another one for feedback.... a godsend this time of year.


NT



Fans are widely used to cool humans by blowing air past them, but this
does not lower the air temperature in any way. This article is
specifically about another use, using fans to lower the temperature of
a room.


==Basic Principles==
Outdoor temperature cycles daily, reaching a peak in the afternoon,
and coldest before dawn. Since daily temperature varies with the
weather, these temperature peaks and minima shift in time to some
extent day by day.

Houses, especially masonry buildings, contain a large quantity of
thermal mass (material that stores heat and cold), and are to a
varying degree insulated from the outdoors. This evens out the
temperature variations, so an unheated house is generally warmer than
outdoors at night, and cooler than outdoors in the afternoon.

We can both heat and cool to a limited extent simply by exchanging
indoor and outdoor air at the right times.
* to heat the house, we run exterior venting fans when outdoors is
warmer than indoors
* to cool the house, we run exterior venting fans when outdoors is
cooler than indoors

The clever bit is that since a house acts as a heat store, changing
its tmeperature this way will still leave it cooled or warmed for the
rest of the time when the fan isn't running. A house still heats up
during the day after it was cooled at night, but because it starts
from a lower temperature it takes longer to get warm and doesn't reach
as high a temperature.

This method can heat and cool a whole house on 200w of fan power...
but:
* it only works with a centrally heated house in summer and part of
spring and autumn.
* the heating effect is limited to several degrees
* the cooling effect can reach 8 degrees C


==Set up==
The system consists of either one whole house fan or several room fans
that exchange indoor and outdoor air when run. The fan is controlled
by 2 things:
# a differential thermostat that measures indoor and outdoor temps,
and runs the fan only when the desired heat gain or loss can be
achieved
# and a single thermostat that stops it if it reaches its target
temperature (which regularly happens in summer cooling mode).

When room fans are used, its best to have separate stats for each
room. This caters to varying indoor temps and varying desired target
temps room by room.


==Design considerations==
There are several point to consider to optimise the design. Since this
is a fairly low powered approach, optimisation matters.


===Common mistakes===
People often assume that manual control (ie with no thermostats) will
get them the majority of the benefit. In fact it doesn't work well
this way, much better results are gained with proper thermostatic
control.

One mistake beginners can make is to hook up several extractor fans
without providing any air intake path. This doesn't work very well

Another common mistake is to assume that one air change per hour is
enough. In fact the level of air exchange needed to be effective is at
least an order of magnitude higher than intuition would suggest.
* 10 air changes per hour gives mild cooling, and can be achieved with
small low cost fans
* 30+ gives a much better effect, but expensive fans are unavoidable
at this level.

The air path should be considered. The most effective is when air
intake and exit are on opposite ends of the room. 2 air ports on one
wall reduces effectiveness significantly.

A common mistake with these systems is to run the fan during the
daytime to obtain cooling. Doing so only increases room temp.

The (non differential) target temp thermostat prevents overcooling in
summer. Without one its quite easy to cool a house overnight enough to
make it uncomfortably cold. Also running the fan into the early
morning wastes electricity and just raises the house temperature.


===Extract or intake===
Fans can run as extractors or intake. Each option has its pros and
cons.

====Intake====
Intake produces much more cooling in the room the fan is in, as its
taking in cold air from outside rather than warm air from the rest of
the house. Intake is well suited to single room systems.

Fans that will run in intake mode are quite a bit more money than
extractors.

Intake fans can be a security issue. The practice of administering
airborne anaesthetics for burglary is sadly a known type of crime, and
an accessible intake fan is ideal for this. To avoid this its best to
avoid single intakes on the ground floor (unless the airflow is fast
enough to blow it out again rapidly), and mount upstairs intake fans
as high up as possible.


====Extract====
Extractor fans are common and cheaper. When using an extractor, the
air intake is usually distributed over many small air leaks all around
a house, so the cooling and warming effect are diffuse. Consequently
extraction suits whole house systems well.


====Loft extractor====
A popular use for a large extractor fan in hot climates is to mount it
in the loft to cool the whole house at night. This is particularly
effective because it cools the hot loft down as well as the rest of
the house. It also produces less noise in the living areas.

An external grill on the ground floor is needed to allow cold air in.

A grill between main house and loft is also needed for airflow. This
requires a little thought at the design stage to avoid a few issues
* the grill should be flame trapping to prevent rapid spread of fire
* the grill must be easy to close and insulate for winter.
* means should be provided to stop dirt and dust falling down.

The first and third points can be addressed by fitting 2 steel boxes
(or buckets etc) above the ceiling grill. The smaller inner one has an
air port cut in the top, the outer one has several smaller ports half
way up its sides. Avoid an exterior fibreglass lining, this will
reduce fire survival times. Don't use soldered containers or aluminium
rivets.

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===Stratification & ports===
Indoor air tends to separate into different temperature layers, with
hotter air high up and cooler air near the ground.

Maximum cooling is obtained by bringing in air near ground level and
venting it at the top of the house. Where airflow across one or 2
rooms is used, best performance is obtained by air entering low and
exiting near the ceiling. Next best is both entering and exiting near
the ceiling.

Maximum heating is obtained with air intake and exit both low down on
the ground floor. However since these systems have more effect for
more of the time in cooling mode, they tend to be optimised for
cooling.

A simple way to provide passive intake ports is to use a window
positioned very slightly open - or in cases where security permits,
fully open (ie sometimes on the 2nd floor and above). Many modern
windows can be locked in the very slightly ajar position. Old ones can
usually have a window lock fitted to make them secure in this
position.

Intake ports on the shaded north facing side of a house give slightly
lower temperatures than south facing ports.


===Thermostats===
Indoor and outdoor sensors must always be out of the sun, and mounted
on surfaces that don't get sun. When there's no other practical option
and the system is to be used for cooling only, the sensor can receive
early morning sun. This will disable the fan during these times, times
when the fan rarely runs.

The target temp (non differential) stat needs 2 way single pole (SPDT)
switch contacts. Not all thermostats have this. Cheap bimetal stats
are more than accurate enough.


===Interaction with central heating===
The 2 systems can sit side by side, independantly, and yet work
perfectly in harmony.

To see how this happens, imagine the worst possible situation. The
indoor temp is below the central heating thermostat temp, but outdoors
is just a little warmer. In this scenario, both systems would run at
first, albeit briefly, if both switched on. The fan warming would only
continue as long as bringing in outdoor air was making the room
warmer, as soon as indoor temp approaches outdoor the fan would stop.
Does this mean that during this time centrally heated air is being
expelled? Yes and no... its only expelled when its replaced with even
warmer air, in other words the fan still gains heat for the room, it
never loses it. The 2 systems can sit side by side, independantly, and
yet work perfectly in harmony.


==Operation==
==Target temp thermostat==
The target temp thermostat is 'not' set to the room's desired target
temp. Why? After a night of cooling the room will warm up through the
day, reaching a higher temperature. Thus the target temp stat is set
to the temp that achieves the room staying within its wanted temp
range all day. The stat setting is soon found by trial. For night time
summer cooling this may be 19C on on uninsulated house.

The same principle applies when using the system for heating, the room
will cool down overnight after an afternoon of heating. So for heating
the stat can be set to the upper bounds of comfort, such as 23C.


===Differential stat===
The adjustment on the differential stat is hysteresis rather than
absolute temperature, meaning it will only run the fan when enough
temp difference exists.

The lower the hysteresis setting:
* the more the fan runs at times when it can only provide a little
cooling
* the lower the system energy efficiency
* the sooner in the day it begins cooling
* the greater its max posible cooling effect
* There is little point setting the hysteresis at under 2C, as the
cooling effect would be little at such times.

Bigger hysteresis, eg 5 or 6C
* ensures the fan only runs when it can provide good cooling effect.
* means the fan doesn't run at all when the available cooling effect
is limited
* the fan starts cooling later in the day
* the system is more energy efficient but less powerful in its cooling
effect.


The situation with hysteresis is different when heating, as fuel
derived heat is many times the cost of 30w of electricity, and the
available temp gains are often only a few C. For heating the
hysteresis is best kept low, eg 1-2C.


===Noise===
Noise is an important consideration when these fans are used in
bedrooms, studies and lounges. Slower rotational speed can be used to
produces less noise and less airflow, requiring a larger fan to shift
enough air. Unfortunately 12" fans are over 10x as expensive as 4"
ones - there's no free lunch.

Fans vary widely in noise figures. Always compare specs before buying,
and pick a relatively low noise model.

See [[Fan noise reduction]] for methods to minimise noise. In practice
I found mounting the fan on rubber washers was fine for a bedroom. The
fan is only noticeable in the background in times of (otherwise)
complete silence.


==Experience==
===The experiment===
The first time I tried this was with an industrial 2kw fan. This was
purely as an experiment, not as a practical approach. A severely
overheated room at about 33C reached under 20C in 2 minutes. The storm-
like airspeeds and industrial scale noise were completely impractical,
but it did demonstrate the amount of cooling available.

===12" room fan===
This setup was a single room system using a 12" intake fan. In summer
it would regularly knock 5 or 6 degrees off the temp in the evening,
and sometimes a fair bit more. That's a massive comfort benefit. On
one occasion 10 degrees C cooling was achieved in half an hour with a
35w fan. This is when I realised how powerful a practical
implementation of this approach was. The fan arrangement meant that
the next room was also cooled.

===5" room fan===
This system was another single room fan, a 5" extractor fan. This gave
far less airflow, and typically achieved 3 degree C cooling in 3 hours
and 8C run all night. Its enough to make the room acceptable in
summer, but a bigger fan would do so a lot quicker.


==Energy efficiency==
This method uses a small fraction of the power of aircon for a given
amount of cooling. However it doesn't produce as much cooling. But it
would be an unusually badly designed house that couldn't be brought
below 24C this way. Even the most severely overheating house I've
encountered, at 33C, was tamed.

Its possible to keep a single room cool on just 20w.


==Costs==
2008 figures:
* single room 5" fan £50, this is about the minimum usable
* 12" fan £200
* thermostat £6 (bimetal)
* differential stats:
** Kits and are low cost if you can solder them
** making your own from a circuit diagram costs next to nothing, eg
using transistors as the temp sensors and opamps to process the sensor
output.


==The free version==
Its possible to just open all upstairs windows fully, and wedge
interior doors open, and get some cooling benefit in the late evening.
However doing this overnight is often unworkable for security reasons,
and it brings in lots of insects. Where it is workable, net curtains
can stop nearly all bugs if fixed to the frame. Loose hung they don't
work. The cooling effect is less than a fanned system unless its
windy.


==The future==
The future of such systems is probably central computer control. A
computer that reads all the sensors and controls the fans and passive
vents can take more factors into account than simple thermostats, and
optimise the system to achieve a better return. It can consider the
temperature profile over the last month, even look up weather
forecasts, and work out the optimum target temperature, hysteresis and
fan use pattern to keep the whole house within a comfort zone as much
of the time as possible on minimum energy use.

A central computer could also perform extra functions, such as
detecting fire by rate of temperature rise, raising the alarm,
summoning outside help, controlling airflow in response to fire to
maximise air quality in non-alight areas, and learning from any false
alarms. It could also detect open windows and alert occupants to the
security risk if and when wanted. It could also detect a fan failure
and switch power permanently off to any such fan and alert someone.

Fitting the fans behind central heating radiators and computer
controlling them could achieve a few more benefits:
* the radiator provides a baffle, reducing fan noise a little
* the radiator makes the fans invisible
* fans can be run at low speed with their shutters closed to increase
radiator heat output, thus slightly smaller radiators are needed, and
the computer can prioritise which rooms to heat up most quickly and
control which need more heat output, giving more even room
temperatures.


==The practical picture==
One of these systems with a much simpler control system than described
here eliminated fuel heating use for about 5 weeks per year. Better
can be achieved with the full system. Of course this is at the mildest
times of year, so the fuel saving is much less than this percentage of
total heating energy use. The savings are about enough to both pay off
the cost of the system over time and provide summer cooling at no
added cost.

One of the barriers to fitting is the amount of work involved running
wiring, both for mains power and sensors. Redecoration, replastering,
and renovation/refits are minimum cost opportunities to bury cables or
conduit that make this system much easier to fit in the future.


==See Also==
* [[Special:Allpages|Wiki Contents]]
* [[Special:Categories|Wiki Subject Categories]]



[[Category:Cooling]]
[[Category:Heating]]
[[Category:Appliances]]
[[Category:Energy Efficiency]]
[[Category:Projects]]