[Greenbuilding] Controlling FLoor Heat

[Laren Corie]

Hello Lawrence;

  Floor sensors have more problems than air sensors,
in a house that has a variety of thermal inputs, such a
direct gain Solar house, which may overheat from the
aun, or go cold from its glass area, and may also
receive enough heat to warm it, from a wood stove.
So, the best way to meassure the living space temp-
-erature is still going to be a wall thermostat. That
is not your problem.  Your problem is that you
are using the wrong wall thermostat.

     Back in the early 1980s, when we were just
beginning to develop the first rubber/plastic hydronic
floor systems, and were the first to use standard water
heaters as our boilers, we faced these problems, too.
I was a consultant in developing the SolarRoll system
which was the first of the EPDM rubber systems.
Those of us who were out there, doing the first instal-
-lations litertally wrote the installation guidelines.What
I and most of the others who involved, found to work
best was to very simply pick a thermostat that had a
hysrtoresis of 1°F or less, instead of the usual 2°F or
more that is preset for forced air systems.  What this
does, is cause the system to shut down much sooner,
(usually less than athird of the time) before the room
overheats, and before the slab is over-charged.
So-called "radiant" floors,especially in an energy
efficient house, express to small of a diferential to
do a huge amount of their heating radiantly. They
are, however, in the perfect location to convective
heating of the space, via it air. If it were really a
radiant heating system, you would be cold setting
on the cfouch, or leaning over a table, out of the
view of the floor.     Basically, in a well insulated
space, the air isgoing to be fairly near the surface
temperature of the floor, when the floor is the
heat source, and warmer than the floor, when
the heat is coming from somwewhere else. So,
use a floor sensor,and you get over-heating.
You problem is not that. Your problem is that
your choice of thermostats, is not shutting the
system off until after it overheats the room.
Get one that has a smaller hystoresis

 I no longer design houses with very large rooms,
that have very high ceilings and large walls of glass,
therefore I see no logic in using radiant floors that
use 65% efficient water heaters and lose 20% of
their heat down into our cold northern ground for
a system efficiency that is luck to break 50%.
I feel that the money is far better spent on super
insulation, and low cost isolated gain Solar, with
effective storage, instead of an overly expensive,
inefficent way to burn fossil fuels. BTW, we used
to do our radiant systems for 50¢/ft², and being
cheap was one of their selling points.

> 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.

-Laren Corie-
Natural Solar Building Design Since 1975
www.LarenCorie.com
www.ThermalAttic.com

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