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I'm struggling and need your input.       I've rather foolishly designed and build a large volume house. The Kitchen / family room, the master and second bedrooms all have cathedral ceilings, as do two of the en-suites!

 

I send my drawings to a number of MVHR suppliers who offer design services and got three quotes back, each one was different  in what was specified and none provided technical detail around calculations. The simplest design was a single MVHR unit and at the other end of the scale, they specified two of the largest units on the market. I'm now thinking I need to do my own calculations and try and clearly identify what is required to meet Part F of Building Regulations.

 

From what I've read in Part F,  the Whole house Ventilation Rate is 25 l/s (based on 4 bedrooms), however, the minimum based on floor area is 114.58 l/s. Having done a spreadsheet (below) it would appear that I need more air extraction to meet the minimum ventilation rate? 

I assume the input air designed to balance the extract rate? Looking at the first of my quotes they suggest a single unit 650m3/h, the other quote suggest two units with a total 985 m3/hour.

 

Am I on the right line here, or am I missing something? All help gratefully received!!!!!!

 

 

image.png.6e33a34bc5a25b71ce37b2c9d81a36b8.png

Edited by Triassic

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Not much help with the specific numbers, but IIRC a number of people have mentioned that the building regs requirement is too high in real life, so it may be best to size on boost rather than the 'optimum' setting. We used Enhabit who didn't seem overly expensive, they mentioned that if the house volume is high for the number of occupants there is an increased risk of the RH being too low. We went for an enthalpy exchanger for this reason. 

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3 minutes ago, Nick1c said:

they mentioned that if the house volume is high for the number of occupants there is an increased risk of the RH being too low

Thanks for the response, a quick look through my quotes and none mention RH being a problem in a high volume house.

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think he meant air change rate being too high?

 

Check the qualifications and if the companies who quoted and designed are indemnified - see if that narrows down the suppliers and layouts?

Ie one who is not qualified/recognised massively over speccing?

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11 minutes ago, Oz07 said:

massively over speccing?

I think this is part of the problem, it took them three attempts to get the design right, they suggested siting one of the MVHR units in a loft space that did not exist, then they moved it to a room without outside walls, making it difficult to run the 180mm intake and exhaust ducts, they would not fit through the gaps in the metal web joists.

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See this Topic in which I give details of the way that my high volume MVHR system design was resolved.  My system volume calculations, detailed by my MVHR designer,  are included for reference. Looking at your volumes, I think my volumes are much higher and I have a single 450m3 MVHR which normally runs at less than 40% capacity.

 

I hope it helps.......

  • Thanks 1

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4 hours ago, Nick1c said:

they mentioned that if the house volume is high for the number of occupants there is an increased risk of the RH being too low. We went for an enthalpy exchanger for this reason. 

 Yes, I was advised this as well.

I have an enthalpy exchanger for the that reason - only 2 occupants usually in a 5 bed house ( 340m2)

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@HerbJ, you have close to double the m2/pp that we will!! I noticed that your dressing room has no supply/extract, ours is specced the same way, I would have imagined circulation around shoes & clothes would be a good thing... As it is next to the ensuite we could have a 'through vent', would this make sense to you?

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On reflection, it may have been useful to have an extract in the Dressing Room, but it is a small room after all the fittings were installed  and the extract volume would have been very low. A through vent is a good idea .

 

We also fitted an extract in the Plant Room, as an afterthought during the installation, as it was very easy. It is not in the original Design calcs but it is on the Commissioning Test Sheets.

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@HerbJ thanks for the link, Sorry it’s taken me a while to get back on topic, I’ve been fixing cladding between rain clouds and airtightness membrane otherwise, Clearly I need to do some reading and more work to get to a more sensible answer.

 

I emailed the two suppliers who quoted and asked for the rational for specifying two MVHR units, in both cases they said it was because it was a large house and needed two units! My house is around 380ms, including a large entrance hall, stairs and landings (60m2) over four floors. On first glance at your approach, am I correct in assuming you really only need to extract from those rooms listed in Part F and put fresh air into accommodation spaces such as bedrooms and lounges and things like stairwell can be discounted?

 

 

Edited by Triassic

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We have GFA 350m2, which Enhabit calculated required 378m3/hr on boost, and are going with their recommendation of a single zehnder Q600 (600m3/hr), with radial ducting.

 

I wonder whether the suppliers recommending 2 units are doing so due to additional factors to size e.g. issues with ducting in a 380m2 house over 4 floors? Or some perceived issue with using a larger unit?

 

 

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Although the house has four floors the top one is a single room, a study, located in the roof space. We only added it due to the spectacular 270 degree views of the valley below.. 

 

8 hours ago, kxi said:

I wonder whether the suppliers recommending 2 units are doing so due to additional factors to size

But why, my house is no larger than others are building on here, all of whom have a single unit. I’m wondering if the MVHR supplier is doing a designed to some sort of worst case scenario, rather than passive house? Clearly I need to do more work as I really don’t need to spend an additional £3,000 on another unit that will according to most here be turned down to the lowest setting once commissioned.

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I would go larger ducts and lower flow rates, single unit 

 

I know that they use radial ducts a lot in mainland Europe but I prefer branched ducts designed in 

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@tonyshouse I'm spending today trying to get to the bottom of the design I actually need. Just as you posted I came across these design and testing guides, which Ive linked to below for anyone else looking for help.

 

I'll take on board your duct sizing suggestion, we live in a quiet rural location and really don't want to be lying in bed listening to some wheezing MVHR system, Id rather listen to the birds! Mind you the triple glazing has put paid to that!

 

 

Edited by Triassic

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@Triassic We also had a quote suggesting 2 x 350m3 units, and TBH I just rejected as appearing to be unecessarily expensive and complicated. However, it would be interesting to find out if there is a good rationale behind these odd recommendations. The obvious answer is they want to sell 2 x (unit + ducts + installation + comissioning), but it would be worth hearing their justification. If "it's a big house" is the only level of detail they are willing to give, I wouldn't be confident in their design anyway.

 

Re. branch vs radial ducting.

In our case, branching was going to be a problem due to our build method & house design. We have:

- a warm roof i.e. ducts can't go in loft

- the roof beams are steel so can't accomodate a 200mm duct through them

- concrete first floor - ducts can't go in the floor

 

This meant a large branching duct would have to run through most of the rooms. We like an industrial look, but that would be too much.

 

Instead we are running the 90mm radial ducts through multiple holes in the roof steel beam webs (SE specified...), which keeps it all out of the way:

image.png.33ad5233ceddf15c7afcc951b6959ebe.png

 

This was the primary reason for radial.

 

But other advantages over branching that we felt were relevant:

- Eliminates risk of cross-talk noise between rooms (which might be small but wanted to be sure)

- More control over each arm i.e. room

- Continious pipes so no airtightness problems at joints over time

- Supposedly cheaper to install (according to https://www.heatspaceandlight.com/difference-between-branch-radial-duct-which-better/ )

 

Radial is less efficient due to greater internal surface area of the piping, but we didn't see that as a big issue.

 

 

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We're in a similar situation, 300m² house (100m² of which is basement with only one room, but all needs to be ventilated).

 

My neighbour designs and installs ventilation systems, and said to me I'd need two units. He says the performance figures and capacities of MVHRs are typically optimistic and while will pass building regs, won't perform that we'll in real life. Much better off with an oversized system running at 50% load most of the time, than a barely capable system running at 75% most of the time and struggles to meet boost demands.

 

I can certainly understand his rational, but I can't justify another £2k so will be going for one unit!!!

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Once the ducting is in surely the unit could be upgraded in future if found to be underperforming. Bit like a boiler swap?!

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17 hours ago, Triassic said:

On first glance at your approach, am I correct in assuming you really only need to extract from those rooms listed in Part F and put fresh air into accommodation spaces such as bedrooms and lounges and things like stairwell can be discounted?

 

Yes, that is correct. Stairwells and corridors, etc (rooms  not normally habitable are not discounted as such, but benefit  (and provide the conduit)from the airflow going from living spaces (supply air) to wetrooms - kitchens, utility rooms, bathroom and toilets.  That's why it's important to balance supply and extract volumes. Also , why all doors should have the ability to allow airflow bypass/transfer pat all the doors when closed, .

 

I believe  stairwells, corridors etc are characterised as" overflow" spaces" in DIN standards and on my MVHR volumetric data sheets as  "Vthrough". 

 

That being said, it is not unacceptable to put a supply/extract duct in a hall. For instance, I am aware that @Jeremy Harris  installed an extract grill in the his hall, outside the kitchen door to assist with minimise kitchen smells being drawn though this overflow space and into the rest of his house.  I haven't found this an issue or need  with my MVHR  design!

 

Edited by HerbJ

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32 minutes ago, HerbJ said:

 

Yes, that is correct. Stairwells and corridors, etc (rooms  not normally habitable are not discounted as such, but benefit  (and provide the conduit)from the airflow going from living spaces (supply air) to wetrooms - kitchens, utility rooms, bathroom and toilets.  That's why it's important to balance supply and extract volumes. Also , why all doors should have the ability to allow airflow bypass/transfer pat all the doors when closed, .

 

I believe  stairwells, corridors etc are characterised as" overflow" spaces" in DIN standards and on my MVHR volumetric data sheets as  "Vthrough". 

 

That being said, it is not unacceptable to put a supply/extract duct in a hall. For instance, I am aware that @Jeremy Harris  installed an extract grill in the his hall, outside the kitchen door to assist with minimise kitchen smells being drawn though this overflow space and into the rest of his house.  I haven't found this an issue or need  with my MVHR  design!

 

 

 

Yes, I did add a fresh air feed (not extract) in the hall, right above the kitchen door.  It was a last minute addition, and not originally planned, but it is very effective.  It provides a sort of "air curtain" of fresh air right outside the kitchen, and because the kitchen is a relatively high extract rate room, this means that air is always flowing into it and out the extract.

 

We do still get some cooking smells in the kitchen, but these never get out of the kitchen door, and tend to dissipate within an hour or so, less if I switch the MVHR to boost.

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I’ve done the opposite and put an extract above the kitchen door to try and also stop smells - this is in addition to the one above the hob

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3 hours ago, HerbJ said:

I believe  stairwells, corridors etc are characterised as" overflow" spaces" in DIN standards and on my MVHR volumetric data sheets as  "Vthrough"

Have you published your data sheet on here? I don’t recall seeing one that included Vthrough?

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4 hours ago, Jeremy Harris said:

 

 

Yes, I did add a fresh air feed (not extract) in the hall, right above the kitchen door.  It was a last minute addition, and not originally planned, but it is very effective.  It provides a sort of "air curtain" of fresh air right outside the kitchen, and because the kitchen is a relatively high extract rate room, this means that air is always flowing into it and out the extract.

 

We do still get some cooking smells in the kitchen, but these never get out of the kitchen door, and tend to dissipate within an hour or so, less if I switch the MVHR to boost.

We did a different approach.  Because the "kitchen" is actually a kitchen / diner (often referred to these days as family room) we put an inlet at the dining end, and an extract at the kitchen end.  When balancing the system I matched the inlet and outlet rates for this room, the theory being air will flow from one end to the other, but not into or out of this room.

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I'm currently calculating the duct size for my MVHR system.

 

What air flow is the 'ideal', what is too high? Some of mine are at a maximum of 3.7l/s in 63mm duct, or 2.3 l/s in 80mm duct.

 

Not sure who produced the attached MVHR sizing spreadsheet (I stumbled across it whilst reading up on MVHR sizing), anyhow here are the results. Any observations?

 

 

MVHR_Sizing.ods

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9 minutes ago, Triassic said:

What air flow is the 'ideal'

 

To minimise noise from airflow aim to keep the air velocity below 2m/s

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