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My house in Cambridge had a low energy renovation around 15 years ago with triple glazing, increased thermal insulation, solar thermal system, thermal store and MVHR. The MVHR unit is a Vent-Axia Sentinel Kinentic BH.
For most of the time since it was installed it has worked very well and I rarely needed to open any of the windows. However, during the recent Amber Heat Alert the house overheated badly. Possibly the high humidity made it worse than in earlier years or my age and, maybe medication, made me less resilient.
My suspicion is that during an extended heatwave with tropical nights the MVHR is unable to maintain comfortable indoor temperatures. With or without a summer bypass, if the air temperature is over 35º C outside then the MVHR will import that air. Even if the air temperature drops lower overnight it can't catch up and the house continues to retain the heat, made worse by the high quality of thermal insulation.
Some form of air cooling may be required! My back garden is nowhere near big enough for a ground-coupled heat exchanger so active cooling may be an option.
I have seen an example where a reversible heat pump was connected to a water/air duct heat exchanger (eg. Zehnder ComfoPost). However, my house has a gas boiler.
Has anyone else experienced similar problems or can suggest possible solutions?
Posted By: john.connettWith or without a summer bypass, if the air temperature is over 35º C outside then the MVHR will import that air.That may depend on the sophistication of the MVHR unit. My Zehnder cools the incoming air down to a set temperature using the cooler outgoing air, if that's cooler than the incoming air. For the summer, I've adjusted the set temperature down to the minimum 12°C, rather than the default 22°C to take full advantage of any cooler outside air - check your MVHR manual as you may be able to do the same.
Posted By: john.connettI have seen an example where a reversible heat pump was connected to a water/air duct heat exchanger (eg. Zehnder ComfoPost).Unfortunately air has a low specific heat capacity, so you need to move lots of air to make a big difference - more than an MVHR unit typically handles.
Posted By: john.connettMy suspicion is that during an extended heatwave with tropical nights the MVHR is unable to maintain comfortable indoor temperatures.If the air is so hot that the building doesn't have chance to cool, as in an extended heatwave, that's a problem that can only be solved by active cooling - typically air conditioning or chilled underfloor central heating. I have the same problem as I have nowhere to install a heat pump.
Posted By: john.connettMy back garden is nowhere near big enough for a ground-coupled heat exchangerA garden isn't necessary; I've seen a vertical borehole drilled through the floor of a plant room in a London home. It would, however, cost several time the cost of an air source heat pump / aircon unit.
Posted By: Mike1I've installed a HomEvap adiabatic cooler before my MVHR but...Another 'but' is that you need to have insulated the air distribution ducts & accessories for this to stand a chance, or the cooled air may well have warmed again before it reaches the ceiling vents. I'm loosing 2 to 3°C on mine, but I haven't yet insulated the manifold nor the main duct from the MVHR unit.
Posted By: CerisyOur 16 year old Titon unit is fine for most of the year but the summer bypass function is useless in our set up. In these heat waves (we live in Normandy and recorded a temperature over 41 degrees!) I switch the MVHR unit off during the day with all the windows and upstairs shutters closed. When the outside temp in lower than in the house I’ll open windows to draw some cooler air through.Summer bypass can't work when it is hotter outside than inside. In those circumstances the best an MVHR can do is to bring the heat exchanger back into the circuit which makes the supply air a little bit cooler than the air outside. Air con is as you say the only real solution, although fans do help somewhat.
Many thanks for the replies. Very useful!
Posted By: Mike1Posted By: john.connettWith or without a summer bypass, if the air temperature is over 35º C outside then the MVHR will import that air.That may depend on the sophistication of the MVHR unit. My Zehnder cools the incoming air down to a set temperature using the cooler outgoing air, if that's cooler than the incoming air. For the summer, I've adjusted the set temperature down to the minimum 12°C, rather than the default 22°C to take full advantage of any cooler outside air - check your MVHR manual as you may be able to do the same.
My Vent-Axia Sentinel Kinentic BH is essentially a stand-alone MVHR. It is located in an insulated loft space with a mains power switch and front panel controls. It does have a remote on/off boost switch which is turned on when the fan in my kitchen cooker hood is running. There may be other controls inside the MVHR but I am reluctant to tamper with them.
A more sophisticated MVHR might have a wider set of controls and report data that might help understand its operation. If, for example, it was linked to an indoor air quality sensor it could reduce the air flow during extreme hot weather while keeping CO2 below 800 ppm. Maybe even stopping if the house was unoccupied.
Posted By: Mike1I've installed a HomEvap adiabatic cooler before my MVHR
I wasn't aware of devices like the HomeEvap Cooler adiabatic evaporative cooler (https://www.homevap.nl/en/overview-cold-humidification/homevap-cooler/). Their website states "The HomEvap Cooler is limited by the duct sensor mounted after the Cooler in the duct. This measures the RH in the duct and will switch off the unit when the RH exceeds 85%.". It's possible that it would be less effective during conditions like the Amber Heat Alert from 22/06/2026 to 28/06/2026 when there was both high temperatures and high RH. Here's a Weather Archive Cambridge International Airport (https://www.meteoblue.com/en/weather/historyclimate/weatherarchive/cambridge-international-airport_united-kingdom_6296643?fcstlength=15&year=2026&month=6) covering that time.
I have a portabe Bosch UniversalFan that runs on a POWERFOR ALL 18V battery which has been very useful! No trailing lead to trip over. However, fans become less effective with high RH and when temperatures approach body heat.
Portable AC might be a challenge with my Passivhaus specification doors and inward opening tilt/turn windows.
I will continue to investigate possible solutions.
Posted By: john.connettThe HomEvap Cooler... ...[may be] less effective during conditions like the Amber Heat Alert from 22/06/2026 to 28/06/2026 when there was both high temperatures and high RH.Yes, that was the case here. And, although the cooler discharges the humidified air outside, the incoming moist air cools so its humidity rises too - it pushed internal HR above 50%, which was undesirable (for comparison, at the moment it's around 35% inside, 24% outside). If that became a regular occurrence then a dehumidifier could resolve both issues (increasing cooling and cutting internal humidity), subject to its heat output being low.
Posted By: john.connettMy Vent-Axia Sentinel Kinentic BH is essentially a stand-alone MVHR. It is located in an insulated loft space with a mains power switch and front panel controls. It does have a remote on/off boost switch which is turned on when the fan in my kitchen cooker hood is running. There may be other controls inside the MVHR but I am reluctant to tamper with them.You should be able to control everything through the front panel controls. Looks like the instructions are here: https://asset.eezybridge.com/nc/ba7b9546-29dd-40c4-87a7-885627281b87/442073.pdf which seem to include an option to vary the target indoor temperature (below) - if you have a summer bypass fitted.
Posted By: john.connettMaybe even stopping if the house was unoccupied.My Brink unit has a minimum setting marked '0' that is actually 50 m³/hr and is meant for use when the house is unoccupied. I think it's designed to stop the air becoming stale. I sometimes use it when I'm alone in the house.
Posted By: Mike1You should be able to control everything through the front panel controls. Looks like the instructions are here: https://asset.eezybridge.com/nc/ba7b9546-29dd-40c4-87a7-885627281b87/442073.pdf which seem to include an option to vary the target indoor temperature (below) - if you have a summer bypass fitted.
Mine looks similar. The Start Up screen displays
Sentinel Kinetic V09
Language: English
Airflow Units: %
Wireless Control: Not Fitted
Humidity Sensor: Fitted
Note there is no "Control Mode". The firmware may be old. No Internet connection for Over-The-Air updates. The User Mode screen displays
Normal Airflow
Clock (set to British Summer Time)
Indoor Temp: 25 C
Note there is no "Outdoor Temp" which, I guess, defaults to 14 C.
Posted By: GreenPaddyJohn,
I've installed a number of these same units ofver the years, and written up "idot's guides" for my clients. Looking back at these, I note that there is NOT an OUTDOOR TEMP screen per-se, however if you follow this it should bring you to the OUTDOOR TEMP settable value...
at INDOOR TEMP screen press SET to choose the required inddor temp.
press SET button again to confirm the set value, which brings you to the OUTDOOR TEMP setting.
The revision I have (maybe 10 years ago or more) says that the range is 5oC to 20oC (14oC default)
Many thanks! The description in the manual of Normal Mode, Evening-purge Mode and Night-time purge Mode is interesting. It looks like the MVHR is trying to respond well given the limited information available to it (time, temperature sensors, temperature setpoints, humidity ). Unfortunately, there appears to be no easy way for it to report these parameters. A smarter MVHR might be able to report more about its operation and, perhaps, use input from a CO2 sensor
Posted By: GreenPaddyTo be honest, I suspect the high overnight temps you were experiencing were probably above 14oC anyway, so would not have influenced the bypass operation, but still worth dropping that OUTDOOR TEMP value, just in case.
Following W-i-A advice to open all doors and windows early morning for several hours till say 10am works well for us, but being in Scotland, the 15oC night to 28oC day temps are quite different to further south being much higher for both.
Although Cambridge was only in the Amber Heat Alert area the combination of high temperatures and humidity together with tropical nights were extremely difficult! The local Heatwave Threshold Daily Maximum Temperature (https://www.metoffice.gov.uk/about-us/news-and-media/media-centre/weather-and-climate-news/2022/heatwave-threshold-changes" >UK Climate Change in action) is 28oC. Today is the first day that is likely to stay below that for some time! Tomorrow is forecast to be above again. If that continues for three days then that's another heatwave.
The recent heatwave has been more manageable! I have opened the windows when the outside temperature is lower and closed them, starting on the south side, once the outside temperature is higher. I have also tried turning off the MVHR during the time when the windows are closed and monitoring the CO2 level. It gradually rose but did not go above 1000ppm. Yesterday, I left the MVHR running and didn't notice any adverse heating.
My house is not certified to EnerPHit but has similar features. I saw https://www.linkedin.com/pulse/how-we-keep-enerphit-home-cool-during-heatwave-richard-dudzicki-qiuie" >How We Keep an EnerPHit Home Cool During a Heatwave. I don't have space for an awning but was doing roughly the equivalent. The author dodges the question of "whether Passive Houses overheat"! I think that the answer is that for EnerPHit the frequency of overheating (over 25oC) must be a maximum of 10% of the time.
I think EnerPHit designates East Anglia as having a Cool-temperate climate (PHPP 9?). Don't know if that has changed in the latest version of PHPP? Wonder if the recent heatwaves in the UK and Europe will result in changes?
Posted By: ArtiglioVery Heath Robinson and only suitable for one room ( handy if you want to make a bedroom a bit more comfortable)
Freeze a five litre container of water, stand it in a washing up bowl or similar ( to catch the condensation) set up a fan to blow over it. Keep door and windows shut. Not going to work wonders , but makes a difference, mainly i expect by lowering the humidity.
That's similar to a swamp cooler/evaporative cooler except using the phase change of ice to water. It might improve your comfort but as it is a closed system it won't change the temperature of the surroundings.
The https://www.homevap.nl/en/overview-cold-humidification/homevap-cooler/" >HomeEvap Cooler adiabatic evaporative cooler is an interesting variation that can produce cooling. It attaches into the extracted air connection before the MVHR and depends on the summer bypass NOT being used. The outgoing cold air chills the incoming air via the heat exchanger. The cooling comes from evaporation and a cold water supply which feeds the evaporator and discharges to a drain. Quite a neat approach! Disadvantages are that it can use up to 2 litres of water per hour (not great if, like me, you live in water stressed East Anglia) and the "wet pads" are a consumable item.
Posted By: stonecoldIt is always at least 5C degrees cooler inside here, even without setting upsummer bypass and the MVHR continuing to do heat recovery for baths/showers and cooking. It is much cooler here since not in SE England although the hottest it got inside was 22.5C at the end of the last (more intense heatwave) when it was in the low 30s outside. Like you we have high embodied energy although in our case that's external insulation so any heat there still has to get through thick some walls to the inside.
That sounds pretty good! In the less extreme heatwaves parts of my house rose to over 25oC, possibly up to 28oC. Enough thermal inertia to stay below the external maximum. Uncomfortable, especially for sleeping, but survivable. I moved out to air-conditioned hotel rooms over the Amber Heat Alert and left the house to its own devices with windows closed and MVHR on. Very hot on my return but gradually cooled down with all the windows and doors wide open.
Posted By: stonecoldOne thing helping us massively is the lack of solar gain, only 2 GF rooms warm up in the morning, the 2 FF rooms have curtains shut and we don't feel it. Outside faces mostly North and South side doesn't seem sunny - the West side is kitchen and bathrooms with few windows and sun absolutely scorches you outside.
My house is a mid-terrace with North and South outside faces and neighbours on the East and West. Windows on the South face are PassivHaus spec triple glazed with a venetian blind on the outside protected by a fourth pane. The ground floor kitchen window is medium sized and the first floor bedroom window is fairly small. The wooden front door is also PassivHaus spec with a triple glazed panel in the top half. No additional insulation in the 1996 cavity walls. The architects for the low energy renovation were of the opinion that the greatly improved windows and door together with the relatively small area of wall would be sufficient. The outer bricks are "Cambridge White" style which are pale coloured. Probably not a lot more that could practically be done to reduce solar gain.
The North face has a Passivhaus spec triple glazed patio door with small matching windows either side in the ground floor lounge. The first floor study has an identical window to the first floor bedroom.
The second floor bedroom and shower room is essentially a pre-built loft extension. The renovation significantly increased the roof insulation. The Velux windows on the North were upgraded with the best double glazing units available. The Velux window in the bedroom has a solar powered external shutter.
Posted By: stonecoldI think shading over the windows seems to be one of the key things suggested. Possibly add some internal wall insulation for key rooms/S or W external walls with a low embodied energy although afaik I think there is only the likes of polystyrene and such. There is much better insulated plaster around now - aerogel plaster and Cornerstone SuperTherm (I'm not sure what level you have now, ours is good for age of building but not new build standard).
I think the windows are doing a pretty good job. I also have heavyweight curtains inside all the vertical windows except for the kitchen. I did consider phase change plasterboard, like Knauf ComfortBoard, under the second floor ceiling but didn't have it installed. Not sure how well it would have performed in a heatwave? It would increase the thermal mass around the phase change range but might not help if it was heated above that range?
The South side faces onto the street so options are limited unless all the houses in the terrace do the same
If there was something that could increase the reflectivity of the bricks on the South face without significantly changing its appearance I would be willing to give that a try.
Posted By: stonecoldCross-ventilation from front to back of house or from one side to another, and no longer making hot meals will probably help it feel cooler.
Already using cross-ventilation on the first floor and stack effect to allow hot air to rise out of the Velux windows on the second floor. Also not cooking hot meals and avoiding other heat generating activities like running the washing machine.
Posted By: stonecoldGiven how it's going with climate change, what about solar PV to run a/c (or even to run a portable A/C unit in the room where the first uninsulated MVHR ducting is?
Most recent addition has been solar PV with a storage battery and gateway. Should be able to run off-grid for a few days, maybe a bit less when running active cooling. Tilt and turn windows are less than idea for portable A/C. Heavy to cart upstairs. Might compromise security on the ground floor.
Posted By: stonecoldOvernight bedroom temperature plus the ability to fully shade a good chunk of outside space seem key - I'm hoping you arent in a flat.
Not in a flat. Lucky to have a small park on the opposite side of the street with plenty of trees and shade. The trees do not block the solar PV in the summer and shed their leaves to let the low winter sun through.
Posted By: stonecoldAir to air heat pumps can do cooling - but doesn't Vent-Axia have options or accessories for warm climates? Zehnder does I think.
Still investigating options. Zehnder have their ComfoPost air to water heat exchangers. However, I have been told there might be issues as some of my MVHR ducts are uninsulated steel from Lindab.
Ideally, I'm looking for a minimally invasive solution. Really don't want a major building project (been there, done that!). The existing MVHR unit is in a loft space on the North side. An external chiller on the outside North wall or in the back garden providing cooling up the wall to something in the loft space might be suitable. Like the idea of using a https://blueheartenergy.com/" >BlueHeart Energy Thermoacoustic Engine! Willing to consider anything suitable that planning and building regulations will allow.
Saw a mention of a device that combines an MVHR and AC in a Facebook group I follow. It's the https://www.nuaire.co.uk/products/residential-fans-and-ancillaries/mechanical-ventilation-with-heat-recovery-mvhr/mrxbox-with-cooling-solution/" >MRXBOX Hybrid Cooling System from Nuaire.
It consists of an AC unit that sits on top of the MVHR and connects to the duct spigots. All of the components of the AC are within the top box so the heat generated must be dumped out of the exhaust connection.
The combined height of the two boxes (and a stand) would be too tall to fit in the height restricted loft space housing my existing MVHR. However, it could be a good fit for others with overheating problems.
If anyone knows of other solutions, please add details to this topic.
Posted By: john.connettLike the idea of using ahttps://blueheartenergy.com/" >BlueHeart Energy Thermoacoustic Engine!
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