Tuesday, April 19, 2011


The passive house concept is built around the minimisation of transmission losses of walls and roofs and maximising the use of adequate thermal insulation. The less the transmission heat losses, the more important are the ventilation losses which is possible by mechanical ventilation with heat recovery. With this system there are some technical aspects to be concerned, for example the energy consumption of fans, the prevention of freezing. Can be approached without mechanical ventilation? Can the passive house standard be approached in more traditional way?

Building Geometry

The surface to volume ratio

The surface to volume ratio is of fundamental importance from the point of view of the ratio of heat losses by transmission and ventilation. At the same insulation level the more compact the form is, the lower will be the heat losses by transmission.

The glazed ratio

The glazed ratio is the subject of a well balanced compromise since on one hand the U value of the window is higher than that of the opaque elements. The problem of solar access is the same for passive and other houses. The “standard” glazing of passive houses is triple, the nominal U value, including the frame is 0,8 Wm2/K and the g value is 0,5. The typical window area is 10 – 20 % of the floor area. 


The ventilation heat losses represent the crucial problem of passive houses. The standard solution of passive houses is a balanced mechanical ventilation system with heat recovery – this way 75- 90% of the ventilation losses are covered by the heat flow from the exhaust air. 

Solar preheating

The traditional solar measures include different techniques of preheating the fresh air. The simplest preheating measure is a wall collector below the window with air inlet under the window sill.  The key problem of the ventilation is the air flow control. Self intended, a passive house and a sample house without mechanical ventilation can be compared only, if the air change rate (and consequently the indoor air quality) are the same in both. Having a mechanical ventilation system the air flow control does not represent a new problem. In the case of natural ventilation demand controlled tricky vents should be applied.


There is no argument against the passive houses with mechanical ventilation and heat recovery but the aversions of the owners should be taken into account. Many of them do not wish to take care of a “machine”, to check and change regularly the filters, they consider questionable the indoor air quality and the noise. The price of the system and the cost of operation and maintenance also need to be analysed.
Can the passive house standard heating energy demand be approached without mechanical ventilation and heat recovery? It is possible if the house complies with the majority of the following requirements:
·         Surface to volume ratio should be as low as possible
·         High quality windows - maximum U value of 0,8 W/m2K and the overall U value of the opaque elements should be between 0,12 – 0,20 W/m2K.
·          Solar gains - the glazed ratio has an optimum – if half of the window area is Equator facing, the optimum is about 15 % of the floor area (ratio depends on the orientation and solar access)
·         Air change rate - not exceed that of the mechanical ventilation – it is of key importance to use demand controlled air inlet and exhaust.


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  2. Good post Luke ! I cannot see how a Passive House could operate without a MHRV system as due to its airtightness there would be problems with "stale" air within the house. Also if trickle vents are installed in each room there would be a problem with thermal bridging, resulting in heatloss. You state that the passive house standard heating energy demand can be approached without mechanical ventilation and heat recovery if the air change rate does not exceed that of the mechanical ventilation, do you think this would be possible in a house dotted with trickle vents ?

  3. Interesting post Luke. in relation to your comment that many users dont want to look after a "machine". I can see where some people would be aprehensive about having a ventilation system rpovide the heating, a phenomenon people in ireland especially are not used to. Most peoploe have no experience of mechanical ventilation systems. However it would have to be saif that the operation and maintenance is no different to a traditional oil or gas boiler and it could be argued that these machines offer greater risks to health through improper maintenance. Also as the passive house does not need a traditional heating system (boiler, radiators etc) the heat recovery ventilation system doent really cost much more. However I agree that peoples initial reaction will be to shy away from it.

  4. Hi Luke
    Your blog is very interesting. In relation to your comment that “many people do not wish to take care of a “machine”, to check and change regularly the filters, they consider questionable the indoor air quality and the noise”. I would have to agree with you, but I feel that the biggest problem with the maintenance is the owners forgetting to maintain them, for example replacing filters. Maybe a system could be put in place that the company that installs the system will keep in contact with the occupiers to remind them to change filters and perform maintenance when required? This could be done simply through an email of text message

  5. Colman that is a good idea about receiving an email when the filter is due to be changed. I think it will take time but the perception that the MHRV system is hard to operate will change, once people begin to see that they can be easily operated and that the settings are set properly from the outset that the system will run smoothly without any hassle. As well as this changing the filters will become just like a daily chore in the house once the filter is easy accessible. In relation to the trickle vents this will surly cause problems with the thermal bridging.