Calculating ducting size

[JohnMo]

  On 10/01/2023 at 15:35, jayc89 said:

would you still recommend doubling up on all the extracts

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None of my extract were doubled up (90mm), longest runs about 12-14m.  No issues

 

Our master bed is approx 18m and 19m runs, we use two outlets and single run to each, which gave a combined flow total of 45m3/h.  You definitely need 2 x90mm ducts at that length, to get 45m3/h over that distance, otherwise you would be turning up the fan speed too much to compensate.

 

Just looking at your flow rates in your bedrooms and something doesn't look right.

 

Master bed rate seems very low?  Would expect to see around 45m3/h when combined with the wardrobe flow for a double room.  Other bedrooms if a guest or single bedroom are fine.  Your living area flow seems very low for a communal area, you have more flow in an under stairs cupboard for example.

[jayc89]

  On 10/01/2023 at 16:24, JohnMo said:

None of my extract were doubled up (90mm), longest runs about 12-14m.  No issues

 

Our master bed is approx 18m and 19m runs, we use two outlets and single run to each, which gave a combined flow total of 45m3/h.  You definitely need 2 x90mm ducts at that length, to get 45m3/h over that distance, otherwise you would be turning up the fan speed too much to compensate.

 

Just looking at your flow rates in your bedrooms and something doesn't look right.

 

Master bed rate seems very low?  Would expect to see around 45m3/h when combined with the wardrobe flow for a double room.  Other bedrooms if a guest or single bedroom are fine.  Your living area flow seems very low for a communal area, you have more flow in an under stairs cupboard for example.

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I've split the master in bedroom/wardrobe/ensuite. The bedroom + wardrobe combined is 9.3 l/s, or 33.5 m3/h, based on 0.3l/s/m2. All 4 bedrooms are doubles. 3 occupied full time, one being a guest bedroom. Similarly the living area was calculated using 0.3l/s/m2, is this not correct?

 

The under-stair cupboard is artificially high, it's part below ground (previously the access into a now, covered over, cellar), so is prone to dampness in the walls. The walls have since been re-rendered in lime, floor dug up and replaced using limecrete etc, so it's just a case of increase ventilation down there. 

[JohnMo]

Some background our houses and what I found

 

Our master bedroom 12 l/s and wardrobe with 6l/s, (just checked the test report, figures in previous post were low) so a combined 20 l/s will get up to just over 1000ppm CO2, with the door closed over night, I wouldn't want a supply rate less than this in a double bedroom.

 

Our living room has 2 terminals with a total of 20l/s combined.  Had 6 in the room for about 6 hours by hour 4 the CO2 was at 1300ppm, so it was put on boost.  With 2 or 3 people it stays in 5-600 region.

 

Both rooms are big, but still need the air changes to keep CO2 low. 

 

Use most of your flow where you want good air quality, the rest of the house under ventilate, to get the correct average over the house.

.

[jayc89]

  On 10/01/2023 at 16:55, JohnMo said:

Some background our houses and what I found

 

Our master bedroom 12 l/s and wardrobe with 6l/s, (just checked the test report, figures in previous post were low) so a combined 20 l/s will get up to just over 1000ppm CO2, with the door closed over night, I wouldn't want a supply rate less than this in a double bedroom.

 

Our living room has 2 terminals with a total of 20l/s combined.  Had 6 in the room for about 6 hours by hour 4 the CO2 was at 1300ppm, so it was put on boost.  With 2 or 3 people it stays in 5-600 region.

 

Both rooms are big, but still need the air changes to keep CO2 low. 

 

Use most of your flow where you want good air quality, the rest of the house under ventilate, to get the correct average over the house.

.

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Thanks, that's really useful. How large is your bedroom/wardrobe and living room for comparison?

 

Assuming my calcs are actually correct, at 0.3 l/s/m2 the per room ACH ranges from 0.36 to 0.432. If I bump that to 0.4 l/s/m2 they range from 0.48 to 0.576. With the master bedroom + wardrobe being supplied with 12.4 l/s (or 44.6 m3/h) and the living area being supplied with 6 l/s (or 21.6 m3/h). Both ACH of 0.576. 

 

My assumption was that I'd dial this numbers down once signed off by BCO, not need an increase

[JohnMo]

Lounge is about 6m x 6m, with vaulted ceiling and at peak about 6m.  Master bedroom 5 x 5m and average 3m tall.  Wardrobe 2m x 2m and 2.5m tall, it as louvre door for unrestricted air flow.

[jayc89]

  On 10/01/2023 at 18:42, JohnMo said:

Lounge is about 6m x 6m, with vaulted ceiling and at peak about 6m.  Master bedroom 5 x 5m and average 3m tall.  Wardrobe 2m x 2m and 2.5m tall, it as louvre door for unrestricted air flow.

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Thanks. Your rooms are slightly larger than mine and not a million miles of my updated numbers.

 

 

In terms of sizing the MVHR unit, what sort of multiplier should be applied to the overall house demand to ensure I have a sufficiently sized unit that isn't being laboured under normal load?

[MikeSharp01]

  On 10/01/2023 at 19:01, jayc89 said:

In terms of sizing the MVHR unit, what sort of multiplier should be applied to the overall house demand to ensure I have a sufficiently sized unit that isn't being laboured under normal load?

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We are looking at this right now and we have been told, 3 sources and here, that you should aim at the unit running between 50 & 65% of max capacity in normal mode.

[jayc89]

  On 10/01/2023 at 19:32, MikeSharp01 said:

We are looking at this right now and we have been told, 3 sources and here, that you should aim at the unit running between 50 & 65% of max capacity in normal mode.

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Thanks. So, assuming the calcs are correct, I'm looking at a unit with approx. 110 l/s or 400 m3/h. 

[MikeSharp01]

  On 10/01/2023 at 19:48, jayc89 said:

Thanks. So, assuming the calcs are correct, I'm looking at a unit with approx. 110 l/s or 400 m3/h. 

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Again I think the units are quoted per fan, so you need a bit less - 75L/s plus you must work the system so supply and extract are handling the same volume otherwise you will get a rise in pressure across the house. You need to decrease the supply or increase the extract - naturally ensuring you stay above the statutory (Building Regs) limits.

[jayc89]

  On 10/01/2023 at 19:57, MikeSharp01 said:

Again I think the units are quoted per fan, so you need a bit less - 75L/s plus you must work the system so supply and extract are handling the same volume otherwise you will get a rise in pressure across the house. You need to decrease the supply or increase the extract - naturally ensuring you stay above the statutory (Building Regs) limits.

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That was based on the max of the supply and extract rates (72.8 l/s) x 1.5 = 110 l/s. Of course in reality, the extracts would be ramped up to match the total supply rate. 

[Post and beam]

  On 10/01/2023 at 11:52, Nickfromwales said:

Depends on the application / room. 
If I want super-quiet I’ll do 2 single runs to 2 valves, or if it’s for flow ( en suite shower ) I’ll just do 2 runs to 1 valve because on boost it won’t be heard anyways. 
Designs should be bespoke, and adapted to each project room by room. 👌

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Thanks Nick. Useful.

[Post and beam]

I could do with a knowledgeable dude to look over a quote for MVHR install i have been given for my project. A 200 square metre 4 bed house. Via DM  preferably

Regards

keith

[Post and beam]

Duct sizing requirements will depend on whether it is a branched or radial design then i guess ? The quote i got today is branched with 125mm semi rigid aluminium.

[Nickfromwales]

  On 10/01/2023 at 22:22, Post and beam said:

Duct sizing requirements will depend on whether it is a branched or radial design then i guess ? The quote i got today is branched with 125mm semi rigid aluminium.

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These series "branched" systems just seem so impractical and impactful from an installation PoV, plus typically start off with larger bore for larger dwellings. I will always stick to radial systems with 92mm or its 'oval' equivalent ( which is almost rectangular btw ).

If you don't want huge posi-joists just for the sake of getting MVHR installed, it's got to be radial.

[Nickfromwales]

  On 10/01/2023 at 21:31, Post and beam said:

I could do with a knowledgeable dude to look over a quote for MVHR install i have been given for my project. A 200 square metre 4 bed house. Via DM  preferably

Regards

keith

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Just post it here, less the company's details if you want some more opinions.

[Post and beam]

Thanks Nick

 Build Criteria

A: New Build

B: Cold Roof Construction

😄 Stud Walls

😧 254mm Metal Web Floor Joists

 

Nervous about upsetting the build company unecessarily.

Edit: I wont put the cost up in a public forum but i am interested in a competent alternative option. The plot is in North hertfordshire.

Edited January 10, 2023 by Post and beam

[john0wingnut]

  On 10/01/2023 at 11:00, jayc89 said:

Took another stab at this, also taking velocity into account.

 

Velocity numbers were taken from https://www.cuidevices.com/product/resource/calculators/airflow-conversion using the minimum flow requirements (l/s) from Column K and the ID of the 75mm ducting (68mm)

 

I'd appreciate someone sanity checking my numbers to make sure I'm not talking out of my arse! (For context, there's an extract in the "Under-stair cupboard" as it's part-below ground so prone to slight dampness in the walls - old houses!)

 

 

Couple of questions though;

- Build Regs state a minimum flow of 0.3l/s per m2 of room area, but that doesn't take into account volume. Our downstairs rooms are quite tall (3m) so should I be looking at more than 0.3l/s/m2?

- Total supply pressure is > 100Pa. For the longer runs where there are 2x or even 3x ducts being run and therefore the pressure drop is greater, should I be looking at using less, but larger ducts instead?

- When sizing the MVHR unit, what multiplier should I apply to the demand to ensure the unit is right-sized. Is 1.3x sufficient? 

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Not sure if it’s me having a moment here, but,  taking your snug as an example - 21.87m3/h through a 68mm ID pipe would be 1.67 m/s via my maths:

 

21.87 m3/h / (Pi*0.034m^2) = 1.67 m/s

John

 

[jayc89]

  On 15/01/2023 at 00:44, john0wingnut said:

Not sure if it’s me having a moment here, but,  taking your snug as an example - 21.87m3/h through a 68mm ID pipe would be 1.67 m/s via my maths:

 

21.87 m3/h / (Pi*0.034m^2) = 1.67 m/s

John

 

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Thanks for taking the time. Interestingly, I've just ran the numbers through a couple of other online calculators and it returned the same 0.42 m/s result.

 

https://www.engineering.com/calculators/air-flow-conversion-calculator/

http://www.calculatoredge.com/optical engg/air flow.htm

 

It's entirely possible I could be doing something wrong though!

[dpmiller]

It's the radius not the diameter you need to enter.

[john0wingnut]

As per dpmiller, it looks like you have put the diameter in, rather than the radius, so you would need to use 34mm

Edited January 15, 2023 by john0wingnut

[jayc89]

Good job I have this forum, think I'd end up with a Wendy house otherwise! 

[jayc89]

Does this look more sensible?

 

 

Some are nearing the magic 1.5 m/s number, should I look at doubling any of those up?

[MikeSharp01]

  On 15/01/2023 at 10:13, jayc89 said:

Some are nearing the magic 1.5 m/s number, should I look at doubling any of those up?

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2.5 m/s is a more common upper limit for air velocity generated noise so provded you are below that you should be OK.

[john0wingnut]

  On 15/01/2023 at 10:13, jayc89 said:

Does this look more sensible?

 

 

Some are nearing the magic 1.5 m/s number, should I look at doubling any of those up?

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Your rooms with 29.16m3/h should that translate to 1.7m/s with 90/78 radial pipe, rather than 1.27m/s

[jayc89]

Other than keeping velocity below 2.5 m/s per duct, are there any other golden rules that dictate when runs should be doubled/tripled up? Length/pressure?

 

  On 15/01/2023 at 22:38, john0wingnut said:

Your rooms with 29.16m3/h should that translate to 1.7m/s with 90/78 radial pipe, rather than 1.27m/s

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Good spot, updated, thanks.