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Is HRV worthwhile in a small build? (90m2)

  • 20-04-2022 8:18pm
    Registered Users Posts: 136 ✭✭

    We're planning a major refurbishment (almost a rebuild) of a single-story dwelling of 90m2 and wondering if we should include HRV?

    Some people have told me that it's not worthwhile to install an HRV system in a building as small as 90 square meters. But even if that is true (is it?) the cost of energy seems to only be going upwards, so it may not be true for much longer. Or does HRV need so much energy input that it can never be cost-effective at this scale?

    The building has a full timber frame construction with timber cladding - no brick or block leaf. The refurbished wall build-up is still being finalized, but pending a proper technical assessment we think it may be something like:

    • 50mm insulated plasterboard
    • vapour barrier / airtightness membrane
    • 150m PIR between timber studs (150x50?)
    • 11mm OSB
    • breathable membrane
    • 50m air-gap (battens)
    • 22mm timber cladding

    Roof will be something similar in terms of insulation but with a metal cladding, thicker OSB deck, air-gap below OSB, etc.

    Windows will be triple-glazed uPVC.

    Heating and hot water will be by ASHP running UFH. We may or may not have solar panels - that's a separate topic. :)

    I don't (yet) know much about airtightness membranes and such - trying to read up on it. We're looking to make the house as energy-efficient as possible but don't have an unlimited budget. We don't yet have any target figure for the airtightness level that we're trying to achieve - but we'll be putting a lot of effort into making it as good as possible.

    So, is it sensible to include HRV in a (re)build like this?

    Oh, and one other question - do windows in a building with HRV typically have "trickle vents"? I'd have thought that you want no vents at all, so that all ventilation is done by the HRV system, but it seems that all the windows we see online have trickle vents...

    Any answers will be very gratefully received! 😀


  • Registered Users Posts: 6,161 ✭✭✭Ubbquittious

    Wow u are really putting in the effort to live up to your username

  • Moderators, Home & Garden Moderators Posts: 10,133 Mod ✭✭✭✭BryanF

    yes mvhr and good air-tightness would be a good idea. Especially if you are forking out for heat pump

  • Registered Users Posts: 2,811 ✭✭✭MicktheMan

    Aim for passive level for air tightness ... definitely worth it.

    Absolutely yes to MVHR and no to trickle vents.

  • Registered Users Posts: 2,553 ✭✭✭Yellow_Fern

    Yes, if I had to choose Id to pick HRV even over things like double glazing, Id always go for HRV. Tech like triple glazing has marginal benefit in Irish climates but mechanical ventilation has enormous benefit.

  • Registered Users Posts: 136 ✭✭CubicleDweller

    Thanks for the replies, folks - and sorry for not saying that sooner. Been busy....

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  • Registered Users Posts: 293 ✭✭spose

    I think you’d struggle to find anyone who has HRV that would say don’t put it in. House size makes no difference. I didn’t put it in originally but was able to retrofit when extending. If I was building again it would be top of my list of requirements

  • Registered Users Posts: 136 ✭✭CubicleDweller

    Thanks, spose. It's amazing how everyone here in this forum says "yes", and all of the "professionals" that I'm dealing with say "no, not in a house that small". I think I'll go with what the forum is saying (and my own online research, such as it is).

  • Registered Users Posts: 293 ✭✭spose

    Sounds counter intuitive to me to say there’s less benefit in a smaller space. My logic would be the same people in a smaller space generating the same amount of condensation from cooking, cleaning etc only makes having the right levels of ventilation even more critical

  • Registered Users Posts: 136 ✭✭CubicleDweller

    Absolutely, ventilation is critical, regardless of how it is achieved - and the problems that would be caused by poor ventilation may well be worse in a smaller space. But the question is whether mechanical heat-recovery ventilation is worthwhile.

    If I may play devil's advocate for a moment...

    Compare a small house with a small HRV system to a much larger house with a larger system. Say the larger system handles four times the volume of air per hour that the small system handles.

    Consider the costs and benefits of each system. Benefits first, as they're simpler. :)

    Assuming the same indoor and outdoor temperatures, the larger system will recover four times the amount of heat - that's the benefit of having this system, the heat that is recovered. So the benefit of the bigger system is four times that of the smaller system

    Now looking at costs. There are two parts to this:

    • the initial cost to buy and install the system
    • the running costs (electricity consumed, replacing filters, servicing/repairs)

    Are the costs of the larger system likely to be four times those of the smaller one? I think probably not.

    • the initial cost of the larger system will not be four times as high as the smaller one - maybe more like double
    • the running costs will also be less than four times, though this may be more linear

    So the cost/benefit ratio is better for a bigger system. I don't know, but I can't rule out the possibility that for a very small system the value of the recovered heat may not actually cover the running costs. Even if the recovered heat more than covers the running cost, I think the payback period (where the ongoing savings accumulate until they cover the initial cost) is likely to be much longer for a smaller system.

    So, I'm not fully convinced yet. I don't know enough to do a proper cost/benefit analysis so in the end I'll probably just wing it and install HRV. It just feels like the right thing to do...

  • Registered Users Posts: 2,553 ✭✭✭Yellow_Fern

    Installation can be a bit tricky for a centralised system and a builder might think it is not worth the hassle. There are a lot of design considerations and ceiling space loss. There are a lot of bad systems out there with bad ducting and noise, but still well worth it overall. It uses so little electricity and has so many benefits. Unlike a heat pump, it is really quite simple and durable.

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  • Registered Users Posts: 4,376 ✭✭✭BoardsMember

    Thoughts on HRV versus a non-heat recovery solution for a retrofit? Aeroco seem to have a nice simple non-heat recovery solution for "dry rooms" to just have a vent on external wall, and only need to have ducting into wet rooms/kitchen. With retrofit the less ducting the better. Wondering what the marginal gains are of a heat recovery above a more basic solution, would it be worth the additional disruption.

  • Subscribers Posts: 40,552 ✭✭✭✭sydthebeat

    You could consider an exhaust air heat pump which covers both ventilation and heating

  • Registered Users Posts: 4,376 ✭✭✭BoardsMember

    Old house, probably would not have the air tightness to run a heat pump efficiently, electricity bills might be alarming!

  • Subscribers Posts: 40,552 ✭✭✭✭sydthebeat

    They're planning a major renovation and installing a HP anyway???

    No reason at all why they can't make it suitable for an exhaust air heat pump

  • Registered Users Posts: 4,376 ✭✭✭BoardsMember

    No heat pump planned. Fireplaces around the house is the main reason. But that probably opens up another can of worms.

  • Subscribers Posts: 40,552 ✭✭✭✭sydthebeat

    Ah I think we're talking about two different things here.

    I was commenting on the OP.

    You are taking about your own separate situation??? Is that correct?

  • Registered Users Posts: 4,376 ✭✭✭BoardsMember

  • Registered Users Posts: 2,811 ✭✭✭MicktheMan

    If your airtightness is going to be poor then wondering about a mvhr vs non-mvhr marginal gains doesn't make sense