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re: erv selection criteria
19 may 2002
larry brasfield   wrote:

>> >the coldest days are when leakage heat losses are at their highest,
>> >tending to vary with the square of inside-outside differential.
>> 
>> the square root.
>
>did you notice i wrote "leakage heat losses"?

no. i was thinking "flow rate."

>...you have to multiply the flow rate by the heat carried
>per unit volume of air.

so the heat loss is the flow rate times the temp diff, ie
cfm = 16.6asqrt(hdt), and btu/h = 16.6asqrt(h)dt^1.5. 
 
>...the flow thru narrow cracks is closer to being proportional to
>pressure than to its square root since the cracks left after doing
>all that (intensive) labor will be more like capillaries than orifices.

laminar flow? maybe you know more about this than i do.

>...the camel's nose scheme depends upon having mostly
>capillary-like vents.)

by "capillary-like," do you mean the flow rate depends on the
pressure, vs "orifice-like" in which the flow rate depends on
the square root of the pressure?

why are "capillaries" needed for an efficient "lung" system? if a fan
makes the flow, why does it matter whether it's proportional to p or
p^0.1 or p^10? 

multiple pinholes would be more efficient than a 2'x2' open window.
what sizes are needed here, and what is the mean size of a house
envelope hole or crack or crevice? what is the standard deviation?

>> >the coldest days are also the ones that will make an erv
>> >useful or not.  days when little air exchange will be driven
>> >by inside/outside temperature difference are not days when an
>> >erv will be better than a fan.

>> it's likely a house will have lots of air leakage on the coldest days.

>what do you believe is added by that comment?

almost all (99.999%?) houses won't need an erv on cold days, when you say
they are most useful, because they will have more than enough natural leaks,
ergo ervs seem useless for all but a very few houses, ie they only provide
a small energy advantage over a ventilation fan. having already agreed that
most houses don't need ervs, we may be about to agree that the fact that
ervs are most useful on cold days in the tiny fraction of houses that need
them makes them even more useless. 

>> >a lot of infiltration control is choosing windows and doors carefully
>> >for leakage. caulking and use of film wrapping are very important too.
>> >i am not ready to declare any of those particularly labor intensive.
 
>> how pompous. you might try talking with people who do this work.
>> it's akin to trying to make a house watertight, like a submarine.

>if you read carefully, you should be able to see that i was
>addressing "infiltration control", not some futile attempt
>to seal a house the way a spaceship would have to be sealed.

a 32x32x16' house contains 16k ft^3 of air... 30 cfm is equivalent to
0.11 air changes per hour. an old house might have 4 ach, an average us
house might have 2 ach, an average new one might have 1, an average new
house with retrofit air sealing and blower door tests might achieve 0.5
ach, and a new "energy star" utility-certified house might have 0.25 ach
(with possible fudging.) a house with 0.11 ach on a cold day is heroic,
a minor miracle. it can be achieved, but it's extraordinarily difficult.
the op described a retrofit project, vs a new house.

>i have seen that work done by others, done some myself, and read about
>it extensively...

me too. i'm no expert, but it took me about 3 days to airseal a 10'x12'
cabin using plastic films and tape and sealants provided by somebody in
the business. (i didn't open the tube of acoustic sealant he called
"black death.") i doubt i achieved 0.1 ach.

>if there is something inaccurate in those comments,
>how about stating what is wrong with them...

imo, you underestimate the effort needed to get 0.1 ach. people who
do this work describe it as detailing, as in detailing a car, or maybe
2 dozen cars. some charge by the square foot. as i recall, 0.1 ach
might add 20% to the price of a new house. sips may be an exception.
some sip builders guarantee 0.2 ach. what would they charge for 0.1?

>> >i think that by the time you get enough of a camel's nose effect from
>> >incidental cracks and crevices to satisfy code mandated air exchange
>> >rates, you've lost the infiltration battle, by quite a margin. 
 
>> would you care to rephrase that in a way that makes more sense?

>i think that if you have enough air volume in all those incidental
>openings to provide enough fresh air to satisfy the 15 cfm per
>occupant standard, then you will have way more {in/ex}filtration
>than is consistent with controlling leakage heat loss.

why do we need "air volume" in these openings? 
we need flow, but why is the volume important?

>> >that camel's nose effect is really just an erv with somewhat
>> >poorer performance.
 
>> would you have any evidence for this article of faith? a few numbers?
 
>...i think the incidental cracks and crevices will vary quite
>a bit in the time that it takes to displace a given fraction of
>their contained air at a given pressure.  this variation will
>cause a large fraction of them to contribute little to ventilation
>or will cause the faster ones to degrade efficiency.

so, iyo, there is a possible problem because these heat exchangers may
be "unbalanced"? but maybe that makes little difference, if they are
"sufficiently balanced" with slow-moving air. how would we quantify this? 
say every 10'x10' envelope section contains a 4x10'x6"x1/32" perimeter
crack, and we want 30 cfm to flow through 4,096 ft^2 of envelope...

>the dual-flow erv can be made arbitrarily efficient (approaching
>the 1.0 limit) by increasing the size of its heat exchanger.  

so can a camel's nose. if you blow through a 3'x1/4" piece of glass
tubing, your breath comes out very close to room temperature, with
some condensation in the first half. the efficiency doesn't drop much
if you blow for a minute and suck for a minute vs blow for an instant
and suck for an instant. after a while, condensation might drip out
the end of the tube vs re-evaporating in the first case, but most
of the winter heat loss is sensible, eg (70f-30f)30 = 1200 btu/h
vs 333 to evaporate 8 pounds of water per day.

>the camel's nose erv, to approach that same efficiency limit,
>has to rely on more capillary-like air paths which, as they
>become better at exchanging heat across low temperature
>differentials, must also be allowing less fresh air to makes
>it way through the system.

i don't understand these words. care to rephrase them? you seem
to be talking theoretically without the sense of proportion that
comes with numbers. 

>if the capillary is completely flushed and beyond,
>so that stale air gets out and fresh air gets in,
>efficiency must be reduced correspondingly.
 
theoretically, but maybe this makes little difference, as in 99.8
vs 99.9% efficiency. then again, a solar house designer may opt to
simply include ventilation air with no heat exchanger as part of
the energy budget, with no backup fuel. solar heat is free...

>another way to look at it is to consider two extremes of temperature
>distribution along the capillary.  if the capillary tends to operate
>at the inside/outside temperature midpoint, (the high flush case),
>efficiency will tend toward 0.5.

why can't a "high flush capillary" have 0.99 effectiveness,
like the glass tubing?

>if the capillary tends toward a constant temperature gradient,
>(the incomplete flush case), it can be very efficient but actual
>air exchange will be limited to diffusion (or mixing) effects.

that may be a better mode. are envelope cracks small in volume all
the way through, or do they admit air to a large stud cavity? 
is this important?
 
>i also see a conflict between capillary materials conductive enough
>to exchange heat in the radial direction and heat loss by conduction
>along the tube, which at the very least tends to create a non-
>linear temperature profile along the tube.
 
sounds like a hard-to-analyze theoretical problem. so what?

>> the fan/partition version might have better performance than any erv,
>> with less hardware.
 
>perhaps you mean "any erv commonly sold".

well sure.

>as for less hardware, by the time you make that fan reversable,
>add the control system that varies turn-around and on times
>appropriately, put an airlock in the middle of the house, and
>carefully control all those cracks to make them uniform enough to
>make that all work, i would say that you have more hardware, not less.
 
fans are cheap compared to ervs, even reversible ones. and electronics
are cheap in volume. we don't need an "airlock" inside the house, just
some sort of partition with doors that are normally closed. it makes
little difference if the partition leaks some air. no energy loss,
just a small loss in effective fan airflow.

a fan can make 50 pa, about 0.2" of water, pushing about 2 ach out of
a house with 0.1 ach of natural air leakage, but we only need 0.1 ach
of fan-forced leakage, so it looks like this might work in a house that
only leaks 0.1/20 = 0.005 ach naturally, ie 1.5 cfm for a 2k ft^2 house. 

>> ornl has done some preliminary experiments.
 
>some experimental results would be interesting
>given my skepticism regarding that approach.
 
talk to them, s'il te plait. they tried this with a very small lung
volume compared to the size of the house or the volume of the stud
cavities, and measured little fresh air exchange. inconclusive, vs
what i'd call your certainty that this is a poor solution. 

>btw, how did you reconcile a 2 degree f temperature
>change over 2 weeks near thanksgiving with feasible 
>thermal mass and 100% solar heating in january? 

bill's window closing served to bias the average concrete temp up 2 f
over 2 weeks, with the usual solar gain and heat loss. that doesn't
require an enormous time constant, nor is it inconsistent with 100%
solar heating in january.

a foot of concrete (about 25 btu/f) inside 12" of styrofoam (r60) makes
rc = 25btu/fx60h-f/btu = 1500 hours (63 days.) phila's deep earth temp
is 54.3 f. if the house were 72 f on day 0, it might reach 68 f when
68 = 54.3+(72-54.3)exp(-t/63), ie after t = 16.1 cloudy days in a row,
assuming no internal heat gain. 

nick, sowing seeds of doubt...




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