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Concrete slab desireable for radiant flooring?

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Posted by Iain McClatchie on November 4, 2005, 10:32 pm
 
We're putting in radiant floors in the new house.  The house does not
have a slab foundation.  We have two choices of radiant construction
(that I know about):

A) Floor is plywood over joist.  PEX tube laid over plywood.  1-2" of
lightweight concrete over that, and flooring installed above that.  Our
flooring will be a mix of hardwood, slate in places, and carpet.

B) Floor is plywood over joist.  Flooring is conventionally installed
over that.  PEX tubes are stapled to the bottom of the plywood from
below, partially covered by malleable aluminum heat spreaders.

I was talking with a radiant engineer who suggested #1.  His claim was
that it was better for two reasons: increased thermal mass in the
building, which evens out temperature swings, and improved lateral and
vertical heat flow.  My principle objection to concrete is just that it
feels somewhat worse underfoot, but I don't want to be dumb about it.

I'm not sure why I would care about thermal mass in the floors.  I
don't think I want more than a few degrees of temperature swing inside
the building envelope, so even a 2 inch slab over all 4000 ft^2 is
going to store around 90000 BTU, which is 10-20% of what the house
loses on a cold winter day.  Aren't the thermostats going to even out
temperature swings?

Heat flow in concrete is 1.8 W/m-C, versus 0.1 W/m-C for wood
(http://www.stanford.edu/~eboyden3/constants.html ).  So if I want 10
BTU/hr/ft^2, it'll take 9.3 degrees C of drop from aluminum to
underside of carpet for wood, and few degrees for concrete.
That doesn't seem like a big difference.  Am I missing something?

I'm also thinking of running radiant loops up the bottom portion of the
walls.  I'm hoping this will help ameliorate cold spots near the
windows -- it's the same strategy people use for placing radiators.


Posted by Q on November 4, 2005, 10:57 pm
 


I think you might have to use both for practical reasons.  You would want
to use the embedded PEX in concrete method for the slate or ceramic tile
areas where you want to have little heat/humidity expansion/contraction
for the flooring substrate. Consider fibercrete (fiberglass strands in
concrete) for added strength and resistance to cracking.  More pourable
than rebar  For the hardwood / carpet areas you want to keep the  tubing
as far from the inevitable nails or carpet tacks as you can.  Most
important in both methods is the use of the reflective radiant barrier
and insulation unless you want to heat what's under the floor too.

Have you considered also using a hot water heat coil in in your air
handler unit like an Appolo or similar model?  This will be about 1/3 the
cost of the slab hydronics equipment.  I don't know where your location
is to judge the applicablility to your climate.

Mark
Gulf Coast Solar, Inc.
www.gulfcoastsolarinc.com

Posted by Iain McClatchie on November 5, 2005, 3:45 am
 Mark> I think you might have to use both for practical reasons.
Mark> You would want to use the embedded PEX in concrete
Mark> method for the slate or ceramic tile areas

Oh great point!  That'll work nicely since the areas with tile
are downstairs on the south side where they get winter sun.

We visited one of our architect's houses under construction,
and noticed that his *outdoor* tile floors were a little "bouncy".
Yow!

Mark> Have you considered also using a hot water heat coil
Mark> in in your air handler unit like an Appolo or similar
Mark> model?

I haven't heard of Appolo.  Can't remember the name of the
units we are looking at.  We're using them for displacement
ventilation.  I'm currently confused about how a fan coil
regulates the output air temperature when the water from the
solar thermal tank varies in temp between 100 F and 170 F.

Any and all suggestions appreciated.


Posted by Q on November 6, 2005, 3:11 am
 

The fan coil is supplied from your conventional water heater, thereby
eliminating the cost of a furnace / heat pump, etc... The conventional
water heater is supplied by solar-heated tank storage.  The circulating
loop goes from the conventional water heater to the fan coil and back to
the solar tanks.  This system also supplies your domestic hot water.  By
using the solar heat for most of the hot water thermal value you can
minimize the times you need to depend on fossil fuels or electricity for
home heating.  The extra solar collectors you put on the roof to supply
the home heat ensure that you won't be paying anything for hot water
during the spring/summer/fall.  Go ahead and put in a hot tub!

Here is a link to the Apollo equipment.  Several other manufacturers fill
this niche also.

www.apollohydroheat.com

Mark
Gulf Coast Solar, Inc.
www.gulfcoastsolarinc.com

Posted by Iain McClatchie on November 6, 2005, 5:50 am
 Iain> I'm currently confused about how a fan coil regulates the
Iain> output air temperature when the water from the solar
Iain> thermal tank varies in temp between 100 F and 170 F.

Mark> The fan coil is supplied from your conventional water
Mark> heater[....]  The conventional water heater is supplied
Mark> by solar-heated tank storage.

Okay, the fan coil sees a constant-temperature water input.
The implication is that as the solar tank gets colder, the
water heater has to burn more gas.  That's not great if you're
shooting for a high solar fraction.

Do you know of any applications that use a passive,
thermostatically constrolled "mixing" valve to regulate the
temp going to the fan coil to something lower than the
nominal temp from the solar tank?  I'm thinking:

  mix valve--->pump----->fan coil--\
  ^     |                          |
  |     |                          |
 hot  cold-------------------------|
  |                                |
  \------------solar tank----------/

I understand these mixing valves are used/required at the
output of hot water heaters to prevent the distributed water
from reaching scalding temperatures.

Mark> Go ahead and put in a hot tub!

Oh yeah... and the tub and shower drains are going into
the greywater system (http://www.rewater.com ), so no
guilt associated with long showers either.


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