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Heat Recovery Ventilation - Air Quality

  • 30-07-2021 8:52am
    Registered Users Posts: 42

    I moved into A3 rated semi-d new build a couple of years ago and have recently been trying to tackle the air quality in the house. The heat recovery unit installed is a Vent Axia Sentinel Plus B and obviously the heat pump hasn’t been on recently in the hot weather.

    Generally it feels quite ‘close’ without the windows open and at night-time with our bedroom door closed it always seems very suffocating. I found out how to adjust the settings of the Vent Axia Unit and in order to get enough air in, the fan speed needs to be about 35% (default is 23%). However this has the effect of significantly increasing the fan/air noise through the duct in the main bedroom disturbing sleep (as the unit sits in the attic above it and the duct seems to be only about 6 foot).

    Has anyone had the same issue or could advise any of the following?

    -         Extending the length of the supply duct over the bedroom to dampen the noise

    -         Install some sort of noise dampeners in the air duct

    -         Give up and sleep with door/window open

    I’ve tried speaking with the installers but they’re not being helpful and only want to change filters.


  • Moderators, Home & Garden Moderators Posts: 5,462 Mod ✭✭✭✭graememk

    The humidity in general over this hot spell has been quite high. - I know the ventilation is meant to replace the house air with fresh outside air with minimal heat loss.

    If that outside air is humid too, your getting the fresh air, but it's still gonna be feel close.

    Have you tried a dehumidifier?

  • Registered Users Posts: 42 Californeeway

    I actually got a device to measure all this and its the CO2 in the room I'm trying to fix really. This is the reading last night at 1am with the door slightly open about 10mm. The more I increase the fan speed the more the CO2 level goes down. But then the noise through the duct goes up!

  • Registered Users Posts: 42 Californeeway

    I'll try the humidifier though, thanks for the suggestion!

  • Registered Users Posts: 355 ✭✭Biker1

    Is there a minimum of 10mm clearance beneath all doors? Was it commissioned with the correct air flow rates for each room?

  • Registered Users Posts: 42 Californeeway

    I'd say between 7mm and 9mm beneath all doors, although I also opened the bedroom door by about 10mm last night and it made no difference really. It had been commissioned by the original installers but I adjusted them slightly since to give more supply to the main bedroom and more extract from the main bathroom (both upstairs). It would be useful to know the correct airflow rates as I also bought an anemometer in my quest to fix this.

    I guess the suffocation/coughing is most likely the high humidity being confused with low oxygen/high co2! So will get a dehumidifier. I'd still like to reduce the sound through the bedroom supply vent though.

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  • Moderators, Home & Garden Moderators Posts: 5,462 Mod ✭✭✭✭graememk

    Must look into what is a "normal" level for co2 in a traditional house (80's bungalow here)

    Looking it up it seems <1000ppm is what is wanted.

  • Registered Users Posts: 42 Californeeway

    Attached is what it says in the app anyways. During the day here and at night with the door wide open it never gets above 550ppm.

    However at night with the door closed (and 9mm gap under it) the reading creeps up above 1500ppm. I've managed to reduce this to about 1200ppm by ramping the fan speed of the HRV unit. i can't ramp it up any more though as its just too noisy through the vents now.

  • Registered Users Posts: 355 ✭✭Biker1

    If there is excessive noise through the vents then it could be that the unit is undersized and the ducting is not adequate to supply the correct amount of air. Seeing that you have adjusted some of the vents then you have put the whole system out of balance. You need to get a NSAI ventilation validator in to check over the whole system. From my experience the majority of installs were never designed correctly.

  • Registered Users Posts: 293 ✭✭spose

    Does it have a summer bypass option and is it tuned on? Will help with the temperature control but won’t change the co2. Is it en suite and if so is there extract in there too? Usually the connections in the ceiling can take 2 ducts…. You’ll get far more by adding the extra pipe than you will by ramping up the fan. Alternatively add an extract to that room as well

  • Registered Users Posts: 42 Californeeway

    Thanks yes the summer bypass is turned on and I've set the indoor temp to 21 from 23 just to cool the house down that extra bit. There is an extract in the ensuite and have left the door open a couple of nights but to no real advantage that I've noticed anyway.

    As well as the dehumidifier I've now increased the fan speeds to run at 30% during the day and 23% at night and it seems a lot better. Originally it was 20% both day and night. 23% is the most amount of vent noise I can handle while trying to sleep and the CO2 levels are about 1200ppm at night which isn't too bad and seems to be normal from any websites I've checked. Ideally it'd be almost silent though like the downstairs supply ducts which have a longer run.

    I realise all this tinkering and adjusting the vents has probably put things out of balance. I do have an anemometer that I can measure the extract/supply rates so I can work these out from section 1.2.3 of the attached building regs guide. However I want to solve the noise through the main bedroom duct first, even if it is just the normal noise of the air flow. I moved from a country bungalow and am used to more or less silence.

    I've contacted two NSAI validators last week and neither of them have returned voicemails as yet. I do see my original installer on the list and he did call out earlier in the year after chasing for a few months but wasn't able to offer any solutions apart from sound absorbers (even though they would block some of the airflow) or care that we already found it suffocating at night. He did charge 135 euro to replace the filters though and said it'd be 170 euro to get the system recalibrated if I wanted to maybe try adding in the sound absorbers. So I'm not in a mood to hound any of them really and would like to figure this out myself.

    That being said if I have to add any extra ducts or add length to a bedroom duct I'm not that handy so will be on the phone then!

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  • Registered Users Posts: 13,462 ✭✭✭✭josip

    Can you try sleeping with the door more/fully open?

    With our HRV there are only ingresses in the bedrooms and the 1st floor egress is in the bathroom.

    I view the whole house as a zone, or at most 1 floor and leave all the doors on that level open.

  • Registered Users Posts: 1,769 ✭✭✭Nedved85

    I’ve the same brand system and having similar problems to you for the warmer months of the year. It’s a disaster to be honest, have to leave the windows open for 4/5 months of the year.

  • Registered Users Posts: 42 Californeeway

    The solution we came to was to leave the windows open at the front of the house and our bedroom door (at the back of the house) open about a foot. Not ideal as this defeats the purpose of a door!

    The only remaining thing I might try is to ramp the fan speeds up and get someone in to install some sort of noise attenuator in the bedroom ducts.

  • Registered Users Posts: 6,191 ✭✭✭SouthWesterly

    While this thread is here. I had to buy one recently and need to get it installed.

    2 questions. I've already a 4 inch vent but the new hev is 5 inches.

    Can I just bore another hole or is it better to just have 1 vent in the room.

    Secondly, my walls are pumped. How difficult will it be to bore a hole and not have beads everywhere.

    I'll probably have to bore the hole myself. Not sure if an electrician would do it or even have the tools.