MVHR air flow rates

[Declan52]

For about the first month or two every one was told that before you went to shower you hit the boost button.  Then being lazy we just stopped using the boost.  In over four years the  bathroom has always cleared within a few mins. 

[Alex C]

We have now lived in our passive house for 2 years and are extremely happy with the air quality. I never use boost on the mvhr. We have unoccupied and  levels 1,2,3. I leave it on level 1 pretty much all of the time and maybe put it on 2 if it is a sunny winter day like the ones we have been having recently when the house has warmed up a little too much, or if the house is full of people. My house humidity is nearly always between 42 and 50% but will drop to the 30's if left on level 2 for long. The bathrooms clear very quickly even on level 1 as the humidity level is so low. I had read somewhere that humidity below about 35% can cause irritation and is made worse if there is dust in the air. The factory settings on the mvhr that we haven't changed are level 1 - 100m3 per hour, level 2- 200m3 per hour. The house is approx 650 m3 but there are only 4 of us in it so quite low occupancy for the volume. I think in general if you have a passive house you can run the air flow volumes quite low and much lower than any building regs values may state.

[joe90]

I am yet to balance my MVHR but because an  imbalance caused the bedrooms to cool a bit too much I turned it off, there are only two of us in a 250 sm house and even with it off the bathrooms clear quite quickly. My wife still prefers to open a window when the weather is mild despite my trying to educate her ?, in fairness she loves to wake to the dawn chorus which you don’t get with MVHR!. I do turn it on when we have guests or do a lot of cooking but don’t find the air quality a problem when it’s not on.

[le-cerveau]

I have installed the Brink RH sensors into my system and told it to get on with it automatically.

I have 2 x MVHR units, one for downstairs and one for upstairs.  The units are both located in the plant room, 1^st floor, with downstairs feeding up into one and out to the attic/outside and the 1^st floor one both top fed.

 

 

below is a sample of the system going onto boost (automatically) during a shower:

This is the first floor unit, it does 6 minute pulses (ususlly 3) until it decides the RH is no longer rising and then the RH will drop back to the baseline over the next few hours.

 

The ground floor unit stays on for about 1 hour then ramps back as RH drops.

 

Don't know why the different behavior, different duct lengths and they are slightly different units (top/bottom feed) but it works.  The steam oven in the kitchen will also trigger it when opened.

 

I run the system at around 66% of BR requirements and that is more than adequate.  The system boosts to either BR (100%) or 125% of BR requirements.

 

[Bitpipe]

Our MVHR is designed to trigger boost when you walk into a bathroom or turn on the light. There's also a humidity sensor in the MVHR itself which will trigger boost.

 

I recall from the time I properly balanced it (some 18 months after moving in) that supply exceeded extract by about 25% so that was the first thing to fix (both tweaking fan settings and the room supply / extract vents).

 

I easily exceeded the BR mins for extract but struggled to meet the min room supply by area which could only be met by cranking up the default fan speed from 30% to 40%.

 

Once I recorded all the correct settings, i turned it back down to 30%, keeping boost at 50%.

 

All seems to work fine - just checked the humidity with a metre and it's about 40%

[lizzie]

3 hours ago, PeterStarck said:

Not that it makes any difference now but I would have had a supply in the dressing area instead of extract and I would have had a supply in the hall as I have here.

Thats exactly what engineer has just said.

[lizzie]

My very grateful thanks to you all for your help.  Engineer has been and swapped exchanger for enthalpy one so fingers crossed for improvement there.

 

We spent the next 2 hours trying to sort the air flows.  My system is shown in Volts (German technology!) so we have had to convert volts to the various other measurements to work back to Building regs, fortunately there was a chart in the handbook that helped with that. It seems my system automatically calculates the flow through each vent back from the manifold so it adjusts each one once you set the benchmark level so no individual adjustment of vents necessary.

 

So we worked out what I should have for the volume of the house using @PeterStarck table (thank you so much) and fractions for conversion  then we measured the flows with the trumpet and converted that back to volts.  We then looked at what I did have relative to what I should have which was a revelation.

 

As suspected my system was hopelessly out of balance....by a very long way.  I was extracting a great deal more than was coming in and I had negative pressure.  Its likely this is what has been causing me the breathing issues, feeling like it is stuffy and I cant get enough air. 

 

System has now been rebalanced - calculated using @JSHarris and @HerbJ helpful info. Spookily it worked to the rough chart I had done following the @PeterStarck formula too.  Engineer said it was spot on.

 

I now have a balanced system and have been advised to run on level 2 and only use 3 as boost as and when necessary.  Level 4 is a blow your socks off job and unlikely ever to be needed.

 

Engineer is coming back in a week or so to hopefully get UFH sorted and give gas boiler it's first service.  We will live with new MVHR  settings in the meantime and if any tweaks needed he will do then.  

 

Thank you all once again, superstars.

[HerbJ]

Well done. Fingers crossed for you?

[newhome]

That sounds great @lizzie. BuildHub delivers in spades yet again and you’ve found someone decent to get the system physically working. Just a pity the previous guy couldn’t do the job he was paid to do. Fingers crossed for the UFH!