Flexibility in MVHR sizing for retrofit?

[SuperPav]

Hi all, I've found a fair few results by searching for MVHR sizing, but they all seem to relate to a new build and meeting ACH requirements.

 

We are (extensively) redeveloping a bungalow. All the upstairs will be newly built and so will the rear glazing/extension. However, the rest of the building will remain as the 1950's solid stoned wall bungalow. We've decided on MVHR primarily for central ventilation, with the heat recovery an added bonus. 

 

The house will be approximately 180sqm. (4 bed, 1 kitchen, 2 1/2 bathrooms), 2 adults and a baby. No new-build ACH requirements from a building regs point of view,  but we need to achieve the necessary background ventilation as none of our windows will have trickle vents.

 

With an eye on the budget, I'm keeping an eye out for reasonably priced MVHR units on ebay and suchlike.

However, beggars can't be choosers, so my question is around how critical "right-sizing" the unit is:

 

So the scenarios I'd like to understand are:

1) If we buy a unit that's a bit too small (e.g. rated for 150sqm)

2) If we buy a unit that's a bit too "big" (e.g. rated for 200-250sqm)

 

What are the implications?

[Cpd]

From what I remember the slower the air is circulated the better the heat recovery also oversized means the fans can go slower and create less noise.

Undersized units will have to work harder and potentially generate more fan noise. 
I cant be sure about these facts but maybe a starting point of the thread is me getting corrected! 

[Marvin]

Our MVHR covers up to 250m2 but we are only 106m2. This was bought larger on purpose to try and reduce wear and noise. We used rigid pipes and insulated all of the pipes, in and out with rigid insulation. We have summer/winter option and humidity sensors for the wet rooms. MVHR very quiet. It uses about 20 watts on normal

 

We have 3 niggles:

 

Cooking smell lingered until I read here that it's a good idea to put the system on boost when cooking.

Sound travels a bit between rooms down the pipes and I am yet to put baffles in. (lazy and so it cant be that bad)

If we leave the en-suite room door shut the extracted air is only replaced with the colder air entering under the door so we have to open the door some time after use.

 

We have realised that if the temperature outside is above the inside temperature we are better leaving the system on "winter" as the cooler air being expelled cools the fresh air coming in. Also we have had no humidity issues. I clean filters about once a year and inspected the condensation trap once so far with no problems.

 

Installed in 2018 and it has worked for us:  

https://www.polypipe.com/sites/default/files/HRX2_MVHR_Technical_Data_Sheet.pdf

Ours system includes the summer bypass. 

 

Hope this helps

 

Marvin