forced air radiant floor heat?

 

I've been mulling over an idea for the past few weeks and thought I'd
run it past the group before I dive in.

A little background on my house:
I have a standard gas-fired central heating furnace with hvac ducts
throughout the house. It's a two-story home with five ducts gong to the
first floor and only one shared duct going to the second floor (I'm
sure it was an afterthought). The house is very old (built in 1935) and
was built without adding insulation to the walls. Also, the windows are
all the old counter-weight style wooden frame single-pane windows.

Anyhow, I am planning on having insulation blown into the outer walls,
adding a ridge vent to the roof line, re-insulating the attic and
replacing all of the windows. I was considering installing radiant
floor heating, but aside from the complications of a less than
desirable situation in the basement (pre-existing plumbing, electrical,
additional supports, etc) there is also the cost to consider. So this
has lead me to the idea of a forced air radiant floor heating system.

The idea is to add a hollow sub-floor to the existing floors in the
house enabling it to act as an extended heater duct. The steps seem
simple enough:

1. Pull up the pre-existing floor grate in each room and outline a duct
path.
2. Raise doors to clear new subfloor (or cut off door bottoms).
3. Install subfloor supports in the shape of the duct paths (using
2x2's).
4. Use a thinsulate material to cover the inside of the new duct paths.
5. Cover the thinsulate with aluminum flashing to direct the radiated
heat upwards.
6. Lay new subfloor over duct paths.
7. Cut a new floor grate hole and re-install floor grate at the end of
the path.
8. Lay new finish floor.

Here are links to some diagrams that I put together to help convey the
idea:
http://www.tobymiller.com/dump/figure1.gif
http://www.tobymiller.com/dump/figure2.gif
http://www.tobymiller.com/dump/figure3.gif

So the whole theory is that the forced air is being pushed up through
the floor vents into the room to heat the space anyhow. Why not use
that same forced air to heat the floor on the way to the room above?

I've already thought of a few issues, but I haven't done the research
to prove or disprove them yet. Any insight would be appreciated.

Possible Issues:
1. Air won't travel far enough to actually reach room.
2. Stronger furnace fan might be necessary.
3. Thermostat may not read true with this modification.

I'm sure there's more that I'm not thinking of ... that's why I'm
posting here before I get started. =)

Any thoughts?

-tm

 

wrote:


One MAJOR issue - you will not have a high enough air temperature to
radiate any appreciable heat into the room.

The floor will absorb what heat is contained in the air - making it
slightly warmer (OK, considerably warmer where the air comes in, and
virtually no warmer at the end of the duct) but will not heat the room
much at all.

*** Posted via a free Usenet account from http://www.teranews.com  ***

 

<clarence at snyder dot on dot ca> wrote:


I disagree. Got numbers?

Nick

 

nicksanspam@ece.villanova.edu wrote:


I don't, but I would disagree on whether low temperature is relevant, too.
We do this by pumping solar-warmed hot air through our concrete slab floor,
but the reason we need a good concrete slab is to store heat through cloudy
periods and night-times.  When you've got a constant heat source, it
shouldn't matter.  If you accomplish nothing more than making the floors
feel warm, occupants will _feel_ warmer.
--
derek

 

I totally agree about the "will _feel_ warmer" comment. I've visited
homes with radiant floor heating before and it really makes a
difference feeling warmth underfoot versus the same temperature
achieved through forced air. I am really hoping for a happy medium
between the two.

The spacing I'm shooting for will be fairly narrow. If I make the
supports out of 2x2's and all of the channels 24" on center then I have
a space of 1-3/4" x 22-1/4". Add some "WHITE Double Bubble Foil
Insulation", which is 5/16" thick, to the bottom and sides of those
channels and you now have 1-7/16" x 21-5/8".

So I guess I could figure out all of my channels and from that I would
know the cubic inches that each duct would be required to travel before
reaching the room above. What I don't have are the formulas to figure
out how to use that information to decide either how much force to add
to the furnace (via a more powerful fan blower) or how much heat to add
to the furnace via a higher btu unit or how much pull to add to the
ducts (via small computer fans wired back to the furnace relay). Any
suggestions on this next part of the puzzle?

-tm