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Radiant Floor Heating Questions

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Posted by yack on February 27, 2004, 7:05 pm
 
I have a couple of questions.

First some background . . .

I have a radiant floor heat installation of around 1800 square feet
installed in south central Missouri, USA.  (poured 1.5 inch concrete
over subfloor with tile floor coverings . . . assume R-19 walls, with
vapor barrier, vinyl siding, and R-38 ceilings, thinwall plaster
interior wall and ceiling coverings, and a pretty high window glazing to
wall ratio)  The current controls are thermostatic based upon air
temperature in two zones . . . north and south.  The northern zone is
expected to run more than the southern zone due to solar gain from south
facing window glazing.

The heat source we currently use is a 75 gallon water heater rated at a
heat input rate of around 75,000 BTU and hour (roughly a gallon of
propane per running hour of burn-time.  I have an additional water
heater of the same size and rated capacity that is in series with the
first on that adds additional storage and heating capacity (if it is
ever necessary).  The water heater(s) provide the heat for both the home
DHW and the heating loops.  On a 0-5 degree day one of the water heaters
will run on again off again at a rate of about 15 minutes on and 10
minutes off.  The second water heater only kicks on when the weather is
especially bad . . . -10 degree days with alot of wind . . . etc.

A mixing valve is used to mix up the return water temperature with
heated water from the water heater(s) before being circulated back to
the floors.   We generally run the input temperature to the floor at
around 100-105 degrees F during the winter and 90-95 degrees F during
the more temperate spring and fall.

The comfort of the heat is wonderful.  But, we are looking for
efficiencies in the process due to the amount of fuel that is consumed.

It would make intuitive sense to me to only heat the water when it is
necessary to heat it.  So, we do our best to keep the pumps running as
much as possible with the lowest temperature feed water possible.  The
temperature differential between inlet and outlet is about seven degrees
F on average.

We have listed a number of things that we know we can do to improve the
efficiency of the heating system . . .

More or better draperies to lower losses through the window glazing.
More or better bandboard insulation to cut edge losses of the slab.
More or better insulation of the piping system itself.
More or better insulation around the water heater(s).
More or better insulation under the slab in the floor trusses above tyhe
basement to cut losses to the spaces below.
Adding solar water heating capacity to cut the overall heating cost of
the system.

But, the questions about the system that are most difficult to answer
are . . . will a differential or set-point controller with the mixing
valve really help with the efficiency of the system and overall fuel
use?  I am specifically looking at the more elusive efficiencies that
can be had by smarter/better controls.  Or should I focus my energies on
just cutting slab losses (edges and below) and solar water heating
options.  Would a different heating source be better.  Big water
heaters, that consume alot of fuel may not be necessary.  A smaller
water heater might handle it just fine.  A small boiler with a lower
fuel consumption and higher efficiency might be better.

Any suggestions would be great!

Thanks

b-b-c-@-m-i-s-n.c-o-m

Posted by Bryan Campbell on February 27, 2004, 7:07 pm
 
Sorry about the goofed up sender address.




I have a couple of questions.

First some background . . .

I have a radiant floor heat installation of around 1800 square feet
installed in south central Missouri, USA.  (poured 1.5 inch concrete
over subfloor with tile floor coverings . . . assume R-19 walls, with
vapor barrier, vinyl siding, and R-38 ceilings, thinwall plaster
interior wall and ceiling coverings, and a pretty high window glazing to
wall ratio)  The current controls are thermostatic based upon air
temperature in two zones . . . north and south.  The northern zone is
expected to run more than the southern zone due to solar gain from south
facing window glazing.

The heat source we currently use is a 75 gallon water heater rated at a
heat input rate of around 75,000 BTU and hour (roughly a gallon of
propane per running hour of burn-time.  I have an additional water
heater of the same size and rated capacity that is in series with the
first on that adds additional storage and heating capacity (if it is
ever necessary).  The water heater(s) provide the heat for both the home
DHW and the heating loops.  On a 0-5 degree day one of the water heaters
will run on again off again at a rate of about 15 minutes on and 10
minutes off.  The second water heater only kicks on when the weather is
especially bad . . . -10 degree days with alot of wind . . . etc.

A mixing valve is used to mix up the return water temperature with
heated water from the water heater(s) before being circulated back to
the floors.   We generally run the input temperature to the floor at
around 100-105 degrees F during the winter and 90-95 degrees F during
the more temperate spring and fall.

The comfort of the heat is wonderful.  But, we are looking for
efficiencies in the process due to the amount of fuel that is consumed.

It would make intuitive sense to me to only heat the water when it is
necessary to heat it.  So, we do our best to keep the pumps running as
much as possible with the lowest temperature feed water possible.  The
temperature differential between inlet and outlet is about seven degrees
F on average.

We have listed a number of things that we know we can do to improve the
efficiency of the heating system . . .

More or better draperies to lower losses through the window glazing.
More or better bandboard insulation to cut edge losses of the slab.
More or better insulation of the piping system itself.
More or better insulation around the water heater(s).
More or better insulation under the slab in the floor trusses above tyhe
basement to cut losses to the spaces below.
Adding solar water heating capacity to cut the overall heating cost of
the system.

But, the questions about the system that are most difficult to answer
are . . . will a differential or set-point controller with the mixing
valve really help with the efficiency of the system and overall fuel
use?  I am specifically looking at the more elusive efficiencies that
can be had by smarter/better controls.  Or should I focus my energies on
just cutting slab losses (edges and below) and solar water heating
options.  Would a different heating source be better.  Big water
heaters, that consume alot of fuel may not be necessary.  A smaller
water heater might handle it just fine.  A small boiler with a lower
fuel consumption and higher efficiency might be better.

Any suggestions would be great!

Thanks

b-b-c-@-m-i-s-n.c-o-m

Posted by daestrom on February 27, 2004, 10:07 pm
 

You don't mention if these are naturally vented 'water heaters' or the newer
condensing 'boilers'.  Some of the newer units will actually condense the
moisture out of the exhaust and need a small drain line to carry off the
condensate.  Although a bit more complicated, they have a higher overall
efficiency.  If the heater is in an unused space, definitely insulate it and
all piping in unheated spaces.

As far as smart/better controls saving you fuel, as with all things in life,
'it depends'.  If you're home most of the time and want the house at a
constant temperature, probably not much savings there.  If you can use a set
back thermostat, they often provide significant savings.  But with radiant
floor system, I would think you'd have to be gone for a while (all day) to
realize savings.  The floor system probably would take quite a while to
cool, and quite a while to rewarm.

Check to see how warm it is in the unheated space under the floor.  If you
feel a fair amount of warmth in between the trusses, you may be losing a lot
there.

Although not 'exotic' and 'hi-tech', basic insulation and draft-chaulking is
almost always a good place to start.  And cement foundation slabs or 'cinder
block' can conduct a fair amount of heat away in cold climates such as
yours/mine (central NY).

daestrom



Posted by Ecnerwal on February 28, 2004, 2:59 am
 

So R19 is a very poor estimate for the wall as a whole...

You don't mention any insulation under the floor. This matters.

Sounds like your water heater is perhaps 80% efficient, there are ones
up to 92% efficient, but that's probably _not_ the first place to go.

Boring items like caulking and air-sealing, and then additional
insulation, usually have more dramatic effects, and shorter payback.
Hunt down drafts and fix them.

It is very worthwhile and instructive to actually calculate out the heat
load of various parts of the building. Try to avoid optomistic
assumptions when doing this (ie, aside from all those windows, at
anywhere from r1 to r3, a stud wall with 6" of fiberglass is not
actually R19, due to all the R5 studs short-circuiting the insulation).

--
Cats, Coffee, Chocolate...vices to live by

Posted by Bryan Campbell on February 28, 2004, 5:54 am
 Thanks a bunch for all the suggestions.  I have only had a couple of
folks chime in thus-far.

As for the walls, I know what you mean.  I was very disappointed with
traditional stick framed walls and bat insulation.  So, when we
insulated we added an R-5 layer of foam insulation first and then our
standard bats.  It compresses the bat a small bit.  But with radiant
heating, the additional, and different type of insulation has performed
well.  2x4 and 2x6 studs are really annoyingly thermally inefficient.

As for the water heater, . . . It is a forced draft unit.  It is rated
as a 90+ efficient water heater -- for what that is worth.  It is 90%
efficient at sucking up a bunch of fuel and converting it to heated
water.  But, it isn't very efficient at thermally storing heated water.
  Not that it is a big problem . . . I have four daughters and a wife at
home all day long.  They like it warm in the house and they use a bunch
of hot water.

The fun part of this is that my wife REALLY wants me to put in solar hot
water heating equipment.  She is very much on board with the more
complex water heating options.  She indulges me . . . knows I like to
build stuff and tinker . . . ANYWAY

Thanks again for your comments . . . it is always good to get
constructive input.

Bryan -

Ecnerwal wrote:

Bryan Campbell . . . bbc@misn.com

STE-MISN    573-775-2111

Key fingerprint:  44AB 0A39 1F4D 0BBE E588  21A7 A4AA B08B AE01 4D39
Key:  http://www.misn.com/~bbc/pgp.txt

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