re: advice needed for cooling residential hydronic system...
29 jun 2001
al youngwerth wrote:
>> >...the worst case cooling load is about 45k btu/hr but we only need
>> >to cool for about 8-10 hours during the day (if we hit 102 in late
>> >afternoon, we're typically 65 by morning).
the large (32 hour?) time constant can work in your favor. you may be
overestimating your cooling need, with an average 74 f july temp...
>i used a program called hv-calc to do this. hv-calc performs a
>manual-j calcultaion based upon parameters you put in about the house.
including heat gain from sun into windows and electrical usage? have
you considered reducing electrical usage with compact fluorescent lights
and more efficient appliances?
>> sounds like the thermal conductance is about 45k/(102-70) = 1400 btu/h-f,
>> (sounds highish) with a 42k/1400 = 32 hour time constant...
>@ 100 degrees: 38826/(100-75) = 1553
>@ 94 degrees: 36067/(94-75) = 1898
looks like it might include solar and electrical heat gains.
>something i didn't mention, this house has almost 800sqft of windows
>(although all eves are 3' and 85% of window area faces south-east).
you might halve that, or build an insulated wall to the north, ie put
the windows over a sunspace vs 24-hour living space for more efficient
heating and cooling. nrel says a 6' tall window in boise wants a 3.8'
overhang 2' above it for min summer heat gain and max winter heat gain.
>> the dew point is about 43 f. with a floor-to-air thermal conductance
>> of 4700x1.5 = 7050 btu/h-f, you only need a 45k/7050 = 6.4 f temp
>> difference for 45k btu/h of cooling.
>> >...i know someone here in boise that has a similar setup but he
>> >is just using a whole house fan to cool his house in the mornings.
works well with lots of thermal mass and thermal mass surface and
insulation and nighttime airflow...
>> >he's happy with it but his house will get to 82 degrees after it's
>> >been 100+ degrees for several days. i want to stay cooler than that.
the cool water temp and tank size would determine the number of 100+
days in a row, which is likely to be a small number, with a 74 f long
term average and 90.2 f daily max. you might estimate the probability
of n 100+ days in a row using a gaussian model (a normal distribution),
or calculate them with nrel's boise tmy2 or 30-year hourly weather data.
in the rare event that you run out of "natural cooling," ie the tank
water rises to 69 f, you could use a small gshp to cool the slab while
warming the tank water.
>> looks like you might keep your house 63 f on an average july day.
>how? i presume you are implying by cooling the floors with water?
yes. keeping the house 70 f or 75 f would make the tank water cooler.
>> how about a reflective concentrating solar trough attic for heating?
>> you can collect about 400 btu of 180 f water per square foot of solar
>> aperture on an average december day and store the heat in a large
>> insulated tank in the house.
>400btu/hour or 400btu/day?
about 400 btu per square foot of aperture per day. nrel says a 1-axis
ew concentrator can collect at most 507 btu/ft^2-day on an average
december day. you might lose 10% through a steep south attic glazing
and another 10% with 90% reflective mylar under the parabolic north
attic roof, with a little more thermal loss from the small hot target
trough to the warm attic for an average of about 2 hours per day.
>i'm starting to put more thought into this option, the problem is
>where to put it without affecting the aesthetics of the house too
>much. the problem is the front of the house faces southeast and my
>wife would never allow me to put a solar collector where it would be
>easily visible from the front of the house.
cherchez la femme.
you might make the attic the collector. the single layer of dynaglas
corrugated polycarbonate over my 32'x16' tall south roof looks a lot
like the north tin roof, except when i turn on a light in the attic
at night. your 4700 ft^2 house might have a 32'x72' roof with a 16'x72'
tall south half with 16x72 = 1152 ft^2 of solar aperture, so you could
collect an average of 461k btu/day of solar heat in december, which
sounds like enough for plenty of hot water and space heating.
>> >...what i was hoping to find is some sort of relatively inexpensive
>> >solution for cooling the water. an evaporator that cooled water
>> >instead of air.
>> looks like trickling water over the roof or a wall at night would work.
>do you mean just trickling my city water over the walls and/or roof
>to provide evaporative cooling directly to the walls/roof?
no. trickle water from a ridge pipe with holes over the north roof (and
maybe the south roof, too) at night and collect it in a gutter and store
the cool water in a large tank inside the house. something like a $400
84"diamx60" tall 1500 gallon ag poly ag tank, or a 4'x8'x8' plywood box
lined with epdm rubber.
or make a shallow outdoor reflecting pool with a large tank (or insulated
swimming pool :-) below it and a parabolic reflector over it. but that
costs more, if you already need an attic. it would be nice to use the
same tank for heating and cooling and put it inside the house to reduce
the heat losses and gains and make those losses and gains contribute to
the house heating and cooling.
>another option you made me think of is running some of the city water
>we'll use for irrigation through the radiant floors. i've got 2400' of
>1/2" pex in the main floor. if the city water is 60 degrees in the
>middle of summer, how much water will i need to run through the floor
>to remove say 35k btu/h?
we figured the floor might be 6 f cooler than the house air, which lets
the water warm from 60 to 69 f... 35k = gpmx8x60(69-60) makes 8 gpm, vs
0.07 gpm (35 pounds per hour) with evaporation. better to recool most
of that water at night...
>i also found a company that makes reverse cycle chillers (an
>air-to-water heatpump), www.aquachillers.com. although in heating mode
>they are not as efficient as gshp (they claim 2.0 cop at 0 degrees
>farenheit), in cooling mode they are 13.5 seer, not that far off from
>a gshp. interestingly enough, with gas in boise @ $.78/therm and
>electricty @ $.0529 i calculated a 2.0 cop ashp is still 10% cheaper
>to operate than a 95% efficiency gas boiler.
but not as cheap as evaporation with a small pump and a 10,000 cop :-)
>some of the other responses are implying the use of a cistern to store
>cool/hot water. it seems they are implying the use of an air-cooled
>radiator, pumping warm water collected from the house during the day
>through the radiator in the cool morning.
just recirculate some cool tank water through the floor. you might use
a filter to avoid clogging the pex.
>i'd love to be able to create a highly-efficient relatively inexpensive
>system that could provide most if not all of my heating and cooling...
sounds good to me :-)