[Greenbuilding] Passivhaus propaganda

[Dan Johnson]

On Thu, Dec 5, 2013 at 7:04 AM, nick pine <nick at early.com> wrote:

> Does the PHPP only allow direct gain solar heat? Is it capable of
> evaluating other designs, with 2 thermal masses at different temperatures?
> Can it even do the equivalent of a single-zone HEED or Energy-10 simulation?
>

You nailed it, PHPP only computes direct gain heating for a single zone.
PHPP is an Excel worksheet that calculates heat balance per EN 13790
Monthly Method and Annual Method. Not an hourly simulation; that is, only
12 timesteps per year, using monthly average climate data. The idea in the
late 1990s was to make this a quick compliance worksheet that would get a
designer in the ballpark and certify a project without requiring a dynamic
hourly simulation. (These days it's often faster to input and run a dynamic
simulation.)

The worksheet also calculates a design heating load, a simple Q=UAdT
including ventilation and infiltration. But here was the important
contribution in the 1990s: PHI claims that Passive Houses [built in
Germany] have at least a 72-hr time constant, so a full 24-hr avg outdoor
temp and 24-hr avg solar and internal gain should be considered for the
design conditions*. They look at two design load scenarios, #1 with a
bright 24-hr and cold temperatures, the other #2 with a cloudy 24-hr but
mild temperatures. The greater of the two loads is the Design Load. Most
load calc protocols (eg ASHRAE or Manual J) disregard solar and internal
gains and size equipment based on the coldest 1-hr interval, as though the
building has no thermal storage and the equipment has to meet load every
hour. I'm curious about the particular logic here:

1. Why arbitrarily pick a 24-hr avg design temp rather than an 8-hr, or
28-hr? Wouldn't it depend on the time constant of the actual building being
analyzed? Is 24-hr conservative for an assumed >72-hr building?

2. By designing for a 24-hr avg. outdoor temp, they seem to be implying
either:
A) the calculated "Q" is only the 24-hr average heating power, or
B) when you have a 72-hr building, you can actually run a heater of this
size for 24-hr continuously and be functionally equivalent to an output
that varies with the hourly load.

The best source material I've found is the work of Carsten Bisanz starting
with his Ph.D. thesis, for example:
1. http://www.solites.de/download/literatur/Carsten%20Bisanz_Heizlast.pdf
2. See also literatur listed at the bottom of this page:
http://www.passipedia.de/passipedia_de/grundlagen/bauphysikalische_grundlagen/heizlast

*However some PHs I've calc'd using PHPP in California can have
conventional insulation levels and still comply with Annual Heating Demand
<4.75 kBTU/ft2.yr due to sunny, mild winters. Therefore time constants are
short and 72-hr assumptions in PHPP Design Load worksheets don't hold.
Furthermore we have 30 degF design temp at 5am and 65 degF at noon, with
bright sun starting at 8am. Even conventional buildings only run heaters in
the morning; load is 0 BTU/hr by 11am. Would I run a heater sized for 48
degF all day long? What about all that sun? :-) Incidentally the North
Coast of California is more economical than Germany for PH, due to mild
design temps and dis-proportionally high HDD.

Note Passivhaus is just an energy standard, any style will work.
>
> Yes, but every actual Passiv building seems to have too much insulation
> and airtightness and triple-glazed direct gain windows vs air heaters or
> low-mass sunspaces. Is this a prescriptive requirement or a lack of
> imagination?


I think it's Status Quo; that is, you don't have to adjust the architecture
to include air heaters or overhead mass with a shiny ceiling :-). Just
increase the wall thickness and buy more expensive windows, and simplify
the would-be complicated conventional geometry a bit (see "starchitect") to
pay for the insulation levels. What is a conventional building with no
reconfiguration for passive solar? A direct-gain building.

Beyond the scope of this email: the prescribed PH HVAC scheme seems to be
electric resistance duct-heaters on the output of an HRV, with flow capped
at only ~18 cfm/person per DIN 1946 (sim. to ASHRAE 62.2). Size the
direct-gain envelope to this heating power, thus "heat with a hairdryer."
Low installed HVAC costs, very elegant. Not necessarily low-energy or
economical. Dan Perunko of Balance Point Home Performance in California
pointed out that a hairdryer is 1500W, and if he provides 1500W to a heat
pump at COP3 he can deliver >15,000 BTH to a more lightly insulated house
with no HRV. The bottom line is the same. Best, Dan

Dan Johnson | Design and Energy | 510.325.5672
Assoc. AIA, ASHRAE, LEED AP, CEPE, CPHC | 5500 Kales Ave. Oakland
California 94618
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/greenbuilding_lists.bioenergylists.org/attachments/20131205/ec9d4c67/attachment.html>