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 PATHChat
| SUBJECT: |
RE: In floor heating requirements
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| FROM: |
DUNCAN WILSON
duncanw@mindspring.com
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| DATE: |
3/19/1999 1:09:00 AM |
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Depending on roof and window details, ICF houses must
have very little infiltration compared to traditional
framing methods, which typically have about 50%
(roughly) of the heat loss from infiltration. I can
intuitively see how the 50% claim could be made.
In addition, radiant floor heating systems
conservatively claim 15% (and often more, in reality)
energy savings over conventional heating systems.
There is no stratification of hot air near the ceiling
(or roof, which is exposed to outside temperatures),
and convective losses and stack effect are both
reduced. Your average sized home can use two or three
small wet rotor circulators which can draw as little as
0.4 amps to 0.75 amps each, whereas forced air blowers
use about 6 amps.
Radiant floor heating in concrete with no floor
covering is about as efficient as it gets, the water
circulating in the slabs can be as low as 90°F.
Solar gain can result in room temperature overshoot in
high mass radiant floors. The slab which is already at
85°F will likely overheat quickly from solar gain. To
compensate for this, a "smart" control should be used,
one with Proportional Integral Derivative (PID) control
logic. It measures how big the difference is between
setpoint and room temperature, how quickly it is
changing, and how long it has been occurring. It then
adjusts system water temperature accordingly. It is
imperative that you have a control that can compensate
for solar gain.
Outdoor reset controls sense outdoor temperature and
adjust system water temperature accordingly. The colder
it gets outside, the warmer the system water, and vice
versa. This also helps, it's like cruise control
rather than on/off response. Tekmar Control Systems in
Vernon, B.C. makes state of the art controls that
provide all of the above features.
The basic formula for heat loss is Q=U*A*delta T, where
Q=btu/hr conductive heat loss, U=conductivity of the
building material (the inverse of additive R values),
A=area of surface exposed to outside temperatures,
delta T=temperature difference between outside
temperature and inside temperature at design
conditions.
Loss through the floor is a bit different, depending on
amount of slab edge exposed, ambient ground
temperature, insulation below slab, and other factors.
Get a GOOD book on the subject. John Siegenthaler's
"Modern Hydronic Heating", Delmar Publishers, ISBN
0-8273-6595-0 is as good as it gets.
Water heaters typically have inputs of 30 to 50
thousand btu/hr. Derate for appliance efficiency and
altitude, and you may end up with a btu output that's
60% of the input, not enough to heat your average home.
Additionally, there may be building code issues with
using an appliance to do something it's not designed
for. Additional safety devices will have to be added.
High output commercial water heaters or multiple hot
water heaters could be an option.
Your starting point is to determine your home's heat
loss.
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Partnership for Advancing
Technology in Housing (PATH)
451 7th Street, SW, Rm. 8134
Washington, DC 20410-0001
Telephone: 202 708-4370 Fax: 202 708-5873
E-mail: pathnet@pathnet.org |
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