Category Archives: Heat Recovery

House Ventilation-Part 1

Controlling the Supply of Fresh Air

It is now time to research the solutions available in order to guarantee a fresh air supply into the house, extraction of the stale air and recover heat before the stale air is sent on its way.

There are a number of options -Use the hole in the wall in each room (with no heat recovery), use a central extract system (with no heat recovery), use decentralised heat recovery or a fully ducted heat recovery system .

From previous experience and research the hole in the wall system does not work well taking into account that we have a wind speed of almost twice that of Germany. The system creates draughts and one has only to view the number of vents blocked up and the dependence on the correct speed of wind blowing to supply fresh air in Irish dwellings to realise it does not work.

The central extraction systems  either using the stack effect (using temperature differences between the inside and outside) and   Bernoulli’s principle (using wind) with no fans is an option. One can use extraction systems that are powered by fans. These do work it appears but they need careful design .

Localised vents in different rooms that use heat recovery such as the Lunos , Aereco and Glidevale iMEV system offer powered heat recovery but when I priced these they were more or less the same price as a fully ducted system and require more holes than I want through the fabric of the building.

Another system recently certified by the Passive House Institute is a heat recovery system that requires two units at either end of a building called (fresh-r). I can see the system working in an open plan environment but the cost is more or less the same as the other solutions above.

If one wants to compare certified heat recovery systems one can use the certified component list on the passive house  web site Certified Heat Recovery Products.  These products are independently tested by the Passive House Institute.

I will be selecting a centralised heat recovery system because I feel if it is specified, designed , installed and commissioned correctly it offers the best solution. I also plan to use it as a clothes dryer in order to get extra value out of it.

What is in a typical centralised Heat Recovery System,

It typically has two energy efficient fans, a plastic heat exchanger, filters to clean the air, enclosed in an airtight box and a means to control the fan speed. It is a simple unit but the cost is high (in line with the price of other systems above except for the hole in the wall or a stack system)

How it works

Passipedia-basics types_of_ventilation

How much does it cost

I was informed recently that the Passive House Institute charges heat recovery manufactures around €60,000 for testing and certification and then there are ongoing yearly costs that need to be passed down to the consumer. A Heat Recovery Company also needs to pay for National test (separate to the Passive House Institute) in different countries which also needs to be passed down to the consumer. There appears to be no European standard test that can satisfy all EU countries.

The product I was originally interested in was from a company called Paul but for some reason the price has risen substantially- I received a price for a Paul 450 unit two years ago and it was €2000 . It is now nearly €3000 -why?. One reason is that that the product was taken over by another HRV supplier Zehnder. I will try not select these products as I see no reason why the price of the unit increased by nearly a €1000. The price above is for large air supply volumes. A three bedroom house may only need a small unit costing 50% less.

Alternative Choices

One of the highest energy efficient certified units available is from a company called Maico-an Irish representative is available. Prices start at around €2400 for a more efficient unit than the PAUL unit .  Other options are from Brink such as  the Excellent 400 plus also available in Ireland. Systemair Gmbh also have units available in Ireland.


I did not think this area was going to be as complex as it is. Lets start-there are different choices of ducts from metal round, plastic round,  flexible round, rectangular and all other types of shapes. The bottom line is that large round metal ducts (150-200mm) are the most efficient and quietest and have the lowest air speed.

The flexible ducts vary between two types either Aluminum or Polyethlene with general sizes between 125mm (steel), 90mm, 75mm and 65mm. The smaller the duct equates to more losses- an increase in noise and possible draughts because of the higher air speed (careful placement and selection of the outlets in rooms to minimise these effects are important). I will be trying to select and use a 90mm semi-rigid duct (outside diameter) or larger steel duct. I want to avoid the use of the 75mm duct and all its permutations such as each room being supplied with two 75mm ducts in parallel.

Larger ducts need mufflers or silencers as room noise can travel between rooms . If one has an attic or one can find a way to install the 125mm steel duct with insulation in particular routes the larger ducts may be the best way to go.

Be aware that installing a false ceiling can double the cost of the HRV. I will review  all possible systems of ducts . If one uses steel I have been advised that one needs to insulate these as it can take a long time to heat up if the temperature drops leading to colder air being supplied for the initial start up.

The flexible ducts have multiple accessories to connect each part and extra cost is associated with this. The weaknesses of extra connections affects reliability. I will try and simplify the system of connections (somehow).

  • Ducts need to be cleaned (design and plan for this)
  • Keep the duct lengths short and straight to minimise losses.
  • Use large radius bends rather than 90 degree bends.
  • Consider placing ducts in the foundation when building new.
  • Some plastic ducts are antistatic (minimises dust collection on inside)
  • Some plastic ducts have antibacterial liners
  • Some plastic ducts are odourless and use physiologically and toxicologically safe polyethylene.
  • Some plastic ducts are smoother on the inside than others (minimises losses and reduces noise).

The non monetary factors for using a centralised Heat Recovery System that I can think of are:

 comfort (help to filter diesel particles , pollen, outside smog etc from the forced incoming air)

building protection and health/hygiene (remove high level of moisture from cooking, showers, baths, and people that could damage the fabric of the building, CO2 from persons in the house and VOC (chemicals from furniture, beds, floor carpets, paints, plastics etc) and supply enough oxygen to get a good nights sleep. Reduced noise because windows do not need to be left open and no holes exist  in the outer walls of bedrooms.

 security (can keep windows closed)

 thermal energy efficiency (recover up to 90% of the heat energy leaving the building)

The Plan

I want to see if I can design the HRV with the 150/125mm steel ducts or use the 90mm polyethylene outside diameter ducts with the antibacterial liner, antistatic liner and low emission polyethylene.

Each of the polyethylene ducts return to either a supply manifold or extract manifold (see example below).

HRV Manifold

If I end up selecting the 150/125mm steel ducts these will use no manifold and the individual rooms are tapped into the main duct.

Extra Functionality Planned For the HRV

In order to get the most value from the centralised Heat Recovery Unit I plan to make a special cabinet to dry clothes using a feed and return air supply. I will add a heater to the cabinet to give the clothes a drying boost where necessary.


An interesting calculator I found is one that calculates the losses of ductwork and air speed. One can experiment using rectangular versus round ductwork and the inner roughness of the ductwork here.  One can see the air speed change as one reduces the size of the ducts. There are another few options on the right hand side menu to calculate other factors.

Example of Duct Losses

HRV Duct
Flexi Duct

Background Research and Notes

  • MVHR are not a fit-and-forget systems
  • Based on European CEN Standard 13779 ventilation for ‘medium’ air quality should be at least 10 L/s per occupant (15 L/s for high indoor air quality).-non residential
  • Ventilation rate of 8 l/s (30m3/h approximately) per person identified in CIBSE Guide A8
  • In order to achieve an air exchange of about 0.33 ach (air change per hour), one would have to open the windows wide for 5 to 10 minutes every three hours – even at night! –Source
  • Biggest complaints -noise  and draughts (over dining area, bedroom areas etc) causing users to switch the HRV off.
  • Balance the air going in against going out-not practical to reach 0%. Aim for less than 10%-allowable imbalance between intake and exhaust air flow for these systems is 10%
  • Filters not changed can increase (double) the cost to run because the fans use more electric power to send air around.
  • Ensure that the HRV is accessible in order to change the filters.
  • A good strategy for the summer appears to be to reduce HRV speed/flow rate and open windows.
  • Total cost of HRV if left on appears to be around €60 for electricity and €60 for filters (Once a year). Check cost of replacement filters for own unit.
  • Ducts need to be cleaned every few years so the design needs to make it accessible.
  • A larger HRV unit than required can be more silent because it does not have to work near its full ventilation capacity.
  • A measured air noise level of less than< 24db for bedrooms (Finnish Guideline) . Can it be specified and delivered?
  • Noise levels up to 30 dB(A) were described as “too noisy” by more than 40 % of respondents. The standard for certified PassivHaus dwellings [13] is a limit of 25 dB(A) in both living rooms and bedrooms. Source
  • Maintaining indoor humidity below 7g/kg should help to reduce the risk of excess mite growth.
  • Note that air speeds greater than about 0.3 m·s–1 are probably unacceptable except in naturally ventilated buildings in summer when higher air speeds may be desirable for their cooling effect.CIBSE Guide A8
  • Significant problems were found with the commissioning of HRV systems, with only 16% of systems being found to have been commissioned correctly with respect to air flow and balancing. Source
  • 44% of kitchens meeting the minimum requirement of 13 litres per second.Source
  • Measured air flow in 88% of systems utilising rigid ducting were equal to or greater than their design air flow values, whereas between only 40 and 44% of systems utilising flexible ducting met their respective design value. Source
  • Any leakage through the dwelling envelope will have an impact on the efficiency of the heat recovery component. Source
  • Lack of appropriate airtightness, lack of complete commissioning, poor air flow and extract rates (and associated lack of compliance with regulatory standards), lack of balance and inappropriate duct types. Source
  • Wolfgang Feist‏@WolfgangFeist-You don’t believe this?The “trick” is:We have a F8 fine filter at the external air (“fresh air”) inlet, therefore supply air is very clean

In another technical paper the following was noted as the cause of excessive noise. The source of the document is here

Extract from document shown below

“The following list of issues are all taken from actual findings on investigations that have been reported. Issues that can lead to excessive noise for occupants are noted under the following headings of design, installation, commissioning and maintenance.

Design issues

• Centralised MEV or MVHR unit located in inappropriate place for break out or structure borne noise, e.g. bedroom cupboard or on rafters in loft above a bedroom.

• Poor ductwork layout – too many bends can lead to additional fan pressure requirement and regenerated noise

• Specification of flexible ductwork

• Inadequate attenuation of duct borne noise

• Installation issues

• Ductwork kinked or damaged inhibiting flow

• Ducts not connected up to supply or extract valves (which will inhibit flow and require higher fan setting)

Wrong type of outlet fitted (using extract outlets for supply air can lead to regenerated noise)

• No anti-vibration mounts used

• Failure to ensure ductwork is clean when installed prior to commissioning

• Use of flexible ductwork where not specified”

Recent Research

I note the above is summarised at the recent passive house conference (see below).


Moisture Control, Supply of Fresh Air, Removal of VOCs.

The Past

Having lived in an house that had single glazing and was draughty, I only realised how inefficient the ventilation was when I purchased a Carbon Dioxide meter (CO2) for the old house. This meter measures the amount of carbon dioxide in a room (as we all exhale CO2). The bottom line is that a level below 1000ppm (part per million) is taken as a healthy starting point. Another factor that is not healthy is Volatile Organic Compounds (VOCs) these are the chemicals, gases given off by furniture, paints, floor coverings, household cleaning agents etc. When one reads the possible health effects from VOCs it becomes clear that one needs to reduce these. See for example

In our last home what was amazing was how little oxygen we were getting especially when asleep (and in turn high VOCs). Within an half an hour the meter would alarm that the levels of oxygen (fresh air) were low.

What I used to do in the old house is determine if there was a wind blowing or a storm due. This then entailed adjusting the window opening to a minimum in order to ensure that we would get some level of oxygen. If there was no wind blowing outside then I would open the window  to approximately 75 mm (The horizontal window was over a meter and a half long) and leave the bedroom door open for some cross ventilation through the house.

I think it is now widely accepted that holes in the wall or opening windows does not work for fresh air and a healthy environment. The only way that appears to work is to blow fresh air in / suck stale air out.

CO2 Meters

CO2 meters for some reason are expensive. The unit I purchased was over €300 a few years ago. I came across a more affordable unit recently on ebay for approximately €100 (allow for customs and excise) that also has a data logger (records the information over time). I feel it is well worth investing in one of these as the true quality of air in a house can only be believed when the CO2 levels are measured.

One seller on ebay was perfectprimetechnology . If one types in the word co2 at their store it should be easy to find or in any other store. 

CO2 Meter
Carbon Dioxide CO2 Meter

While there is no direct link between VOCs and CO2 I have read that if one is breathing air in a room with high CO2 levels it gives  a good indication that the VOC levels are also high.

Going Forward

In the new house we plan to use controlled ventilation which will supply fresh air and in turn recover some of the heat blown out of the house .  It is a mechanical system called a Heat Recovery Ventilation Unit (HRV). I do not know why they don’t call it a fresh air unit as this factor has to be more important than recovering heat. In order for this fresh air supply system to work one has to control all drafts in the house as one does not want to be drawing air in around windows and doors and hope that the fresh air ends up in the correct room. This can only be achieved by eliminating drafts by sealing the building in an air tight membrane/system and blowing fresh air in and extracting stale air out using two ducts.

In order to carry out airtightness in a home one appears to have a few options -On a block house this is achieved by internal plastering (and some preparatory work) or for a wooden frame house one can use a membrane or take a chance on using OSB (Oriented Strand Board). Even a block house will still need air tight membranes on some structural details.

Another reason for using this special air tight system is to control moisture generated in the house from showers, cooking, drying clothes etc. This moisture can have a detrimental effect on the building fabric and reduce the insulation levels and in a worse case scenario lead to mould growth.  I feel the mould issue is going to be a health issue for generations to come where insulation is added ad hoc to dwellings without a proper design using building physics. I am not aware of any building physicists in Ireland guiding the construction industry. A worrying trend I am hearing of is cases where the consumer believed that adding insulation was a good thing not expecting the creation of mould and health issues for themselves.

The Airtight Target for a Passive house

In order to control the ventilation and heat loss the passive house standard requires less than 0.6 air changes per hour at their test pressure. The best analogy I could find for this figure was at the web site. One must achieve no holes or less than one 18 mm hole for every 5 m2 of the building envelope. 

Passive Haus
Airtight Standard Passive House

Practical Experience of the first steps in airtightness

As I was installing counter battens on the ceiling in front of the airtight membrane I know from experience I missed the rafters in 3 locations in the roof. I am using serrated nails with a nail air gun. Once these nails are fired they are almost impossible to remove.

Serrated Nails

The strategy is not to try and remove them as this would leave a hole other than a nail in a hole behind the batten.

Battens on Ceiling

Installing battens on the ceiling

Another factor is to ensure that the insulation does not sag below the rafters before the airtight membrane is installed.

When one is laying out the membrane one needs to work in a triangle when stapling. For example staple 3 metres  or more forward on one side and then find the centre of the other side and staple from this point to either side.  Ensure that stainless steel staples are used as there is very little in the price difference.