.</blockquote>

I find this surprising, others have said drying clothes in a room with extract works well, I wonder what is different or have I missed something?
- CommentAuthorPeter_in_Hungary
- CommentTimeFeb 15th 2017
Posted By: ringiI think to much fess is made about getting all the airflows correct between rooms etc, and provided they are reasonable the building will just sort itself out. (Assuming you donâ€™t have close fitting auto closing doors on your rooms!)

Isn't that the basis for which the freshR is supposed to work - just on an even cruder basis, one input and one extract.
- CommentAuthorEd Davies
- CommentTimeFeb 15th 2017
Posted By: joe90I find this surprising, others have said drying clothes in a room with extract works well, I wonder what is different or have I missed something?
I don't think it's surprising, the RH will be lower in a room with a supply. That's likely more significant than a slightly lower temperature.
- CommentAuthorMarkyP
- CommentTimeFeb 15th 2017 edited
Posted By: Peter_in_Hungary
Posted By: ringiI think to much fess is made about getting all the airflows correct between rooms etc, and provided they are reasonable the building will just sort itself out. (Assuming you donâ€™t have close fitting auto closing doors on your rooms!)

Isn't that the basis for which the freshR is supposed to work - just on an even cruder basis, one input and one extract.

possibly for a small dwelling. for my house they proposed 3 fresh-r units and I think 5 room to room fans to achieve adequate airflow which made the product very expensive compared to ducted MVHR systems.
- CommentAuthordjh
- CommentTimeFeb 15th 2017
Posted By: ringi
Posted By: djhMake sure the areas without vents (e.g. halls) are on a transit path for air

In most cases this is not important, as gases do defuse about and mostly sort themselves out.

I don't want to get into an argument so I suggest anybody reading this thread reads the research for themselves and forms their own opinion.
- CommentAuthorRobL
- CommentTimeFeb 15th 2017
<blockquote><cite>Posted By: ringi</cite><blockquote><cite>Posted By: RobL</cite>I originally had an MVHR extract in our drying room, and found it took days to dry stuff.</blockquote>

Was your drying room cooler then the rest of the home?</blockquote>

No - the whole house is a similar 16-20degC, depending on the sex of who you ask :-)

I think it's just low RH air dries stuff faster than 50% RH air.
- CommentAuthorwookey
- CommentTimeFeb 16th 2017
Posted By: gravelldGreat idea to work on an openly visible sheet!

Mine has been openly visible for a year, of course :-)

http://wookware.org/files/MVHR.ods
http://wookware.org/files/MVHR.xlsx

And has the exciting feature of working offline. Bit quaint in this day and age, I know, but I actually think that's an advantage. But if Marky wants to make a google sheet rather than improve mine, that's cool too.

What you _really_ want for this is a visual design tool where you draw the layout and indicate segment sizes/types/connections, then a sheet does all the sums. It'd be a lot easier to drive than filling in lots of error-prone bits of sheet that bake in the layout. Not sure what the easiest way to do that is, so I didn't :-)
- CommentAuthorMarkyP
- CommentTimeFeb 16th 2017
Posted By: wookey
Mine has been openly visible for a year, of course :-)

http://wookware.org/files/MVHR.ods
http://wookware.org/files/MVHR.xlsx

And has the exciting feature of working offline. Bit quaint in this day and age, I know, but I actually think that's an advantage. But if Marky wants to make a google sheet rather than improve mine, that's cool too.

What you _really_ want for this is a visual design tool where you draw the layout and indicate segment sizes/types/connections, then a sheet does all the sums. It'd be a lot easier to drive than filling in lots of error-prone bits of sheet that bake in the layout. Not sure what the easiest way to do that is, so I didn't :-)

your sheet has been a great help. I've already lifted some of the basic ideas and conversion calcs into mine. Rather than try to make a new version of yours, I'm finding it easier from a thought process point of view to build my own. And I had no easy way to share an Excel, where it's a doddle with google. Also I use google for a couple of reasons - anyone with a modern browser can view it, no need to download a sheet and open in bloated MS software. Another feature (admittedly not helpful here) is you could see me editing it in real time if you were viewing at the same time as I were working on the sheet, this is a powerful feature of google's Docs suite. In fact, if I changed the permissions to allow it, we could all be editing the sheet in real time. This can be really powerful in a buisness setting. Also it's worth adding that Google sheets is available offline, you just sync your google drive locally and work in a browser as normal.
- CommentAuthorSteamyTea
- CommentTimeFeb 16th 2017
Posted By: wookeyWhat you _really_ want for this is a visual design tool where you draw the layout and indicate segment sizes/types/connections, then a sheet does all the sums.
There is this to help with the drawing:

http://sketchup.engineeringtoolbox.com/hvac-components-to_16.html

And stuff of interest on this page:

http://www.engineeringtoolbox.com/ventilation-systems-t_37.html
- CommentAuthorEd Davies
- CommentTimeFeb 16th 2017
Posted By: MarkyP: â€œâ€¦and open in bloated MS softwareâ€¦â€

No need for bloated MS software. Bloated open-source software (LibreOffice) will do the job equally badly. I doubt Wookey used anything from MS to create these sheets and I didn't need any to read and play with them.

I too worry that spreadsheets are too difficult to read and audit for anything which is not very simple. A visual ventilation design tool would be great, particularly if it could extract the geometry from a more comprehensive building design system, but often a simple program can make these sort of calculations quite readable:

https://edavies.me.uk/2014/04/heatloss/

(Readable enough that, on a quick look just now, I noticed that the window U values seem to be swapped: the bedroom windows will be 2G as they're escape windows (top hinged) whereas the remaining windows (in the warmer parts of the house) will be 3G centre hinged. You can get 3G top hinged but they're very expensive. Still, that overestimates the heat loss very slightly so it's a â€œsafeâ€ error.)
- CommentAuthorMarkyP
- CommentTimeFeb 16th 2017
so a little progress on my sheet. As suggested by DJH, I've worked out the overall 0.3 requirement and converted this m3/h - it's a fairly eye popping amount of air to shift about at 279m3/h. The extracts at regs boost only amounted to 162m3/h, so an adjustment factor was applied to scale them up to match the 279 supply. this seems like a silly amount of air to shift, but if this is needed for signoff then I will use this as a worst case and expect to tune things down later.

I'm thinking of using the ubbink radial ducts, the semi cricular AE55SC looks good in terms of air speed for a given volume so using that to generate some more numbers. I've worked out the number of ducts and their lengths and and now looking at the air speed figures for each duct. I looked at the manufacturer data sheet, the air speed relationship to the volume of airflow is linear and works out at 1m/s per 19.6m3/h. So am I on the right track to use this to work out the air speed in each duct? I have the m3/h value for each duct and I've divided by 19.6 to get the airspeed. Unless I've missed something, the numbers look good - even with the system at full chat the worst duct is just over 3m/s. Only doubt is my crude calc doesnt allow for duct length which I have a nagging feeling I should have factored somehow, or is this just a case of balancing using restrictor rings to balance the flow across each duct at commissioning? Main goal at this point is to know the duct air speeds will be low from a noise point of view. next step is to work out the pressure side of things and then I can size the MVHR unit
- CommentAuthorringi
- CommentTimeFeb 16th 2017
You may find it is worth doubling up on ducts on some long runs even if the air speed is OK, so that you need less pressure. Ideally you want the pressure drop on all duct runs at the required flow rate to be about the same.
- CommentAuthordjh
- CommentTimeFeb 16th 2017
Posted By: MarkyPI'm thinking of using the ubbink radial ducts, the semi cricular AE55SC looks good in terms of air speed for a given volume so using that to generate some more numbers.

My system uses the AE35 ducts and we only had to double up for the kitchen (with hindsight I probably would have put two extract vents in the kitchen instead of doubling up). My design meets the passivhaus design criteria, which are stricter than Part F when it comes to speed and noise, so I'm surprised that you need to go to AE55. Do you have enough vents?

Duct length, and the number of bends, is critical. With the Ubbink system, you don't balance anything at commissioning time, it is all done at design time. Which is just as well because changing a restrictor ring after everything is installed could well be extremely difficult. Ubbink have a spreadsheet that calculates everything.

Note that whilst the semi-rigid ducts provide a lot of flexibility in routing the ducts, you do need to consider the congestion at the distribution boxes and how you will route each duct out of that area. I was able to cross one duct over another in the space between the metal webs of my joists, which was essential.
- CommentAuthorMarkyP
- CommentTimeFeb 16th 2017 edited
Posted By: djh
Posted By: MarkyPI'm thinking of using the ubbink radial ducts, the semi cricular AE55SC looks good in terms of air speed for a given volume so using that to generate some more numbers.

My system uses the AE35 ducts and we only had to double up for the kitchen (with hindsight I probably would have put two extract vents in the kitchen instead of doubling up). My design meets the passivhaus design criteria, which are stricter than Part F when it comes to speed and noise, so I'm surprised that you need to go to AE55. Do you have enough vents?

Duct length, and the number of bends, is critical. With the Ubbink system, you don't balance anything at commissioning time, it is all done at design time. Which is just as well because changing a restrictor ring after everything is installed could well be extremely difficult. Ubbink have a spreadsheet that calculates everything.

Note that whilst the semi-rigid ducts provide a lot of flexibility in routing the ducts, you do need to consider the congestion at the distribution boxes and how you will route each duct out of that area. I was able to cross one duct over another in the space between the metal webs of my joists, which was essential.

the AE55 ducts gave me a reasonable air speed of 3m/s of less, albeit at the max regs requirement of 279m3/h. I dont expect to need to run boost at that rate in practice but thought I should cater for the worst case. I will have a play with the numbers of the smaller duct and see. Not sure how adding more vents would help me, I have enough in terms of coverage for supplies and extracts, but would be adding duct runs to allow me to use a narrower ducts, think I'd prefer fewer bigger ducts over more smaller.

The distribution boxes are proving to be a bit of a headache. I think I can make it work by buying the 24 port boxes which mean I can take ducts into three sides of each box and avoid a spagetti situation.

How do you access the Ubbink calculator? sounds interesting..
- CommentAuthorringi
- CommentTimeFeb 17th 2017
Posted By: MarkyPThe distribution boxes are proving to be a bit of a headache. I think I can make it work by buying the 24 port boxes which mean I can take ducts into three sides of each box and avoid a spagetti situation.

There is no reason you can't use more then one distribution box and connect them back to your MVHR unit with large rigid ducts.
- CommentAuthordjh
- CommentTimeFeb 17th 2017
Posted By: ringi
Posted By: MarkyPThe distribution boxes are proving to be a bit of a headache. I think I can make it work by buying the 24 port boxes which mean I can take ducts into three sides of each box and avoid a spagetti situation.

There is no reason you can't use more then one distribution box and connect them back to your MVHR unit with large rigid ducts.

Interesting; I hadn't seen those new distribution boxes before. Do get the noise reduction option. I see they call the calculator 'a commissioning tool'. I think it will be much easier to use a single 24-port box as you suggest if you can make a feasible layout. Using multiple boxes would mean getting back into the bad old days of balancing calculations etc, as well as taking yet more space and extra expense.
- CommentAuthorwookey
- CommentTimeFeb 17th 2017
'Just sync my google drive' to use the sheet offline.

Hmm, pretty sure I haven't got one of those, and also sounds a lot like being dependent on a very nosey corporation (which exists by selling my data) for my software. Not at all keen on that. But you are right that the option does improve the functionality.

And yes of course I don't have MS office - that was written with Libreoffice calc. (I normally use Gnumeric, but that sheet found a major bug in named cells exported then imported via ODF).

I quite understand about having to re-write these things to actually understand them. Spreadsheet is pretty-much a write-only language, that only very recently acquired named-variables. It's vile, but handy.
- CommentAuthorringi
- CommentTimeFeb 17th 2017 edited
Posted By: djhUsing multiple boxes would mean getting back into the bad old days of balancing calculations

Not if the ducts to them where oversized so they had no real effect on air flow rates.

Posted By: djhas well as taking yet more space

That depends on the layout, I expect that for a lot of homes, having separate boxes on each floor may save space as some of the boxes will fit between floor joists.

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Ventilation: DIY MVHR design and calculations