Are these MVHR numbers correct?

[SBMS]

So we’ve got a fairly large sqm required for MVHR - circa 400sqm. A couple of other MVHR suppliers (including BCP) have suggested large units such as airflow and komfovent to shift up to 900m3. 
 

However Paul heat recovery has come back with the zehnder comfoair q600 with a much lower estimate required for airflow. 
 

Does the below look right? In particular it looks like they exclude the hallway and landing areas from their estimates (cascade ventilation)?

 

 

 

[Ben1984]

Following with interest. Currently experiencing something very similar - had 4 quotes for MVHR install (~400m2). 2 are adamant I need two units and the other 2 are saying the Zehender 600 will be sufficient. 

[SBMS]

7 minutes ago, Ben1984 said:

Following with interest. Currently experiencing something very similar - had 4 quotes for MVHR install (~400m2). 2 are adamant I need two units and the other 2 are saying the Zehender 600 will be sufficient. 

Who have you had quotes from? Have you tried Paul heat recovery (what did they say if so?)

[Ben1984]

Just now, SBMS said:

Who have you had quotes from? Have you tried Paul heat recovery (what did they say if so?)

CVC and BPC are both saying I need 2 units (or a large commerical unit that is not cost effective)
The Air shop and Paul heat recovery have both specified the Zehender 600. (It's worth noting that Paul Heat Recovery missed some rooms on the initial design, but even after amending it they are sticking with the Zehender 600).

[Ben1984]

BPC's quote for 2 x Vent Axia units is by far the cheapest @ £4700. They also quoted for 2 x Zehnder units (Q350 and Q450) @ £8000 this is also cheaper than both quotes i've received for a single unit. I prefer the idea of one single unit, to simplfy the design and reduce maintenace costs but that is currently more expensive and I am concerned it would struggle when the house is busy.

Edited Thursday at 21:19 by Ben1984

[SBMS]

Same dilemma. And Paul heat recovery have over estimated mine as the top floor bedrooms are rooms in roof so not 2.4m high for the whole room…

[SBMS]

The thing for me is the fact they’ve excluded the volume of the hall and landings as that is quite a bit of volume. 

[Ben1984]

Whilst I know very little on the subject, my concern is that Paul heat recovery and the Airshop have manupilated the figures a bit to try and make one unit 'work' when in reality it could struggle in certain situations. I have plenty of space in my plant room, so I'm currently inclined to go with 2 x Vent Axia units. But I won't be placing the order for a few months so I'm keeping my options open.

[Nick Laslett]

This is a complicated topic. My memory from the research I did here back in 2019/20, was that for building regs the MVHRs were turned up quite high, but once the dwellings were being lived in, the MVHR was turned down. This was stated by at least three members here. 
 

This is what they imply with the statement commissioned on Level 3 for ventilation requirements.

 

The supply requirements are less stringent, which is why you see the two ventilation numbers, one for BR using the number of bedrooms, the other is a Passive House standard. 

Edited Thursday at 21:30 by Nick Laslett

[JohnMo]

For building regs compliance you need a given volume flow, this is calculated differently in England and Scotland. So go to the relevant page in the building regs.

 

But passivhaus work on approx 0.3ACH. 

 

The issue with a big house (you have a big house) is over ventilation and the house air getting very dry in winter. 

 

1 minute ago, SBMS said:

over estimated mine as the top floor bedrooms are rooms in roof so not 2.4m high for the whole room…

Ventilation is nothing to do with bedroom volume - it's all about how much CO2 and humidity is being generated. Two people in a big or small room generate the same amount of each.

 

3 minutes ago, SBMS said:

fact they’ve excluded the volume of the hall and landings as that is quite a bit of volume. 

They have include volume and the ventilation mode is cascade. Cascade is a pretty standard way of ventilation. So imagine air being pumped in to a bedroom and being extracted via bathroom, the hall is being ventilated via cascade.

[SBMS]

1 minute ago, JohnMo said:

Ventilation is nothing to do with bedroom volume

If ventilation is nothing to do with volume why do all the calculations and sizing reference volume and why do the units have sizing related to volume (genuine question as this repeatedly confuses me. If ventilation is to do with number of occupants generating co2 then shouldn’t mvhr units be based on number of people in the dwelling?)

[JamesP]

I had a problem with being recommended and purchased  a Vent Axia based only on area and not volume. All based on 2.4m ceiling heights.

Due to exceptional ceiling heights in 50% of our dwelling the MVHR could not reach the minimum requirements for building control even at 100% boost.

 

I had to be a little creative with figures for BC to pass.

 

However our MVHR runs 24/7 at 30% and seems adequate for our needs.

 

The Zehender 600 has an air volume of up to 600m3/h at 200Pa.

 

 

 

 

[Ben1984]

1 minute ago, JohnMo said:

 

Ventilation is nothing to do with bedroom volume - it's all about how much CO2 and humidity is being generated. Two people in a big or small room generate the same amount of each.

 

This is an interesting point, I remember reading something about this before. If this is true, why does everyone seem to use the size/volume of the house to specify the MVHR unit? Is this just to ensure it meets building regs?

[Ben1984]

4 minutes ago, JamesP said:

I had a problem with being recommended and purchased  a Vent Axia based only on area and not volume. All based on 2.4m ceiling heights.

Due to exceptional ceiling heights in 50% of our dwelling the MVHR could not reach the minimum requirements for building control even at 100% boost.

 

 

I had to be a little creative with figures for BC to pass.

 

However our MVHR runs 24/7 at 30% and seems adequate for our needs.

 

The Zehender 600 has an air volume of up to 600m3/h at 200Pa.

 

 

 

 

Interesting. Based on your experience it would seem that if the unit can meet building regs requirements it should be perfectly fine with day-to-day ventilation. 
So the single units should in fact cope just fine on my build (assuming they have been designed to meet building regs)

[Nick Laslett]

24 minutes ago, SBMS said:

If ventilation is nothing to do with volume why do all the calculations and sizing reference volume and why do the units have sizing related to volume (genuine question as this repeatedly confuses me. If ventilation is to do with number of occupants generating co2 then shouldn’t mvhr units be based on number of people in the dwelling?)

BR requirement for supply is based on no. bedrooms. That is what you see in the first line of the Paul quote. 
BR uses a different criteria for extract. 
There is possibly an “and/or” requirement in BR to satisfy the criteria. 
 

MVHR are not designed to satisfy UK building regs, but to give you a pleasant house environment (forgive gross oversimplification).

 

But if you install the right MVHR unit it will comply with both Building Regs. 
 

The MVHR thread here does explain it, and the spreadsheet notes cover the calculations in detail. Just like the heat loss topic, you can do your own calculations. I will find the link. 
 

 

First post has the MVHR_sizing spreadsheet, and ventilation building regs document. 

This is what I posted in 2020 to help myself to understand the building regs requirements circa 2020. This was from the Vent Asia website at the time. 
 

Quote

Take the total floor space x 0.3 l/s = Minimum Supply amount. Then add up the building regs minimum for the Extract rooms, e.g. Kitchen=13 l/s, Bathroom=8 l/s, WC=6 l/s, Utility=8 l/s. This is the Minimum Extract amount. The Extract amount must be at least equal to the Supply, but can be greater. Add the two together and this is the total l/s the MVHR must be able to supply to satisfy building regs. 

0.3l/s equates to 1.08m3/hr per metre of floor space. As most manufacturers quote in M3/hr then it's probably easiest to use those units.

 

Edited Thursday at 21:56 by Nick Laslett

[JohnMo]

10 minutes ago, SBMS said:

If ventilation is nothing to do with volume why do all the calculations and sizing reference volume and why do the units have sizing related to volume (genuine question as this repeatedly confuses me. If ventilation is to do with number of occupants generating co2 then shouldn’t mvhr units be based on number of people in the dwelling?)

Because building regs tell you too. It's also based on number of people that could be living there. 

 

Passivhaus have a figure for single bedroom and a different one for double rooms. Wet rooms have have there own figures. BR sort of do the same.

[SBMS]

3 minutes ago, JohnMo said:

Because building regs tell you too. It's also based on number of people that could be living there. 

 

Passivhaus have a figure for single bedroom and a different one for double rooms. Wet rooms have have there own figures. BR sort of do the same.

Would You agree with Paul heat recovery’s assessment of the size of unit required or do you think the unit would run undersized?

[SBMS]

I asked ChatGPT to do me an estimate and it was surprisingly useful. It asked me bedrooms, volume, sqm etc and concluded with the following:

 

 

🧮 Here’s what that means in practice:

 

• Normal continuous mode: You can expect around 400–460 m³/h (≈ 110–130 L/s), depending on your ducting layout ().
• Boost or peak mode: Up to 600 m³/h (≈ 166 L/s), assuming your duct system doesn’t exceed 200 Pa pressure drop  .
• Free-air (no duct resistance): Might get up to 700–750 m³/h, but this isn’t realistic once ductwork is in place ().

 

✅ So: Is the Q600 enough for your needs?

• Continuous operation (~430–470 m³/h needed): ✅ Yes, Q600 handles this well, especially with low-pressure ducting.
• Boost mode (~650–720 m³/h ideal): ⚠️ It’s a bit below ideal, maxing at ~600 m³/h. You might see slower clearing during showers/cooking, but this is a common and workable compromise.

 

[Nickfromwales]

The world is doomed......

[Nick Laslett]

This is the thread I was thinking of with an excellent post from @HerbJ breaking down the MVHR numbers for another member. 
 

 

[JohnMo]

9 hours ago, SBMS said:

Would You agree with Paul heat recovery’s assessment of the size of unit required or do you think the unit would run undersized?

If the quoted unit is capable of 600m³ and it's being asked to sub 400m³ at boost I see zero issues.

 

I would expect to drop to level 1 (220m³) once the house has dried out - unless there are 10 people living in the house.  Also get a humidity sensor, in winter you don't want to drop below about 40% RH.

[SBMS]

11 hours ago, JohnMo said:

being asked to sub 400m³ at boost I see zero issues.

It’s coming out at around 650-720m3 according to the ChatGPT estimate… who knows which is right?!

[JohnMo]

Without fully check AI answers, I wouldn't trust them at all. The answers just change depending how you ask.

 

Go to the core document - building regs - what does that say? Understand BR, it makes life way easier, no one can pull the wool over your eyes. I could read it for you - but...

[SBMS]

9 minutes ago, JohnMo said:

Without fully check AI answers, I wouldn't trust them at all. The answers just change depending how you ask.

 

Go to the core document - building regs - what does that say? Understand BR, it makes life way easier, no one can pull the wool over your eyes. I could read it for you - but...

To be fair the AI calculated the BR value which I checked and it was correct (111l/s). The occupancy figure was around 120-130l/s. So fairly similar. 

[JohnMo]

13 minutes ago, SBMS said:

To be fair the AI calculated the BR value which I checked and it was correct (111l/s). The occupancy figure was around 120-130l/s. So fairly similar. 

as I said you need to read building regs, have assumed you are in England. Not sure where your figures align with what BR asks for?

 

From building regs 

Mechanical ventilation with heat recovery

Ventilation rates

1.67 For dwellings using mechanical ventilation with heat recovery, each habitable room should have

mechanical supply ventilation. The total supply air flow should be distributed proportionately to

the volume of each habitable room.

1.68 Mechanical supply terminals should be located and directed to avoid draughts.

1.69 The minimum total continuous rate of mechanical ventilation with heat recovery is the whole

dwelling ventilation rate in Table 1.3

1.70 For dwellings using mechanical ventilation with heat recovery, each wet room should have a

minimum continuous mechanical extract ventilation high rate as given in Table 1.2.

 

Tables are