re: new mobile with hydronic heat??
22 feb 1999
>please further explain what your talking about.
to me, this sounds like a home that gets heat from plastic pipes
in the floor that have warm water running through them. that kind
of warm-floor heat is said to be very comfortable.
it might be done in another way: put a mobile home running east-west
inside a commercial plastic film greenhouse (say a 30' wide x 100' kit
that costs about $2,000 and takes 3 people 1 day to set up.) most of
the greenhouse floor might be covered with playground mulch over black
plastic film on the ground. the home might have a strawbale skirt.
cloudy polyethylene greenhouse film with uv protection additives comes
in folded rolls up to 40' wide, costs about 5 cents per square foot,
and has a 4-year guarantee. it might last 10 years if covered with
greenhouse shadecloth in the summer. two people can replace the film
in an hour or two on a calm day. bayer's new urethane film is clearer
and costs about 35 cents per square foot, with a 10 year guarantee.
a 14x72x8' tall us r11 home with 100ft^2 of r2 windows and an air
infiltration rate of 0.5 air changes per hour has a heat conductance
of 14'x72'/r11 = 92 btu/h-f for the ceiling, 50 for the windows, 125
for the walls, and 0.5x14x72x8/55 = 73 btu/h-f for air leaks, a total of
340 btu/h-f, so keeping it warm for 24 hours on an average 30 f day
where i live takes 24h(70f-30f)340btu/h-f = 327k btu. keeping it warm
for 18 hours takes 245k btu.
we might use a couple of fans to collect some solar-warmed air from
the greenhouse on an average day and store some heat in some sealed
containers of water inside the skirt to keep the home warm at night.
if we let the temperature drop say, 10 f from dusk to dawn, we need
a thermal capacitance c = 245k/10 = 24,500 btu/f, eg 24,500 pounds
of water, eg 54 55 gallon drums full of water.
january is the most difficult month for solar house heating where
i live near philadelphia. the average temperature is 30.4 f according
to the national renewable energy laboratory (nrel.) on an average day,
1,000 btu of sun falls on a square foot of south wall and 620 falls
on a horizontal surface, with a standard deviation of 42.
the south wall of the greenhouse might collect about 15x100'x1000x0.9
= 1350k btu of sun on an average january day (more if it has a frozen
pond to the south.) the south half of the roof might collect another
837k, a total of 2187k btu, about 7 times more than the home needs
to stay warm for a day.
over a 6 hour solar collection day with no ventilation or heat storage,
the 2,350 ft^2 of r0.8 greenhouse plastic film wall and roof would
lose 6h(t-30)2350/r0.8 = 2187k btu, making the interior temperature
t = 154 f, but the poly film ir loss limits the max to about 100 f,
and we might make it 80 f by using some shadecloth and venting.
collecting 245k btu of heat over 6 hours, ie 41k btu per hour from warm
air with a 10 f temperature difference means moving about 4100 cfm,
which might come from a couple of $26 grainger 4ch71 20" box fans used
to exhaust air from the south side of the skirt, with a duct at the
back to bring down warm air from the top of the greenhouse. these fans
can each move 2100 cfm at 87 watts on low speed. some one-way plastic
film dampers might prevent reverse airflow at night. with an uninsulated
floor, 327k/24h = 13.6k btu/h of heat might flow with a skirt/home air
temperature difference of 13.6k/(14'x72')/r1 = 13.5 f. a few holes in
the floor with small fans or $11 automatic foundation vent registers
with bimetallic springs could reduce this temperature difference and
better regulate home air temperature.
the home needs 5x345k = 1725k btu of heat to stay warm for 5 cloudy 30 f
days in a row. this might come from some more water cooling from 120 f
on an average winter day to a minimum usable temperature of 80 f, with
c = 1725k/(120f-80f) = 43k btu/f, another 96 drums in a us r50 strawbale
solar closet off one end of the home. the closet might be 14' deep x 15'
tall x 16' long. only 8' of the length need be filled with drums stacked
4 high in a 4x6 array. the rest of the space might be a sauna, a place
to dry clothes, etc.
the strawbale south wall of the closet might be covered with a layer of
polycarbonate plastic over some dark window screen with an air gap to
collect about 15x16x0.9x0.9x1000 = 194k btu of sun on an average january
day, with an average amount of sun (or more, if there's a reflective
surface to the south.) on an average day, it would provide no heat for
the home. it would just store heat for a few cloudy days in a row. on
an average january day, we'd have
194k btu = 6h(t-80f)15'x16'/r1 for the south wall during the day
+ 18h(t-30f)15'x16'/r51 for the south wall at night
+ 6h(t-80f)1124ft^2/r50 for the rest, during the day
+ 18h(t-30f)1124ft^2/r50 for the rest, at night
= 1795t - 139k,
so t = (194k+139k)/1795 = 186 f.
again, ir reradiation limits the max to something closer to 120 f.
on a cloudy day, another exhaust fan might move room air from under
the skirt into the bottom of the closet, and closet-warmed air might
flow along the length of the roof of the home through a duct and down
down into the home and back through the floor vents into the crawlspace.
the greenhouse might also be used to grow plants, and it might have
an in-ground pond on the north side to store rainwater (an average of
about 240 gallons a day in pennsylvania.) part of the space under the
home might be a sawdust "compost furnace" that provides more backup
heat and recycles water and takes care of sewage treatment. the top of
the home might have an insulated water tank connected with a warm water
convection loop to something like zomeworks big fins or some pvs with
hydronic heat sinks below for solar water heating.
it often seems to me that heating bills matter more to people who live
in mobile homes than people who live in million dollar homes, and they
might better appreciate having more floorspace, and they might have
fewer rigid ideas about what houses ought to look like.