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Despite the 100+ years of
formal research
and over
10,000 years of recorded history, radiant heating
and cooling is still for some strange reason treated as
a popular science experiment.
To address several
discussions stemming from people not knowing if they
could just heat the basement and let ‘heat rise’ to
misunderstanding about radiant cooling, to the
definition of comfort - I’ve penned a few words of
wisdom to address these and other issues.
Comfortable buildings or bodies that
are comfortable?
Once again, we have to
get over the fact that
we don’t condition buildings we condition people
which means we have to understand comfort is a state of
mind interpreted not by what the ‘room temperatures’ is,
but what the 166,000 +/-
thermal sensors in the skin are
picking up and communicating to and
interpreted by our
brain (see work by
Egan,
Johnson, Farrell et al).
What does this mean?
It means comfort exist
both as a physical and psychological reality measured
not by a building or by its thermostats but by the occupant,
in other words,
 people do not feel the heat loss of the
building they feel the heat loss from their bodies...and this is the most
important part – people don’t work on 1’s and 0’s
…people work on grey or what we call fuzzy logic
(see Table 1).
What does this mean?
It means you can’t
dictate someone to ‘be comfortable’ based on digits like
72 deg F dry bulb temperature (see work by
Rohles et al). There are ranges of comfort based on
environmental and physical factors and there are ‘fuzzy’
thresholds of discomfort (see PMV work by Fanger et al).
Here’s a few simple
examples; a numerical based room heat thermostat
satisfied at 68 deg F (contacts open), creates
perception of discomfort in some people, yet place the
same person in a car with color coded controls in the
presence of similar thermal conditions they are likely
to express comfort.
Just from relocating from the house
to the car there is a perceived change that exists
psychologically due to cultural
 conditioning (numbers on
house thermostats, colors in cars). That same effect
can be demonstrated with furniture, color and light (Rohles,
Gagge et al). In one study that Fanger did, subjects
preferred lower ambient temperature in red light than in
blue light. Comfort can also happen with a slightly different
twist in offices when people think they have control
over their environment with a window, fan and or
thermostat - when they have control they tend to
perceive higher levels of comfort - this has been
demonstrated in research and in the field…just ask the
controls contractors who have installed ‘faux’
thermostats or have recalibrated thermostats to serve
the clients psychological needs instead of the client’s
physical needs. You can even have the phenomenon where
someone has paid for something like floor heating in the
basement only and will then claim the main floor is
heated with radiant heating even though it is several
degrees cooler than their physical desire for greater
floor comfort. Their psychological denial over what
they paid and thought they were getting allows them to
exist with their physical discomfort due in part to what
they don’t have….all weird and wonderful stuff in the
world of thermal comfort.
So first thing, as
a psychological phenomenon, designers must appreciate
they are dealing with as many subjective perceptions as
there are to be occupants in the space; and each of
these occupants consciously and unconsciously interprets
thermal sensations from localized parts of their body
which then leads to both general and specific
interpretations known as perceptions about the
environment expressed as discomfort.
So now that you have an extremely abridge
discussion on weird comfort stuff let’s answer in
general terms one of the
22 myths we feature at the site...can you heat the
entire home with just the basement floor?
Assuming the occupants fall under the
bell curve defined by a combination of
environmental and
physical factors satisfying something like 80% to 90% of
the population and assuming the only thing separating
the occupants feet from the floor is socks and assuming
in the first instance that they have installed a hard
conductive
low VOC flooring
for indoor air quality
and assuming they are in
contact with the floor for anything other than a
temporary period of time (for giggles say more than 10
to 15 min); extrapolating from research, suggest (all
other factors being equal) that if the
floor
temperature is within
79 deg. F. to 83 deg. F less than 10 to 15% of the population will complain
(Olesen et al) about the temperature of
the floor (give or take 5%)…again there is no 1’s(ones)
or 0’s (zero’s) in comfort …only grey, fuzzy and
frequently distorted perceptions. For a second example,
if they use a textile based floor such as your grandmas
old groovy shag carpet or your new-age cousin who jumped
into the recent trend to grandmas’ carpet choices on
steroids -
monster shag, the perceived localized foot comfort is based
on a range between 70 deg. F. to 82 deg. F. We’ll get
back to groovy carpet choices in a minute.
So lets’ use 79 deg. F. as our floor
surface temp for comfort with socks and the space
temperature is controlled to 72 deg. F. operative
temperature (Top).
Simplified Top is the average of the
dry
bulb (Tdb) and
mean radiant (Tmrt),
= ((Tdb + Tmrt)/2)
(for those in the know – know that I know
its a bit more detailed than that but it’s close enough
for this conversation).
At 79 deg. F. and a
heat transfer
coefficient of a nominal 2 Btuh/sf/deg. F., the floor
under maximum load could deliver a combined radiant and
convection output equivalent to:
= (79 deg. F. - 72 deg. F.) * 2 Btuh/sf/deg.
F.
= 14 Btu/hr/sf ,
or for 1800 sf of floor area a load of
25,200 Btu/hr.
[The heat transfer coefficient of a
nominal 2 Btuh/sf/deg F is approximately 50% radiant and
50% convective. The split is influenced by the MRT, room
geometry and any external influences such as fans.]
So here’s the thing, in a 6,500+
heating degree day year climate zone, 14 Btu/hr/sf
exceeds the required output for any home built to high
performance standards such as Passivehaus or R2000. But
let’s have a bit of fun and go with 79 deg F and 14
Btu/hr/sf for giggles and take a trip into the basement.
Lets TRY a hypothetical load in the basement based on
40% of the main floor load (if you don’t like 40%
use your own ratio – it’s my party and I can TRY
40% if I want too…), 40% of 25,200 = 10,080 Btu/hr or
roughly 8 Btu/hr/sf if you take out the unheated areas
(assume 1300 heated basement floor, i.e. 500 sf
unheated).
The required basement
floor surface temperature becomes ,
=
72 deg F + ((8 Btu/hr/sf)/(2
Btuh/sf/deg F))
= 76 deg F,
which in this hypothetical case would be
3 deg F cooler than the required main floor temperature
of 79 deg F. ergo you won't be heating the main floor to
79 deg F if the basement only needs to run at 76 deg F.
Let's for a moment, set aside choices of flooring in the
basement and agree at this point it would be a bad choice to rely on the
basement for comfort conditioning of the main floor. One
would have to run both a CFD and an
FEA
model to see the
convective , radiative and conductive transfer to
establish the actual required basement floor temp to
deliver the main floor surface temp (ergo resulting in
the operative temp) but without taking several days to
model this I can say with reasonable accuracy to heat
the main floor to 79 deg F the basement floor would have
to be just a tad on the hot side…even if we forgo the
main floor temp of 79 deg F and just work with 72 deg F
operative temp the basement floor would have to produce
a combined load of 35,280 Btu/hr or roughly 27
Btu/hr/sf.
The 27 Btu/hr/sf equates to app. 86 deg F surface temp which
exceeds slightly the comfort level for socks on tile or
grandmas groovy shag carpet. But hey, If you just
cranked the surface up to say 95 deg F you could forgo
the carpet or tile, sprinkle kitty litter on the floor,
pretend you’re at the beach with your fancy umbrella
drink and depending on which way you swing, watch the
half naked volleyball girls and boys on Sports Net.
But I digress…let’s go back to the
question…IF hypothetically the high performance home had a
main floor design flux
requirement of say 6 Btu/hr/sf at maximum load (10,800
Btu/hr) the required floor temperature becomes,
= 72 deg F +((6 Btu/hr/sf)/(2 Btuh/sf/deg F))
= 75 deg F.
75 deg F. meets ASHRAE Standard 55,
however if one had bare feet on tile at 75 deg F it may
for some to feel neutral to slightly cool but it would
be ok for for
carpets, linoleums and wood - but put this into perspective -
75 deg F is going to be warmer than the floor in a forced air heated
space.
So
one solution if you want a warmer floor, is to reduce
the heated floor area (i.e. from 1800 sf down to 900 sf) which boost the flux load
(from 6 Btu/hr/sf up to 12 Btu/hr/sf) and thus
the surface temp (75 deg f up to 78 deg F.) to improve localized foot comfort
perceptions without overheating the occupants at maximum
load.
Anyhooo…at maximum load of 6 Btu/hr/sf
and tiled floors, using say 8” tube spacing the average
fluid temperature is appx. 77 deg F (using ASHRAE Fig. 9
Nomograph).
If you designed the system for a 10 deg F
differential the supply temperature would be,
= 77 deg F + (10 deg F / 2) = 82 deg F.
Sans an academic physiology debate, that
is roughly 16 deg F cooler than your
blood
temperature. Which is a
very weird construct for some since the floor whilst in
it’s heating mode is actually cooling your body so you
can remain comfortable….bizarre eh? I can say that cause
I’m Canadian eh?
Now here’s where radiant shines from an
energy perspective. What radiant has over other systems
is its innate ability to enable boilers, heat pumps, and
geo or solar thermal system to achieve close to their
maximum engineered performance. Most of the radiant
claims on efficiency are caribou candies but you can’t
argue the COP’s and combustion efficiencies achieved
with low return temperatures in heating and high return
temperatures in cooling. Rather than spoil the fun, why
don’t those who know…check out the COP of a heat pump or
combustion efficiency of a boiler when the return
temperature at maximum load is,
= 77 deg F – (10 deg F delta t/2)
= 72 deg F,
…yes for those who raised their eye brows
– the return fluid temperature is equivalent to the
designed space temperature…watch what that does to
efficiencies.
So let's go back again and look at the basement
assuming the same split of 40% the basement load becomes
roughly 4300 Btu/hr or 3 Btuh/sf. If we add back in the
upstairs load of 10,800 Btu/hr we’re up to 15,120
Btu/hr
for a basement flux of,
= (10,800 Btu/hr + 4,300 Btu/hr) / 1300
sf
= 12 Btu/hr/sf,
this results in a floor surface temperature
of,
= 72 deg F + ((12 Btu/hr/sf)/(2 Btuh/sf/deg F))
= 78 deg F.
78 deg F is within comfort conditions
(see Table 2)
and with some convective help would stabilize the
basement space while conditioning the entire home
without
 heating in the second floor. The convective help
could be small fans or architectural features to create
natural drafts…sloped roofs to a cooler wall with
transfer grills to and from the basement etc…
Again the above is a general overview
based on hypothetical cases…for the snipers in the
readership…change the numbers to your own delight and
accuracy; and understand that I understand my picture
changes as your numbers change.
If we agree on that then let’s continue.
Some other considerations:
Thermal lag and heat rising
Radiant energy is not slow
nor does heat from a radiant panel 'rise'...radiant
energy travels at the speed of light from
hot to cold. The thermal lag associated with startup is
not the same as the instant surface response based on emissivities and differential temperatures.
Lag at start
up is dependent on the differentials between the various
temperatures of the outdoors, in-space, slab/panel and
back temperatures (earth and in-space),
enclosure performance; the fin
efficiency of the slab including the tube, spacing,
depth, specific heats, conductivity and, resistances in,
on and under the floor, and the controls employed to
‘wake up’ the system. Again as a reminder,
in a high
performance home the fluid temperatures are not hot –
they are tepid at best and with modern low mass
conductive flooring options and basic analogue weather
compensating controls there is similar controllability
one would find with other heating systems.
Put it into
perspective…we’re talking about heating the floor under
maximum load to 75 deg F +/- …that’s only three or four
degrees above space temperatures. For lower loads (say
95% of the year we hardly need any heat delivered to the
floor) – that’s why you don’t need sophisticated
expensive systems - one heat source, once circulator,
one analogue weather compensator and one thermostatic
non electric radiator valve with remote actuator and
spring loaded bypass valve across the radiant manifold
and you have a simple elegant system.
Affordability
Affordability is
relevant; expensive to one is affordable to another…I
learned a long time ago not to be the financial advisor
by saving money on
IEQ systems for people
who’ll then spend what they saved on outdoor furniture and boutique
coffees.
Solar loads
With regards to solar
loads: in the presence of shortwave energy (solar), the
heated floor at 75 or 76 deg becomes an absorber not an
emitter and can be used to distribute or shed excess
heat gains. If people are concerned with overheating due
to solar gains then
keep the shortwave energy off and
out of the building.
Radiant split and latent vs. sensible
loads
The radiant component in
a floor heating system is long wave energy and
represents approximately 50% of the heat transfer from
the floor which is very similar to the
ratio of sensible
heat lost from the body at low level activities
(watching TV, reading etc.) and wearing light
clothing. This long wave energy from the floor is not
absorbed by the body as most think but rather heats up
the cooler room surfaces which reduces the
radiant
losses from the body…ergo it’s not the heat you are
gaining but the heat you are not losing that contributes
to your comfort (recall you already produce excess heat
to the tune of 400 Btu/hr again sans any academic debate
on activity, clothing, gender, physical attributes).
In a high performance
home the interior surfaces are warmer in winter and sans
discussion on internal gains, cooler in summer…i.e. in
winter you wouldn’t need any heat from a mechanical
system if heat produced by the body augmented by heat
from fridges, freezers, lights was sufficient to
maintain comfort, but unlike winter, you ‘ll need to get
rid of the sensible heat in the summer….but unlike the
summer latent loads (with moisture); heat created from
the sun, lights,
motors, compressors and some of our body heat is sensible
(no moisture) which
radiant
cooling is perfectly
suited.
Radiant cooling
In
radiant cooling, the floor is lowered to a
temperature based on a lower heat transfer coefficient
since the convective component is essentially reduced to
zero and the heat absorption is almost all radiant. The
cooling coefficient for a floor is a nominal 1.2 Btu/hr/sf/deg
F. +/- 0.1.
For a hypothetical
cooling load of say 10 Btu/hr/sf the required surface
temperature becomes,
= 78 deg F – ((10 Btu/hr/sf)/(1.2 Btu/hr/sf/deg F))
= 70 deg F.
(66 deg F
floor surface temperature is considered
the lower end for people wearing normal footwear).
Going back with our tiled
floors and 8” o.c. spacing the average fluid temperature
for cooling is approximately 68 deg F., for a 8 deg F
differential, the return temperature to the heat pump
would be,
=
68 deg F + (8 deg F / 2)
= 72 deg F,
….now again...go back and
look at the
cooling plant efficiency with 72 deg F
return temperatures.
Regarding latent loads:
they have to be considered - maintaining a lean air
mixture is required to prevent condensation on any cooled surface,
regardless of the HVAC system, i.e.: the ventilation air has to be introduced to the
space dry enough to absorb the latent loads from people,
infiltration, grooming, cooking, cleaning etc.
A 70 deg
F surface temperature for comfort cooling and dew point
limitation corresponds to app. 75% RH at 78 deg F space
temperatures. Evaluations like this are done using
the psychrometric chart, a sample shown below (l) with a
finite element analysis for a floor cooling system shown
on the right.
75% RH doesn’t serve
any building or health science needs whereas 50% does.
At 50% RH and a space temperature of 78 deg F the dew
point is app. 58 deg F. leaving a huge safety
margin….normally 2 to 3 deg F is adequate (as
shown above in the left hand side image). So determine
your space dew point at 78 deg F and reduce the absolute
moisture in the incoming air (based on moisture produced/deleivered by the occupants, infiltration etc.)
and deliver that lean air mixture to the space through
the ventilation system or provide in space
dehumidifiers. If you live in a dry climate like I do,
dehumidification is not typically our problem but
humidification is...so again your mileage may vary
depending on the geography, occupants and building
performance.
If you haven’t yet
checked the performances of 72 deg F return temps
through heat pumps and boilers I would encourage you to
do so for your own entertainment….I can tell you
this...every engineer at the manufacturing plant will
kiss your feet when you can make his/her stuff do what
it was designed to do…
Sustainability,
energy and exergy efficiency
One of the messages in
this is direct coupled earth systems (without
compression or combustion) become attractive at 72 deg F
fluid temperatures…even if the earth system could supply
enough cooling and heating for 75% of the year, the
balance could be made up with solar for a truly
sustainable exergy and energy efficient system. If solar
isn’t an option at least the non renewable is limited,
the efficiency is close to perfection with entropy
losses minimal. |
