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Mechanical heat recovery ventilation system

  • 12-11-2022 12:05pm
    Registered Users Posts: 530 ✭✭✭

    I'm looking to improve the air quality in my house. Our BER rating is C1. For the mechanical heat ventilation system does the ber need to be improved or does your house also have to be airtight? In that case would I better off to go with something like a passive input ventilation system? Is that any better than just leaving your windows open for 20/,30 mins a day? TIA


  • Registered Users Posts: 862 ✭✭✭redlough


    I was just about to ask something similar

    I am wondering do you have to ventilate every room or can you get away with a hall/bathroom/kitchen etc? leaving the rest?

  • Registered Users Posts: 2,527 ✭✭✭Yellow_Fern

    Smaller halls and landings often wont have outlets.

  • Registered Users Posts: 2,760 ✭✭✭MicktheMan

    The answer is of course, it depends.

    Taking a general view, it might be instructive to break the MVHR system up into its component parts. First you have the MV part which, imo & ime, every house should have in our mild climate. On the other hand, the HR bit is really only beneficial if you have very good air tightness AND the house is a new build or bungalow due to the complexity of the ductwork.

    Finally, I would install an input ventilation system (commonly known as PIV) due to the risk of damp / mould / decay within the building structure. A cMEV is far superior.

  • Registered Users Posts: 522 ✭✭✭mike_2009

    Installed a PIV in the landing and the heating coil generated a lot of moisture in the attic as it was at the unit end, not at the point where the duct enters the plasterboard of the landing. There are different models so watch out for this if installing. Worked great otherwise but it's a 400w heater so you'll have bills...!! Did keep the kitchen smells downstairs though.

    In the end I took it out due to moisture issue and attic has been dry since. Retrofitted a Zehnder Comfoair 350 for the upstairs only and the bedroom air quality is massively improved. 3 x bedrooms and 2 x wetrooms covered. It's slightly audible as the lowest setting expects to do a whole house so velocity is a bit higher than it would otherwise be but if you can sleep in a hotel room ok it's no worse than that.

    You can always crack open a window but over the course of a night, this may not be desired / possible so air quality decreases over time. I've 2 x F7 filters on the unit and an air quality sensor in my bedroom.

    I'm a B1 but no air tightness done - I've Z profiles in the passive air vents still and not blocked them up yet (these cut down sound but narrow the air passage significantly). Ideally you'd do air tightness first as air will start flowing in different ways. Not seen any issues after 18 months and no lingering damp in wet rooms after showers etc.

    Doing downstairs would be a significant challenge though as a retrofit but was the next step in the plan....

  • Registered Users Posts: 530 ✭✭✭mullinr2

    My house suffers from really high humidity. It's usually between 65 to 70% sometimes close to late 70s. We ventilated the house first thing in the morning and when we come home from work. Windows wide open for 15 mins. The heat from rads and stove brings the humidity down but it'll go back up next morning. Will any of the above systems help with humidity?

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  • Registered Users Posts: 2,527 ✭✭✭Yellow_Fern

    Both should but PIV tend to underperform. Go with cMEVs, but do your research and plan well if you get irritated by noise, as cause some can be noisy.

  • Registered Users Posts: 2,760 ✭✭✭MicktheMan

    Apologies, this should read "Finally, I would not install an input ventilation system ....."

  • Registered Users Posts: 2,760 ✭✭✭MicktheMan

  • Registered Users Posts: 1,879 ✭✭✭Citizen  Six

    What air quality sensor do you use? I'm looking for one for a couple of rooms, to test. We have a vent axia MVHR system, but often have condensation in the bedroom windows in the morning, and mould growth in the our wardrobes. Really need to get it serviced and balanced, but want to figure a bunch of stuff out myself first.

  • Registered Users Posts: 522 ✭✭✭mike_2009

    sensorpush - Robin Clevet used it in a recent youtube video on his channel to test a new build house. About the size of a box of matches, battery lasts a year, bluetooth and just sync your phone / app with it every 2 weeks or more often. Mine is in my attic (see below) after the condensation issue I had with the PIV unit. There is a gateway device you can also buy to internet connect the sensorpush but it's hard to find in stock. Get one and move it around daily to measure different rooms.

    MVHR have two types - Enthalpy models can cause problems with moisture but I doubt you have that type. Yours is too high, not too low!

    Do you have any commissioning paperwork for the MVHR? Flow rates etc? Be good to see if its balanced or not? You're on the right track.

    How do you dry your clothes, how many showers are active / day - you also have humans adding to moisture, how many bodies vs what is the floor area of your house? Opening windows in the morning is a good idea, having some data on humidity will help too. Cooking / extract, opening windows after showers, ventilating bedrooms with fresh air in the morning should all help. Be good to identify rooms with highest levels of moisture and what times it builds up.

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  • Registered Users Posts: 1,879 ✭✭✭Citizen  Six

    I'd need to check, but I doubt we got commissioning paperwork. They guys the builders hired, while being from a well known company, were total cowboys. They messed up a lot of stuff during installation, which was a nightmare to deal with.

    Haven't had a proper chance since to research other companies, and get someone reliable to service and balance the system.

    Our clothes are mostly dried outside, so that's not a huge issue. Showers are ok, as confined to the bathroom, and the extract seems to clear the moisture fairly well, so no issues there. The main issue is the wardrobes in both rooms, and moisture in the main bedroom window in the morning. And we have a skylight, where warm moist air rises to, and gets trapped. There was a bit of a cold bridging issue with that as well, which I've since sorted.

    For cooking, the engineer and architect suggested a recirculating extractor, but I wanted one that vented externally. So I fitted it with a motorised baffle that automatically closes when the extractor isn't on. We usually open a window slightly to avoid negative pressure, which would pull air in via the 'sealed' stove.

    Currently working on a plan for an air exchange system, that would draw air from the wardrobes, which would be heated via my stove, then returned to bedrooms. Might be a bit crazy to do. But trying to find a solution to the wardrobe issue, as well as a way of using the stove to heat other parts of the house. Should have got a back boiler, but stupidly listened to the engineer on that one.

  • Registered Users Posts: 522 ✭✭✭mike_2009

    What is your heating type? Any gas heating? Just wondering where all the moisture is coming from? The humidity measuring will be interesting. I'd guess it's the bedroom and not the wardrobe that's the issue, solve it there, the wardrobes should be ok. You're just missing humidity data to help trace the severity and cyclical timing of the issue.

  • Registered Users Posts: 1,879 ✭✭✭Citizen  Six

    Yeah, gas fired central heating. Haven't had to use it yet this autumn though. Duct for the incoming air should be on the far side of the room to the door, in my opinion, but I could be wrong.

    We also have a cellar that's 4 sqm. Had similar mould issues early on, as it get's no airflow really. So I've setup a dehumidifier there at 55%, which has controlled the issue.

  • Registered Users Posts: 2,527 ✭✭✭Yellow_Fern

    It is a nice idea to have a valve in a large wardrobe, but I am sure it not essential, would rather be a fun advanced DIY project. You prob dont have enough flow into your bedroom. How much is the valve open? Post a pic or a measurement.