re: natural ventilation
29 apr 2002
>i moved to snowy new england and am looking into what to do for more
>efective heating & cooling the whole house. i am thinking about
>setting up a natural ventilation system between floors. creating an
>open stair from the basement to the second floor, and adding open
>holed vents between floors. creating large sliding doors in the
>stiarwell and vent covers which would be used during the winter.
you might add on an inexpensive plastic film low-thermal-mass sunspace
>any of you frugal homeowners have an opinion? the ac guy says this
>won't really work -- something about unbalanced air pressure, heat
>rising, etc. is he wrong?
yes. sounds like a typical hvac criminal.
>is this a good way of cooling?
i think so.
i'm working on a smart whole house fan controller for a homeowner now.
it will turn on the fan vs ac (or furnace) when the outdoor temp is
cooler (or warmer) than the indoor temp and the outdoor humidity is low.
nrel says the average daily minimum temp in phila is 67.2 f in july, vs
59.2 in burlington, vt, 56.5 in concord, nh, and 54.5 in caribou, me.
we might not need much ac at all near phila, with smart nocturnal
ventilation. large buildings have "enthalpy economizer controllers"
made by honeywell and others, but many hvac people consider this
too complex and expensive for houses.
in "smart controls for whole house fans" (at the 10/4-7/01 arizona state
u cooling frontiers symposium), pablo la roche and murray milne of ucla
described experiments with a microcomputer-controlled thermostat with indoor
and outdoor temp sensors that can control a whole house fan as well as a
furnace and air conditioner. they said no such thermostat is currently
available commercially. they wrote:
...the greatest single source of free cooling energy for homeowners is
outdoor air. most homes are not designed to effectively "harvest" this
resource, and most homeowners do not understand how to manage or control
it. proof of this is the common situation where a home's air conditioner
is running when the outdoor air temperature is quite comfortable.
they were more concerned with cooling than heating, and didn't measure or
control humidity. they tested 4'x8'x8' tall side-by-side cells containing
thermal mass with various night ventilation strategies.
say our phila family has a 1988 3.5 ton lennox ac, a 4k cfm whole house
fan, and forced-air gas heating, and wants to reduce the electric bill
below the 600 kwh/mo baseline, and they've already replaced all the lights
with cfs and keep the house 65 f in the winter and 75 in summertime. how
about a smart "economizer control" for cooling and heating?
the fan might draw air from basement windows equipped with plastic film
one-way dampers (ie sensitive barometric dampers) that only allow air to
enter the house. warm air rises, so we might leave the fan louvers open to
allow natural stack effect cooling and lower fan speed and noise. humid air
also rises, but it may diffuse down into the house through open louvers,
so they might need a $50 2 watt damper motor, especially in wintertime.
what are the potential savings, and if they are significant, why don't
people do this more? allergies, maybe, but we have good filters for that.
or humidity problems. a tmy2 simulation might answer the first question.
houses with shorter internal and longer external time constants and wider
acceptable internal temp ranges could save more.
what's a good controller for this, and how can it rule the ac and heating
systems? la roche and milne used a $300 tattletale tfx-11 data logger-
controller from onset computer corp (the hobo people at 1-800-loggers)
containing 68hc11 and pic 16lc62a processors, 19 analog channels, a tfbasic
language, 128k of ram, a hardware real time clock, a 472k flash eeprom,
and a serial port to a pc. thermistor accuracy was +/-0.9 f at 70 f over
50' wires. onset also sells humidity sensors. we might put a clear plastic
box around a digital heating and cooling thermostat and turn on a 24v bulb
inside to enable the ac and disable the furnace.
in cooling mode, it might work like this (in courier font):
ti to |fan ac
<73 | - | - | -
>75 | <70 | on | -
>75 | >70 | - | on
it would also be nice to avoid making the house too humid:
ti rhi to rho |fan ac
<73 | - | - | - | - | -
>75 | <60 | <70 | - | on | -
>75 | >60 | <70 | low | on | - "low" means outdoor air will reduce
>75 | >60 | <70 | high| - | on the indoor rh. "high" will not...
>75 | >60 | >70 | - | - | on
adding a heating function:
tm rhi to rho |fan ac furn notes
<65 | - |67, last resort
<68 | <60 |>tm+5| - | on | - | - | until tm>70, limit ti<75
<68 | >60 |>tm+5| low | on | - | - | until tm>70, limit ti<75
<68 | >60 | - | high| - | - | on | until rhi<55, dehum need
>72 | >60 | - | high| - | on | - | until rhi<55, dehum need
>72 | <60 |65
>72 | >60 |65
>75 | >60 |>tm-5| - | - | on | - | until tm<73, last resort
in this scheme, tm is the inside wall (thermal mass) temp, which we try
to herd in the direction of 70 f from above and below with ventilation
while maintaining 65 f as the lower comfort limit for room air and 75 f
as the upper. (these limits would be programmable by the occupants.)
as a last resort, we use the furnace or ac to nudge the house wall temp
2 f back up or down.
this truth table/state diagram might be enhanced a) to favor opening
2 watt dampers before turning on 300 w fans (before 4 kw acs), and
b) to keep the tm average temp higher in winter than summer, with
ventilation, if we can tell whether we are in "winter" or "summer,"
eg whether the average outdoor temp has been more or less than 70 f
for the last 48 hours. for instance, we might try to use ventilation
to make the average tm = 70+(70-tab)/8, where tab is the average ta
over the last 48 hours, so tm = 75 f if tab = 30 and 66 if tab = 100,
which might make better use of the house thermal mass for heating
and cooling in winter and summer.
another enhancement c) might turn on an indoor fountain in a hot and
dry house in phoenix. what's the optimal fountain control algorithm,
combined with these other functions? would the performance exceed
a conventional swamp cooler?