Part Thirty – MVHR Details

This thirtieth entry was published originally by JSHarris on the 29th April 2014 and received 2,012 views on the closed forum

Just a short update with some details of the MVHR install, and a brief explanation of how I drilled some big holes through the airtight walls of the house………………….

One job I’ve been putting off was cutting the fresh air inlet and exhaust air outlet duct holes through the walls of the house. Ideally I’d have put these in before the insulation was pumped in, but at that stage I hadn’t purchased the MVHR and so wasn’t quite sure where I wanted the holes.

The technique I used was this. First I made up a 600mm long 6mm diameter drill, by drilling a 3mm hole in the end of a bit of 6mm bar in the lathe, turning down the shank of a 6mm wood drill bit to 3mm and then silver soldering the two together. This very long drill allowed me to accurately drill a pilot hole through the wall, in a location free from any internal timbers. The secret to getting it to go straight was to get a block of 70mm square timber and drill a 6mm hole through it that was square on one axis and had a 1 deg angle on the other axis. Holding this block against the inside of the wall and using it as a drill guide ensured I got the long drill through square, with a slight downward angle to ensure no moisture could ever run back into the house via the duct. Once drilled right through I undid the drill chuck and left the drill in the hole as a guide.

Next I used a short bit of batten with a 6mm hole in one end and another hole in the other end the right size for a pencil. I spaced these holes about 80mm apart, so that when I slipped the 6mm hole over the drill I could draw a 160mm diameter circle on both the inner and outer walls (the latter from the top of a ladder). Next it was out with the jig saw to cut the wall skins out (fiddly for the one shown below because it’s the fresh air intake and is right in the corner of the eaves).

In the photo above you can see that the inner airtight/vapour block skin has been cut away and the warmcell insulation (the grey stuff) is packed solidly in the cavity, even in this corner at the very top of the wall and around a metre or so from the nearest injection point.

Next, a plastic funnel was taped over the end of a bit of 150mm PVC duct (pointy end out, and the nozzle taped over) and the whole thing pushed through the wall from the outside (only so that any displaced warmcell fell inside). The cone of the funnel tended to compress and displace the warmcell as the duct slid in, ensuring that very little is lost and that it is packed tightly around the duct. Before pushing the duct right in, I fitted the external terminal to the duct and squirted expanding foam around it before pushing it right home, to seal up around the duct exit in the outer skin. I then pushed the thing home and screwed the terminals to the cladding, then fitted the grills to them. They are dark brown and blend in reasonably well with the larch.

To seal the inside, which is far more critical, I first used low expansion foam (the pink stuff) to fill around the duct and bond it to the inner skin. Before the foam cured I wiped around it with a gloved finger to get a fairly smooth inner surface. As a belt and braces seal, I then added Siga airtightness tape to seal all around the joint. The photo below is the exhaust duct, but shows the same sort of seal (the exhaust duct is easier to photograph, as it’s higher up on the gable wall, rather than very low down on the rear wall behind the MVHR unit).

Having got these ducts fitted, I could then shift the MVHR unit (all 78kg of it…….) on to the mount I’d made for it. I’m keen to minimise noise that may be transmitted from this unit to the house structure, so as well as using very short lengths of flexible duct to connect it up as vibration isolators, I also used four rubber anti-vibration mounts to support a base frame that the thing sits on. This raised the MVHR up (which makes it easier to service and allows room underneath for the condensate drains). Here’s a photo of one of the rubber bobbins I used (the keen eyed will recognise these as Mini exhaust mounting bobbins……….):

I’ve kept all the duct runs from the MVHR to both the outside fresh air/exhaust terminals, and the house extract and supply manifolds, as short as I can to reduce losses. The system uses radial ducting (the Swiss HB+ sytem) so every vent in the house comes back to a common manifold. This is the extract manifold, fitted to the wall to the left of the MVHR:

And (tucked away in the tight space on the floor behind the MVHR) this is the fresh air feed manifold:

Finally, this is a view of the installed MVHR with all the ducting connected up, showing the house feed and extract end:

And another one showing the fresh air feed and exhaust end (the fresh air feed is behind the MVHR box down at floor level, on the rear wall):

The next job is to finish the wiring (pretty simple, just power and a four way control cable that goes to the controller already fitted down in the hall), add some insulation around the fresh air inlet and exhaust ducts and then test and commission the unit. I suspect that adjusting flow rates may be the most tedious bit, as this has to be done by measuring the flow at each extract and fresh air feed terminal (using a low air speed measurement unit) and then adjusting the flow rate by swapping restrictor plates over in the fittings where each radial duct fits to the manifold.

Hopefully the above may be of use to anyone else contemplating their own MVHR install. So far it’s taken me about a day to fit all the first fix ducting and perhaps 3/4 of a day to fit the unit itself and connect up all the ducts. In addition to doing this today I filled the primary ASHP circuit and leak tested it (hence the damp patch on the floor under the small PV for this circuit) and tested the ASHP briefly (seems to work, but I need to read the manual tonight to programme it).

I’ve edited this post to add a photo of the intake and exhaust ducts, taken from the North East corner of the plot. The exhaust is on the left (on the East gable wall) and the intake is on the North wall, right under the eaves.


ProDave 29 Apr 2014 06:47 PM:

 Great information as always.

 One question abut drilling the holes through the outer wall:

 Did you cut the outer skin in one go with the timber cladding in place? (long jig saw blade to cut both the cladding and outer skin of the wall together)

 Or did you remove a section of the timber cladding, Drill the inner and outer skins of the wall, then drill the cladding to match and replace?


 jsharris 29 Apr 2014 07:15 PM:

 I did it in two stages, Dave, but had I had a longer jigsaw blade I’d have done it in one go. I ended up using the jigsaw to cut through the cladding, then fiddling around with a Bosch multitool to cut out the outer OSB skin (pretty fiddly when doing this up a ladder).

 The outer terminal covers I used (these ones: http://cart.vacuumsd…d=377&parent=46 ) had a surround that’s a fair bit bigger than the duct, making it easy to have a bigger hole in the outer skin and use squirty foam to seal it up.


 couplands 01 May 2014 05:53 PM:

 Just wondering if you have to worry about the build-up of static in the ventilation system. Ive seen with air extraction systems in workshops (albeit at higher flow rates), that the plastic pipes need to be grounded using copper wire along their length.




 jsharris 01 May 2014 07:07 PM:

 Not as far as I’m aware. Lots of MVHR systems use plastic ducting and none use any form of conductive coating or wire, as far as I know.

 I fired up the MVHR unit today and it seems to work OK, nice and quiet when in passive mode at low fan speeds, but there is some noise when it’s in active mode. This worried me at first, as when I turned it on it defaulted to active heating, with the fresh air feed running at around 40 deg C, the exhaust at around 4 deg C and a fair bit of noise coming down the ducts. An added problem was that all the menus were in Danish (because I bought the unit direct from Denmark) so it took me a while to work out how to switch it to English and turn the temperature down so it wasn’t in heating mode.

 In passive mode and with the fan speed at the lowest setting it’s pretty much inaudible in the bedrooms and living room, but can be heard in the utility room and downstairs WC. I didn’t fit silencers to the fresh air feed and extract ducts, before the manifolds, but I’m sure it does really need them (the manual says they must be fitted). I’m now going to make up some silencers and fit them tomorrow, should be easy enough, just some big expansion boxes with some acoustic lining.

 The really good news is that the vibration isolation seems to work very well indeed, the only noise I’m getting is through the ducts. One advantage of a sealed and well insulated passive house is that it is very, very quiet inside with the doors and windows closed, although this does mean that any installed system has to run quietly.


 SteamyTea 11 May 2014 10:18 AM :

 When it comes to commissioning and balancing the system, how do you cope with the permutations of doors in the house being open or closed. Some doors you know are usually closed (bathroom, toilet (or is it lavatory, I never know), utility room. But other doors can be left open, or closed, depending on mood.


 jsharris 11 May 2014 10:55 AM :

 For the purposes of the report for building control I just left all the doors open, as that reduced the variation I was getting from the wind outside (wind is a real problem – I was seeing about +/- 30% flow variations second by second on pretty much every terminal on a very slightly windy day).

 There are three things to do when commissioning the system, I think, plus the final tuning after you’ve done the building regs tests.

 The first is to balance the fresh air feed in and the exhaust out, by restricting all the ducts (at the manifold end) for the higher flow side (in my case the sum of the extracts started off around 40% more than the sum of the fresh air inlets).

 Next is to set the MVHR to it’s highest speed (in our case fan speed 4) and check the extract flow rates against building regs (6l/S in a WC, 8/lS in a bathroom or utility room, 13 l/S in the kitchen). This needs a bit more balancing, and may involve opening up some extracts (like the kitchen) and closing down others (like the bathroom, WC and utility), double checking that the sum of the flow from all the extracts after doing this is the same as it was before.

 Next you set the MVHR to it’s manufacturer recommended occupied house running speed (in our case that’s fan speed 2) and measure the total flow out of, and into, the house to ensure it exceeds the building regs minimum continuous ventilation rate. For us this gave a flow rate that was a fair bit higher than building regs needed. Our habitable floor area is a bit under 130m², so using the building regs minimum of 0.3 l/S per m², we needed about 39 l/S. I found that I was getting over 50 l/S on fan speed 2, so switched the unit down to fan speed 1 (the unoccupied house level) and found I still got more than 40 l/S total, with no noise at all in most of the rooms.

 That completed the building regs compliance bit, so I then went around and shut the WC, bathroom, bedroom and kitchen doors and checked the flow rates again. On the lowest setting they didn’t noticeably change, but on fan speed 4 the extract rate in the kitchen, utility and WC dropped a fair bit. This is because the only source of air into those three rooms is via the 8mm gap under the kitchen door (which was howling a bit when I did this). Opening the kitchen door got the flow rates pretty much back to where they should be, and reduced the noise a fair bit.

 In practice the flow rates will vary a lot from whatever they’ve been set at during the building regs compliance testing, as differential wind effects on the external exhaust/intake terminals seems to have a big impact. In my case this impact is made worse as I have the exhaust on the East facing gable and the intake on the North facing wall, right under the eaves (I’ve just edited the post above to show the arrangement of these terminals on the outside). I’m bound to get fairly wide variations of dynamic pressure from the wind between these two locations, so a fair bit of variation is to be expected. I’d imagine that situations like this must cause problems when doing measurements in a lot of installations, as, in terms of avoiding cross flow from the intake and exhaust, it makes sense to position the exhaust on the wall opposite the prevailing wind and the intake as far away from it as possible.


 SteamyTea 11 May 2014 05:47 PM:

 Now I am lodger free again I can use the prevailing wind to ventilate my place again. Suppose it is worth checking again when wind is in different direction, just for fun mind.

Good that you are compliant at the lowest, and quietest, setting.


 Calvinmiddle 21 May 2014 11:27 AM :


 Has your BCO come back yet saying he is happy with the method you used to commission your system?


 jsharris 21 May 2014 08:43 PM:

 I’ve not heard a thing from the BCO, which usually means that he’s OK about things. If there was a problem I suspect he’d have let me know by now, as in the past he’s been pretty quick at getting back to me with anything that wasn’t clear.

 I won’t seem him again for a while, as our next inspection is the final completion one, which won’t be for a while yet as I’m now doing everything else on the house myself, which is turning out to be a slow job!


 coopers 25 May 2014 07:21 PM:

 Hi Jeremy, Did you implement the “poo-smell-direct-ducting” that we were discussing a while back?


 jsharris 26 May 2014 02:45 PM:

 Yes, I’ve put the ducting in and looked inside the plastic built-in cisterns we’re using to make sure there is a direct air path out from the flush pipe. The answer is that I was right, there is a good direct connection from the airspace above the water in the cistern to the flush pipe, so there’s no reason in theory that this system shouldn’t work.

 Whether it does or not depends on whether there’s enough airflow to overcome the tendency for smells to disperse. The biggest connection I can practically make to the cistern is with a 32mm flange fitting, but I’ve run 40mm pipe along the eaves space from the extract manifold, and that does seem to have a fair bit of “suck” at the end, especially when the MVHR is in boost mode. The biggest restriction is inside the cistern, the internal overflow pipe that connects the air space to the flush pipe is only about 20mm, so will only allow a limited flow rate. I suspect that the flow rate needed is probably pretty low, though, you don’t really want a massive draft down there!


 Calvinmiddle 27 May 2014 03:05 PM:

 Have you more details of this “poo-smell-direct-ducting” as it is something that interests me. I have seen this product advertised, is it this or something like this?

 I find the search function on the site hard to use and can’t really find anything


 coopers 27 May 2014 07:31 PM:…_+odour +buster


 Calvinmiddle 29 May 2014 12:03 PM:

 Thanks Coopers

 Jeremy – if I understand this you are running the extraction from the pan (via the cistern) all the time. Is there a 40mm pipe conected to the manifold directly for each of the 3 toilets – so each pipe is acting like the extract from another room in a radial system (abet with a 40mm pipe reducing to 32mm) Or does the 40mm pipe split and and serve all 3 toilets.


 jsharris 29 May 2014 05:46 PM:

 Unfortunately it’s impractical to fit this to the downstairs WC, as I only had the idea after the boxed in and insulated soil pipe stack had been plasterboarded and plastered, so I’ve no easy way of getting an extract duct down there now. For the bathroom WCs, what I’ve done is make a home-made adapter to connect a 40mm waste pipe to the extract manifold. This 40mm pipe runs along the top of the eaves service space, and both bathrooms have their WCs fitted against this stud wall. I’ve fitted a swept tee off the pipe to the first WC and a bend to the second WC, with both reducing down to 32mm flange fittings on the cisterns.

 Yes, the extract is continuous, but at a pretty low rate. I have a feeling that the air flow rate needed to disperse odours down and out only needs to be pretty low, just enough to counter natural dispersion that would occur otherwise. The system will undoubtedly work better (assuming it does actually work at all) if the toilet lid is closed quickly after use, something that will, at least, please the female member of the household……………….


 Calvinmiddle 01 Jun 2014 12:10 PM:

 Thanks Jeremy

 I’m finding this interesting as due to the limitations of the plot we only have clerestory windows in the bathroom and its three metres to the bottom of them. My wife is insisting that the windows need to open but I’m telling her there is no way to reach them and the MHRV to get rid of odours in the bathroom – also showed her the odour buster video showing smoke getting sucked down as this makes sense as opposed to dragging the smell across the room to the extract vent.

 She is from Australia so airtight houses with closed windows is something of an alien concept as windows are there to let breeze in to cool the house in her eyes.


 jsharris 01 Jun 2014 01:17 PM:

 I can empathise with overcoming the “we must have windows that open” problem! Not only is my other half convinced that opening windows is “healthy” it was also a recurring theme from questions asked during our open weekend.

 The idea of drawing odours directly from the toilet pan seems obvious, to me, and makes me wonder why on earth we didn’t think of doing it years ago. Apparently the odour buster units have been marketed in Malaysia for years and the story I heard was that they were invented there to overcome this problem in a relatively sealed house that relied on air conditioning to keep the temperature and humidity within reasonable limits.

 I’m just hoping that the MVHR will have enough oomph to do the job, but I do have a fall back option if it doesn’t, and that’s to get a couple of the cheap ebay fan units, discard the charcoal canister and connect them to the MVHR extract ducting as booster fans.


 vk2003 16 Aug 2014 04:31 PM:

 The only thing I would worry about with connecting your mvhr to the toilet pans, is the bacterial aerosol load that will get drawn up into your system/manifold. Have you put an inline filter in to prevent this?


 jsharris 16 Aug 2014 05:40 PM:

 I’ve answered this question here:…dea/#entry95090 , as best I can, but I don’t think it will present a problem at all, as other systems are sold that do pretty much the same thing, like this unit: and they include an activated charcoal cartridge, which would be a breeding ground for aerobic bacteria. Luckily, faecal coliform bacteria are anaerobic, so won’t survive for long in a dry, aerobic environment like an air duct.


 iSelfBuild 13 Mar 2015 06:02 PM:

 Just nearly had a heart attack when I saw how much your MHVR unit was in your cost breakdown… but it doubles as your ASHP! Very interesting.


8 thoughts on “Part Thirty – MVHR Details”

  1. Hi,
    I just tumbled about this when I searched for possibilities to monitor a Genvex MVHR. We are using the same MVHR (labelled Alpha-Innotec, not Genvex) and I am wondering if you were able to decode the 2wire protocol that is used between the main unit and the Genvex Optima panel.

    1. Hi Michael,

      Sorry for the delayed reply, for some reason the blog didn’t send me an email. I didn’t really work hard at trying to decode the data protocol for the Genvex, I’m afraid. I found that I could do all that I needed by just using the dry contacts available in the MVHR, as these over-ride the commands from the Optima 300.

      Hope this helps,


  2. Hi Jeremy, I’ve got a couple of MVHR questions you might be able to help me with. After the house was signed off did you keep the Building Regs. ventilation settings or did you change them to PHPP settings? On the Genvex do you know if it is possible to set up the Optima controller to automatically increase the fan speed to number three when the heat pump is providing warm air or if there is another way to achieve that?
    Best regards

    1. Hi Peter,

      Yes, I did turn the settings down after the house was signed off, as I found the ventilation rate was a bit too high for just the two of us.

      I don’t use our Genvex to provide heating, but I can confirm that the Optima controller does gradually increase the fan speed as needed to deliver the required cooling output. Often it starts out without changing the fan speed, with just the heat pump coming on, but if the cooling demand increases it will just ramp the fan speed up.

      I would have thought it might do the same when heating, so perhaps it’s just a question of how much heating the unit believes you need.

      One thing I did find with our Optima was that I had to trim the indicated temperature by about 1.5 deg or so, as the sensor in the Optima wasn’t reading the room temperature very accurately. I found this a bit of a slow process, as the Genvex can often take a minute or more to register a change for some reason. This fooled me at first into thinking the adjustment process wasn’t working. I also adjusted the switching differential to as low as it would go on the Optima, which I think is 3 deg C for cooling, I’m not sure what it is for heating.

      All told I didn’t find the Optima manual that easy to decipher, and spent quite a lot of time adjusting things, waiting, then switching to the diagnostics screen to show what was really going on. Even that can be slow to respond, I found, with the indications often lagging by a minute or more. I have a feeling that the communications link between the Optima and the Genvex works in some sort of burst mode, where data is only transferred perhaps one or twice a minute, but I’ve not been able to confirm this.

      It might be worth turning the set point temperature up to a high value, and seeing if the fan speed automatically increases after a few minutes. That would indicate that it does the same when heating as ours does when cooling.

  3. Thanks Jeremy, I also had to recalibrate the indicated temperature by 1.5C. I didn’t spot that until the second time of reading the manual. I’ll try out your suggestions tomorrow, but as you say it is a slow process with the Optima.

    1. Hi Peter,

      Interesting that you found a similar room temperature error, makes me wonder whether there is something that effects the sensor. I believe the sensor is below the Optima, near the SD card slot, but perhaps there is some degree of self-heating from the unit itself.

      I’d be interested to find out if yours does ramp the fan speed up when there is sufficient heating demand, just in case we decide to use ours for supplementary heating at some time.

      Pity that the system is so slow to respond to changes, though. It had me fooled into thinking nothing was working properly when I first installed it.

  4. Hi Jeremy,
    In my Optima User Manual, Section 16 of the service menu is called T2 Fine Tuning and indicates it is possible to fine tune the room sensor to make it possible to view the correct current room temperature. The default setting is -3C. I assumed it was just a way of calibrating the sensor, but did think it a little strange. It certainly takes some time for any alterations to have any effect. I calibrated mine against my Testo Anemometer as I thought was probably my most accurate thermometer.
    I did check my fan speed percentage when the heat pump was running and it had increased so I guess it does operate as you thought.
    As you say it is very slow to respond to changes, which is annoying.

    1. Hi Peter,

      Glad to hear that your unit seems to behave in the same way as mine does on cooling. I also found the -3 deg initial offset a bit odd, especially as it takes so long between making any adjustment and that adjustment actually being acknowledged by the MVHR.

      I would love to have a better understanding of the way the unit works, then write some additional notes to make the manual a bit easier to understand.

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