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Heat Recovery / MHRV DIY Install, initial impressions

  • 20-02-2023 1:06pm
    Registered Users Posts: 33,128 ✭✭✭✭

    So i thought i would put together a brief thread on my experience with installing an MHRV system in the house.

    I am not a professional, none of this is advise you go out yourself and start. I am but a keen well read DIY'r with a silly amount of tools picked

    up over the years to crack on with projects. Life is short, you might learn something new if you start.

    MHRV has been one of the various items that i wanted to complete on our renovation journey since purchasing our renovation ~ 5 years ago.

    There are numerous items i want to get to, But as always time and money are the key considerations for each project.

    Having already done an EWI install DIY 4 years or so back in which i wanted to achieve as high a standard I could and also make the house comfortable

    to be in with lower future running costs. I had to obviously provide ventilation due to the higher air tightness. The stop gap being cored and Piped

    vents in each room.

    Around that time i plugged all 3 chimneys (2 fireplaces and an aga chimney in the kitchen) So the house was night and day in terms of airflow being more refined/ controlled. I had the walls pumped with bead (cavity wall) under SEAI and i DIY'd the EWI. The garage end of the house had cavity block as it was a later addition in the 90s But i wrapped that and did not bead pump. Its attached to the house but has cavity wall Gable divide which i had pumped also during the SEAI work.

    So background over, Why MHRV? Having done the work above and gone over airtight taping, replacing windows doors etc throughout Air Quality is a serious concern. You can end up with Worse Quality air but a more insulated living environment. (not good) . I did find that the house always had consistent high Humidity levels, No mould problems but some of the windows in areas did have water droplets in spots at the end. Nothing too obvious but Humidity was always 70+. Heating would take a while to get going as the air was heavier. Some rooms cooler than others. And the constant battle between should i open the windows or close them for freshness. Also the vents themselves whilst refined were blowing in gales and general waste of warm air flowing out of them. So it was not the best long term solution if you are looking at an overall collective improvement in comfort quality of life in the house (which i am)

    I started to get quotes for installations around Oct last year, And they were varied. Some quite reasonable. Some less so, The best thing about this type of work is the fact its a bungalow, less ideal is its not small and has large enough duct runs 210 SqM area so materials costs are always higher. I went over the quotes a few times and as one does researches solutions. I found that depending on what you want you can get a very effective basic system that you install and walk away from and it will do fine. And then again you can get systems with better efficiencies because they have a modicum of smartness about them. E.g Remote management (App/Web) Multiple Sensors which automate the systems fan speeds / pre-warming. (temp sensors/ CO2 sensors) You obviously have to consider the system sizing (Is it big enough to cover the volume of air space in the property and what about future room additions) The smaller system will run more often and at its higher output meaning it uses more energy.

    All of these things to wrestle with I decided that I did want to have a system that had Temp and CO2 sensors, I needed it to be easy to maintain from a service standpoint, I needed it to be large capacity both from energy cost operation but also so that it was running at lower fan speeds to mitigate sound problems in sleeping or living areas. And i wanted to be able to login and monitor it because im a nerd and i cant let things just be installed i have to see what they are up to.

    All of this unfortunately as usual for me meant that my requirements came at a higher cost. So whilst initially i was going to go with Professional installation. The budget stretching was beyond my willingness and i slowly chose the DIY route. I would recommend to go Pro though unless you are slightly lunatic and are capable of pushing on even if the work is getting incredibly frustrating (missing / wrong parts dealing with issues on the fly)

    Started to get Materials prices from 3 suppliers who do supply DIY market some also install too. Ended up going with a company that have a NI office and UK office. I found them to be very engaging, extremely quick on shipping and helpful. Note i think their detailed Plans option are possibly not worthwhile as the original basic plan with the quote was nearly exactly the same as the 'paid' engineer plan. But that was my experience could be different for others.

    The unit i went with was an AirFlow DV145 Adroit.

    I went with 75mm Radial Ducting, Supply Manifold covering 15 supply duct feeds, Extract manifold covering 6 extracts (wet spaces). The ducting came to around 350m of ducting. The Fresh supply and Exhaust were 200mm rigid metal Ducting, which would be wrapped in insulation.

    At a very high level view, You need to ensure working with your supplier that there is enough Vents and duct runs to each room to provide/remove enough volume of air. It is vital to get this properly done at the planning/design phase because if you don't get this right adding additional duct runs later on is painful and costly. Getting this wrong can result in louder operation to rooms, worse balancing across the system and inefficiencies. So spend sometime going over this. There are calculators available online to work out a reasonably balanced system. What you will end up with is in some rooms x2 Vents and can have multiple duct runs to a single vent for good balancing. e.g I have a long run to a bedroom which has 2 supply vents, one with 2 Duct runs and one with 3 Duct Runs. This is due to large size of that room and its length of the duct run.

    I started first by getting the planning and design down, its going to be different for everyone. But one thing i definitely wanted (for my system) was the Unit itself to be located inside my living envelope. I felt that putting the unit in the attic was not for me, main reasons being 1) Accessibility, 2) efficiency in operation . I needed to be able to get good access to it for servicing and the units work less well if they are installed in a Cold Attic space (or warm depending on winter/summer) My attic goes through those two extremes. So for my purposes I wanted it inside the house so the temps are fairly level all year round. I had that space in our rear hall entrance, however not everyone will have this luxury of location.

    Also due to the attic space being a cold attic, I would have to insulate all duct runs but the plastic 75mm feeds and the rigid metal supply and exhaust. I chose to wrap the metal and to cover the plastic ones in attic insulation which i have a good few left in the attic to roll out.

    So where to start the work! Ducting!!

    I built a cable reel puller out of left of timber in the garage and made a spindle for the center out of old gun-barrrel plumbing pipe. This allowed me to sit the large 60m rolls of 75mm duct on the reel spindle and pull it easily (relatively) into the attic space. Leaving the reel at the foot of the attic stairs. Pulling each duct run to their end position over the relevant room and then back to the manifold location, cutting and doing the same for the next duct run and so on. I placed my duct runs to the edges of the attic space and out of the main central area where possible in case we decide to build out the attic to living space in the future. Pulling the ducting is a 2 man job, but im doing this solo so it took longer. I thing maybe 2 days to get all of mine runs in and basic length cut. I then installed the manifolds to their correct final spot. These are basically large metal boxes with a large hole on one side to take the 200mm metal pipe and x15 75mm holes the other side to feed or extract depending on the runs. I had 2 manifolds.

    Next up room duct Plenums's. These are basically like an inverted plastic leg brace shape. (best way i can describe them) with single,double or treble feeds for ducts depending on your design. They have a 125mm large hole that goes through your ceiling which will take the vent and 75mm at a 90`angle hole/s the other side to take your ducts. These Plenums are generally made of plastic. They will need to be braced which means you will have to fix them to ceiling joists. It may require using 2x1 or Ply to cross the joists and the plenum fixed to that with x2 screws either side. I used 2x1 on either side of the plenum and fixed into the joists. cordless 1st fix and cordless circular saw made this job much quicker (in confined location) . You will need to cut the ceiling to take the 125mm Plenum hole. I bought a large adjustable plasterboard hole saw on Amazon bargain price. Bit finicky as had a habit of readjusting itself to different size after every second cut. So had to make sure to tighten the gromits holding the blades on.

    Cutting ceilings, dropping plenums in and fitting brackets to them was finicky work again probably 2 days in that if you are going solo.

    Next up was fitting in the ducts to the plenums and the manifolds. You want to brace the ducts to the ceiling joists every 1.5M or so and refrain from any hard 90 turns in the duct. All turns should be gradual in nature. It mitigates sound of the airflow later and also prevents damage to the duct walls from cracking. This is a long labour pain in the .... . You will be making lots of adjustments to the length at either end, bracing the duct with steel band and also in my case covering the duct as you go with wool insulation to protect from Temperature change. This stage i would say is solid 4 days.

    Fun 210mm Core out through garage gable for Intake Duct, had this one and a second for Exhaust.

    Now with the Plenum to Ducts to Manifolds done, You move on to the 200mm Metal ducts to the Unit itself and to the outside world. In my case I chose to go out the garage wall gable end for the Exhaust and Intake. rather than going with the much closer straight out the Roof. My sole reasons for this is in my opinion always avoid going out your roof for any purpose if you can. It opens up too much complexity for water ingress. Its always a risk. Plus the roof fittings for exhaust and extract are incredibly pricey. The much simpler but longer exhaust and intake runs are Core drilling Gable walls and adding covered vent outside. Less complex and less risk with roof damage and water ingress.

    (Metal Ducting - pre taping insulation wrap and final fixing) Can see the manifolds in the rear left here.)

    The metal duct work is not too bad, measuring cutting with grinders minor adjustments. There is less leeway with metal ducting obviously so make sure your routes to manifolds and outside are well mapped out. Roof trusses, walls, any other attic stuff like water tanks are key considerations. This process is 2-3 days. And insulating can be added to that maybe 1-2 days. For these metal ducts you will be using specific airtight sealer and tapes. You have to be very perfectionist about this so spend the time to avoid air leakages. The same can be said for the rubber fittings on the 75mm Ducts. make sure your angles are good when connecting to plenums or manifolds and in my case i used a spray of silicone on the rubbers to make them easier to fit but offered some lubrication for the seals.

    Install work done, the system is to be commissioned. Ideally get this done by a professional, I will be making some slight adjustments to mine and presently doing that (adjusting the vents in each room for flow) But i have the system in and 'on' for around 8 days now.

    Summary of my initial thoughts .

    It is expensive to get those higher end systems with more functionality. - you can definitely get close for less.

    A painful enough DIY, it will test your limits so not for everyone.

    The system has already knocked 20% off my humidity levels and going lower.

    The fan spends much of its time in the 4%-15% range. it kicks up to 34% when detects moisture increases or CO2 increases, showers/cooking. I have to put in the boost switches that came with it, there are 5 of those. I may only need x2 bathrooms and kitchen though will see. at the moment the automation is working well.

    The house feels much much fresher without the cold associated with it. Its very hard to describe unless you lived here. But the best i can come up with is its like having windows open all day long but the breeze and draught that comes with that is not there.

    The heating works better, the rooms get quicker faster and stay warm longer. Ive knocked off 25% of the daily heat run time already. *But that is a very very very short time frame to see longer term impact (*i fully realise this). Its been between 6-9degrees outside all of last week and constant 20.5-21 inside.

    The x2 windows that did have water drops on the bottom are completely bone dry.

    I wfh mostly and previously i would get a bit of a chill sitting in the same spot for a long time, that's completely gone now. It is like the house no longer go through its up and down fluxes of change*. Its very much consistently similar all the time.

    Again this is my immediate 8 day impressions, I don't think its the expense of it all sullying my opinion. I am genuinely happy with the system after a short time frame.Once fully commissioned and setup this should improve. I think it was the right thing for us based on the work we are doing and have done on the house up to this point.

    I will be exploring Solar PV next, to target operational costs of running these systems as i do want to get an A2W installation next. My ultimate goals are to achieve little wins and newer skills through the projects. Be capable of servicing and knowing everything in the house myself. And end up with a house that is very low cost to live in and operate comfortably through smart operation decisions. Time will tell if i am making the right decisions but for now its been positive so far.


  • Registered Users Posts: 1,483 ✭✭✭hesker

    Fair play. Great post. Will have a good read of this.

    Did you do all your air tightness measures also. Did you fit vapour barrier membrane also.

  • Registered Users Posts: 33,128 ✭✭✭✭listermint

    No vapour barriers it's block construction with cavity plastered inside and plastered outside to the block. So airtightness from the block perspective to ceilings already. The windows and doors were primary concern when we switched them out for airtightness measures. Used tapes and Orcan F for this purpose.

    I do however have to seal up my cored temp ducts. They have duct closures on them but will be Sealing up. Easy enough job as they're all piped through.

  • Registered Users Posts: 1,483 ✭✭✭hesker

    Not too dissimilar to my own. I have single leaf. EWI, heat pump & MVHR to be done this year. Company doing it is very vague about air tightness claiming EWI will provide it which I know is BS. Only hope that SEAI process will drive it. Gearing up to do it myself and identifying all penetrations.

    Did you seal all pipe & electrical cable penetrations into your attic

  • Registered Users Posts: 12 smyth79

    Great post.

    Just wondering if you did a post about your DIY EWI install a few years ago? I am planning on doing the same this year.

  • Registered Users Posts: 33,128 ✭✭✭✭listermint

    Yes made a point of doing that above or below depending on access availability. It will differ depending on the room and electrical locations. But overall it was quite good as whomever original builder was in 1970s they encased everything in conduit. Decent enough job all round.

    As for air tightness. You definitely want a reasonably high standard for mhrv for you to get max value. You can get a air exchange test done on the house by an engineer. The blower results will tell you how good or bad the house is.

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  • Registered Users Posts: 33,128 ✭✭✭✭listermint

    I did, I'd have to see if I can locate it for you.

  • Registered Users Posts: 1,483 ✭✭✭hesker

    Would you mind sharing any links to sites that show you how to calculate the number of vents per room. Searching online there and having to wade through a lot of basic FAQs and not finding anything.

  • Registered Users Posts: 355 ✭✭Biker1

    TGD F 2019 gives the calculation methodology. Download it from the gov website.

  • Registered Users Posts: 33,128 ✭✭✭✭listermint

    As above there are government guidelines on minimum Ventilation for dwellings. They guidelines are mainly concerned with the extracts rates l/s airflow based on treated air flow and occupancy. Number of people.

    At a high level you have a minimum extract figure l/s for each type of wet room. These are totalled up and you have a general airflow requirement. This then gives you the sizing of the unit you need. Your supply side should match the extract sides and you divide up the Ventilation across the sqM of those rooms, where high traffic liveable spaces get a higher l/s as people in them are creating moisture and use up the air.

    There are various calculators available online. But best bet is to get your house plans drawn up either using digital draught tools or by a professional. Any mhrv suppliers will need plans to work from and they will be extremely helpful to you. Laying stuff out.

    I used floorplanner as a basic starting point for design. It's an app but has a browser version too

  • Registered Users Posts: 99 ✭✭kevinc565

    What are your CO2 levels like? Especially the poster who runs the system at 4-15%?

    Some thoughts:

    the regulations don’t take ‘actual’ occupancy into account. They will suggest you pump equal air into equal sized bedrooms regardless of occupancy. A room with a couple will need more airflow than a singly occupied room.

    CO2 concentration will be higher in a smaller bedroom compared to a large bedroom for the same occupancy. Suggests higher airflow needed in small room than suggested by regs.

    the regs mean you pump/extract unoccupied rooms (ground floor) at night when really the bedrooms require more air.

    I have a MHRV system. It’s recently been ‘rebalanced’ although it’s not actually ‘in balance ’

    we found we need to run the system far higher than 4-15%. what make/model are I using? Are you achieving reg airflows with that very low setting?

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  • Registered Users Posts: 33,128 ✭✭✭✭listermint

    CO2 levels are changing all the time obviously they go up when cooking so have to use boost.

    But I'm getting average 500 to 800 over the course of a week.

    My system mostly runs around 15 percent. With that it's at the minimum regs and positive pressure. I purposefully oversized mine my house size is just under 70 percent of the units actually capacity capabilities.

  • Registered Users Posts: 33,128 ✭✭✭✭listermint

    Finished sealing my old Vents over the weekend, And hooked the unit up to the Cloud. I can access the content from anywhere.

    With the balancing done, the old vents closed up i am seeing more improvements in the PPM CO2 and the humidity has dropped probably another 6%. Could be just the house drying out though over the few weeks since install.

    Great to be able to graph the data though from the Cloud service. I will be installing a basic power monitor inline with the unit so see how its performing.

    One thing is though dryer lips , the air is crispy obviously lol/