Posted By: Mike1Yes, that's the limit for the house as a whole - but based on climate data that is rapidly becoming obsolete. Better to use future climate data instead - see https://colbe.bath.ac.uk/.

I don't disagree, but once you have that data you can also test this in the Passivhaus PHPP model.

There is a built-in 'Stress Test' for summer overheating that lets you turn off certain measures like night purge, to assess the implications for overnight heatwaves like the one we had recently. You can also adjust the peak temperatures used in the (localised) climate file to add an extra few degrees above the present-day model.

4th August edit: See my additional post today - this chart is probably too pessimistic...
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Posted By: john.connettI took hourly readings

For comparison, I did the same last week - a rare period when I didn't have the MVHR switched off during the day. These show temperature & RH of the :

- External air, near-ish to the MVHR air intake
- Bedroom air
- Bedroom MVHR ceiling vent (the air being cooled at the MVHR by the previously mentioned cooler)

The MVHR manifold & short pipe leading to it aren't yet insulated; when they are the MVHR vent temperature should drop a bit more. I took theses MVHR readings at 15:00 on the 29th:
- Intake air = 36°C (36.4°C on my external sensor)
- Exhaust air = 32°C
- Extract air (cooled by the cooler) = 23°C
- Supply air = 24°C (warmed to 26.4°C by the time it reached the ceiling vent)

Some notes:
- The 'bedroom' is actually an all-purpose studio while the works are ongoing, so I was working in it most days, which would have increased the temperature.
- The windows were open elsewhere in the apartment from very early morning until breakfast.
- When the windows were closed the MVHR was boosted to well over 1ACH.
- The entrance 'air lock' is outside the insulated zone but, since there's no door yet between it and the rest of the apartment, there's more heat coming in that way than there will be long-term.

Looking at the above chart, the 30th and the 31st - the 2 days after the peak of the heatwave - are interesting; despite the external air being cooler, the inside temperature was higher both days.

The explanation seems to be that the external RH was higher on those days, which pushed up the internal RH, which meant that the cooler couldn't cool (evaporate) as much before triggering the cooler's maximum 80% RH. I'll have a think about upping the setting, but condensation & mould risk need to be avoided.

The same seems to have occurred on the 26th & 27th.

blocked filters...

much more important than for heatwave situations is that a blocked filter would put the supply and extract out of balance, and so in winter may well cause A LOT of cooling in the house. eg, if the filter on the extract side is more blocked than the filter on the intake side, more fresh air will come in than stale air going out, so the fresh air will not receive it's scavanged heat energy.

I manually adjust my fan speeds in winter to return the balance, but I'm familiar with installing/commissioning. The constant volume units ofcourse maintain this balance, and so no need for intervention, but then they are more expensive. In truth, I suspect MVHR's should all be constant volume, as it's very likely filters will block, maintenance won't get done, and so unheated fresh air will be supplied, cooling houses in winter.

BTW, here's a picture I just found showing the concentration of ducts approaching the distribution boxes. Just for interest.

Also to emphasise that cold winter ventilation air carries in a lot less humidity than warm summer air, so is much more effective, so the MHRV flow rate should be much less in winter than in summer.

Our cheap units don't have humidistats so have to be manually turned up in summer (or open the windows when it's not too hot)

I attended the recent Building Test Solutions webinar "Too Hot to Ignore: Understanding and tackling overheating in buildings" presented by Ben Roberts of Loughborough University. Now available on YouTube (https://www.youtube.com/watch?v=SSJE7QwN_9Q).

Some interesting information from their twin test houses where different approaches can be directly compared on two semi-detached, 1930s(?) houses with synthetic occupants.

Impressive results with various types of external shading (but not including brise soleil). The shading seemed to be mainly to reduce solar gain through the windows although some areas of brickwork were also shaded. In one example there was a maximum operative temperature difference of 6.3°C between the shaded and unshaded living rooms.

It was not clear whether external shading would be permitted development or not. The two test houses are in a conservation area and have not (yet?) had planning problems. However, the shading installations have been temporary. I'm not sure permanent shading like brise soleil would be permitted on my house even though it is not in a conservation area. The south side front faces onto the street so is more likely to receive objections than if it were hidden at the back. I intend to investigate further.

Also, some interesting results from dynamic thermal models. Often the models predicted worse overheating than was actually measured. Different models, and modellers, produced different predictions. The models used were commercial and proprietary. They haven't tried the Passive House Planning Package (PHPP). It would be interesting to know if they have tried the open-source Home Energy Model (HEM) (https://home-energy-model.co.uk/).

Posted By: john.connettI attended the recent Building Test Solutions webinar "Too Hot to Ignore: Understanding and tackling overheating in buildings" presented by Ben Roberts of Loughborough University. Now available on YouTube (https://www.youtube.com/watch?v=SSJE7QwN_9Q).

Thanks - I've just had a listen and it's an interesting methodology. I've not looked through it yet, but Ben's (the presenter's) thesis which, from the title, looks to be the basis for the webinar, is available at https://repository.lboro.ac.uk/articles/thesis/Ventilation_and_shading_to_reduce_overheating_in_UK_homes_an_evaluation_using_matched_pair_test_houses_with_synthetic_occupants/13281293

Since the 5th heatwave seems to be more-or-less over, here's a chart of the readings, un-smoothed this time. The coffee shop closed for the holiday and the filter is OK!

Similar to before, measurements are from:
- Near-ish to the MVHR air intake
- Bedroom air
- Bedroom MVHR ceiling vent (the air being cooled at the MVHR by the previously mentioned cooler)

I've also added the forecast temperatures for several of the days (thermometer temperature, not 'feels like' temperature). I checked the night-time temperature several times and they were within 1°C; the peak temperatures seem to be over-estimated.

Inside temperatures remained stable at 29.9°C ±1.25°C, which should improve a little once outstanding works are done + with the effect of the ceiling fans. There appears to be a heavily damped 6-hour decrement delay.

File attached - download & remove the .txt from the end of the file name to view.

How is moisture being condensed from the air in the homevap? Isn't the idea to evaporate some moisture into the air and so absorb latent heat, which would make the air cooler?

If 1.2 litres were condensed, that would give off around 2400 x 1.2 = 2900kJ or just less than a kWh of heating into the air - not what you want in a heatwave!

We have a Vent Axia Sentinel Kinetic BH plus, perhaps the same unit as the OP. We got it in 2012, and I added an external CO2 sensor to it almost as soon as we got it. It's still working fine - it's a 0-10V analogue output unit, link below, and is powered directly by the vent-axia itself - there's a wired interface on the front of the mvhr with 0V, 0-10V input, and 24V that it uses.
The unit maps 0-10V to 0-2000ppm CO2, and I think is now programmed to control fan speed from nominal to max over a 500ppm to 1000ppm range. We leave the nominal fan speed fairly low (15%), and so it often controls on CO2 or humidty.

https://www.digikey.co.uk/en/products/detail/amphenol-advanced-sensors/T8031/5774483

I think VA offer an external CO2 sensor as well, at the time it was silly expensive so I went with an alternative compatible unit.