[Strawbale] (not) Re: Blower Door test set-up

[Rob Tom]

On Mon, 09 Apr 2007 17:25:00 -0400, Mark Bigland-Pritchard  
<mark at lowenergydesign.com> wrote:

>  For getting reasonably accurate predictive
> numbers for heat loss, I stick with my description of problematic.  What
> factor do you divide the 50Pa ach value by to get the "actual" average?
> - it depends on a number of factors which generally cannot be adequately
> accounted for in the best thermal modelling suites let alone in a simple
> assessment tool (like HOT2000, or the British SAP procedure, for
> example).  Having a maybe 30% error in your air exchange heat loss can
> make a big difference to design of a near-net-zero dwelling.  And it's
> not insignificant even for say a standard sb building in mid-Canada when
> you come to assessing the financial viability of HRVs or solar preheat,
> maybe even for sizing the heating system.

Sorry Mark, I'm confused. (It seems to happen a lot to me these days, so  
nothing new there).

Blower door test ACH50 numbers are something that are acquired after the  
house is built.

Predictive heat loss numbers (as a result of air changes whether natural  
or forced) for the purposes of design (ie at the design stage where the  
house is not yet existing) would, as a minimum, be based on the minimum  
air changes as prescribed by ASHRAE or if one felt that the  
ASHRAE-prescribed air change rate was too low to be healthy, a  
mechanically-induced higher target rate.

Once the house is built, testing would confirm that the house is either  
tight enough or not tight enough to perform as per the design specs.

ie No house that is built to be air-tight should have a natural leakage  
rate that is anywhere close to being as high as the ASHRAE minimum air  
change rate.

And any house that aspires to be near net-zero energy would have to be  
built air-tight since it would necessarily be superinsulated and if  
super-insulated would need to be air-tight or else it would experience  
moisture problems and/or rot in the envelope materials. (Or least that  
would be the case for a house built in Canada. Maybe not for one built in  
the desert.)

Given the above, I don't follow how there could be the possibility of a  
30% error in heat loss due to air exchanges (forced or natural) nor any  
major errors in sizing of a heating system.

In all likelihood, I think that the Code-required automated heating system  
would be oversized simply because the units avaialable on the market are  
designed/made for conventional homes with conventional heat load  
requirements and the capacity of even the smallest unit made will likely  
be more than is required (assuming the SB house is designed & built to be  
an energy-efficient/low-energy structure.

So more often than not what one ends up doing is installing an electric  
furnace or electric baseboard units (to satisfy (Canadian) Code  
requirements, the easiest/simplest/cheapest route) and which seldom get  
used if at all and a woodstove is fired up for those occasions when  
auxilliary heat is necessary.

Fortunately, they're relatively cheap (ie big toasters),easy to install  
and take up minimal space. (ie the space under the stairs usually will  
suffice for a whole-house capacity electric furnace and the ductwork would  
be there anyway for the ventilation system that is necessary to ensure the  
delivery of fresh air to every liveable space (again required for Canadian  
homes, don't know about elsewhere. Perhaps in David's Texan (?) locale, a  
exhaust-only/passive inlet ventilation system is feasible and permitted.)

If anything, tools like HOT2000 (don't know anything about the Brit SAP)  
are conservative so if there is any error, it is typically on the safe  
side ("safe" from a homeowner PoV). ie The actual house performance will  
be better than predicted if it was built to be as tight or tighter than  
designed (as confirmed by testing.)

I just noticed that there is no "question mark" (in quotes so as not to be  
confused with Mark, who is not a question) anywhere in the above but this  
whole posting  should read as one big "question mark".

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
winnow the chaff from my edress in your reply