Tuesday, June 8, 2010

PHPP design process in detail

Why is PHPP more accurate for energy efficient buildings than other tools?

PHPP was systematically developed by aligning the utilization rate function with the results of dynamic simulation models [AkkP 13]. For this development only such models were used as had been validated against monitoring results of built passive houses (see fig. 2 in the left column). By this method was the standard for Passive Houses aligned, as well as a standard for buildings with low, but not as low, energy requirement for heating. However, for such buildings the calculation differs slightly from what is given in the European standard EN 832 (ISO 13 790). But the difference is not important for conventional buildings - it is only of influence for buildings with very long time constants. In this class of buildings the ISO 13 790 tourns out to be a little bit too optimistic.

The results from PHPP-calculation have been repeatedly compared with monitoring results of sufficiently large samples of built Passive Houses (see fig. 4 on the left side). These comparisons have always shown a very good correlation.

The PHPP clearly uses boundary conditions that are significantly different from the calculation process used for the German Energy Conservation Ordinance (EnEV). There are important reasons for this - these are discussed in detail in [Feist 2001] and given in short here:

For internal heat sources in residential buildings using efficient appliances, during the heating season values of some 2.1 W/m² (±0.3) are realistic (and not 5 W/m², as frequently assumed). In the PHPP, there is an additional calculation sheet to determine the internal heat sources of given building projects. However, if internal heat gains are assumed to be higher than realistic, this will result in significantly lower heating energy requirements and may even lead to the illusion that a "zero heating house" can be built with a building envelope of mediocre quality. Practice shows this to be untrue.
The average indoor design temperature in German dwellings can be assumed to be 20°C. This is more realistic than the 19 °C given in the German ordinance. The PHPP user can adjust this indoor design temperature to his or her specifications.
To calculate solar gains it is important to take into account realistic shading factors (the environment, balconies, etc.) and also to account for ever-present dirt and dust on surfaces.
Temperature-correction-factors (F-factors) very often were chosen too optimistically for super-insulated buildings. E.g. for insulated ceilings under uninsulated roofing, the F-factor values are not in the range of 0.8, but nearer to 1.0.
The assumption to add an "additional air exchange rate" due to user-opening of windows is given by EnEV to be 0.15 h-1 for exhaust systems; 0.2 for balanced ventilation systems with heat recovery. Those values are assumed far too high. To be correct, one needs to base values on achieved air-tightness; which means based on actual measured n50-value, as in the PHPP and DIN EN 832 / ISO 13 790.
These and additional topics result in differences in calculation results, which are significant for energy efficient buildings.

More than just an Energy Calculator

The PHPP was not primarily developed just to calculate energy requirement verifications. Much more, the PHPP is a design-tool, which can be used by the architect and the engineers to design and optimize their Passive House project. In the PHPP they will find dimensioning tools for the windows (with attention to optimal comfort), for the heat recovery ventilation system (with attention to good indoor air quality and sufficient relative humidity), for the mechanical systems and for summer comfort. Within PHPP, the building and the mechanical equipment are treated as one overall system.

The PHPP-handbook is not restricted to explaining the use of the spreadsheets and the compilation of the input data. Rather, the handbook gives advice on how to optimize the design (e.g. how to build very air-tight, how to avoid thermal bridges, how to minimize construction costs). All this is very useful during the planning phase and for quality control work as well.

This link leads to the main site of the Introduction to Passive Houses.


  1. This comment has been removed by the author.

  2. The Passive House is not an energy performance standard, but a concept to achive highest thermal comfort conditions on low total costs - this is the correct definition:

    "A Passive House ist a building, for which thermal comfort (ISO 7730) can be achived solely by postheating or postcooling of the fresh air mass, which is required to fullfill sufficient indoor air quality conditions (DIN 1946) - without a need for recirculated air."

    This is a purely functional definition. It does not need any numerical value and it is independend of climate. From this definition it is clear, that the Passive House is not an arbitrary standard enacted by somebody, but a fundamential concept. Passive Houses have not been "invented", but the conditions to use the passive principle has been discovered. One could argue about, whether the noun "Passive House" is adequat to denote this concept. Well - there is no better one. Thermal comfort is delivered in a Passive House by passive measures as far as reasonable (insulation, heat recovery in the temperature gradient, passive utilized solar energy and internal heat loads). To use only passive measures might be possible in some climates - but it will not be reasonable in most of them.

    An even better understanding we get from the following practical considerations:

    1) In airtight houses one always needs a ventilation system (ask the Sweds). All really energy efficient houses have to be airtight. That means, that with the Passive House concept the technical component "ventilation system", which one needs anyhow, will be sufficient to heat (and to cool) the building without additional ducts, hugher duct diameters, additional ventilators,...

    Remark for readers from America: You are used to have air based heating and cooling systems (thats why you call it "air conditioning"). But the systems used in America are almost all just recirculating indoor air at a quite high rate (> 10 ach, but the air is not "changed", it is just recirculated). The system discussed here is something very different: It replaces the indoor air with a very low rate (0.3 to 0.6 ach) with external air to maintain a good indoor air quality. There is no recirculated air. The airflows are much lower, there is almost no noise and no draft at all. Well, the use of such a system might be very similar to the ones you are used to - but quite more comfortable.

    2) This concept makes it possible, to construct buildings with a very efficient heat recovery and to do that cost-effective. This is difficult in other cases, because heat recovery systems form a quite expensive additional investment to the heating system - normally it is difficult to have a reasonable pay-back-time. Therefor it is a good idea to reduce costs of at least one of the two systems: The ventilation or the heating system. If one reduces costs for the ventilation systems by choosing e.g. just an exhaust fan ventilation, then the ventilation heat losses will be quite high and the building will need a conventional heating system - in this case the result could be a low energy house. Or the heating part is simplified in a way, that it can be integrated into the ventilation system - in that case the building will be a passive house.

    The extraordinary low consumptions of passive houses are just a direct consequence of the concept given above. To deliver all the space heating just by heating with fresh air can only work, if the overall heat losses are very low. Therefor the insulation of the building envelope has to be very good - at lest in cold climates. But the same holds for hot climates, if the fresh air supply has to be sufficient for airconditioning.

  3. That notion of heat recover surely appeals to me.
    There are several canadian built heat recovery units available, usually rotating drums with plastic screens, which return warmth to the incoming airstream.
    They tend to be low volume as they are meant for the whole house venting system at 50 CFM or thereabouts.
    This technology makes a lot of sense in cold climates, I would couple it to a Co2 switch to only activate the fan when there is occupancy, or a need.
    My work is building the heat scavenger to extract heat from warm solar heated air into my DHW pre heat tank, a very low cost, low impact, no copper tubing, no differential thermostat, no imported collectors, no fancy aluminum sealed boxes with tempered glass, i have a hollow sheetmetal box much like a hotwater radiator, but air flows up and out by natural thermosyphon, gives up heat to a fin coil heat exchanger, the cooler air returns to the collector if outside temp is below air temp, otherwise air is exhausted through cupola in ridge.

  4. http://www.passivhaustagung.de/Passive_House_E/passivehouse_definition.html