MVHR DOs & DON’Ts: Location of MVHR units

Novus 300 at Golcar PassivhausA common question asked by many our MVHR customers is where is best to site my MVHR unit? For newbuild properties this is relatively straightforward as we can state our preferences early on in the design process, which can be incorporated into the design early on to provide plenty of plant space. It gets trickier with retrofits as our traditional construction methods and design were never intended to include a mechanical ventilation system and therefore often don’t have much plant/utility space.

So where is the ideal location?

In our opinion, the ideal location to site your MVHR unit in a home is in a utility/plant room on the north side of the building close to an external wall. The reason we prefer it on the north elevation is to improve the efficiency of the automatic summer bypass function that all our MVHR units have built in. In summer, the sun will obviously  be at its hottest on the south elevation and so the north elevation will be cooller. When the summer bypass operates one of the air streams will by-pass the heat exchanger meaning the intake air is not recovering the heat from the extract air, it is being distributed straight to the supply rooms at external temperature. Locating the intake grille on the north side and bringing in shaded air it will help improve summer cooling, which is desirable. NOTE: this will not prevent overheating!!

‘Out of Sight, Out of Mind’

Another reason for locating the MVHR unit in a utility/ plant room is that it is easily accessible and will not be forgotten about. Access will be required to the MVHR unit every 3-6 months to change the intake and extract filters inside the unit. If the unit is tucked away in the eaves of the loft, the building occupants are less likely to climb up there and change the filters.

Can my MVHR unit go in the loft?

Despite our reservations, in some cases we have sited MVHR units in lofts, commonly in retrofits and smaller projects. When positioning them in lofts it is important to create a decent access route to the unit to allow for filter change so a hatch with a ladder and a walkway or floorboards up in the loft, otherwise the filters will not be changed and commissioning and servicing will also be a lot harder.

Siting the intake and exhaust grilles

If the unit is in the loft,  it is a good practice to get the intake and exhaust grilles on a gable wall if possible (ideally north, east or west, for reasons explained above). If there is no gable wall  it is likely that the intake and exhaust outlets will need to go through the roof using roof terminals. These can add cost onto a system, as they require extra parts and drains to deal with condensate running back down the duct and into the MVHR unit. As we know a roof can get very hot in summer,  so again the summer bypass function will not be working as efficiently as the intake air will be hot air radiating off the roof.

Cold roofs & MVHR

If the unit is located in a cold loft, the supply and extract ductwork will need insulating to protect from the risk of condensation, It is important that the roof insulation should be laid over the ducting, it is bad practice to run the duct above the roof insulation and insulate it as there will always be a temperature gradient between the cold roof and the room below.

Leaving space for MVHR maintenance

Figure 1.1 shows an MVHR unit which at first appears to be in a good position in a large plant room. When commissioning the system we need to remove the heat exchanger from the unit. This was made extremely difficult as there was a water cylinder in front of the unit, as seen in Figure 1.2. It is important to ensure that there is enough space to stand and work on the front of every MVHR unit and to be able to remove the heat exchanger and filters. This space varies depending on which unit has been specified for the project however as a general rule of thumb you could leave space at least the depth of the unit again.
 

fig. 1.1

fig. 1.1

fig. 1.2

fig. 1.2

 

 

 

 

 

 

 

 

 

fig 1.3

fig 1.3

Figure 1.3 on the right shows a horizontal unit located in a warm loft with the intake and exhaust ducts passing out through the gable. The problem we encountered here was removing the front cover of the unit to get to the heat exchanger. As you can see,  the end of the unit is butted up against some boxing hiding the intake and exhaust ducts and primary silencers. In order to remove the cover it needs to slide slightly to the left then lift off. However, in this instance, we didn’t have the space to slide the front cover.

Condensate drains

As you can see in Figure 1.3, a hole has been filled in on the plinth that the unit is resting on. This hole was drilled to allow us to check that a dry seal trap had been fitted to the condensate drain below the unit, which is essential.  It would have been good if the installer had taken a picture before boxing it in so we could have confirmed the trap was present. Ideally any condensate trap should be accessible for future maintenance.

Your project

The location for the MVHR unit is job specific and there are often barriers and a variety of factors to consider at the design stages. Some projects require special attention such as locating the MVHR unit in a basement or a garage where fire dampers may be required. As part of our MVHR design service we can can advise on the best location for the MVHR unit on your specific project.

 

Tom Heywood Senior MVHR Design Technician Green Building StoreTom Heywood,MVHR Design Manager, Green Building Store

 

 

27th February 2018

46 responses to “MVHR DOs & DON’Ts: Location of MVHR units”

  1. blank Aled Owen says:

    What would the ideal size for a plant room for a two storey house of about 2000 sq ft

    • blank Tom Heywood says:

      Hi Aled,
      A ground floor utility/plant room on the North side of the building would be ideal if you have the space for that. If you allow a space of about 850mm wide by 1200mm deep (600mm for the unit 600mm for access) with floor to ceiling height that should be fine. If you were to box the unit in the cupboard you could have a cupboard 650mm deep providing you have full access to the front of the unit.
      I hope this helps.
      Tom

  2. blank Jenny says:

    Does the MVHR unit need to be against an external wall, or could it be housed e.g.in a hall cupboard? We were hoping to install it in the utility room or loft room of our new house (currently under construction) but these are both on the south side of the house, which doesn’t sound ideal.

    • Hello Jenny,
      It is not essential to locate the MVHR unit on an external wall. It can go in a cupboard providing there is a simple route to get the intake and exhaust ducts to outside. The shorter these ducts are the more efficient the unit will be and the less heat loss there will be through the duct. The optimal length is <1.5m.
      If you’d like us to look at this with you and advise you on the best location please send your plans over to [email protected] and ask for a designer to call you.
      Thanks for your enquiry.
      Tom

  3. blank Doug Unkles says:

    Hi, Is it feasible that a heating coil can be installed within a MVHR system as the only active heating source within a certified PassivHaus standard new-build dwelling? I am looking my own private project and want to avoid putting any gas central heating, stove, etc. into the dwelling. I’m hoping that the MVHR can be designed to keep the building warm enough during coldest winter temperatures. Having visited a number of PassivHaus projects, it is apparent that most of them hardly ever turn on a back-up heating system; however, I don’t want to leave this to chance.
    Any advice you can provide would be appreciated.

    • Hi Doug,
      This is possible however you need to be very careful that it will provide adequate heating levels in those colder months. I would suggest having PHPP done for your project, this will give you an annual heating load and tell you if supply air heating via the MVHR system is a feasible solution. In the past some of our clients have allowed electrical points to some rooms to allow for electric towel radiators to be added in the future if necessary.
      So in summary, it all depends on the building fabric, levels of insulation, airtightness and no thermal bridges. It’s also best to get PHPP done.
      If you’d like any further information on MVHR or PHPP please don’t hesitate to give us a call.
      Kind Regards

  4. blank Richard Blake says:

    Hi,
    Can an MVHR system me used in a non passive hays property. We live in a very well insulated Victorian property and about to undertake an extension & major overhaul and looking to install a system. Dust is a massive problem in our property and both me and my children have asthma issues. Do you also recommend a specific unit for asthma sufferers?
    Thanks, Richard

    • blank Tom says:

      Hi Richard,

      MVHR is great to use in any property where either there is a high level of air tightness or internal/external air quality is an issue. Our MVHR systems are designed to keep relative humidity in the building down to between 40 and 60% which reduces the population of dust mites and therefore asthma sufferers will see a benefit. MVHR will also remove a lot of the air borne dust and keep a constant supply of fresh filtered air to the habitable rooms. If you’d like a quote or would like to discuss this further please get in touch with us [email protected] or 01484461705.
      Thanks
      Tom

  5. blank YASH says:

    Hello
    We are making a passive haus in India having 3 floors. I thought of putting one unit on each floor. Is there any restriction with the duct length with air distribution? I mean to say if I put MVHR in the end corner of the building, it will work efficiently?

    • blank Tom says:

      Hi Yash,
      The limit on the length of duct work depends on the size and spec of the ductwork used and the air flow flowing through it. This would be down to the designer to check the pressure loss in that run and ensure that the air will make it to the air valve.

      Kind Regards
      Tom

  6. blank Vivek Singh says:

    We are buying a house, built in 70s. There is an accessible loft. The house has heating through air vents and not radiators.
    Does having vents already mean it will be easier to retrofit MVHR, and cost lesser.

    I am aware that the house may be leakier than desireable.

  7. blank Tom says:

    Hi Vivek,

    Its unlikely that the vents currently in the house are suitable for our MVHR systems. Its important that the vents have dampers built in and that they are designed in such a way as to minimise sound levels while ensuring a good circulation of air in the room.

    Kind Regards

  8. blank Murray Mackay says:

    I was told you had to have a poo massive house for MHRV. After we upgraded our boiler fitted a wood burning stove and changed the windows we are experiencing mould in the bed rooms and high levels of CO2. Would this indicate that our house would benefit from it. Is there a definitive test ?

    • blank Murray Mackay says:

      Passive not poo massive. Auto correct gone mad 🙂

    • blank Chayley Collis says:

      Sorry for the delay – you don’t need to have a Passivhaus (aka ‘poo massive’) to have an MVHR system but it starts making more sense the more airtight the house is. Although MVHR could be installed in all types of building, we would only recommend it for buildings with good airtightness levels. As a rule of thumb, this would be where the air permeability of the thermal envelope is at or below 3 m3/m2.h @ 50 Pa. If this level of airtightness is not achieved, then the heat recovery and consequent energy saving benefits of MVHR would be lost. It definitely sounds like you need more ventilation though (whether MVHR or not). To discuss the suitablility of MVHR for your project further, please ring the MVHR team on 01484 461705.

  9. blank Tina says:

    I recently visited a ‘Dan Wood’ house where I noticed MVHR grills on the floor. Is this best use of an MVHR system? Before this, I always assumed the system was placed to come through to the ceiling, so I’m now somewhat confused! Thanks Tom.

    • Hi Tina,
      Its best not to locate them on the floor, you will feel the draught from supply valves and you extract valves will become blocked easily with dust. Also warm moist air rises so its best to get them all at high level, at least above head height.

      Thanks!
      Tom

      • blank Shawn says:

        What about displacement ventilation in spaces with higher ceilings, say a kitchen or workout room at 3m high? With enough height for stratification of temperature above the head, would this be a good strategy?

  10. blank Cormac says:

    Hi tom,

    Have a mvhr installed. Seems the installer has not insulated the ducting as it is just left sitting on top of the attic insulation. Recently I’ve noticed a gargling sound when ever the unit is on boost. Would putting insulation over the ducting make any difference/solve this issue. Thinking the installed may have cut corners with the install and now trying to fix the issues myself. Thanks.

    • Hi Cormac,

      Running the ducting in a cold loft is not a good idea, it will hugely reduce the efficiency of the heat recovery as all of the heat from the extract air will be lost in the ducting in the loft before it hits the heat exchanger, any heat that is recovered will be lost in the supply duct in the cold space. Also condensation becomes an issue which it sounds like you may be experiencing. I would usually suggest laying the insulation over the ducts however it sounds like you are past that point so wrapping 25mm of foil-backed mineral wool around all of the ducts in the cold space will certainly help. It’s important that this is wrapped tightly and taped securely at all of the joints.

      Thanks!
      Tom

  11. blank Bart Hommels says:

    Hi, I am making good progress installing the MVHR unit & ducting I bought from you. One question regarding the condensate drain/run off: I reckon the water is equivalent to demineralised water? If I collect it, can it be used for the steam iron etc?

    • blank Tom Heywood says:

      Hi Bart,
      I cant see an issue with that however there wont be much water and if it is drained into a bowl it will need emptying frequently in the winter.

      Kind Regards

  12. blank Chris Cohen says:

    Hi, our new build is backing into a 3m bank albeit with separate retaining wall 300mm away and so the 2m depth plant room will be effectively basement. would the MVHR be OK against a wall with ‘periscope’ venting. Alternative is in loft on west facing gable. Thanks, Chris

  13. blank Tom Heywood says:

    Hi Chris,
    The unit could go in the basement, periscope venting to outside would add cost on due to having to run the ducts underground and the addition of extra parts to deal with potential condensate issues in the duct. A simpler option would be to locate the unit in the basement then if possible rise into a cupboard or boxing in on the GF then out of the wall above ground level, this obviously depends on the layout of your GF as to if there is something we can rise into. Alternatively yes the West gable would be great if that’s easier.
    Hope this helps, do contact me if you need further info.
    Kind Regards

  14. blank Emily says:

    Can a mvhr be used in a twin skin (2 layers of 22mm wood with 144mm insulation between) 4 bed log cabin?

  15. blank Jax Stephen says:

    Hi, I am installing a MVHR system in to a new build house. The plant room is situated on the north side of the property and has a flat roof. Roof space is 2m x 3m. The boiler is also positioned within this room with the flue penetrating the roof.

    What is the minimum distance you would suggest a boiler flue can be positioned next to the MVHW intake and extract??

    Many Thanks in advance.

  16. blank tom heywood says:

    Hi Jax,

    It is good to get a separation of 2m between the MVHR intake and the boiler flue if they are both exiting on the same level. If the boiler flue is located higher than the MVHR intake they can come closer, if this isn’t possible internally you could use a plume kit externally on the boiler flue. I would still suggest trying to achieve maximum separation.

    Kind Regards

  17. blank Paul says:

    Hi Tom,

    In a new build, residential building with four floors and fifteen apartments in total, what impact will the plant room located on the ground floor with MVHR and a boiler have on the flat directly above it, on the first floor? Is there risk of heat/noise/vibrations travelling to the flat above? Thanks

    • blank Tom Heywood says:

      Hi Paul,

      Thanks for your enquiry.
      In relation to the MVHR there wont be a problem, the units should not vibrate or rumble away like a washing machine, the breakout noise from the unit will be a dull whirring of fans at around 50dB(A). Other plant may cause an issue and it would be worth installing sound insulation to the floor above and allowing for some purge vent in the plant room whether this is manual or automatically opening windows or an extract separate to the MVHR system that can operate at a high level to dump heat outside.
      Regarding the flats I would suggest looking into a decentralised system so that all flats have their own MVHR unit, this can work out cheaper and means each occupant has their own individual control.

      Kind Regards

  18. blank Brian Kennedy says:

    Hello

    In nearly all literature online, MHRV providers detail extracts from shower rooms and kitchens with supply into living rooms and bedrooms. Kitchens and showers typically provide warm damp heating load for a only a very short amount of time during the day. A bedroom with two people will provide a significant amount of heat and moisture vapor for up to 8 hours a night.
    Why are systems not designed to take this into account ?
    Brian

  19. blank Garth Johns says:

    If locating the MVHR on a ground floor, is it acceptable to have the fresh air intake and stale air outtake on the same wall? if so have far apart should they be?

  20. blank Stead says:

    Is there any issue with trimming the MVHR exhaust & intakes flush with the outer wall and then fitting a louvred grille, the installers have left ours with 250mm protruding.
    Boxing this in seems a little mad.

  21. blank Claire Baxter says:

    would a cellar work to house the unit rather than the roof? North side of house.

  22. blank Liam says:

    Hello, great article!

    Can I just ask, how does the unit manage the humidity in a property? For example on a very wet day in the winter the air coming in will have high humidity. I’m guessing the unit does something about that?

    Kind regards

  23. blank Tom Heywood says:

    Hi Liam,

    Outdoor air usually varies in relative humidity from about 60-100% throughout the year regardless what time of year it is. When cold outdoor air goes through the heat exchanger, the heat from the indoor air is transferred, warming up the incoming air. As you warm air up, the relative humidity falls (warm air can hold more moisture than cold air can). This effect is bigger in winter than summer as the outdoor air is colder. For example, fully saturated (100% humidty) outdoor air at 0 degrees, when heated to 20 degrees reduces in relative humidity from 100% to about 30%. For spring or autumn, fully saturated air at 10 degrees, reduces in relative humidity from 100% to about 60% when heated to 20 degrees.

    This means that in winter, when the inside surfaces of the external walls are coldest and at most risk of condensation and mould, then the relative humidity of the house is at its lowest, i.e. the air is dry enough to prevent condensation on cold surfaces and therefore mould growth. In the warmest months of the year, it may not be possible to get the relative humidity of the house below about 60% but it won’t cause a problem at that time of year because there’s not a condensation risk caused by cold indoor surfaces

    Hope this helps.
    Kind Regards

  24. blank Jane says:

    Thanks for all the information that you have provided. I did not see an answer to Brian’s question above. I forsee the same problem except instead of just 2 people in a bedroom, there would also be 4 dogs. Should i consider a different option to MVHR , the dogs stay! Also I am in Merseyside, can you provide the design drawings for a local person to fit, as l guess we are too far away for you to do the work? Lastly, it would be retrofitted to a 1975 bungalow with cavity wall insulation. Would it be best to have external insulation fitted. We are having new windows etc.

  25. blank Tom Heywood says:

    Hi Jane,

    With regard to Bedrooms, the designer should always consider occupancy levels. We will supply around 35m3/hr into a room with double occupancy whereas if the room is single occupancy, around 22-25m3/hr will be sufficient. The fresh air supply puts the room under negative pressure and the air is drawn out by rooms with negative pressure, such as the Bathrooms so the bedrooms will have a really good air change rate. It is important that the designer ensures the system can run at a level that is providing a good ventilation rate for the occupants of the bedroom while also ensuring it is inaudible, otherwise the occupants simply turn it off in the night due to noise disturbance.
    Regarding dogs, yes it will be important that you notify the designer of this at design stage as they also have an airflow requirement.
    We would indeed provide a design and supply service for another to fit, it might be worth having a look at the list of approved installers on our website.
    Regarding insulation, this is worth looking at but also and probably more importantly is the airtightness of the building. If a building has lots of insulation but isn’t airtight, the cold air can simply go around the insulation. Our consultancy team can help you with making the best decisions here.

    I hope this is helpful, do get in touch if you require any further information or a quote.
    Thanks
    Tom

  26. blank John Kaye says:

    Hi
    I’m having a MVHR unit installed. The supply and exhaust are on the same wall, one above the other about 1.5 metres apart vertically. Does it matter which way around the supply and exhaust are positioned, top or bottom?

    • blank Tom Heywood says:

      Hi John,
      There are arguments for and against both options. BPEC advise exhaust above intake however Passivhaus good practice guidelines suggest exhaust below intake being a good solution too. However, with a distance of 1.5m both options should be okay.

      Many thanks

  27. blank Michael Ray says:

    Hi,
    I’m concerned about piercing the airtight envelope when running ducting into and out of a cold attic
    Would your design drawings provide a solution to this?

    • blank Tom Heywood says:

      Hi Michael,
      Our design does not typically show a detail for this, however it can be fairly simple with the use of an airtightness grommet. You shouldn’t, however, run the supply and extract ducts serving the rooms outside of the airtightness envelope and certainly not outside of the thermal envelope. The intake and exhaust to the outside, however, are fine to run in the roof.

      Many thanks

  28. blank Danny says:

    hi is it possible to have inlet and outlet pipes falling from main unit and exiting roof, or must they rise from the unit thanks danny

    • blank Tom Heywood says:

      Hi Danny,
      The Intake and exhaust can either rise or fall. Both options are fine and this is determined by the unit position and where the intake and exhaust are penetrating the fabric to outside.

      Many thanks

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