Posted by Gary on December 13, 2006, 1:38 am
Sounds like the house is pretty well insulated except for the floor.
I'm not clear on whether the roof insulation is 1 3/4 foam or 9+ inches of loose
The 1 3/4 over the walls would give you about R9 just for the foam board
insulation with no thermal bridging -- probably R12 or so with the rest of wall
even if there is no insulation in the stud cavities -- seems pretty good for MS.
Windows also sound good, but thermal shades would help.
It seems to me the crawl space would be worth going after. You might consider
making the crawl space a conditioned space, and insulating the crawl space walls
rather than the floor. You can find some info on converting to a conditioned
crawl space here:
You might also want to read the sealing links.
The "Insulate and Weatherize" book by Harley is excellent.
I assume you are talking about a staple up installation (in the crawl space).
I am in the process right now of adding radiant heat to about 1000 sqft of my
house. I am pulling up the existing floor (which we wanted to get rid of
anyway) and putting down sleepers with PEX tubing and alum heat spreaders
running in the grooves between the sleepers. I have to say its time consuming
and not particularly cheap. I think that doing the staple up from the crawl
space would fall into the same category.
If you are not going to do solar heating, then I would say it is not worth the
effort. If you are going to use solar, then it may be worth the effort -- ask
me in another month after I recover :)
As you know, the radiant floor heating is a good way to make use of solar heated
water, in that it can use low temperatures, and that improves the efficiency of
the solar collection.
The heating coil in the forced air furnace duct will probably take much warmer
water to work. Most forced air furnaces put out 130F ish air because anything
much cooler feels to cool on people due to the moving air thing.
I think that one of the big challenges for making use of heat from solar water
collectors is how to distribute it efficiently and cost effectively to the
house. The radiant floor solution is efficient, but kind of expensive and labor
intensive. Nick has proposed some ideas for other methods -- maybe he would
like to explain them here? Or, others might have some good ideas on this??
If you were doing a new house, a radiant floor concrete slab seems a good way to
go (to me) -- you get thermal storage in a floor you needed to build anyway.
An alternative you might consider if you have the geometry for it is to use some
form of solar air collectors. These are simpler and less expensive to build and
you could just add enough collector area to use the thermal mass of the house
itself as storage. Water collectors, heat storage tank, and radiant floor have
the potential to achieve a higher solar fraction than air collectors without
added storage, but the cost in money and effort is also MUCH higher.
As Jeff said, some air collector stuff here:
For solar, yes -- as explained above.
One option might be to add the heat exchanger AND a 2nd fan that circulates air
a much lower rate. This way the exchanger, might be able to use lower
temperature solar heated water, and not have it feel cold coming out of the
I'd say no.
Radiant heat is said to be more "comfortable", but it seems like a lot of work
for a little more comfort?
I ran simulations for my system, which come out as follows:
240 sqft of collector.
House heat loss is about 450 BTU/hr-F
Percentage of heat from solar (from the 240 sqft of collector) -- about 35%
I live in Bozeman, MT -- 8000 deg-days.
I would guess you would do much better in MS.
My system is described here -- it includes "how-to" on building the collectors
and the storage tank.
You mention sealing -- this would probably be a high payoff thing to work on.
Read some of the stuff at the links above -- you may be surprised where the
major sources of air infiltration are.
In a similar deal, I've heard that our local place that manufactures SIPs has
scraps for free.
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Posted by nicksanspam on December 13, 2006, 2:32 pm
Are the sleepers 2x4s lying flat on a concrete floor, with plywood over that?
No foamboard on the concrete? I can imagine wide flat black PE ag piping
instead of the PEX and heat spreaders. Maybe something like this:
plywood strip ag tubing 1/4" plywood strip ag tubing
1/2" foil-faced foamboard
plywood strip 1/2" plywood strip
----------------------------------------------------- plastic film
The strip layers could be at right angles...
Harry Thomason put a large horizontal cylindrical heat storage tank in
the basement and surrounded it by rocks and let warm air flow up from
the rocks and exit via a slot in an inside wall, near the first floor
ceiling. Floor returns were near outside walls. The rocks had lots of heat
transfer surface and some mass, and the inside wall acted as a radiator
and a thermal chimney to increase thermosyphoning airflow and reinforce
the natural pattern of room air moving down along cool outside walls
and up near warmer inside walls.
We might do something like this with fin-tube pipe or a $00 MagicAire
SHW 2347 duct heat exchanger (about 800 Btu/h-F with 125 F water and
1400 cfm of 68 F air) or an auto radiator near the floor in a closet.
Most fin-tube baseboard radiators have very short thermal chimneys, eg
a 6" tall housing vs an 8' closet. One equation says they don't do well
with small temp diffs. Starting with a measured 690 Btu/h-ft with 1 gpm
of water at an average temp of 200 F and 65 F air, with 690/(200-65)
= 5.1 Btu/h-F-ft, using 100 F water at say, 2 gpm would lower the output
to 690*2^0.04*(9.6865E-04*(Tw-Ta)^1.4172) = 106 Btu/h-ft, ie 106/(100-65)
= 3.0 Btu/h-ft, but that probably doesn't apply in a closet, which might
increase the standard output by 40%.
The 1994 second edition of Passive Solar Energy by Anderson and Wells has
a nice chimney flow formula: cfm = 486Asqrt(H(To-Ti)/(Ti+460)), where Ti
is the lower temp. If To and Ti are unknown but other temps and thermal
resistances are, we can solve this in a 2-loop iteration, starting with
Ti as the mean temp, then estimating the heatflow, then adjusting Ti
based on the estimate.
Then again, concrete is heavy, and it's hard to turn off the heat, and
it's hard to heat floor vs ceiling mass with warm air, and living above
the floor makes for a smaller temp swing and less heat storage.
Water collectors and radiant floors are expensive, but a heat storage tank
can be cheap if it's site-built and shallow, eg a 2'-tall plywood box with
an EPDM liner for a cloudy-day heat store, for a new house with lots of
airtightness and insulation. On an average winter day, we might store and
distribute overnight heat in a closet with a modest amount of mass with lots
of surface. A 1'x1'x8' closet with 200 pounds of water in a Coroplast air-
water heat exchanger (a sandwich with 4 mm water and 8 mm air spacers) might
heat an 8' D-cube with 100 F air from a sunspace or solar siding and cool to
60 by dawn. We could pump tank water up through the closet on cloudy days
and heat the tank with the closet for a short time on sunnier warmer days.
Posted by Solar Flare on December 14, 2006, 12:31 am
I was always afraid of the nail through the top sheet of flooring
puncturing the PEX tubing with the sandwich you have designed. I have
waffled back and forth so many times on my new house design to date.
The last thought I had was to pour an upstairs floor out of concrete
of some kind to protect the tubing, and it looks easier for new
construction compared to stapling up tubing underneath. The other
aspect is the heat temperature requirements of the upper floor and the
basement should be similar then. Underneath wood flooring may require
more temperature than the slab tubing and this concerns me.
Is this black, cheap agri PE pipe good for hot water though?
Posted by nicksanspam on December 14, 2006, 11:36 am
Making all the panels the same and careful marking would help. And making
the floor easily removable. A carpenter nailed the Warmboard PEX in
a friend's Vermont house. The leak was hard to find, but it wasn't
the end of the earth.
You might waffle more about its thermal conductance.
Fin tubes and thermal chimneys come to mind.
At least 130 F, I would guess. I'd keep it filled with water,
vs letting it drain and refill often, which might wear it out.
Posted by Solar Flare on December 15, 2006, 3:07 am
Some type of subflooring still has to be used and usually nailed down
on a fine nailing pattern to do ceramics, vinyl or urethane sheet
goods and/or carpet. It is not likely to nail anything after that is
down...except maybe the forgotten water pipe to the dishwasher etc..
It can be difficult seeing those "nail here" markings on the subfloor
through the underlayment required for most flooring.