Multiple fans off one isolator

[aj_london]

Hi folks,

Homeowner here. I'm in the planning stage of a project, seeking advice from professionals.

I need to upgrade the ventilation in my home following advice from a surveyor. The plan is to replace my one bathroom fan with two fans (one shower extractor and one MVHR unit). In addition, I need to add a MVHR fan to my dining room.

The isolator for the existing bathroom fan is just outside the bathroom. Would it be acceptable to have all three fans coming off this one isolator, despite the fact it would be controlling devices in different rooms?

This would be achieved by taking the existing bathroom fan feed (with switched and perm live) and splitting it into three with a junction box. Two of the split feeds would stay in the bathroom. The third one would go outside the house (via whiska boxes and conduit) and back into the dining room a few meters away.

The thinking here is to be minimally invasive in terms of messing up my freshly decorated interior, and take advantage of the fact that we are punching some big holes right through the wall anyway, so it's not that onerous to route cables via the outside.

Thanks in advance for your advice!

 

[goasis]

If you're installing an MVHR, surely you don't want to be extracting air to the atmosphere with a regular fan as well, that kinda defeats the point doesn't it?
From my limited experience, MVHR units tend to have the fans built into them and just ducting to and from the various rooms.

 

[aj_london]

Sorry, I should have made clear that when I say MVHR here, I am talking about "single room MVHR" devices. Which are basically always-on extractor fans (i.e. standalone units).

 

[Mike Johnson]

What problems have you experienced to justify the Surveyor recommending MVHR is installed.

 

[aj_london]

What problems have you experienced to justify the Surveyor recommending MVHR is installed.

Humidity, mainly.

 

[Mike Johnson]

I assume fro that you mean condensation, which is a ventilation problem, sorry to ask some basic questions, but have your recently:
1. Blocked up a chimney.
2. Had new windows installed with double glazing.
3. Had new doors installed.
4. Have a new Tumble dryer or washing machine.
5. A new oven with steaming facility.

All of these could add to a condensation problem that was not apparent until they where installed, if none of the above applies, why did you have a surveyor around and what did their report say?

 

[aj_london]

I assume fro that you mean condensation, which is a ventilation problem, sorry to ask some basic questions, but have your recently:
1. Blocked up a chimney.
2. Had new windows installed with double glazing.
3. Had new doors installed.
4. Have a new Tumble dryer or washing machine.
5. A new oven with steaming facility.

All of these could add to a condensation problem that was not apparent until they where installed, if none of the above applies, why did you have a surveyor around and what did their report say?

Yes indeed, we have a condensation problem. 2 and 4 apply. In addition, the condensation was more apparent in recent months, because we were spending so much more time at home due to Covid restrictions - doing probably double the usual amount of cooking and showering at home.

 

[Mike Johnson]

Also breathing out water ladened air.

The new double glazed windows I assume have trickle ventilation slots at the top of the frame are these open??????

Tumble dryer and/or Washing machine are they condensation appliances or pipe discharge?

How old is your property? Current building trends hermitically seal the building hence condensation is rearing its head again, used to be quite common prior too the 50's.

 

[aj_london]

Also breathing out water ladened air.

The new double glazed windows I assume have trickle ventilation slots at the top of the frame are these open??????

Tumble dryer and/or Washing machine are they condensation appliances or pipe discharge?

How old is your property? Current building trends hermitically seal the building hence condensation is rearing its head again, used to be quite common prior too the 50's.

Trickle vents are indeed open (where possible, a couple of the windows are special acoustic ones with no vents).

Washer/dryer is a single unit which is plumbed in. It has a grey water outlet pipe, but no vented air/duct outlet... not sure if this answers your question, sorry.

Building is from the 30s, converted into flats sometime later. We are on the ground floor and added an extension a couple of years ago.

 

[Mike Johnson]

Difficult to go much further with the diagnostic without seeing the situation, but it does sound as though some kind of ventilation is required if only to offset the stagnant air in the room with the non vented windows, the room with the washing machine should have some kind of ventilation to offset the heat from normal operation, a 30's building will have a chimney, is this vented or has it been taken down?

Only other consideration is why a Heat Recovery system and where are you going to recover the heat too?

 

[aj_london]

Many thanks, I'm happy enough to follow the surveyors recommendation for MVHR-type fans rather than MEV. I guess either could work in our situation.

Do you have a view as to whether it's OK to run all 3 fans off one standard fan isolator switch (which is in a different room)? And no particular worries with running the cabling externally?

 

[brianmoooore]

The two fans in the bathroom can run off the one isolator. The third should have it' own, suitably located.

 

[Mike Johnson]

Cable capacity and load will have to be checked to ensure compatibility, most extract fan manufacturers suggest their fans have an inline 3amp fuse, this would have to be checked, I would be tempted to put in a new circuit to control the ventilation system, I would be more concerned with the route of the ductwork for the recovered heated air, if a false ceiling is needed with this cover plaster details usually found in buildings of that age, ceiling rose's and corniche details come to mind.

As far as I know there is no legal requirement for isolation switch's to fans, its a convenience thing for electrical maintenance to allow work without the lights being out.

 

[aj_london]

The two fans in the bathroom can run off the one isolator. The third should have it' own, suitably located.

Many thanks!

Just so I understand, would having the third fan on the same isolator be a "it's not best practice but you could do it if you really want" thing, OR "it's a regs violation, you definitely can't do that" type of thing?

 

[aj_london]

Cable capacity and load will have to be checked to ensure compatibility, most extract fan manufacturers suggest their fans have an inline 3amp fuse, this would have to be checked, I would be tempted to put in a new circuit to control the ventilation system, I would be more concerned with the route of the ductwork for the recovered heated air, if a false ceiling is needed with this cover plaster details usually found in buildings of that age, ceiling rose's and corniche details come to mind.

As far as I know there is no legal requirement for isolation switch's to fans, its a convenience thing for electrical maintenance to allow work without the lights being out.

Thanks Mike. The fans are all under 10W so current load is unlikely to be an issue. The fans would all be venting straight outside so no ductwork to speak of.

My initial concern was the potentially non-obvious isolator location, and whether there were any regulatory issues about cabling going outside the house and back in again.

(I say non-obvious isolator location, but if the cables are indeed routed outside, I guess it would be pretty clear to anyone looking at the side of the house.)

Interesting point about why fans have isolators in the first place. I assumed it was a regulatory thing, since you always see them. But what you say does make sense.

 

[Mike Johnson]

So the heat recovered would be vented straight back into the room the air was taken from, is this what you want?

Isolation switch's can be located in a bathroom if they are above 2.25M high, would seem preferable to running cables outside the building.

 

[aj_london]

So the heat recovered would be vented straight back into the room the air was taken from, is this what you want?

Isolation switch's can be located in a bathroom if they are above 2.25M high, would seem preferable to running cables outside the building.

Such is the way these single room MVHR units work. It's either that or just install an MEV. Some heat recovery seems like a decent idea, in order to keep the room warm and therefore keep the dew point as high as possible.

 

[brianmoooore]

Isolation switch's can be located in a bathroom if they are above 2.25M high

This only applies in the zones (1 and 2). Anywhere else in the bathroom they can be at any height, or they can be mounted immediately outside the door.

 

[Mike Johnson]

Europe is way ahead of us on this subject using multi ducted domestic systems of heat recovery, unfortunately not very viable in a flat without a false ceiling to accommodate the return ducting see this French system: https://www.leroymerlin.fr/v3/searc...pleted=true&keyword=vmc double flux connectée

 

[Mike Johnson]

This only applies in the zones (1 and 2). Anywhere else in the bathroom they can be at any height, or they can be mounted immediately outside the door.

If you want them at all, or put all the fans on one MCB in the CU, but being in the UK we tend to just join onto the lighting circuit (hence our wont for isolators) for the fan power and put in an inline fuse.