115: What’s the Difference between HRV and ERV?

Source: http://fantechhhv.co.nz/
Source: http://fantechhhv.co.nz/
Grant Anderson is National MVHR Manager for Fantech NZ
Grant Anderson is National MVHR Manager for Fantech NZ

What’s the difference between HRV and ERV? Grant Anderson is the National MVHR Manager at Fantech. (What’s MVHR? Listen in to find out!)

More detailed shownotes to come. For now, please feel free to contact me if you’d any help with choosing a ventilation system.

You can also find Grant and his team at Fantech.

For information about how Zehnder heat recovery systems work, check out the informative UK Zehnder Comfosystems page.

See Fantech for more on true HRV - Heat Recovery and Ventilation
See Fantech for more on true HRV – Heat Recovery and Ventilation
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  • Matt

    Balancing Act.
    Great podcast on MVHR and the differentiation from the pseudo-science of ‘HRV’. We’re about to start building an airtight house with MVHR and the extent of the balancing act required to do it properly shouldn’t be underestimated. For starters, us Kiwis tend towards putting the air tightness layer on the outside of the building (it’s easier) which leads to interstitial condensation – so we’re building with SIPs to eliminate vapour permeable insulation. Then there are all the air leaks you want – or indeed need in your house (Especially down south here): Log burner? ok, Ethos with external air source. Clothes dryer? Condensing model without external air vent. Bathroom steam extraction? Specify an MVHR that can handle the steam and do this job. Range hood? No idea. The recirculating models have no place in an eco home as you have to continually throw away and replace the carbon filters and I’m yet to find a ducted one that seals properly when not in use. If anyone has a great solution to this one, I’d love to hear it!

    • mcutlerwelsh

      Yes, it is a balancing act, and things get harder at the higher performance end. It sounds like this is where you’re headed, which is great. From talking with Grant, I think an integrated system from Fantech should be able to handle the kitchen extraction as well as bathroom. I suggest contacting him directly.

      Also, I totally agree regarding recirculating range hoods. However, I say that have no place in any home. ‘Recirculating’ does not equal ‘extraction’.

      • Matt

        Thanks for the responses – I’m glad to hear that my main outstanding problem in this area is due to it being a commonly unresolved one. At this stage what I’m looking at doing is getting an off-the-shelf range hood and adding a Honeywell 6″ electrically actuated duct damper for around US$100. (incidentally i believe this is available from Fantech who I have already used extensively via work and can absolutely vouch for). I have to admit, I also found the old Zehnder catalogue online that showed the non-powered hood connected to the MVHR system. I had my doubts about the impact on the duct work if the filters weren’t kept perfectly clean though so thanks for confirming that those doubts were founded. Also Grant, – the first line of the design spec we sent out to builders was “The building code is a bare minimum, not a target to be aspired to”!

        • Lanthanide

          This is what I’m also considering doing, since I think the recirculating range-hoods are crap, and I really want to get away from a loud fan in the kitchen.

          It would seem to me, though, that if you’re exhausting air at (over) 700m3/hour from the house through the range hood, that the MVHR system would have trouble replacing all that air? And you are going to be sucking out all of that warm air in your house, especially in winter.

          • Matt

            Absolutely right. If you choose your MVHR well then you can get ones that you can ‘unbalance’ forcing more air in than it takes out specifically for things like a range hood (I was looking at using this feature for lighting the fire too before I decided to go to a fire with external air source) in Europe it would appear that an ‘air make-up valve’ is a common solution for a range hood (or fire) in an airtight house. This is literally a 6″ hole in the wall (vent) that opens automatically when you turn the range hood on because onviously if there’s nowhere for the air to get INTO the house then the range hood can’t suck ANY air out of the house. In a perfect system all that would happen is that when you turn the range hood on the pressure in the house would drop and your ears would pop! On the point of sucking all the nice warm air out – yep it does that. But you’d rather waste a bit of heat than get grease from cooking into you MVHR ductwork and core! 🙂

          • mcutlerwelsh

            Things obviously get a little more complicated at the high performance end. Good to know there are solutions, and smart people out there who can help!

          • It really gets easier at the high performance (passivhaus) end, it’s doing something halfway between code minimum and high-performance where it gets complicated.
            Recirculating cooker hoods have a simple job and that is to filter the grease out of the cooking air. They are not for extract or ventilation purposes so arguably may not be the right solution in a low to medium performance house where typically a proper ventilation system is missing!
            A house needs a proper ventilation system, though and this is a separate function from filtering grease from cooking air. If the house does have a proper ventilation system (eg balanced MVHR) it will remove all smells and moisture as it should do, and at a sensibly low & quiet rate. It will also provide adequate fresh air inside the house while not chucking all the heat away. In this case, the recirculating hood filters the cooking air at a much higher rate but doesn’t chuck all the heat out of the building at the same time, either.
            And for balanced ventilation to work properly a house needs to be effectively airtight so the MVHR isn’t competing with air leaks elsewhere. (Unrelated to opening & closing Windows whenever you want though, which of course you can do.)
            If you have a wood burner in an airtight house with a proper ventilation system, the wood burner should be room sealed (ie airtight to the room) and have a dedicated make-up air supply direct from the outside to the inside of the wood burner. So the fire isn’t competing with you for the air in your house, nor is it competing with the ventilation system.

          • mcutlerwelsh

            Thanks Elrond. Good point that it’s the ‘middle of the road’ that’s hard. At the high performance end it sounds as though cooking smells are usually dealt with pretty well, either by filters in the reticulating hood, or by the MVHR – in your experience.

          • It is very rare that in a high performance building a cooker hood needs to actually extract air from the building. There are 2 examples that I know of:
            The first is where there is a commercial catering kitchen – eg in the Passivhaus schools we’ve designed in England there is usually a catering kitchen. The solution here is to have a separate MVHR just for the kitchen with additional grease filters etc. It only runs when the kitchen is in use, runs at a higher speed to match the extract requirements, and is balanced so provides enough make-up air to match the extract requirements. This means it works in within the same building envelope as the main MVHR without upsetting the overall balance of fresh & exhuast air. Teaching (food tech / Home ed) kitchens effectively used for domestic scale cooking still have recirculation hoods though.
            The second is where a family has a regular massive cooking event, in this case an Asian multigenerational family that did open pan cooking in considerable volume and on a regular basis (but not every day). Such cooking events combined with many more people being in the house generate far too much heat to want to keep it all inside! Throwing open the windows each time wasn’t a suitable solution! So the solution here was a dedicated cooker hood extract system that dumped the hot moist cooking air direct to the outside (no heat or moisture recovery) with a localised air inlet valve that ducted make-up air back to the cooking area. This extract hood & air inlet valve were only used for such events where the extraction, including throwing the heat away, was desirable. The make-up air matches the higher rate of extraction so the balanced MVHR providing background ventilation to the house isn’t affected. For everyday cooking the simple recirculation hood is used (along with the main MVHR) and the extract hood and air inlet valve a sealed shut & airtight.

    • ZehnderNZ

      Hello Matt,
      Good to see your are building to the standard. A term bounced around recently at ‘the Experts tour’ was “the building code is a minimum, not a target”. How true and yes there is a balancing act.
      We are so fortunate in some regards that we are so far behind other countries. We have a pool of knowledge available to us which has been tested and demonstrated in terms of real world results.
      Unfortunately the Range Hood remains as the great debate. The recirculating type range hood is the only option (at this stage in history) for Passive House projects due to airtightness. Zehnder did manufacture a range hood to perform as you describe and is likely to be developed further. Hopefully we will see these as a standard option some time soon.

      • Lanthanide

        Hi Grant,

        I’ve been talking to a designer about a passive house, and when I mentioned the rangehood, he said it would be connected up to the ERV system, just with grease filters in front of the intake.

        It sounds like that isn’t really a workable solution – at least not yet?

        • ZehnderNZ

          There are a couple of things to consider which can eliminate this as an option.
          The design air exchange rate for Passive House is 0.35 – 0.4. A heat recovery ventilation system will therefore be operating somewhere between 130m3 hr – 250m3 hr. A small range hood operates at over 700m3 hr.
          The other consideration is the practicality of air tightness. To divert volumes through the exchanger will effectively require 2 airtight mechanicaly operated dampers. This allows all return air volumes to divert to the range hood, then change back once it is turned off. This can be done (at a price) but volumes are still low for a range hood especially in any home less than 250m2.
          What has been described here was in Zehnders 2013 catalogue and has been discontinued. I suspect there have been a number of concerns.
          Lastly I would like to point out that the Zehnder system will take care of moister by removing it at the source. The kitchen has extracts there for this purpose. To eliminate moister related concerns, control odor, and to recover energy, then recycle that energy back into all living spaces.
          We are very happy to work with anyone who wishes to explore any, and all options. We enjoy a good challenge.
          All the Passive house projects I have had the privilege of visiting have used recirculating type hoods, the Zehnder system takes care of odor, and moister with ease.

    • Kara Rosemeier

      I don’t actually see the problem, here. What’s the purpose of the range hood? To prevent the spread of smells, moisture and fat. The HRV will contain moisture and smells by extracting pretty much at the source. A recirculating range will deal with the fat. Why is that not: problem solved?

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