MVHR specification?

[Philxyz]

After some wildly differing MVHR specifications/quotes I have attempted some basic calculations myself and wonder if they're of any use to help me choose installers. It's a small cottage (91m2) of four rooms: kitchen/living, bathroom, bedroom, office/bedroom. It will be airtight.

 

To calculate the whole house l/s flow rate I multiplied total m² floor space by 0.3: 91 * 0.3 = 27.3. Rounding up, I reached a whole house flow rate of 30l/s.

At 5l/s per duct* this means 6 supply ducts and 6 extract ducts.

 

Area	Floor m2	l/s(floor*.3)	Target l/s	No. ducts
Kitchen/living	42	12.6	20	4 extract
Bathroom	8	2.4	10	2 extract
Bedroom	12	3.6	15	3 supply
Office/bed	29	8.7	15	3 supply
TOTAL	91	27.3	30	6 extract/supply

 

*I have not attempted to calculate ducting resistance. Instead I have simply calculated using a 5l/s flow rate for 75mm semi-rigid ducting which is a lower figure I reached from searching the internet. The MVHR unit will be centrally located and all ducting can be installed with minimum bend and distance. Is this reasonable or does it make my whole attempt nonsensical?

[JohnMo]

Seems a bit of a strange way to do duct runs. Looks way too many ducts.

42 minutes ago, Philxyz said:

have not attempted to calculate ducting resistance

That's not right, you need to calculate duct length and pressure drop.

 

Your kitchen needs extract, but your living area is very likely going to need supply.

 

Use this to help decide the room flow rates required.

 

[Philxyz]

Thanks JohnMo.

 

The kitchen/living is one room. I was working on the idea that it is not good practice to have supply and extract in the same room. Is that wrong? Could I have extraction on the kitchen side and supply on the far end of the room?

 

I suspected this was over doing the amount of ducting but figured that being cautious and over engineering would allow low flow rates down each duct. One of the specifications I have been quoted had way more ducts. Others had less. My shot here falls in between.

[JohnMo]

8 minutes ago, Philxyz said:

was working on the idea that it is not good practice to have supply and extract in the same room.

You need to work out how the air will move around the house, so if air enters room A, B and C, where does it travel to get to the extract terminals, you are aiming to have air wash through the whole house. 

 

Plenty of good practical advice in the attached.

 

 

2018 MVHR Good Practice Guide rev 1.1.pdf

[MikeSharp01]

1 hour ago, JohnMo said:

That's not right, you need to calculate duct length and pressure drop.

and there be dragons - Mr Bernoulli’s Equation is no fun to apply and this has been discussed before:

 

 

[Kelvin]

8 hours ago, Philxyz said:

Thanks JohnMo.

 

The kitchen/living is one room. I was working on the idea that it is not good practice to have supply and extract in the same room. Is that wrong? Could I have extraction on the kitchen side and supply on the far end of the room?

 

I suspected this was over doing the amount of ducting but figured that being cautious and over engineering would allow low flow rates down each duct. One of the specifications I have been quoted had way more ducts. Others had less. My shot here falls in between.

We are open plan too. There’s a double extract behind the hob area and a double supply in the living room area. 

[JohnMo]

1 hour ago, MikeSharp01 said:

Bernoulli’s Equation

You don't need to get down to that detail, just get pressure drop data from the chosen manufacturer and they will list pressure drops for duct and fittings. You just need to do the most onerous route for a semi flex manifold system, you then add the plenum and the duct between MVHR and plenum to get the complete pressure drop. Get that as low as practical. 

[JohnMo]

12 hours ago, Philxyz said:

75mm semi-rigid ducting

I would look at 90mm also. Found it worked out way cheaper, as all runs became single ducts, for our house which is long and thin. With 75mm most would have ended double 75mm ducts to each terminal.

[Philxyz]

5 hours ago, Kelvin said:

We are open plan too. There’s a double extract behind the hob area and a double supply in the living room area. 

That is very helpful thanks Kelvin.

 

John and Mike, thanks very much for the PDF and thread link. I will respond later after some reading.

[Philxyz]

On 16/06/2024 at 20:48, JohnMo said:

That's not right, you need to calculate duct length and pressure drop.

 

I have calculated for my most difficult run (all the others are very simple of a few meters and a plenum)

Pressure drop calculation

For Ubbink Air Excellence 90mm ducting @40m3/h

• Drop per meter = 1.7Pa here
• Drop per 90° bend = 3.4Pa here
• Drop Plenum = 4.5Pa here
• Drop dbox = 5Pa here

My most difficult duct run:

5m length + 2 bends + plenum + dbox = (5 * 1.7) + (2 * 3.4) + 4.5 + 5 = 24.8Pa

 

But now what do I do with this? I presume it should inform my calculations of ducts per room but I cannot quite get my head around how!

 

Advise appreciated. Thanks.

[JohnMo]

13 minutes ago, Philxyz said:

My most difficult duct run:

5m length + 2 bends + plenum + dbox = (5 * 1.7) + (2 * 3.4) + 4.5 + 5 = 24.8Pa

It's basically tells you that you only need one duct run to each terminal point. As your max target is 100Pa and you are well below that.

 

On 16/06/2024 at 20:00, Philxyz said:

Kitchen/living 42 12.6 20 4 extract

Bathroom	8	2.4	10	2 extract
Bedroom	12	3.6	15	3 supply
Office/bed	29	8.7	15	3 supply
TOTAL	91	27.3	30	6 extract/supply

So your number ducts is now decreased to one to each room. You now need to look at the kitchen living space and assess if you need supply and extract. If the living space doesn't get a cross flow from elsewhere, you need a supply duct there and an extract in the kitchen as far from each other as possible.

[Philxyz]

16 hours ago, JohnMo said:

So your number ducts is now decreased to one to each room

OK, this I do not quite understand. I'll explain my calculations and perhaps you will be kind enough to point out where I am wrong.

 

Whole house air flow

The min requirements for kitchen/bathroom/bedroom are lower than m² by .3 so I use the latter.

91m² * 0.3 = 27.3l/s = 98.28m³/h = [99m³/h]

 

Calculations at 35m³/h per duct

 

Area	Floor m2	Floor*.3 m3/h	Extract m3/h	Supply m3/h	Extract ducts	Supply ducts
Kitchen/living	42	45.36	70	35	2	1
Bathroom	8	8.64	35		1
Bedroom	12	12.96		35		1
Office/bed	29	31.32		35		1
TOTAL	91	[99]	105	105	3	3

 

Pressure drop using Ubbink 90mm ducting

• Drop per meter = 1.3Pa here
• Drop per 90° bend = 2.6Pa here
• Drop Plenum = 3.5Pa here
• Drop dbox = 5Pa here

5m length + 2 bends + plenum + dbox = (5 * 1.3) + (2 * 2.6) + 3.5 + 5 = 20.2Pa
 

Ah. OK I think the penny may have dropped as I write this out. 

17 hours ago, JohnMo said:

your max target is 100Pa

My calculations above have a very low pressure drop. Should I readjust my calculations to have a higher flow rate whilst keeping Pa < 100? Let me try that now. The largest single room requirement is the kitchen (45.36m³/h) so I will use 50m³/h.

 

Calculations at 50m³/h per duct

 

Area	Floor m2	Floor*.3 m3/h	Extract m3/h	Supply m3/h	Extract ducts	Supply ducts
Kitchen/living	42	45.36	50		1
Bathroom	8	8.64	50		1
Bedroom	12	12.96		50		1
Office/bed	29	31.32		50		1
TOTAL	91	[99]	100	100	2	2

Pressure drop using Ubbink 90mm ducting

• Drop per meter = 2.7Pa here
• Drop per 90° bend = 5.2Pa here
• Drop Plenum = 7Pa here
• Drop dbox = 5Pa here

5m length + 2 bends + plenum + dbox = (5 * 2.7) + (2 * 5.2) + 7 + 5 = 35.9Pa

 

To summarise: both these calculations would work. The 35m³/h system has a massive headroom and uses more ducting than needed. The 50m³/h system has less but adequate headroom and uses fewer ducts. Go with the 50m³/h system.

 

Is this correct?

[JohnMo]

30 minutes ago, Philxyz said:

My calculations above have a very low pressure drop

Which is good. Means the fan in the MVHR unit has to do less work so makes less noise.

 

But think you are generally confusing yourself.

 

Step one

Set you room flow rates, these become fixed. Use the chart I posted the other day as a guide.  Make sure extract and supply figures are equal for the whole house.

Step two, plan you duct runs - for your most onerous run for both supply and extract) calculate the pressure drop with a single duct, at your normal flow rate for that duct. If below 100Pa, all good. You can use single duct runs everywhere.

 

Size your MVHR unit, really it needs to have a capacity of about double your normal flow rate, to keep noise down.

 

 

[G and J]

I wonder if an MVHR design service offering independent advice would make money.

 

Send in your plans, fill in a questionnaire, get back a list of requirements for each element, suggested vent locations, maybe a couple of suggested products to make searching easier to get into, etc.

 

I’m currently swamped with offers of ‘free (but biased) advice’.  Sigh. 

[JohnMo]

40 minutes ago, G and J said:

wonder if an MVHR design service offering independent advice would make money

Possibly no, as they would need to sell something, not many are willing to pay for a well thought through design. I was getting quotes of £10k for MVHR, they are building in an overhead for the design.  Decided to research and do it myself. Spent £2.5k.

 

Most properties could do a reasonable cascade system with coanda nozzles for a descent cost and very little duct. Our house was quoted for by Fresh-R on a cascade system and only had about 20 to 25m of duct all in. House is 25m long and about 7-10m deep. 

 

But designs are given out to sell product. 75mm ducts support 40 to 50% less flow than 90mm for a similar pressure drop, but most designs will use 2x75mm instead of 1x90mm, they sell twice the length of duct, more expensive double entry plenums etc.

[Kelvin]

1 hour ago, JohnMo said:

 

 

Size your MVHR unit, really it needs to have a capacity of about double your normal flow rate, to keep noise down.

 

 

This is great advice. I was in a house recently where the MVHR was really obvious and the owner admitted they’d undersized it. 
 

I had a design done that I threw away (and got my money back) it was impossible to install in the house. In the end we used half the ducting they’d originally specified. 

[Philxyz]

Thanks for bearing with me. Below I try to follow your steps as best I can but I am not quite there and have more questions:

 

4 hours ago, JohnMo said:

Step one

Set you room flow rates, these become fixed. Use the chart I posted the other day as a guide.  Make sure extract and supply figures are equal for the whole house.

• Straight from your chart I enter 47m³/h and 29m³/h for the kitchen and bathroom extractions respectively
• This gives a total of 76m³/h for extraction
• The bedrooms is less clear. Your chart says "2bed/3 person: 61m³/h". So I enter 30.5m³/h supply for each bedroom. Is this correct?
• To bring the supply up to equal the extraction I add a 15m³/h supply into the living area of the open  plan kitchen/living.

Area	Extract m3/h	Supply m3/h
Kitchen/living	47	15
Bathroom	29
Bedroom 1		30.5
Bedroom 2		30.5
TOTAL	76	76

 

Is this room flow rates table OK?

 

The total reached of 76m³/h is well below the whole house flow rate (91m² * 1.08) of 98.28m³/h.

Don't I need to raise both extract and supply up to reach the whole house rate?  

 

 

4 hours ago, JohnMo said:

Step two, plan you duct runs - for your most onerous run for both supply and extract) calculate the pressure drop with a single duct, at your normal flow rate for that duct. If below 100Pa, all good. You can use single duct runs everywhere.

 

What is 'your normal flow rate for that duct'? I don't know what that means. Where do I get it, or how do I  calculate it?

 

Thanks very much for your time.

[Kelvin]

We have two bedrooms master and guest. Smaller one (guest) is 20 and the larger one is 40. For comparison. 

[JohnMo]

6 minutes ago, Philxyz said:

Area Extract m3/h Supply m3/h Kitchen/living 47 15 Bathroom 29   Bedroom 1   30.5 Bedroom 2   30.5 TOTAL 76 76

You concluded at the start of the thread you needed 30L/s (108m3/h) whole house flow based on area. So your final supply and extract both need to hit that figure.

 

So main bedroom (use passivhaus rates) 40m3/h, second bedroom use 20m3/h, the rest into the lounge, so 48m3/h. So that equals 108m3/h.

 

Extract, kitchen 47m3/h (building regs most onerous), bathroom 37m3/h. So that falls short, so increase these, so kitchen say 60m3/h and bathroom 48m3/h. That will give you 108m3/h.

 

18 minutes ago, Philxyz said:

your normal flow rate for that duct

So for example your kitchen normal flow would be 60m3/h. Boost would be about 15% above that. So use the 60m3/h for that duct pressure drop calculation.

[Philxyz]

OK, putting together what I've learnt on this thread:

 

Area	Extract m3/h	Supply m3/h	Extract ducts	Supply ducts
Kitchen/living	60	48	1	1
Bathroom	48		1
Bedroom 1		40		1
Bedroom 2		20		1
TOTAL	108	108	2	3

 

Most erroneous duct run @ 60m³/h air flow rate:

• Drop per meter = 3.9Pa here
• Drop per 90° bend = 7.6Pa here
• Drop Plenum = 10.1Pa here
• Drop dbox = 5Pa here

5m length + 2 bends + plenum + dbox = (5 * 3.9) + (2 * 7.6) + 10.1 + 5 = 49.8Pa (<100 so OK)

 

Requirements:

• 90mm ducting (eg Ubbink Air Excellence)
• Coanda effect supply vents (eg Zehnder)
• MVHR unit with a flow rate of at least 216m³/h (eg Zehnder ComfoAir 350)

How does that all look?

 

Edited June 19 by Philxyz
incorect Pa calculation

[JohnMo]

24 minutes ago, Philxyz said:

How does that all look?

Looks good.

 

25 minutes ago, Philxyz said:

Coanda effect supply vents (eg Zehnder)

These go in a wall near the ceiling and will push air across room 4 to 6m. They also do a flow adjuster for these, it inserts into the 90mm duct. Looks like a bunch of short straws and various washers.

 

https://www.epicair.co.uk/products/comfotube-8960-90mm-adjustable-airflow-regulator

 

You will need the following also

2 port manifold for extract and 3 port manifold for supply. Ideally an attenuator or duct that does it between MVHR and each manifold.

 

90mm to 125mm adaptor for the extract terminals - such as UBBINK AE48C VALVE ADAPTOR 90 DEGREE 125MM.

 

 

[Philxyz]

Fantastic. Thanks very much for your great help John. And thanks to others.

[Philxyz]

Having got a design I now think I may install it myself. Maybe you would comment on my vent positions (see attached image). These are currently more positioned for ease of install. I suspect the far end away from the vent in Bedroom 1 might be lacking flow.

 

How do they look to you?

 

 

 

[JohnMo]

33 minutes ago, Philxyz said:

 

How do they look to you

I would expect the air flow would be quite good. Something like the attached.

 

But looking at the layout, you could eliminate the lounge supply as you will get good cross flow from the bedrooms, just divide the supply flow to both bedrooms equally. I had just assumed it was an upstairs and downstairs configuration.

 

 

Once you living with it you may be able to tweek the flows down also.

 

[Philxyz]

In fact there is a mezzanine above the bathroom and bedroom 2. It will be used for utility and storage (the MVHR unit for example). Not a living arera but will be regularly accessed. Is it worth moving that supply vent onto the mezzanine?