Bathroom extractor fans / Positive input ventilation

[take2]

I’ve been struggling with this one and thought I may get some good answers here.

I’m currently undergoing a house renovation and want to ensure I allow for proper ventilation (both extraction and input) given all the work I’m doing to insulate and improve air tightness.

Originally I was thinking of a mechanical ventilation heat recovery (MVHR) unit but given retrofit restrictions I gave up on that idea

I’m now considering using a positive input ventilation (PIV) system for input and humidity controlled bathroom extractor fans for output, plus a manual kitchen extractor fan that’s externally ducted (not one of those carbon filter recirculates).

I’m currently trying to decide on the type of bathroom extractor fan - I can’t use an axial type as I have no external walls on the bathroom (terraced house). I got a (very high) quote from envirovent to use their “cyclone 7” centrifugal fan (see here: https://www.envirovent.com/products/ext ... cyclone-7/)

However, after some initial research I think I could accomplish the same thing (or even better) with an in-line fan that’s wired to a humidistat. See here for inline fan: https://www.extractorfanworld.co.uk/man ... -246-p.asp
and here for humidistat: https://www.extractorfanworld.co.uk/man ... -270-p.asp

I also got a (fairly high) quote from envirovent for the PIV system - see here: https://www.envirovent.com/products/pos ... piv/atmos/

I haven’t done a ton of research into different brands but I have found considerably cheaper options.

My questions are:

1) Am I going about this the right way / does my understanding above make sense?
2) The standard way to do bathroom extraction appears to be to put in a cheap axial fan on an external wall that’s wired to the light switch but has a [15 min] timer on it to continue extracting after the light is switched off. I can’t use an axial fan but I could just get an inline with a timer vs trying to wire up a humidistat. Would that be a better solution?
3) Where should the humidistat go in the room? The particular one I found is quite ugly and I’m not sure DW wants it ruining the decor. Are there other options for humidistats?
4) Does the quality of envirovent make up for the higher cost? I’ve seen some impressive videos and the sales guy was full of bluster but his main selling points was the built in humidistat. However it’s on the fan itself so if it’s ducted away from the bathroom and used like an inline (which I would need to do) it seems counter productive?

[chenda]

I have a similar internal bathroom with no external walls, it has a small axial fan in the ceiling which I assume ducts out to an external wall somewhere. It's wired to the light switch with a stay-on timer as you indicate is standard. To be honest it's perfectly adequate. Mould is sometimes a problem in the winter but its easy to remove. This is the model I have:

https://www.screwfix.com/p/manrose-mg10 ... lsrc=aw.ds

[AxelHeyst]

You're in the UK right?

I like the idea of having an inline fan so that you can place it far away from the bathroom itself. The sound of cheap axial fans ceiling mounted struggling away is obnoxious to me. Also that fan noted since you can put the fan somewhere else you can have the register directly above the shower to hoover up the moisture source. Nifty.

I don't like the idea of using a humidistat. This might be a style preference, but I like my stuff as manual as possible. I distrust automation until it proves itself. I expect sensors like humidistats to break, to need calibration, to quietly malfunction without me knowing it. I like the timer idea, except I'd have a separate switch for the fan. So I can go in the bathroom and turn the lights on without triggering the fan if I don't want it to. If I do want the fan, I turn the fan on, and when I leave I turn it 'off' but it stays on for 15min. That way I can also decide to leave the fan on continuously without the lights being on.

How tight is your house / is your house going to be? If it's not tight, you don't have to worry about air balance (I think*). If it is tight, you need to think about whether these fans are going to fight each other, slam doors, blow doors open, etc.

I'd look at getting the speed controller for that fan. 68L/s might be a lot/noisy depending on grille size.

I do not recommend that you use flexible ducting. Only use it where necessary. I'd spend more $ on straight duct.

Also: just because the fan inlet and outlet are 4in does not mean the duct should be 4in! At that airflow I'd want to go no smaller than 6in, and if I had space I'd go 8. Off the top of my head and looking at a quick velocity and pressure drop calculator.


*(I did commercial HVAC design. It's all the same basic concept, but I don't have experience with residential ventilation design and specific issues... so don't take my post for more than it's worth. Also I live in an arid climate and it takes great efforts of creativity to get things wet enough long enough to mold.)

[Ego]

I can’t use an axial type as I have no external walls on the bathroom (terraced house).

I may have missed something obvious but did the quotes include ducting to the outside or is there already an existing duct? If a duct exists, you can remove the switch from the circuit and wire directly to an inexpensive retrofit fan that has a humidity sensor integrated. They make them sized to slip into the area where a standard fans fits. If there is currently no duct then that is probably the expensive part of the job. If that is the case, be careful as retrofitting ductwork can look terrible.

Companywide we are replacing the user controlled fans with the humidity sensor version because people tend to shut off the noisy fan before it has finished clearing the moisture and end up with moisture issues.

[take2]

@Ego: It’s a whole house renovation + loft conversion so I’m running new ducts. I think I’ve figured out the best way to get them to the eaves which should minimise any ugly boxouts but I will need some as I need to cross a steel beam which is in line with the joists.

The link you sent is a similar style to what we have here in the U.K. The only issue with those is the fan itself needs to be in the room and thus needs to be placed further away from the source of steam/moisture. It also runs in the room which can be noisy.

@AH: My thoughts on humidity sensor is to take human error out of it, same with wiring to the lights vs separate switch. I would likely be conscious enough about it (though not always) but not sure it would be the same for my family. It’s more common here to wire with the overhead lights + timer to make it idiot proof. Many bathrooms have secondary lights over the vanity that can be used without turning the fan on.

Re: sizing of the fan - I think the fan I selected is already far too big for what I need. I have three bathrooms - one is c. 7.5m3, one is c. 14m3, and one is c. 8m3. From what I understood common practice is to multiply this by 10 to get a suitable m3/hr figure. That would range between 75-140m3/hr and the fan I linked is capable of 180-245m3/hr, depending on the speed (I’m getting one fan per bathroom). See here: https://www.manrose.co.uk/wp-content/up ... orange.pdf

If anything I’m more afraid this is oversized but maybe I’m missing something? There will be some drops with the duct but I’m not sure how to calculate that. I’ll also need flexible ducting as the distance is relatively short (maybe 4-5m max) but I need to get under a beam and back up into the loft with the final exit running vertically out of the roof.

What do you mean by a speed controller? The fan notes it has 3 speeds but I thought that was something that was programmed when it’s wired and can’t be changed easily? Maybe that’s a dumb thought?

I’m not sure how air tight the final house will be but I’m being very anal about making sure all potential leaks are sealed, plus I put in new windows and I’m adding additional internal insulation on the exterior walls (50mm). I doubt it will be as good as a new build but I suspect it’ll be airtight enough that i need to worry about proper ventilation.

[AxelHeyst]

I hear you, humidstat makes sense.

Yes that fan is roughly 2-3x oversized seems like? The way we calc exhaust rates for single occupant bathrooms in the states is 85m3/h if it's intermittent and 34m3/hr if its continuous, we don't go by area or room volume.

Regarding pressure losses and airflow, I *think* how it's listed is they're going to assume some typical amount of pressure drop the unit will see based on duct length and registers. So it says on the product page "duct up to 10m without loss of power", that should mean the fan will move about 245m3/hr as long as your duct system isn't much longer than 10m or has high pressure drops for other reasons (kinks in your flex duct, etc).

This particular model; the MF100S is the basic On/Off version. It can be paired with Manrose 1361 remote humidistat and Manrose 1350 speed controller.

ETA: as stated I'm no expert, but in the states I've only seen continuous exhausted residences, I haven't seen PIV. Conceptually PIV makes way more sense to me because you get to filter (and possibly condition) the air, unlike continuous exhaust which just sucks outside air in through nooks and crannies or inlet grilles.

[ffj]

If the fan is just for the bathroom I wouldn't overthink it too much.

Here in the States the bathroom exhaust has to be at least 4 inch diameter in the piping and it has to be insulated. The exhaust fan has to be sized for the size of the bathroom. This is all easy to figure out so I won't bore you there.

One issue you need to consider is accessibility. A wall or ceiling mount fan is easy to install and replace. An inline fan maybe not so much? These fans will eventually fail at some point.

The other questions are easy to answer as it comes down to user preference. Some people like a noisy fan as it covers up unpleasant sound, haha. Some like automatic switches, some like a choice. Personally I wouldn't over-complicate it with a humidistat as I understand when humidity is created, but you may have small children.

Now if you are looking for a whole house system that is an entirely different discussion.

[ducknald_don]

@take2 I assume you know you need to be qualified to do electrical work in a bathroom or kitchen in the UK.