MVHR and the heat wave

For those with MVHR, can you please feedback on your experience with the recent heat wave. What sort of internal temperature gain did you experience or not ?

I have a Zehnder ComfoAir and live just north of London.

I kept a close eye on the ventilation during the heatwave and fortunately the app shows incoming and outgoing air temperatures, also supply and extract temperatures. At the high temperatures we were experiencing I could see that the heat exchanger was being bypassed, but the incoming air was still getting slightly warmed as it passed through the unit and extract air was still getting cooled. I suppose it was unavailable when the airflows were so close.

Example below

Typically the air supplied would be 1-2°C warmer than the outside air. I would minimise airflow when the house was warming up (inflow warmer than outflow) and purge ventilate at night when the air was cooling (inflow cooler than outflow).

The house was clearly getting a lot of energy input other than the warm air, as during the day the extracted air could be 5 °C warmer than the inflow.

Unfortunately I cannot access historic data from my system.

We live in Rochdale. At around 2pm on the 19th July the temperature was close to 40 outside and around 30 inside.

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Could you describe the MVHR setup related to those figures please? I suspect you have good wall insulation and shaded windows.

“The house was clearly getting a lot of energy input other than the warm air, as during the day the extracted air could be 5 °C warmer than the inflow.”

The energy input being chiefly from solar gain presumably, and with some from the internal sources of occupants and equipment.

Exactly. The fridge freezers were on and my son’s PS5. The walls are not insulated yet and were warm to the touch, plus direct insolation through windows. We were probably lucky to get away with a 5°C gain.

Being the measured air temperature at the MVHR unit means that I am getting an average temperature, no doubt there were hotspots.

We have a Titon HRV 1.35 Q Plus with Aurostat controller. The intake/exhaust is on the east gable of the house. We have (regretably) 60mm Kingspan kooltherm EWI, Green Building Store performance range windows and internal blackout blinds. Our roof overhang offers some shading, but on the 19th, not enough!

Sorry we don’t use the MVHR in the summer just the windows.

We have a reasonably well insulated home and normally use blinds and cross ventilation to control solar gain. On the hottest day in question, blinds were down and the windows closed. By 1.30pm the internal temperature was 24.5℃. The trees and shrubs were swaying, and I (mea culpa) opened the windows. By 2.30 pm internal monitor showed temperature 37.2℃ and low humidity below.

We can cope with solar gain but the hot air dome needs a lock down and controlled ventilation strategy as your feedback has indicated; MVHR is going to play a part in our future strategy and maybe a cooling system as we progress towards 1.5℃+.
Thanks for the responses as it is only by sharing that we learn.

" The trees and shrubs were swaying, and I (mea culpa) opened the windows."

We presume that you did this in the interests of scientific enquiry.

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:face_with_symbols_over_mouth:All good experiments come from observation!

I don’t have MVHR, but survived two days of heat on the Monday and Tuesday reasonably well.
While the loft bedroom and bathroom reached about 35, the same temperature as outside, the ground floor only got to about 25 degrees, and had no trouble sleeping on first floor where it was about 26/27 when I went to bed.
I managed the heat by checking HA regularly to compare external temperatures with ground/1st/loft temperatures, keeping all doors and windows if room cooler than outside, keeping curtains/blinds and otherwise covering windows on inside to keep sun out of south/east/west facing windows.
For instance opened loft windows at around 9:30 pm on Tuesday, 1st floor windows about 11pm, and maybe downstairs for half an hour before I went to bed at 12:30.
My kitchen/living room is N facing so kept N facing windows closed but uncovered.
The E facing utility room has no door/window coverings and after observing big morning spike in temp on the Monday, I covered the window with a piece of hardboard before sunrise Tuesday.
I do have IWI on main south facing rooms and triple glazing and am sure this helped, but no idea how much.
After two hot days during which internal temperatures continued to rise, it took days for them to reduce to ‘normal’ summer levels.
A similar heatwave that lasted longer and I would be figuring out how to get out onto the dormer roof to sleep, or getting down into the underfloor crawl space. In fact I should probably put another TH sensor in the crawl space to check below floor temperatures.
Having ‘live’ data to refer to is proving extremely useful, but you need to have some good coping strategies as well.

Seems like you did pretty well. We are intending to add blinds / shades to control solar gain on south facing windows but controlling warm air infiltration needs to be part of the mix, so airtightness / MVHR has a wider role to play in futureproofing against climate change and heatwaves!