Expectations around MVHR

[IanofEpping]

Hi,  As background we have recently completed a deep retrofit of my 1960’s family home housing three adults and three children. We added an extension, new roof and are aiming for an EnerPHit standard of thermal performance and airtightness. Measures included new triple glazing for half the windows, IWI and CWI insulation, lime parged walls, air tightness taping throughout the house, new air source heat pump and an MVHR system. A recent air tightness test has shown we have gone from a leaky 15 ACH to 5. The test was done with 14 year old double glazed PVC windows and an air conditioning unit left untaped, knowing they are a little leaky. So far so good. 

 

Three months ago, after a long wait, we switched on the MVHR system: A Zehnder Comfoair Q350 and steel ducts from 21 Degrees who also designed and commissioned the system.  The commissioning process didn’t go to plan as a young engineer connected up the waste pipe to the wrong side of the machine, causing a leak a few days later. Fortunately, this was found quickly, and the waste pipe was switched over. 

 

Unfortunately, the system, hasn’t performed as well as I had hoped. I did ask 21 Degrees if it had been damaged and to their credit, they sent another engineer over who found it hadn’t.

 

Here are the issues:

 

The kitchen and utility rooms seem to be clear of condensation but the two bathrooms, after hot showers, remain damp after two hours and longer, so we have reverted to opening bathroom windows until the condensation has cleared. We chose not to have extraction fans fitted in these rooms, believing the MVHR extraction vents would suffice. Sadly, increasing the ventilation rate from the normal ‘2’ setting to the high ‘3’ setting makes no difference. I noticed the downstairs bathroom outlet has less suction power – perhaps a third lower than the others - unable to hold a paper tissue on the normal or high settings.  I should say here, adjusting the outlet vents doesn’t make a difference.

  

Questions

0. Are my bathroom extraction vents not working properly?

 

1. Should an MVHR system be able to clear bathrooms of condensation? 

 

2. Speaking to 21 Degrees I was surprised to hear that they don’t expect the system to clear the bathroom condensation.  Am I right to feel disappointed here?

 

3. Is it a balancing issue, whereby the supply and extraction vents need adjusting by someone, unlike me, who knows what they are doing and has the right tools? 

 

4. You can feel the soft gentle supply of air from the supply vents but, unlike some MVHR users on this website, the circulating air flow sees lame, i.e. it doesn’t whistle under closed doors. My air supply flow can only be felt close up at the outlet ducts. Is something wrong here?

 

I am about to lay my carpets and cut my doors to keep the recommended 10mm air gap beneath. If there any issues with the MVHR system I would obviously like to address them before this. I appreciate any thoughts and help with this.

 

Thank you Ian

[JohnMo]

You should have been given a commissioning certificate, this should show the room by room flow rates - if they haven't handed over ask for it now.

 

When commissioning was taking place, did the person go around the supply and extract terminals (room by room) to set flow rates, they should have used a certified flow meter - if they didn't the unit wasn't commissioned correctly.

 

Test for you to do - In the bathrooms, utility and kitchen, tear off a 2 square strip of toilet roll and place it over the extract terminal - it should stay in place. If it doesn't and falls off the flow rate is just too low.

[TerryE]

1. Yes

2. 🫣

3. Probably

 

As @JohnMo says you should have been given a commissioning certificate and report which details how their installation meets BRegs.   We have an energy efficient new build with an ACH around 0.5.  We did a lot of the internals ourselves where we had suitable knowledge / experience to save costs and guarantee quality.  This included the MVHR design, commissioning and installation.  That was over 8 years ago and we've had no problems at all with the MVHR including the bathrooms. 

 

Yes, we do get a little condensation on the mirrors in the en-suites after a shower, but this clears after a few mins.  However, (mainly because evaporating off condensation dumps heat due to the latent heat of evaporation) I do wipe off glass and tiled surfaces with a flannel and wring this water into the waste outlet, so in my case the en-suite is bone dry within 5 mins.  My wife doesn't bother, so it takes another 5-10 mins in her case.  The extreme case is that after we take a bath I leave the hot water in for 30 mins or so to dump most of its heat into the house rather than losing it down the plughole, and this does steam up the bathroom.   This is me being anal, but even doing this, the bathroom is still totally dry within 15 minutes.

 

TBH, I am shocked at their reply on (2).  If this is the case, then their installation almost certainly doesn't meet BRegs.  Doing the design calcs and commission certification requires you or someone you trust (such as a competent professional) reading and understanding the relevant Regs then doing the proper calculations based on them.  You can do the commissioning yourself, you just need a flow anemometer.  We used to have a couple available for members to borrow, and there are threads on how to do this going back 8 years or so if you want to browse and review.

 

OK we have an airtight house, but the general design principle is that fresh air goes into living spaces which people occupy. Extraction is done from wet rooms such as bathrooms, utilities, cooking areas.  In / Out placement should allow clean flows between them.  The system should be properly sized and balanced so that the as-designed and as-installed system should meed BRegs.  There are dozens, maybe hundreds of members here that have done this successfully themselves, so IMO any company that claims to offer a professional design and installation service here should be capable of meeting the regulations.

 

Edited yesterday at 15:47 by TerryE

[MikeGrahamT21]

First off, you can continue with carpets and doors as you've suggested, this is normal practice for MVHR ventilated homes.

 

What sort of temperatures are your bathrooms at currently? My bathroom tends to sit around 22C and any condensation clears really quickly, its quite a large space so have 2 extract vents in different locations with 1x 75mm flexible pipe into each.

 

The soft gentle airflow from the vents is how its supposed to be, its an incredibly low throughput system that does its job by doing both the push and pull of the air on its own, rather than traditional extract which creates low pressure which has to be balanced from air leaks. I would not expect an MVHR to be whistling under the doors, that in itself would be wrong.

 

Please take some photos of your MVHR install so we can spot any errors!

[TerryE]

BTW, there is an entire sub-forum here dedicated to this MHVR,  perhaps one of mods could move this topic to it?

 

Edited yesterday at 15:43 by TerryE
Fix link

[Redbeard]

@TerryELink does not work for me...

Edited yesterday at 15:16 by Redbeard

[mike2016]

Do you leave the bathroom doors open after a shower or keep them closed? Just curious about air flow volume. Each door should have 80 cm2 of space beneath it (80cm wide, 1 cm high gap). 

Link to sub forum here: https://forum.buildhub.org.uk/forum/79-mechanical-ventilation-with-heat-recovery-mvhr/

 

[jack]

3 hours ago, IanofEpping said:

0. Are my bathroom extraction vents not working properly?

 

If you don't get an answer you're happy with from the suppliers, we hire out an anemometer. I think you might need 10 posts or something like that to see the Tool Loan sub-forum, but you'll get up to that easily enough just replying to this thread.