Calculating ducting size

[jayc89]

Given a list of room volumes and distances back to the MVHR unit location, is there a calculation I can use to determine the optimum duct sizing? I'd prefer to use 75mm everywhere, but given the size and distance away from the unit some rooms are, I'm not sure they will be suitable. 

[JohnMo]

A lot of the duct manufacturer will give clear guidance in the duct datasheets.  Most ducting are pretty much the same, so would be worth going through some datasheets and taking a look.  Zehnder for example usually have decent information.

[Conor]

I went by the pressure drop guide in the Lindab catalogue. They also gave figures for bends etc. Flow rate on boost setting and duct diameter determine hghest airflow velocity and therefore pressure loss. I was aiming for a pressure loss of about 100kpa across the system. The lindab 76mm ID single ducts will be OK for small rooms up to 7m away from the unit. I doubled up everything beyond about 8m or so. Also to larger bedrooms and the kitchen unit where you want a higher flow rate. I didn't even entertain the 63mm ID ducts as they'd all have to be doubled.

 

Remember that the cross sectional area of a circle increases by a factor of 4 as diameter doubles. So a 100mm duct has 4x the "capacity" of a 50mm.

[jayc89]

Using 90mm (OD) ducting gives me a total pressure drop of 133 Pa which I assume is too high, meaning some need to be larger ducts.

 

Is there a target max pressure drop per run?

[Iceverge]

Yikes, we used 75/63mm ducts everywhere on a suck it and see basis apart from the kitchen which is doubled. 

 

Seems to work fine. 

[jayc89]

  On 09/01/2023 at 17:29, Iceverge said:

Yikes, we used 75/63mm ducts everywhere on a suck it and see basis apart from the kitchen which is doubled. 

 

Seems to work fine. 

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What was the max run distance? Because mine's a retrofit, my max run is approx. 20m. In terms of pressure drop, the worst looks to be the master bedroom run. 40m3 volume and a run of approx. 16m.

[Nickfromwales]

I've got runs of 35m in my current PH job and I have simply doubled up with 92mm ducts. I expect zero issues / elevated sound levels etc.

Trickle flow rates are equivalent to a decent sized hamster coughing. People seriously over-think this. 

[jayc89]

  On 09/01/2023 at 17:41, Nickfromwales said:

I've got runs of 35m in my current PH job and I have simply doubled up with 92mm ducts. I expect zero issues / elevated sound levels etc.

Trickle flow rates are equivalent to a decent sized hamster coughing. People seriously over-think this. 

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What would you consider to be the longest run length before you need to start thinking about doubling up?

Edited January 9, 2023 by jayc89

[Nickfromwales]

  On 09/01/2023 at 17:46, jayc89 said:

 

What would you consider to be the longest run length before you need to start thinking about doubling up?

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Depends on the room / space, and what it's doing ( fresh or extract ) and so on.

[Iceverge]

  On 09/01/2023 at 17:34, jayc89 said:

 

What was the max run distance? Because mine's a retrofit, my max run is approx. 20m. In terms of pressure drop, the worst looks to be the master bedroom run. 40m3 volume and a run of approx. 16m.

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About 15m was the longest one via some sweeping bends. 

 

From memory about 30m³/HR was the rate I planned for it. 

 

 

 

 

[Miek]

   BPC gave me this general advice for 75mm ducts

 

  Do not place valves above showers (Outside shower area)

· Place the Kitchen Extract Valve near the sink, not above the cooker.

· Kitchen, Bathroom, En-Suite and Utility Valves require a double duct run.

· If the duct run is greater than 15m, double the ducting.

· If the duct run is greater than 20m, triple the ducting.

· If the room is greater than 20m2, the valve requires a double duct run.

· Do not exceed runs of 30m.

Edited February 10, 2023 by Nickfromwales
75mm instead of 75m

[Post and beam]

Apologies for jumping into this thread. When you say 'double up' does this mean 2  outlets in that room or is there a 2 into 1 junction box just before the outlet?

Thanks in advance

keith

[Miek]

2 pipes Into one plenum/vent

[MikeSharp01]

  On 09/01/2023 at 16:51, jayc89 said:

Using 90mm (OD) ducting gives me a total pressure drop of 133 Pa which I assume is too high, meaning some need to be larger ducts.

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How did you work this out - did you include the incoming / outgoing ducts feeding fresh air in and stale out to the atmosphere. Also remember that runs in parallel don't just add up.

[jayc89]

Assuming my calcs are correct, that would mean I need 32x duct runs, resulting in a total pressure drop across the system of 234 Pa, which I thought was too high? 

[jayc89]

  On 09/01/2023 at 20:40, MikeSharp01 said:

How did you work this out - did you include the incoming / outgoing ducts feeding fresh air in and stale out to the atmosphere. Also remember that runs in parallel don't just add up.

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My last post includes some of the workings out. It doesn't include Supply/Extract ducting to/from the MVHR, just the ducts to/from the rooms. How should parallel runs be accurately accounted for?

[MikeSharp01]

  On 09/01/2023 at 20:45, jayc89 said:

My last post includes some of the workings out. It doesn't include Supply/Extract ducting to/from the MVHR, just the ducts to/from the rooms. How should parallel runs be accurately accounted for?

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I think you only need to do the extract side of the system as the supply has another fan and to keep things in balance will deliver the same volume of air (so you will need to adjust it down to 57). So each side can be independent and that will sort of halve you extract pressure drop to 90 (ish) much better and your supply at about 140 but each fed by their own fan. Least I hope I have that right as I have always assumed there are two systems and the pressure unit ratings are for the two independently not added together.

Edited January 9, 2023 by MikeSharp01
correcting figures

[MikeSharp01]

PS there is also the old adage of keeping the flow velocity in any duct below 2.5ms for reason of noise.

[jayc89]

Urgh, it's been a long day - that would make more sense

 

I'll tackle crunching the flow velocity numbers tomorrow, I already have brain ache

[jayc89]

  On 09/01/2023 at 21:03, MikeSharp01 said:

PS there is also the old adage of keeping the flow velocity in any duct below 2.5ms for reason of noise.

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That looks to be around 30m3/h for 75mm ducting. Due to the distance, or room size, I'm already planning on doubling up the ducts anywhere that's nearing 30m3/h, so each duct should be well below 2.5ms

 

 

That's based on 0.3 l/s per m2 for Supply and Building Regs minimum per room type for Extract (i.e. 13 l/s for kitchens etc)

Edited January 9, 2023 by jayc89

[jayc89]

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? 

[Nickfromwales]

  On 09/01/2023 at 20:14, Post and beam said:

Apologies for jumping into this thread. When you say 'double up' does this mean 2  outlets in that room or is there a 2 into 1 junction box just before the outlet?

Thanks in advance

keith

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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. 👌

[JohnMo]

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

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?

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I just found it easier to use 90mm throughout, instead of 75mm.  Most runs in our house would have been double runs in 75mm, but are a single run in 90mm.  Less hassle running and ended up cheaper overall.

 

The other thing to think about, and this why you are managing the pressure drop.  You manage the pressure drop, because a low pressure drop allows the MVHR fans to run further down their performance curve, i.e. they run slower, so makes less noise and does less work (less kWh ultimately) for a given flow rate.

[Nickfromwales]

Yup. You should be making the runs as least resistance as possible, to ensure fan speed is minimal and audibility is low / non existent. 
Remember to include box attenuators at the machine to kill the noise at source. Install them on the ducts to atmosphere also if you have neighbours / other noise close by. 

[jayc89]

90mm ducting certainly reduces the pressure drop across the whole system to below 100 Pa. It looks like I'd only need to double up on the 2x longest supply runs (living area and master bedroom), would you still recommend doubling up on all the extracts?