Mechanical ventilation design

[giacomo_z]

I am in the process of designing mechanical ventilation for my 20-year old house, which is not particularly airtight. My goal here is to improve air quality and reduce condensation automatically - without having to worry about opening trickle vents or windows. I am aware this will have a cost in terms of running cost of the unit itself.

 

Note the exercise below assumes a perfectly airtight house, with no leakage. This is hardly true for my house but again the goal here is not improving airtightness but rather air quality.

 

Now, part F requirements, as I understand them, say that:

1. There should a be a minimum whole house flow rate of 0.3 l/(s.m2)
2. Specific rooms should have a flow rate according to Table 1.2:
   1. Kitchen 13 l/s
   2. Utility room/bathroom 8 l/s
3. Also from Table 1.3, I understand bedrooms should have flow rate of 6l/s

 

I don't understand requirements 2 and 3. Taking my kitchen as an example:

• Area: 12.77m2
• Volume 30.64m3
• Using the flow rate above of 13l/s -> 46.8 m3/h, I would end up with 1.53 ACH (assuming the kitchen itself was balanced, either by equal number of supply/extracts or by a suitable door undercut).

 

However a house built to the exact same airtightness and thermal specs with a kitchen double the size, would end up with half the ACH figure. Clearly this is cannot be right, can it? What's more, for my very small ensuite I would end up with just over 4 ACH - practically a clean room! 

 

Just for fun, I tried a different method:

• Requirement 1 states whole house flow rate of minimum 0.3 l/(s.m2), that is 1.08 m3/(h.m2).
• Say I want to design for double this flow rate for the kitchen, that is 2.16 m3/(h.m2).
• For kitchen area area and volume stated above, this leads to 55.14 m3/h, or 0.9 ACH.

Importantly, this method would give equal performance no matter the size of the room.

 

Now, applied to the whole house:

• Requirements 2 and 3 give a total flow rate for the house of 306m3/h. Requirement 1 applied to the whole house gives 105m3/h.
• Using the alternative method with 2x multiplier for kitchen, 1.5x multiplier for bathrooms, 0.8x multiplier for all other rooms (where the (arbitrary) multiplier is applied to the whole house flow rate 0.3 l/(s.m2)), I get a total flow rate of 124.86 m3/h. Requirement 1 applied to the whole house obviously still is 105m3/h.

 

So, how do I make of this? Spreadsheet is attached for your perusal, if you wish.

 

 

20240220 - ventilation_calcs.xlsx

[JohnMo]

I think you are confusing yourself.

 

Trickle vents, just add humidity activated trickle vents to dry rooms (close wet room ones); they look after themselves. In wet rooms and kitchen just add dMEV fans (such as Greenwood with automatic boost for humidity) set to required min flow rate. Job done.

[Iceverge]

Agreed, I bought a Greenwood CIV2gp on ebay. 

 

Keep an eye out and they can be had very reasonably. 

 

 

@JohnMo do you have any recommendations for humidity sensitive vents? 

 

[JohnMo]

13 minutes ago, Iceverge said:

do you have any recommendations for humidity sensitive vents

The one that @HughF found and is using, but not sure what make or model, but he seems happy with them.

[giacomo_z]

@Iceverge and @JohnMo thanks for your comments.

 

The problem I have is that most rooms are wet. My house is as follows:

• The wet rooms - kitchen, bathroom, ensuite and possibly cloakroom
• 3 bedrooms - I get condensation overnight and  air is very stuffy in the morning. DO I treat these as wet as well then?
• Dining/living room - this is the only place where I can hang clothes to dry on an airer in winter. Obviously, condensation forms on the windows
• Hallway and landing are possibly the only dry areas

 

Regardless of the route I decide to take for my particular house, my doubt stands. Have I interpreted Table 1.2 and 1.3 requirements wrongly? I would appreciate if anyone had any pointers on my calcs. Thanks

[JohnMo]

3 minutes ago, giacomo_z said:

3 bedrooms - I get condensation overnight and  air is very stuffy in the morning. DO I treat these as wet as well then

No, wet rooms have water in them, like a sink, shower etc. the solution above give ventilation across the room when required. The trickle vent opens and the dMEV fan which run all the time provide cross ventilation. Ideally you will have 10mm undercut at the doorways so air is free to flow when doors are closed.

 

4 minutes ago, giacomo_z said:

Dining/living room - this is the only place where I can hang clothes to dry on an airer in winter. Obviously, condensation forms on the windows

As above

 

[HughF]

2 hours ago, JohnMo said:

The one that @HughF found and is using, but not sure what make or model, but he seems happy with them.

Brevis Higroster - polish….. very nice vents.

[Iceverge]

@giacomo_z

 

Bregs are a crude device. They tend to be written for worst case scenario rather than the actual demand and most of us find they massively overventilate a house. 

 

Work on 0.3ACH for the whole building as per passivhaus and you'll be closer to a comfortable standard. 

 

Say you need 150m3/hr, about 40l/sec. Then 3 X of those above fans should be ok. 

 

Install one of them, suck it and see. You'll be surprised given a few days it'll reduce the dampness significantly. 

 

Do what you can in the meantime to stop adding to the damp, dry clothes outside if possible for example. Use extractor fans for cooking etc. 

 

[giacomo_z]

@Iceverge I like your practical approach in my case, as this is a retrofit and I can take a piecewise approach.

 

For a new design, perhaps I can answer my own question. Although the ventilation requirements for wet rooms don't take into consideration the size of the room, the relative humidity of a big room will be less than a small room because the sources of humidity (shower, bath, hob, etc) are constant - ultimately a bathroom will normally have up a 22mm hot water pipe supplying it.

 

Therefore, the absolute amount of air to be changed per hour to bring the relative humidity down in a big room will be less than in a small room.

 

For bedrooms, once again it's unlikely than more than 2 people will sleep in a single bedroom, regardless of the size.

Edited February 20 by giacomo_z

[JohnMo]

1 minute ago, giacomo_z said:

don't take into consideration the size of the room

They need to really take account of room size, a room with a shower is still a room with a shower, big or small. But extract is doing several jobs, clearing excess humidity at source and dragging moisture from other rooms like your bedroom when you are sleeping.

[Iceverge]

The reality of any house is that it's far to dynamic a system to ever be modelled accurately. 

 

Factors include, occupancy rates, amount of breathing, number of drying socks, cooking pasta or rice , wind direction, temperature, humidity. How long the door was open or how long the shower ran.

 

The list of variables is massive.  Calculations are completely best guess. 

 

Humidity levels tend to equalise within a house pretty quickly anyway so if you sort one or two rooms then the rest will follow. 

 

The beauty of the above fans is they respond to humidity so get it exactly right in theory. And they're cheap and easy to install too. 

 

 

 

 

 

 

 

 

 

 

[JohnMo]

We have a Greenwood dMEV fan (set to minimum speed and is inaudible) in our summer house, outside it's currently 4 degs and 79% humidity. Inside it's 19 degs and 39% humidity. Over the last month the max humidity inside hit 44%. For comparison in our house we have MVHR and over the same period the max humidity in the house hit 46%. So overall pretty similar.

 

To lower the humidity in a building you need a fresh flow of air and heat. You need both together, doing one without the other humidity stays high.