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Atze Boerstra1, Peter Op'Veld2, Herman Eijdems3
1 BBA Indoor Environmental Consultancy, the Netherlands (bba@binnenmilieu.nl)
2 Novem Netherlands Agency for Energy and the Environment, the Netherlands
3 Cauberg Huygen Consulting Engineers, the Netherlands
ABSTRACT
A literature review was conducted that aimed at
mapping the impact of Low Temperature Heating (LTH) systems on Thermal Comfort,
Indoor Air Quality, and Safety. With LTH systems defined as systems that use
"low exergy" heat sources, with heat supply elements like floor heating, wall
heating, enlarged radiators/convectors, or enlarged heating coils for air
heating. Low Temperature supply elements turned out to have many advantages
over their "high temperature counterparts".
Thermal Comfort: diminished vertical temperature
gradients, less radiant heat asymmetry, more comfortable floor temperatures,
less temperature fluctuations, and reduced draft risk; Indoor Air Quality: less mites, fewer airborne
particles, lower average air temperature thus improving the Perceived Air Quality; Safety: minimized burning risk, less changes on
injuries when falling.
Note that a couple of the aspects named only apply for
floor and wall heating systems.
KEYWORDS Heating, Thermal comfort, Air Quality, Safety.
INTRODUCTION
Due to better thermal insulation of new and retrofitted buildings and
thanks to new techniques for reducing ventilation losses the heating demand of
modern buildings is decreasing.
A further reduction of the energy use of buildings (needed to meet future global
targets for emission reduction) implies that we look at ways to improve the
heat generation process.
The heating system of the "average" Dutch dwelling consists of a high efficiency
natural gas boiler, a water pipe system, and radiators or convectors as heat
supply elements. The system operates at a water temperature of around
70-90 deg. C under "winter conditions" (outside temperature between around -5 and
+10 deg. C), therefore we speak of high temperature or HT systems.
Note that also a large amount of smaller utility buildings use comparable systems.
On the long run it will be inevitable to shift towards so-called "low valued energy"
or heat sources at "low exergy levels". It is available from residual heat, ambient
heat and renewable sources. It can be used for Low Temperature Heating (LTH)
systems in residential and utility buildings. For this purpose todays new
buildings and installations should be designed for (future use of) Low
Temperature distribution systems (NOVEM definition, see table 1). The generally
used (water based) heat distribution systems (pipes) in buildings have a life
cycle of 40 to 50 years. In order to allow for a broad introduction of low
exergy heating systems in the next half century it is necessary to start
changing our heat distribution systems into systems suitable for Low Temperature
water distribution soon.
Aware of this need the NOVEM initiated a program of feasibility and theoretical studies,
field experiments, and demonstration projects to stimulate the introduction of
Low Temperature heating systems in general and Low Temperature distribution
systems in particular.
The overall goal of this study was to identify the (proven) advantages and
disadvantages of Low Temperature Heating systems (heat supply elements) in
comparison with their "high temperature counterparts" (e.g. wall and floor
heating were compared with conventional HT radiators, enlarged LT radiators
with HT radiators, LT air heating with HT air heating). These "qualitative
aspects" of LTH systems were identified on the basis of a review of national
(Dutch) and international (scientific) literature.
In the context of this study Low Temperature Heating
systems were defined as systems that use "low exergy" heat sources, with heat
supply elements like floor heating, wall heating, enlarged radiators or
convectors, or enlarged heating coils for air heating (see also table 1, "Low
Temperature" and "very Low Temperature" combined).
System |
Water temperature |
| Supply flow |
Return flow |
|
High temperatures (HT) |
90 deg. C |
70 deg. C |
Medium temperatures (MT) |
55 deg. C |
35-40 deg. C |
Low Temperatures (LT) |
45 deg. C |
25-35 deg. C |
Very Low Temperatures (VLT) |
35 deg. C |
25 deg. C |
Table 1. NOVEM Definition of heating systems based on operative water temperature under "winter
conditions" (outside temperature below + 10 deg. C) RESULTS - THERMAL COMFORT
Radiant heat transmission
The radiant heat transmission component of LT-systems is higher than for
High Temperature systems. For example, due to large surfaces and Low
Temperatures the radiant component of floor and wall heating is about 50 to
70%. For conventional HT-radiators this is only 30 to 50% and even less for
conventional HT-convectors [3]. Therefore the amount of heat "transferred by
air" (by conductivity) is reduced with LT heat supply elements, which implies
that air temperatures can be 12 deg. C lower at the same comfort level (with the
same operative temperature). Field study results indicate a high appreciation
by building occupants for heating systems that work primary on radiant heat
[4]. It is assumed by some authors (e.g. [4]) that a relative large contribution
of radiant heat transfer (e.g. in a room with warm surfaces but relatively cold
air) better fits comfort needs of human beings because it is more "natural"
(compare this with being outside on a sunny day in winter). Note that the
advantage named especially applies for wall and floor heating.
Vertical temperature gradient
Laboratory and field experiments alike show a clear difference between the vertical temperature gradients (temperature difference in a room dependant upon height above floor level) between LT floor heating and HT-radiator heating. With floor
heating practically no gradients are found, assuming well-insulated spaces (e.g. [2], [5], [6]). For radiators, convectors and comparable heating systems the extent in which gradients appear is much more sensitive to the quality of the heating design. Normally, with HT heating, gradients
range from 2-3 deg. C between floor and ceiling; but "less well" designed HT systems show gradients up to 7 deg. C. (Compare - NEN-ISO 7730 demands that the vertical temperature
difference does not exceed 3 deg. C per meter; preferably it stays below 2 deg. C).
Even though no data where found on LT-heating systems other than floor heating,
the general assumption is made that for LT-heating in general gradients are
considerably lower than for HT-systems. Window temperature asymmetry
Cold window surfaces sometimes cause discomfort through radiant heat losses, which
are not in balance with radiant heat flows in other directions. Complaints
occur when differences exceed 20-25 W/m2 or when a temperature
asymmetry of 10 deg. C occurs [7].
Conventionally compensation was provided by placing hot radiators/convectors under windows.
Removing HT radiators/convectors from facades and replacing them with for
example wall or floor heating is often thought to introduce extra thermal
discomfort. Due to the development of high insulation glass however, this
aspect has lost importance. Given Dutch average outdoor temperatures in winter,
at U-values under 1.5 W/m2.K no significant differences in discomfort between heating systems occur near windows, under the condition that windows are less than 1,5 to 2 meters high[8].
Surface temperature of heated floors
Floor heating offers specific users a comfort level that is not achievable with
regular facade mounted heating systems (with or without textile floor
coverings). Where people walk bare foot a lot (e.g. in swimming pools, but also
in dwellings), or where some sit on the floor a lot (e.g. in nurseries) floor
heating is the obvious choice. Research shows that the optimum floor
temperature lies between 20 and 28 deg. C with shoes and between 23 and 30 deg. C bare
foots depending on the material the floor is covered with [5]. Without floor
heating, floor temperatures normally lie between 15 and 20 deg. C in winter, which
is out of the "comfort" range described (especially when walking bare foot and
sitting). A study for increased bacteria growth on feet (in shoes) due to
exposure to heated floors showed no significant impact [9].
Air velocities and turbulence intensity
Too fast temperature fluctuations around a constant mean air velocity (a too high
turbulence intensity) might result in draught complaints. LTH-systems in
general are more "inert" than HT systems. Which is explained by the fact that
the driving forces are smaller due to relative large surfaces and lower
temperature differences between heat the supply element and it"s surroundings.
Both laboratory and field results show that LT systems and especially floor
heating systems generally have a lower turbulence intensity "about 20% - and
therefore lower risk for draught complaints [5] [11] [14]. Note by the way that
the mean air velocities for floor heating are in the same order as those for
conventional HT radiators within the living zones assuming limited influence
from ventilation through facades on indoor air flows".
When a building is ventilated "naturally" (through vents or windows) the design of the
facade openings in combination with the location and type of heat supply
systems need special attention. Especially with LT systems like floor and wall
heating systems. Otherwise LT systems might in some situations (vents too wide
open, high wind pressure, low outside temperatures) indeed result in more
draught complaints near windows.
Warming up period & general inertia
A disadvantage of LT systems, and especially of wall and floor heating systems,
that one often hears is the relative long warming up period needed.
The warming up period first and for all is related to the amount of "thermally
active" building mass. So in cases where (like in many new example buildings)
the actual floor and wall heating systems (upper package) are well insulated
from the rest of the building mass, and the heating package itself is
relatively light (the case with some of the newer floor heating systems), the
warming up time of LT systems too can be rather short.
Moreover, heating up is often associated with increasing air temperatures. Looking at
overall thermal comfort or the operative
temperature the so called differences between LT and HT-heating systems become
less pronunciated.
Also, for high-insulated buildings the energy gain from a night setback is small due
to the fairly limited reduction at night of the building structure temperature.
Therefore, for LT systems in general only a minor night setback (around 2-3 deg. C)
is recommended [1] [12]. So warming up time becomes less of an issue.
Another aspect often talked about with wall and floor heating systems is their inertia with incoming solar heat or with sudden changes in internal heat loads, resulting in an unwanted (uncontrollable) increase in temperatures. This effect
however in practice is much less spectacular than often thought, LTH-systems
and wall and floor heating in particular take profit of the so called "self-regulating"
effect. Due to the small temperature differences (delta T) between heat supply
elements (e.g. a heated wall) and the inside air (often only a couple of
degrees), a sudden increase in heat load (e.g. solar input in one room)
instantly results in a considerable reduction of the delta T; because this is
the main driving force for the heat exchange, the declined delta T in it"s turn
instantly decreases the amount of heat supplied through the LT system (in that
room) therefore "self-regulating" the amount of heat that is being delivered to
that specific space [5] [10]
Cooling abilities
Increasing the insulation of buildings together with reduction of ventilation losses and increased
utilization of solar gains introduces the risk of overheating during summer
time [13]. In that context LTH systems
offer an interesting advantage namely opportunities for cooling (both in
commercial buildings and in dwellings). Especially when combined with a ground
collector (and heat pumps) a limited capacity of (high temperature) cooling can
be accomplished with a rather limited energy use [5]. Other expensive measures
against overheating thus can be avoided.
RESULTS - INDOOR AIR QUALITY
Many studies show a positive
effect from floor heating systems on reduction of mite populations in dwellings
resulting considerable health improvements of allergic building occupants (e.g.
[17]). The underlying mechanism is a lowered moisture content in the upper
boundary layer of the floor (of the floor covering) due to the heating of the
floor.
The mite survival threshold is a
relative humidity level RH under 45 % during a prolonged period (at least
several weeks). The influence of floor heating system on the RH in the boundary
layer is calculated to be in the order of 10 % points. Given a Moderate Western
European See Climate, this reduction is just enough to bring the RH in the
boundary layer under 45% for most of the winter, thus decimating mite
populations [21]. Note that with floor and wall heating systems the change for
mould growth on interior surfaces is also decreased.
A Finnish field study
concluded that heating elements of the "low surface temperature type" result in
less eye-irritation and throat and other mucous diseases (e.g. they found most
symptoms with the "hottest" heating elements, electrical heaters) [15]. Also a
correlation was found between the temperature of the heating surface and the
amount of particle deposition. It is assumed that the lower grade of air
fluctuations from LTH systems causes a lower quantity of suspended particles in
spaces [16].
It is well known that inhaling dust can cause allergic reactions [20]. The sensitivity of humans for inhaled particles is not only dependent on the amount of particles inhaled, more important is the quantity of suspended matter [18]. At temperatures exceeding
55 deg. C the process of dust singe starts. Some suggest that particles get more
reactive and irritating after contact with the relatively hot surfaces of HT
heating elements [15].
Conclusion, indications
exist that the application of LTH systems not only results in less suspended
particles, but also leads to less reactive (less irritating) particles due to
the absence of dust singe.
Temperature effect on SBS symptoms & PAQ
As a result of the high
contribution of radiant heat the room air temperature can be 1-2 deg. C lower in spaces with LT systems. Studies show that the amount of complaints about stuffiness increase and the perceived air quality (PAQ) decreases at higher
air temperatures (e.g. [18]). Also the amount of mucous irritation complaints
and general SBS symptoms increases significantly with higher air
temperatures (indoor air temperatures > 22-23 deg. C) [19].
Because LT systems allow for
slightly lower air temperatures the risk for SBS symptoms is lower and the
expected Perceived Air Quality better.
RESULTS - SAFETY
Hand Burning
Field experiments (e.g. [5]) show that heat supply elements introduce a risk for hand
burning when surface temperatures exceed 40 to 45 deg. C. The surface temperature
of supply elements with water based piping systems is approximately the same as
the mean between water supply temperature and return temperature. So with
conventional HT radiators and convectors the surface temperature of the heating
supply element can be as high as 60 to 80 deg. C under winter conditions. Only when
the outdoor temperature is above 10 deg. C - not the case during about 5 months a
year - the HT supply element will be at or below 40 deg. C [1].
With LT radiators water temperatures lie substantially lower (see table 1), but still
with outdoor temperatures under about -3 deg. C the surface temperature will come
below 40 deg. C.
With wall and floor heating systems surface temperatures of heating elements seldom
go over 30 deg. C, so hand burning is not an issue (also the case with air heating
systems in general).
Physical Injury
Traditional radiators and convectors often form substantial obstacles in living and working
spaces. These are a potential cause for injuries. No specific statistics were
found on the yearly amount of people injured after falling against radiators
and convectors, but anecdotal evidence exists that the amount of accidents with
especially the elderly and children is substantial. This explains why e.g. in
design guidelines for buildings for the elderly one is often stimulated to
avoid the "conventional" radiator or convector. Wall and floor heating systems
and also air heating systems (HT or LT) have the advantage over (HT or LT)
radiators that they are embedded in the building structure (system) so possibly
dangerous obstacles are avoided [1].
DISCUSSION
The literature review showed
that Low Temperature Heating (LTH) systems have many advantages over High Temperature systems in terms of safety, health and comfort;
With LTH systems Thermal Comfort increases on
many aspects (more radiant heat transfer, less temperature gradients, more
comfortable floor temperature, less draught and air turbulence);
The Indoor Air Quality is positively influenced
(less dust mites, better Perceived Air Quality through lower air temperatures,
and less SBS symptoms due to less suspended particles and decreased dust
singe);
Safety is improved - Less risk for hand burning
and for physical injury when falling.
The disadvantages found (e.g. warming up time, radiant
asymmetry near windows) in most cases only apply in case of improper design. The
original study also mapped energy advantages. This was not dealt with in this
article due to space limitations, We narrow ourselves with just a short statement
about energy savings. Given the Dutch climate (moderate Western European see
climate) the energy reduction of LT heat supply elements on their own is about
2 to 10% compared to "conventional systems" depending upon the type of system
used (highest with wall and floor heating). It can be up to 30% or more when
the LT supply element is combined with alternative heat sources selected for
optimum combination with LT supply elements e.g. heat pumps [1]. The energy
advantage of Low Temperature Heating systems generally are well known by
parties involved in building projects. The literature review resulted in a
broad range of new arguments that can be used to convince clients, consultants
and architects to chose for Low Temperature Heating.
ACKNOWLEDGEMENTS
This study was financed by the Netherlands Agency for Energy and the Environment
NOVEM, section EV&B. We thank Arjen Raue (BBA) for reviewing draft
versions.
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