MVHR Commissioning of a Zehnder ComfoAir Q600 with ComfoCool unit completed

14 June 2017: Today we finished the final stage of what was a complete design, supply, install and commissioning of a Zehnder ComfoAir Q600 Mechanical Ventilation with Heat Recovery (MVHR) system – that also features cooling air-conditioning.

Although we’ve commissioned many Zehnder and PAUL MVHR units in the past, this was the first time we have designed a system with active cooling to deal with potential summer overheating here in the UK.

Here’s a snapshot of the 3D detailed ductwork design:

Complicated ducting for an MVHR system with ComfoCool

The ComfoCool unit sits right on top of the MVHR unit, so it’s very easy to integrate into an MVHR system. The supply ducts have to be wrapped in foil-backed insulation to preserve the “coolth” that the unit is providing to the supply air on hot days.

The MVHR and the cooling unit are linked by a simple communications cable:

Comms cable to link the MVHR to the cooling unit




The cooling unit will kick in automatically when the main unit detects that the internal temperature of the home has reached 25Deg C or above. It does this by measuring the extract temperature.

The Q600 unit also has variable summer bypass, so on warm days it can vary how much heat it recovers back into the home and how much it exhausts outside. This is important on spring and autumn days when it’s very sunny (so lots of internal solar gains) but the air is still very cold.

Using an integrated unit such as a Zehnder ComfoCool is a better alternative to air conditioning. It won’t provide that immediate high volume blast of cold air that air conditioning units provide, but it will provide constant background cooling on the warmest days, which is much more comfortable than a noisy AC unit blasting out cold air.

Cooling through MVHR ventilation

We’re really pleased that we’ve completed our first MVHR system with integrated cooling technology. As the UK weather moves generally warmer, these cooling options are going to become more and more important in the future as part of working out how to prevent overheating in homes during UK summers.

If you’d like any help, guidance or pricing for your MVHR with cooling system, please contact me at via the Contact Us Page at the top of the page, or via our free MVHR sizing form here.


27 thoughts on “MVHR Commissioning of a Zehnder ComfoAir Q600 with ComfoCool unit completed”

  1. Hello,
    I have a Zehnder comfocool system in our flat in London and I’m looking to get it cleaned. Do you offer this service or know of a company that do? Many thanks in advance.

    • Hi Charlie,

      Yes, I think we can help. I’ve emailed you from my work email address so we can talk more.

      Best wishes,


  2. Hi
    Is the “Zehnder ComfoAir Q600 Mechanical Ventilation with Heat Recovery (MVHR) system” recommended for the warm climates i.e hot summers (up to 36 degree Celsius) with cold winter climates (down to -10 degree Celsius)? Does the integrated cooling system with MVHR meet the cooling demands in this climate condition?

    • Hi there,

      An MVHR system is first and foremost for ventilation, so it’s suitable for all buildings across all climates that require good indoor air quality.

      The climate you describe sounds like Central or Western Europe, where thousands of MVHR systems are installed and working very well. If the house has been designed with limiting the risk of overheating in mind then the ComfoCool and MVHR system will be a welcome addition – but it’s not as powerful (or as noisy!) as air-conditioning. I’ll contact you from my work email address to discuss your project more.

      Best wishes,


  3. Hi, I’m researching for a new build house in Chichester. We are interested in the Zehnder Comfocool Q600 system with the MVHR as we are worried about overheating in the summer. Can you give me a ball park figure for this unit so we can decide if we can add it to the (tight) budget?

    • Hi Alice, the ideal way to minimise the risk of overheating is via external shading (eg, trees, canopies, brise soleils, window awnings, etc). That way the solar radiation doesn’t hit the windows in the first place. Internal blinds don’t work as well because 90% of the heat energy has already passed through the glazing. The second-best option to mitigate over-heating is passive/cross-flow ventilation, ie, opening at windows at night (although sometimes this isn’t possible due to security, noise or pollution).

      I imagine you’ve considered all this, but I wanted to highlight that the ComfoCool is often the “third-best” option in mitigating the risk of overheating.

      The Zehnder Comfocool has a roughly 1.5kW cooling unit which will lower the supply air temperature by up to 8 degrees. Due to the relatively low flow rates in each room however, this won’t cool the room as powerfully as an air-conditioning unit – the ComfoCool would cool by around 1-2 degrees depending on the room volume and amount of glazing and solar gain. But compared to an air-conditioning unit the MVHR system is more energy efficient (a lot more), it’s quieter and it’s better for the environment. The act of the MVHR changing the internal air constantly and extracting humidity will also help limit overheating.

      I will email you separately on my work email address with pricing for the Zehnder Q600 + ComfoCool unit – this will also include detailed ductwork design, supply and commissioning.

  4. We are interested in the Comfocool system. We have an MVhR system installed at present but it seems a poor quality system with no summer bypass and flexible ducting. It is a chalet bungalow so all areas are easily accessible.

    • Hi James,

      We can certainly retrofit an MVHR system that features automatic summer bypass (to exhaust warm indoor air straight to outside in hot weather) and some form of passive or active cooling.

      Self install is quite straightforward, especially if all of the walls and ceilings are accessible.

      I’ve emailed you back from my work email address so we can talk more there.

      Best wishes, Patrick

  5. Hi,
    Our house is about 300 m2 and we have a 12-year old MVHR system. We have been thinking about replacing it with a ComfoAir Q 600 ST / ComfoCool Q600 ST. A “full” A/C solution for this area would require about 30 kW. The ComfoCool Q600 ST only has 2.3 kW. Would this even be noticeable?
    You write “ComfoCool would cool by around 1-2 degrees”. Is that also what you think we would get?
    Thanks, Martin

    • Hi Martin,

      The ComfoCool is limited by its maximum flow-rate as air is a poor carrier of “coolth”, so yes, I realistically think you’d only get 1-2kW of useful cooling in the height of summer. This might be enough for some properties, but if you’ve calculated 30kW then it will struggle.

      However, upgrading to the Q600 would be appropriate for your property given its large floor area, and the ComfoCool is then an additional £2,502 ex VAT – so it’s a cheaper way to get a little cooling than a dedicated AC system at £7 – 11k ex VAT. But it won’t likely be enough on the warmest days. The Q600 also has a lot of very good design features – variable summer bypass, internal humidity and temperature sensors and touchscreen controls that show a lot of data.

      If you’d like to discuss more please contact me at patrick [at] and I’ll run through the best options from a design perspective.

      Best wishes,


  6. Hi Patrick, we are Eva and Alex from Vilanova i la Geltru, 50km south from Barcelona.
    We are in our life project of living in a passive house. We have the architect’s plans already finish but still deciding installations for ventilation.
    The constructor is recommending us to install the Comfo Air Q350 and we agree with that but as we live in a coast relatevly hot and humit area in summer we think to install the ComfoCool as well.
    The engineer is suggesting us to install another cool/heat aerothermia machine with a pair of dehumity annexes, but we are not sure about that…
    We will appreciate any suggestions about your experience or feedback of other customers,
    Thank you very much,
    Bests Regards
    Eva & Alex

    • Hi Eva and Alex,

      Great news about your dream project and I wish you every success.

      The key to controlling overheating is external shading and cross-flow ventilation – this should be your priority above all else. European homes tend to get this right – external shutters (closed in daytime) and inward-turning windows are the best in the fight against overheating. You can also consider a green roof.

      In order to fit the Zehnder ComfoCool you would have to upsize the MVHR unit to a Zehnder ComfoAir Q600. This is not a good idea as it’s a lot of extra money for only 1.5kW of additional cooling, so don’t do that. The ComfoCool is only for very large homes (more than 350m2 gross floor area).

      I would suggest you install the Q350 and assess how the home performs during the next summer. If it is very hot you can look at additional cooling such as a small AC unit. Hopefully it won’t be overheating, because AC is expensive, noisy and very bad for the environment.

      But I must stress – external shading is key. External shading is key. External shading is key. Shutters, parasols, trees, walls, canopies – it doesn’t matter how you do it, but stopping the solar radiation getting in the first place is how you stop overheating.

      I hope this helps. Best wishes with your project.


      • Hi Patrick,
        Thanks a lot for your response and above all for your recommendations and wisdom, regarding the subject.
        We will take your advice very much in consideration,
        Best regards,
        Eva and ALex

  7. I’m about to start building my dream house . It’s just over 200M2, two story,flat roof.
    A couple of aspects have extensive glazing. Whilst there will be some shading from canopies which are part of the design, and trees, I’m concerned about possible overheating. Especially in the upstairs bedrooms.

    Living in Manchester, there are few nights each year where the Summer outside temp stays much above 20C. Is it likely that the Q350 will be enough to draw down bedroom temperatures close to external?

    To be honest, the comfocool sounds ideal for being able to reduce summer temperatures by a couple of degrees, but I understand this isn’t suitable for a house of my size?


    • Hi Mark,

      If we assume your room heights are 2.6m average, your internal building volume is 520m3. If your MVHR is running at 260m3hr in standard mode, then every two hours the entire volume of your property will have been replaced with outside air. If the summer bypass mode is running because the internal air temperature is above 24 – 26 DegC, and the outdoor air is cooler, it should cool the internal air temperature down at night. Opening windows at night does the same thing, but in UK summers the air is often really still, so no wind carries in the cooler air. Plus crime and noise often prevent the use of opening windows at night.

      It’s important to consider the sources of internal heat gains though – hot water pipes in walls and floors, AV rooms with media kit and general cooking and showering will all contribute to internal heat gains. As soon as the sun rises, solar radiation will begin to heat up the rooms with lots of window glazing. I’d look at overhangs over the upstairs bedrooms or maybe a higher Ug value on the window glazing themselves. If you want more guidance contact me at patrick [at]


  8. Hi
    I’m planning a major renovation of my house and am thinking of an MVHR unit. We will be moving into a loft conversion and I am concerned that the space will overheat in the summer. We plan on having up to date builder regs insulation. Would the comfocool system be satisfactory in providing enough cooling to this space? I am worried as I’ve heard that rooms in loft conversions are like an oven in the summer

    • Hi Simon,

      Yes, lofts tend to overheat, mainly because the roof takes the bulk of the sunlight falling on the building. The ComfoCool system will provide limited cooling potential, but it’s not likely to be powerful enough to compete with a hot roof in summer. There are other solutions – if it’s possible to email me at Patrick [at] I’ll be able to advise more information if you have plans available.

      Best wishes,


  9. Hi
    I have a single story bungalow which I recently extended so it has an old part (1960s) and a brand new part. The new part consists a sun room / kitchen 6mx8m with an open pitched ceiling and a 6mx6m lounge. Both of these rooms suffer/ benefit from considerable thermal gain reaching temperatures in excess of 25 (winter) to 32 (summer) degrees. At the same time the old part of the house, which has no thermal gain but is well insulated and relatively airtight is a lot cooler even in the summer (often less than 18 degrees). Is there a cost effective way to transfer the heat to this cool area?

    • Hi Colin,

      That sounds like a good set-up actually, with a cool area of the house to retreat to when it gets too hot in summer.

      It’s difficult to shift warm air from hot rooms to cooler rooms, but an MVHR would help “spread” the heat around the property as it runs into the heat exchanger and heat is transferred to the incoming filtered air to all rooms around the house. In summer the heat exchanger would become a “cool exchanger”, as it pulls cool air from the original part of the house and transfers the coolth to the incoming summer ambient air, cooling it passively.

      You could also look at the heating system setup for the house, and for each room, and the control that it gives you to zone areas between the new and old house which would allow you to run heating in the old house whilst it’s off in the extension.

      Hope this helps.

      Best wishes,


  10. Hi,
    I live in a 70sqm passivhaus flat that significantly overheats in the summer. I was thinking replacing the Zehnder Comfoair 200 with a Q600 with Comfocool. But you mention that would only be suitable for homes >350sqm. Is there no other cooling system that can be integrated with a Zehnder MVHR? If not, what type of air-conditioning system would you recommend for a flat of this size? Thanks.

    • Hi Greg,
      There are options here, but over-sizing the MVHR unit may not be the most straightforward given the space restrictions, need for larger ducting, increased airflow causing humidity issues, noise, etc.

      Zehnder only offers the ComfoCool in the Q600 size, so that would be out, but I’ll email you directly and I can find out more about the flat and we can run through the options to mitigate overheating.

      Best wishes,


    • Yes, Phil, you can, but there are some important pieces to install straight away – eg, additional insulation and ducting that can be cut away. Please contact me on Patrick [@] if you’d like to discuss further.

      Best wishes,


  11. Hi Patrick just came across your article and read with interest the comments and your responses, so thanks for those. We have just moved into a “new” build (2018 but we’re the first ever occupants!) and we’re struggling with the heat in the house right now (18/07/21). So I believe Zehnder only do a ComfoAir q600 and nothing smaller? Our Comfoair Q

    • Hi Rob,

      Overheating is a severe issue in UK new build homes, so you’re not alone. The Q600 is likely too large a unit for your home, but there are a few smaller options depending on if you have a heat pump, and there is more general advice such as cross-flow, external shading, purge ventilation via the MVHR, etc.

      I’ll send you a direct email to discuss cooling options in more detail.

      Best wishes,


  12. Hi Patrick
    We’re renovating our house before we move in and have already designed an MVHR system to be fitted when we realised we wanted air con as well.
    I’m stuck between a new aircon system as well, or the Q600.
    I am worried that the Q600 wont cool enough on the hottest nights, but will be great the rest of the time.
    Also, we will be fitting the units in the loft, so I wont get to see any of the controls.
    Does the Q600 have app connectivity?
    My final question was going to be – we are having 6 bifolds fitted across the back of the house, looking into the garden.
    Obviously, when the bifolds are open, the temperature in the house may get to 26-28 degrees. Is there any form of detection with the Q600 or would it just kick in, as in, not a smart system?

    Great answers so far, thank you so much!

    • Hi Salih,

      An MVHR system should always be the base solution for ventilating/cooling a home, as an MVHR removes humid, hot air in summer automatically which helps to lower indoor heat temperatures, plus it’s ventilating, which is very useful if windows are closed in summer (eg, due to pollen, security or noise) and it keeps the airflow running at night when the outdoor air temperature is cooler but the air outside is still.

      The Q600 is top of the range, and can be fitted with an internal cooling unit to further cool down the air – but it’s not quite as powerful as air conditioning.

      The MVHR units we specify are compatible with separate air con systems as they have an ability to auto-detect cooling, so they won’t fight against it by trying to recover heat – most MVHR systems can’t detect cooling, so they won’t work well in summer.

      I’d be happy to have a proper chat with you as it’s quite a complex set-up, but we can specify an MVHR system with cooling that will work for your home – I’ll be in touch directly.

      Best wishes,



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