Cooker hood to mvhr extract plenums

[CC45]

Hello,

 

Suspect a few of you have faced this challenge with your mvhr system.  Is there a certain distance the kitchen mvhr extract plenum to the cooker hood/hob?  Is this the direct distance? Or the horizontal distance from directly above the hobs? Would 1m be enough?  I've tried finding an answer on line / in my various folders but no luck.  What have you all done about extractor hood? Just use a recirculating one?

 

Cheers

 

CC

[Jeremy Harris]

Recirculating seems the best bet; I've made provision to fit one, but we're waiting to see if we need it before fitting it (I suspect we won't need it, as we don't use the one in our old house much).  Any extractor can't be connected to the MVHR, because it would both massively imbalance it and also because there is a high risk that the ducts will get fouled up, along with the MVHR filter and perhaps even the heat exchanger.  Fitting an extractor with a separate duct to the outside creates a big air leak, and also creates a massive MVHR imbalance when it's running - it would be enough to almost stop the MVHR extract system from working when it's on, I think.

[ToughButterCup]

Hmmm, @JSHarris, I wish you'd been with me when we were talking to the kitchen rep. Passiv has a long way to go before it sinks into the consciousness of many in the building sector.

In terms of recirculation, is there anything we should look for in terms of cooker hood design? What are the design differences which assure filtration as opposed to expulsion? Any particular spec of filter? Or grill size? Or flow rate?

 

BTW, thanks @CC45 for tagging accurately.

Ian 

[PeterW]

18 minutes ago, recoveringacademic said:

What are the design differences which assure filtration as opposed to expulsion? Any particular spec of filter? Or grill size? Or flow rate?

 

Good record hoods have two filters - first set in the hood itself where they catch the grease and muck and ideally these are stainless mesh so you can bung them in a dishwasher. Second filter is either just before or just after the fan and are activated charcoal to catch the smells. @ryder72 probably has some good examples. 

[Jeremy Harris]

I'm pretty sure there are some other discussions on extractors here, or may be they were on Ebuild.  The better quality recirculating hoods work pretty well, and I've found that I can clean the stainless mesh grease filter on ours in the dishwasher, and the activated carbon "smell filter" can be opened and the carbon granules can be replaced cheaply, using fish pond filter carbon granules, which you can buy in bulk (much cheaper than buying new filter elements).

 

I haven't looked at newer models; the one in our old house is around 20 years old, and as I mentioned, we don't use it that often.

[curlewhouse]

As our ground floor is pretty much one open plan space this is a concern for us too with no longer having the option of venting outside for the very reasons JHS give re MVHR. So I'm also currently looking for the most effective recirculating hood I can find, and there are some models about which look promising. Hoods I've had in previous houses, even venting outside, often seemed quite underpowered really, and allowed a significant amount to "escape" into the room. But I also plan to put a "wall" either side of the hob itself to assist and minimise any spread of the rising steam/smoke/odour laterally. I've seen that done elsewhere and it looks OK and seems to assist. Trouble is course that we'll never know how effective it is until doing a "kipper test" one day! 

Edited June 6, 2017 by curlewhouse

[Stones]

We fitted a Siemens extractor with re-circulation kit (additional charcoal filters on the fan).  We don't use it if just boiling veg on the hob, but do use when frying food or using the grill plate we have for the hob.  Judging by the grease that's trapped in the initial stainless steel filter, I would say that they do provide some meaningful filtration and odour reduction.

 

 

[Bitpipe]

56 minutes ago, Stones said:

We fitted a Siemens extractor with re-circulation kit (additional charcoal filters on the fan).  We don't use it if just boiling veg on the hob, but do use when frying food or using the grill plate we have for the hob.  Judging by the grease that's trapped in the initial stainless steel filter, I would say that they do provide some meaningful filtration and odour reduction.

 

 

I have a similar arrangement - I put an additional extract duct midway between the ovens and the cooker hood (on opposite walls). Works fine and if I burn the toast I just open the window a little to increase ventilation.

[CC45]

Thanks all - what about the distance from the mvhr plenum? Need to get them fixed in on Thursday evening ready for the pipework install this Saturday.

 

Cheers

 

CC

[Jimbouk]

I too am fitting my first fix Mvhr on Thursday....

 

Wondering what the best approach is to mounting the plenums? Screw them to the joists and then try to cut the plasterboard to fit, or cable tie them roughly in place and let the plasterers align them to the holes they cut in the plasterboard?

 

Also have the same question regarding how far from the hob.

[Jeremy Harris]

56 minutes ago, CC45 said:

Thanks all - what about the distance from the mvhr plenum? Need to get them fixed in on Thursday evening ready for the pipework install this Saturday.

 

Cheers

 

CC

 

 

The position of the MVHR plenum chambers isn't relevant, so stick them in the most convenient location.  We have both the extract and fresh air feed plenums up in the service area, fairly close to the MVHR unit, in order to minimise the length of duct from the MVHR to them, but this isn't really critical.

Edited June 6, 2017 by JSHarris

[CC45]

Hi @JSHarris, I wasnt talking about the manifolds - they are in the loft, I meant the plenums in the individual rooms, any advice?

[Jeremy Harris]

5 minutes ago, CC45 said:

Hi @JSHarris, I wasnt talking about the manifolds - they are in the loft, I meant the plenums in the individual rooms, any advice?

 

The plenums are the chambers/manifolds where the ducts connect, before being connected by larger ducts to the MVHR.

 

Do you mean the extract terminal in the kitchen?

 

If so, then all the terminals should be positioned to maximise the path length of air in the room.  We have our kitchen extract terminal in the corner of the ceiling that's almost diagonally opposite the kitchen door into the hall, as that door is where the air comes into the kitchen from the rooms that are fed with fresh air.  The same goes for the fresh air fed rooms, but in reverse, so, for example, the living room fresh air feed terminal is in the ceiling corner almost diagonally opposite the door into the hall, as this door is the extract path.

[Declan52]

33 minutes ago, Jimbouk said:

I too am fitting my first fix Mvhr on Thursday....

 

Wondering what the best approach is to mounting the plenums? Screw them to the joists and then try to cut the plasterboard to fit, or cable tie them roughly in place and let the plasterers align them to the holes they cut in the plasterboard?

 

Also have the same question regarding how far from the hob.

Fix them in place and cut them so the bottom of the tube is level with the top of the plaster board. Don't cut holes in the ceilings prior to plastering the ceiling much easier to skim the ceiling without them. Have all the measurements of where they are and then cut through the ceiling and fit the cover on.

I have my extract 1300mm from the hob.

[ProDave]

3 minutes ago, Declan52 said:

Fix them in place and cut them so the bottom of the tube is level with the top of the plaster board. Don't cut holes in the ceilings prior to plastering the ceiling much easier to skim the ceiling without them. Have all the measurements of where they are and then cut through the ceiling and fit the cover on.

I have my extract 1300mm from the hob.

Exactly what I am doing right now.

 

If the room above is boarded, I am screwing them to the underside of the floor above. If not I am fixing various bits of wood in place to screw them to.  In the case of the loft, which is boarded, they are going above the boards, so drill a big hole in the loft floor and screw them down from above.

 

Pictures on my blog in a few days when I am finished.

[Declan52]

Think due to the depth of my truss I had to use a 50mm batten to bring the bottom down to where I needed it.

[CC45]

@JSHarris bpc call the terminals in the room plenums.  

 

@Declan52 good tip - bit worried that I may not be able to locate them accurately after skimming! I thought about marking their centres on the floor using a plumbob - should make finding them easier. Not too sure what to mark them with - needs to be fairly resilient or it will just rub off. Is there a min distance? Is 1300mm enough?

 

CC

[CC45]

@ProDave exactly how Ive fixed mine - just the kitchen ones to go in now.

[Bitpipe]

Just now, Declan52 said:

Fix them in place and cut them so the bottom of the tube is level with the top of the plaster board. Don't cut holes in the ceilings prior to plastering the ceiling much easier to skim the ceiling without them. Have all the measurements of where they are and then cut through the ceiling and fit the cover on.

I have my extract 1300mm from the hob.

 

Extract similar distance for me. 

 

Depending on your joist type (we had metal web) you may be restricted in how neatly the duct arrives at the plenum and you'll then have to position the plenum to best receive the duct and allow a snug connection (buy WD 40 silicone spray, a must for getting the duct with rubber washer snug into the plenum). It's easiest to have the duct arrive straight at the plenum (at least 1m) to give you the room to position and insert it home, avoid very tight bends immediately before the plenum as you'll find it tricky to get the duct in.

 

I used 50mm batten offcuts to create two noggins across the underside of the top web of the joist and screwed the plenum into this. There was initially quite a bit of fiddling about to get them where I wanted and consistency spaced from walls, webs always seemed to get in the way but you get the hang of it after the first few.

 

I then ran the duct from the manifold location (leaving plenty of slack) to the plenum and secured it (rubber washer and clips). Be prepared to reposition duct a few times, especially if you have a few coming together through a penetration in wall or floor - can take a few tries to get the flattest arrangement.

 

Two people are better than one when running duct as it can sometimes need a pull and a push to get round bends - usually gets caught on metal webs (I used bits of cardboard to minimise this). Making a lasso from poly rope can help pull it along from ground level and avoids hopping on and off a ladder. Keep your duct loosely coiled  - I used a batten across a door way as a cable spool, if it totally unwinds then it will knot.

 

Last thing I did was connect duct to the manifolds, be cautious when trimming to get a neat fit, suggest you get them roughly connected first and then trim each to get rid of excess, if you over trim, you may struggle to get any additional length in the duct to get a tight fit to the manifold. I zip tied these when final as my manifold only had two spring clips per duct.

 

If you have the ceiling mounted type of manifold that has ducts on either end and the main duct to the base, you need to ensure it's flat otherwise the duct down to the unit will be crooked. Can take a bit of trimming of duct either side to achieve.

 

After I was done, Sparky moved a few plenums to line up more neatly with downlights and other roof furniture, provided you have a bit of slack in the duct, this is not an issue. 

 

I left my plenums untrimmed and had the tackers cut board to fit - they were pretty neat, only a few mm of gap.

 

Left them untrimmed for plastering too, probably should have trimmed with a multitool to get a neater finish, however the cowl extends an inch or so past the plenum edge so ceiling looks fine.

[Declan52]

21 minutes ago, CC45 said:

@JSHarris bpc call the terminals in the room plenums.  

 

@Declan52 good tip - bit worried that I may not be able to locate them accurately after skimming! I thought about marking their centres on the floor using a plumbob - should make finding them easier. Not too sure what to mark them with - needs to be fairly resilient or it will just rub off. Is there a min distance? Is 1300mm enough?

 

CC

Take measurements of the walls both ways. Any mark on the floor will have disappeared by the time the plasterers have finished.

[CC45]

@Bitpipe like you we have metal web joists - so much easier.  Was planning on doing virtually the same as you suggest. Got 12 fitted so far - no probs. Mate due to help on Saturday so it would be good to get the ducts in - how much slack did you leave on each run?  For the loft runs I was going to measure the runs and then cut the pipe (downstairs).  Long runs first then progress to the shorter ones in order to avoid joints if at all possible.

 

I liked @JSHarris idea of a silencer - intend to do the same if I can figure a way of sealing the pipe to them.  Having said that perhaps spending £80 on the ones available would save time and hassle - I'm not short of jobs to do.

 

CC

[Bitpipe]

1 minute ago, CC45 said:

@Bitpipe like you we have metal web joists - so much easier.  Was planning on doing virtually the same as you suggest. Got 12 fitted so far - no probs. Mate due to help on Saturday so it would be good to get the ducts in - how much slack did you leave on each run?  For the loft runs I was going to measure the runs and then cut the pipe (downstairs).  Long runs first then progress to the shorter ones in order to avoid joints if at all possible.

 

I liked @JSHarris idea of a silencer - intend to do the same if I can figure a way of sealing the pipe to them.  Having said that perhaps spending £80 on the ones available would save time and hassle - I'm not short of jobs to do.

 

CC

 

Not a massive amount of slack but enough to be able to move each plenum 500mm or so if needed. Duct should not be super taut, if you keep your bends fairly lazy then you'll have some wriggle room if you have a problem.

[Jeremy Harris]

36 minutes ago, CC45 said:

@JSHarris bpc call the terminals in the room plenums.  

 

 

It's still incorrect terminology - a plenum is defined as an air chamber where ducts meet in this context, not a vent or terminal, no wonder there's communications problems on here when even professionals make such a fundamental error.  Perhaps someone should put them straight, as it's the sort of error that would put someone like me off doing business with them, on the basis that if they can make errors like this, what other errors could they be making?

 

https://en.wikipedia.org/wiki/Plenum

[Jeremy Harris]

7 minutes ago, CC45 said:

 

I liked @JSHarris idea of a silencer - intend to do the same if I can figure a way of sealing the pipe to them.  Having said that perhaps spending £80 on the ones available would save time and hassle - I'm not short of jobs to do.

 

CC

 

I'd definitely just buy silencers, it's really not worth the hassle to make them.  I only had to make ours because I foolishly left insufficient room to fit off the shelf ones.  They are very effective, though, and I think they are well worth fitting.

Edited June 6, 2017 by JSHarris

[Declan52]

Connect one end of the ducts all up at the room end and leave the ducts 1m longer than you need. Have them all tied together to keeping them neat where ever you are connecting them up. Once you know they are all in and don't need moved it's a 2 min job cutting the duct and connecting it up.