[Greenbuilding] Controlling FLoor Heat

[Alan Abrams]

> From: "Alan Abrams" <alan at abramsdesignbuild.com>

> We're trying to figure out if it's "worth it" to do radiant floor
> heat in a 300 sf addition to a forced air heated house.  The
> estimate for the Warm Board grooved subfloor is about
> $2000 just by itself.

Hi Alan;

  I have a general guideline for such additions.
It is that the addition should have no more heat
loss, than the wall that it is replacing.  That way
no additional burden is place on the heating
system, or your heating bills.   Then, it should
also be able to collect enough heat from Solar,
to (on an annual basis) supply its own heat,
plus as much as reasonable, to the rest of the
house. Rather than putting your money into
an inefficient fossil fuel burden, put it into the
addition and house being able to share their
heat and heating loads. The addition should
reduce your heating bills. If it increases
them, it can be designed better.

> Other questions remain--
>
> Should the hydronic heat source be combined with
> the domestic water heater,
> or should it have its own separate water heater?

A high efficiency tank type gas water heater will
usually be about 65-70% efficient.

> Which of the above would work best if supplemented
> by solar thermal panels?

  You are building. Let the addition be a collector.
A domestic water heating system doesn't even collect
enough in winter to heat the water. It sure doesn't have
anything left over for space heating, especially at the
higher temperatures that hydronic floors require.  In
winter (at least in the north east) solar water heating
just heats the water between ground temperature
and maybe room temperature.   If you use smart
integral (passive) strategies, you can do that with
just an uninsulated tank as an ambient preheater.
It will also work in summer.

<<<<<<<<<<<<<<<

Thanks for the feedback, Laren--

A few comments--your points are well taken, however the very first thing I
did when I took on this project was a careful solar analysis, building
computer models using Sketchup, including the house next door and nearby
tree cover.  I took advantage of as much solar gain as possible, with
carefully calibrated overhangs, but can only get a few hours starting around
1:30, without gaining too much heat in the summer.  Windows are also
designed for cross flow, and take into consideration the prevailing wind
direction, and also include my experimental butterfly casement
configuration.  (generally, they would open like this:

 \/                       
----    
to induce a venturi effect, instead of  

 / \  
-----
where each window blocks the other)

There is however good solar exposure on the existing structure to place at
least 64 sf of collectors.

Also, we make a practice of analyzing energy use of existing structures and
strive to not increase loads as we build on.  Here is a case study where we
accumulated enough data to prove we accomplished this goal:

http://abramsdesignbuild.com/lempert.ricci.htm

In the subject case, the wall we are adding to is uninsulated masonry, so it
should be like shooting a fish in a barrel.                  

Still, these folks are convinced that they want a radiant floor.  We
considered doing it over a slab, but that would require removing existing
soils, importing structural fill, and related reinforcement that is hardly
resource efficient.  So we would do a frame floor on a crawl space,
insulated with foam.  An easy, infiltration free r-30 compared to r-10 for
2" of foam under a slab that may wind up feeding some ants.

Your point about max 70% efficient water heaters is compelling.  Given the
owners' prejudice for a radiant floor, it suggests that it would be more
efficient to just run some electric resistance mat on ordinary plywood and
let the owner write the big check to Potomac Electric, and to the devil with
tubing, valves, tanks, and such.

-AA