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Ventilation: MVHR sizing - Part F and retrofits

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    • CommentAuthorwookey
    • CommentTimeDec 2nd 2015
     
    No, I don't understand their very confusingly-labelled set of speeds either. I think I might just measure the airflows I get from twiddling the switches and knobs.
    • CommentAuthorGreenfish
    • CommentTimeDec 5th 2015
     

    Posted By: wookey Now I have a (non-room-sealed) small woodburner, so this definitely isn't actually true here. Not sure how to allow for that.

    I'm definitely not an expert on wood burners so hopefully somebody who knows will come along, but I don't think wood burners and MVHR mix well. The problem is the need to ensure there is always greater pressure outside the stove than inside it, to prevent blowback of nasty gases and particulates. I have this horrible fear that the regs say you need a huge hole in the wall of the room. What does your BCO say?


    Not an expert either, but with a WBS and MVHR I have some experience to share in this area.

    Under the regs the BCO should demand that you either have a certified room sealed WBS or a hole in the wall. But assuming you get past that somehow, and dance with the devil of possible blowback of nasty gases, please have a CO detector fitted!!

    Other than possibly poisoning yourself if you don't have a hole in the wall, the other thing that could be effected is flue draw. I have found that air tighness makes more impact than MVHR. How air tight is your build Wookey? Mine is well tight (current gale giving a better work out than any blower test), and I have a room sealed WBS with external air supply. When the stove door is open (for loading more wood) I have found it is unable to draw air from the room, hence it can belch smoke into the room. The flue works well and draws nicely, but not from the open door. The MVHR makes no difference on, off, or boost smoke can drift out. I think it is the fact that the house is so air tight there is nowhere in the house for the flue to pull the air from.

    Could I even suggest that flue behaviour is a DIY air tightness testing method? Mine seems to be. To get air to draw from the room and up the flue I have to open a window. Then again perhaps my house id odd?
    • CommentAuthorwookey
    • CommentTimeDec 16th 2015 edited
     
    So, I am progressing slowly, have bought a lot of pipe (ducting-online.co.uk do galv piping for slightly less than I could find plastic (but stock plastic too) and have a good range of fittings and make their stuff in the UK so I picked them in the end. You do have to pay 20% restocking on returns though, unlike i-sells, for example.)

    As someone else already said. If you have the chance, allow plenty of space in your machine room. Fitting it into a cupboard has been interesting. I ended up doing some CAD drawings to work out exactly how much offset pairs of 45/30/15 degrees bends give, for example, and the lowest-resistance design would only fit in the cupboard if I binned the silencer, so a slight efficiency compromise has been forced by what will physically fit. Worst run is 7Pa, which I think is OK.

    Anyway, to the point (s):

    Simple question first:
    1) I need to insulate about 1m of 150mm pipe. (I have kept the external-going pipes commendably short). But duct-wrap comes in 17m long rolls costing about 75 quid. Anyone got some leftovers they want to send me? Or a better suggestion for bodging with some other insulation? Maybe I should just go and buy a couple of (unbranded, obviously) karrimats to cut up?

    I've just discovered this ubbink foam pipe. Should have used some of that!: http://www.phstore.co.uk/ubbink-insulated-foam-ducting.html doh!

    2) What to do about having a normally-unheated extension?

    We have an extension incorporating the garage/workshop and 'potting shed'(a proper brick room). It's all pretty well insulated (U=0.2 or better, thermal bridges mostly removed), but is not normally heated (although it does have UFH so it could be heated one day if we (or future occupants) chose to).

    But it does need to have some ventilation, if only to satisfy the building control man. (In practice it's generally fine as it is, although I'd like purge ventilation for smelly shed jobs, like painting and angle-grinding).

    So the question is - does one use the nice (and expensive) MHRV unit I've already bought, or ventilate it entirely separately?

    Treating is as a normal room as part of the system is just going to bring a lot of cold air in for half the year and make the heated rooms draughty. It's also quite well-sealed from the rest of the house so might need its own supply _and_ extract. On the other hand, a future occupant might expect it to operate with the rest of the house, and it seems a waste to not use the great big manly fan I've already bought.

    The obvious thing to do is have a separate system - e.g. a single room MHRV but that's moderatly expensive, and another hole though the wall.

    So I wondered about connecting it to the MHRV (supply _and_ extract), but using a (pair of) shut-off valves so that this is only enabled when needed, especially for purge. Manual ones are not too expensive (http://www.ducting-online.co.uk/shut-off-gate-c2x14641788 ) but I've not found an electric one yet that could be remote-controlled (and they probably _are_ expensive). How bad a thermal bridge is a closed-off metal pipe? This is only to a 'cool' area, not outside. It never goes below 10C no matter how cold outside is. It's usually 13-16C.

    Should I just get a plain purge fan and fit that for independent use? (I noticed this which seems nifty: http://fans4less.co.uk/heat-recovery/wall-mounted-heat-recovery-86/twhr150es-upto-90-efficient.html )

    Someome help me undither... :-)
    •  
      CommentAuthordjh
    • CommentTimeDec 16th 2015
     
    For purge, windows are the obvious choice if there are some. Extracting through a heat recovery unit is not a good idea for anything that's creating dust, like angle grinding. Or sawing, planing etc etc. You'll be forever changing filters.

    I think I'd treat it as a separate system, rather than extend the house system to it, whatever you do.

    Posted By: wookeyThis is only to a 'cool' area, not outside. It never goes below 10C no matter how cold outside is. It's usually 13-16C.

    I was going to ask why it stays so warm, but I suppose it is the relative insulation levels of its external walls as compared with those of the wall it shares with the house?
    • CommentAuthorwookey
    • CommentTimeDec 18th 2015
     
    Right - it's just a 65mm cavity-with-fluff to the house, but 200mm dritherm or 100mm PUR or 270mm PUR in the roof, and fairly good airtightness.

    Very good point about contaminating filters. Hadn't thought about that.

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