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  1.  
    I'm looking at a new build which will be as airtight as it can reasonably be made with a masonry construction. I'm contemplating specifying MVHR and also a central vacuum system, but I'm not sure how these two will work together.

    Normally central vaccum systems expel the waste air outside to make sure any residual dust/pollen doesn't remain. This is going to mean a lot of air sucked out of the building to the outside world.

    If I have a (reasonably) airtight structure is the MVHR going to provide the air to replace the expelled air? If not, how can I make this work?

    Thanks very much,
    Mark.
  2.  
    Posted By: MarkBennettIf not, how can I make this work?


    Open a window if you're doing a lot of vacuuming ... if not, don't worry about it. No building is airtight and so the pressure will equalize eventually. Just don't run a sealed fireplace (with its own airsupply) with the fireplace door open otherwise you'll suck air into your house down your chimney (for fireplace read any sealed woodburning appliance). If it's not sealed, then you'll certainly suck in air down the chimney.

    Paul in Montreal.
  3.  
    Opening the window. Hmm, why didn't I think of that (sound of forehead being smacked). :shamed:

    I'd already thought about the making sure the fireplace door was closed, but it was worth pointing out.

    Thanks for the answers,
    Mark.
  4.  
    ...images of Mark pleasantly hoovering in a Freddy Mercury sort of way, perhaps whistling a tune....

    ...meanwhile family slowly keel over one at a time...

    ...Bennet Junior found crawling across floor, his arm forlornly raised to try and reach the window handle before he takes his last gasp...

    :wink:

    J
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 3rd 2009
     
    Posted By: MarkBennettOpening the window. Hmm, why didn't I think of that
    because it's daft! (sorry Paul) All that trouble to minimise losses and seal airtight so you can make do with little or no fuel-burning, then in mid winter you open and suck a freezing draught through the place, a loss that you've no fuel-burning way to make up! Like cooker and (be damned) tumble drier exhausts and open-flue woodburners, central vac is fundamentally incompatible with airtight/MHRV.
  5.  
    Posted By: fostertomLike cooker and (be damned) tumble drier exhausts and open-flue woodburners, central vac is fundamentally incompatible with airtight/MHRV.


    Actually, it's not that bad. Central vacuum systems are not high flow rate (look at how small the pipes are) plus they're used very intermittently. My comment about opening the window was flippant, but for people who are scared of being asphyxiated by a MHRV failing, there is always this option should the need desire. Just entering the building will take care of any depressurization quite quickly. If you look at the hot2000 report I did for Tony: http://www.tonyshouse.info/Hot2000FullReport.htm you will see that 71 litres per second of exhaust will cause a 10Pa depressuriation - but the MHRV is already supplying 60l/s. A quick google found this page http://www.canavac.com/page1.htm which shows suction power versus airflow - the maximum is around 60l/s for a pipe of 50mm (the typical size used). So, in fact, the MHRV system will be able to keep up with this without any depressurization at all. Same thing for a tumble drier. Open flue woodburners are different because they're not intermittent devices.

    At the end of the day, we have to be pragmatic and focus on the big loss issues. Of course, if one was really paranoid, the exhaust from the central vacuum could be piped through the MHRV - but I'd want to be sure that the exhaust would be clean enough before attempting this so as to not clog up the heat recovery core. Another advantage of doing this would be to recover the heat of operation of the motor - but, again, it's very intermittent and is probably not worth worrying about.

    Paul in Montreal.
  6.  
    Posted By: Paul in Montreal
    A quick google found this page http://www.canavac.com/page1.htm which shows suction power versus airflow - the maximum is around 60l/s for a pipe of 50mm (the typical size used). So, in fact, the MHRV system will be able to keep up with this without any depressurization at all.


    This would them mean the MVHR became imbalanced, almost no air coming in, but none going out. Obviously this will mean that incoming air isn't warmed significantly (I guess almost equivalent to opening a window) but otherwise will the MVHR system cope?

    Posted By: Paul in Montreal
    At the end of the day, we have to be pragmatic and focus on the big loss issues. Of course, if one was really paranoid, the exhaust from the central vacuum could be piped through the MHRV - but I'd want to be sure that the exhaust would be clean enough before attempting this so as to not clog up the heat recovery core. Another advantage of doing this would be to recover the heat of operation of the motor - but, again, it's very intermittent and is probably not worth worrying about.


    This is also an interesting idea, but I agree that it could need an additional level of filtration before feeding into the MVHR. Would MVHR's cope with effectively forced air on the stale air intake?
  7.  
    For the tiny amount of time per year a central vacuum system is used, I wouldn't worry about either depressurization or heat loss. It really isn't worth worrying about. You might lose 50p worth of heat per year - the cost of putting in all the extra duct connections, filters and the like will have such a long return on investment that it's just not worth it. If anything, a MHRV will reduce the frequency of vacuuming anyway since the incoming air is filtered (to some extent) rather than just coming in through the window or trickle vents. Of course, this doesn't address human-created dust, but even the MHRV will help with this as some dust will be exhausted by the system.

    Paul in Montreal.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 3rd 2009
     
    OK Paul, that's valuable putting-into-perspective. If easily done, and if filtration good enough, then 85% of half hour's motor (2kW?) once a week (?) plus less cold air pulled in by imbalance could be worth saving? Unless the motor heat isn't cooled by the airstream but is added to useful room heat, then there's little to be saved.
    • CommentAuthorMarkBennett
    • CommentTimeFeb 3rd 2009 edited
     
    Posted By: Paul in MontrealFor the tiny amount of time per year a central vacuum system is used, I wouldn't worry about either depressurization or heat loss. It really isn't worth worrying about. You might lose 50p worth of heat per year - the cost of putting in all the extra duct connections, filters and the like will have such a long return on investment that it's just not worth it.

    Agreed, I'll stop worrying about it and get my Freddie Mercury outfit ready...

    Posted By: Paul in MontrealIf anything, a MHRV will reduce the frequency of vacuuming anyway since the incoming air is filtered (to some extent) rather than just coming in through the window or trickle vents. Of course, this doesn't address human-created dust, but even the MHRV will help with this as some dust will be exhausted by the system.

    Most of the dust in our house comes from the cat. We had carpets in the previous house and never noticed it, now with wooden floors we keep finding piles of cat fluff gathered up in corners/under sofas/beds etc. Doubt the MVHR will help much with this!
    • CommentAuthorStuartB
    • CommentTimeFeb 3rd 2009
     
    MarkBennett - why do you want a central vac system in the first place?? I think these things are completely OTT and will probably cost you thousands. What's wrong with a £150 Dyson?
    • CommentAuthorMarkBennett
    • CommentTimeFeb 3rd 2009 edited
     
    Posted By: StuartBMarkBennett - why do you want a central vac system in the first place?? I think these things are completely OTT and will probably cost you thousands. What's wrong with a £150 Dyson?

    Two main reasons, convenience and noise.

    We're looking at four stories (basement, 2 stories plus attic) which is a lot to lug a Dyson up and down on a regular basis. The central systems also provide skirting board vents to make it easy to sweep up floor dust without having to get the whole system out.

    Our cat and the two young children currently get quite disturbed by the noise of the Dyson, particularly the really high-pitched whine it makes. A central vac puts this noise outside and would allow us to clean up at a time convenient to us, which at the moment is pretty much only after the kids have gone to sleep.
    • CommentAuthorStuartB
    • CommentTimeFeb 3rd 2009
     
    Mmmm - would agree it is maybe marginally more convienient than a hoover but would not agree that it justifies the extra expense.

    WRT the noise - if your kids are young enough then you can train them to sleep through any kind of noise, they get used to it very quickly. Means when they go to bed you can play music, have footie loud on the TV, have friends around for a drink and a laugh without worrying about the kids waking up.

    All cats are disturbed so can't help you there.:wink:
    • CommentAuthorludite
    • CommentTimeFeb 3rd 2009
     
    I agree that kids will sleep through anything - EXCEPT when hubby's alarm goes off at 5:45 every morning. They are now programmed to wake at this time. I think 7:15 would be far more civilised, however, after months of me being grumpy, I have decided that if I can't beat them, I may as well join them and have started rising at the same time as the rest of the family - after all, who needs sleep?
    • CommentAuthorTheDoctor
    • CommentTimeFeb 4th 2009
     
    i would avoid the Dyson like the plague

    the concept is excellent.
    the actual functionality of the bagless system works

    BUT

    never in the history of design and sentient thought has one man so over-designed, over-engineered a single piece of equipment.
    so many daft wee bits of plastic, tools ,attachments and add-ons, of which 90% probably never get used.

    they break down, they break, they clog.

    Even the super-duper pet hair specials can't cope after a while


    motors keep going, and suction remains, but it is packaged in such a ridiculous frame.


    A Dyson system in a plain and simple, minimal case would be the eureka moment.
    Mr Dyson missed it by a Country mile.

    time he launched the 'retro Dyson'


    hijack / rant over!
    • CommentAuthorCWatters
    • CommentTimeFeb 4th 2009 edited
     
    Posted By: MarkBennettI'm looking at a new build which will be as airtight as it can reasonably be made with a masonry construction. I'm contemplating specifying MVHR and also a central vacuum system, but I'm not sure how these two will work together..


    I wouldn't worry. Our MVHR system manages to supply enough air for two wood burners that burn room air. We managed to convince our BCO that the MHRV system meant we didn't need air bricks for the wood burners either. If I open the ash pan door on the big fire insert I can create a huge raging inferno in the fireplace that sends vast quantities of air up the chimney... far more than any central vac system would ever export. We also have a cooker hood extractor in the kitchen that works fine.

    IF funds were tight I would rank MVHR above all else, for example higher than a central vac (which we don't have) or wired computer lan (which we do). The improvement in air quality has been very impressive compared to all previous houses we have lived in. It feels like you are camping and we have zero condensation so no window sills to wipe etc. We even have some old fashioned "bronze" metal framed windows in two rooms and even they don't get condensation on them despite not having thermal breakes. Would certainly install one again if building another house, no contest.
    • CommentAuthorPDobson
    • CommentTimeFeb 4th 2009
     
    Just a quick note on the CVac + MVHR - we have both in our recently completed new build and haven’t noticed any problem. We decided to vent the vacuum into the boiler room/workshop in which the CVac is situated. It does seem to pump out quite a lot of heat when running - enough to heat up the workshop which relies on background heat by a couple of degrees, but we haven’t noticed any dust coming through – not the cleanest environment anyway just at the moment though.

    Re the noise -the cats still don’t like it and hide under the bed as with the old upright - it makes a surprising amount of sucking noise at the nozzle end - having said that its certainly a lot quieter than a Dyson.
    • CommentAuthorStuartB
    • CommentTimeFeb 4th 2009
     
    Doctor - wash your mouth out! Dysons are fantastic. Our motor went after about 7 years (because my wife forgot to put the filter back in after cleaning) and the guy came out and replaced it within 2 days.

    They have a set call out fee of £39 (I think) which covers all labour and parts. Because ours was quite old he replaced nearly every other part - brushes, hose, flex, filter etc all for £39. It is like a new machine. Fabulous service and saves many machines from going to landfill.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 4th 2009
     
    Yeah - why can't a central vac recycle its own air?
  8.  
    Posted By: StuartBDoctor - wash your mouth out! Dysons are fantastic.


    I have to agree. Our Dyson is by far the best vacuum we've ever had. We also had a motor burn out (our fault for not cleaning the filter ever) and it was all sorted by a fixed fee repair (where they also replaced the hose). Excellent product and excellent service.
  9.  
    Posted By: fostertomYeah - why can't a central vac recycle its own air?


    One of the selling points (justified or otherwise) of these vacs is the reduction in fine household dust and pollen. The idea is that anything fine enough to escape the filter is still ditched outside the house compared to a normal vacuum where it is effectively injected back into the air inside the house.

    This is quite a big thing for Asthma/hay fever sufferers and the like.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 4th 2009
     
    So even with a fixed unit, filtration can't be that good?
  10.  
    Posted By: fostertomSo even with a fixed unit, filtration can't be that good?
    It can be as good as you want - you can install a HEPA filter if needed. Some central vacuums are cyclone based (same principle as the Dyson) others are just like ordinary vacuum cleaners with regular bags and filters. At least with an external exhaust you're not putting fine dust back into the house (as you would be with an ordinary cleaner). Probably one advantage of a central vac is that you can use a larger fan and thus get a higher airflow (and thus better cleaning power). The incremental cost of installing one at house construction time is pretty small. All that said, we did not install one when we built the new house a couple of years ago.

    By the way, if you want a DIY cyclone vacuum cleaner, take a look at this kit from Lee Valley Tools in Ottawa: http://www.leevalley.com/wood/page.aspx?c=1&p=30282&cat=1,42401 This converts a rubbish bin into a cyclone if you have a regular shop vac - great for people with workshops with lots of wood tools.

    Paul in Montreal.
    • CommentAuthorgyrogear
    • CommentTimeApr 3rd 2018 edited
     
    <blockquote><cite>Posted By: MarkBennett</cite>The idea is that anything fine enough to escape the filter is still ditched outside</blockquote>

    Daft question, Nine Years Later...

    Could the Central Vac vent pipe be plumbed into a soil vent stack (*above* the level of the highest WC, naturally ) :shamed:
    the idea being it is ejected through the roof 3 feet later...
    (it is the lower one in the photo)

    This is to avoid having to penetrate the envelope (we have EWI).

    Otherwise, could the vent be just "dumped" into the crawlspace, such as into a mosquito-netted corner enclosure etc.

    gg
      mushroom heads-soil lower and extract-ventilation above.jpg
    • CommentAuthorOtterbank
    • CommentTimeApr 3rd 2018
     
    The central vacuum we had was fitted with a filter on the exhaust to avoid the need to duct to the outside. Seemed to work fine. However we decided against having one in this build, didn’t think we could justify the expense.
  11.  
    We were all set to put a central vacuum into our new build, but in the mean time got a Dyson cordless for the old house which was so good we didn't bother!
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