I've a couple of questions that I was hoping someone could help me out
I currently leave my central heating system on - constantly (during
winter). When I'm out of the house I turn the room thermostat down to
20 degrees, when I return I pop it back up to 22-23 degrees, at night
I put the system back down to 20 degrees.
My theory is that because the water (in the system) is actually
already at temperature and the house never goes below 20 degrees and
that this is a more efficient way to heat my house and keep the bills
down. When the room thermostat is not calling for heat the boiler
only comes on every now and then for approx 30 seconds, I assume just
to keep the water which flows through the HW cylinder at the required
Would anyone agree with this?
This question may seem silly but if my central heating system is
switched off all day the house temp drops to approx 15-16 degrees,
this takes quite some time to get to 22-23 degrees and I guess alot of
Also, if the above holds true, would it be more economical if I close
the valve to the HW cylinder and just use the electric immersion
heater and a timer for the required hot water?
Thanks in advance
If your system is wired correctly, it should *never* come just to keep the
boiler hot - unless either the room stat or cylinder stat is calling for
The heating system has to replace the heat lost through the walls, ,
windows, roof and floor. The hotter the building, the greater the heat loss.
It is always more efficient to allow the building to cool down when you
don't need it to be hot - but better still if you have very good insulation
so that it only cools very slowly when the heating is off.
It is *never* more efficient (cost-wise rather than energy-wise) to heat
water with an on-peak electric immersion heater rather than by gas.
According to the instructions for my Danfoss TP9 Programmable Timer/Thermostat..
if you have a gravity hot water system and pumped central heating you may well
a H/W tank thermostat. Basically the boiler operates as you describe.
If you have Honeywell C plan then the boiler would only function when either the
C/H or H/W require.
One advantage of the TP9 is that it only supplies power to the boiler when
either the C/H pump is on
or the H/W timer is on.
With a conventional programmer the boiler is trying to keep itself warm whenever
the timer says the C/H is on which in your case would be always.
The other advantage of the TP9 is that you can make the C/H follow a temperature
profile with 6 changes per day. - different at weekends if you want.
When I was a school I recall 'rate of loss of heat is proportional to the excess
temperature'. i.e. the warmer your house is compared to its surroundings, the
more heat it will loose.
The problem in practice is that can take a while for the house to heat up and
cool down - as you mention!.
Then you have a large boiler that modulates down. The warm up will be quite
quick. If it is a condensing boiler then you will be running it efficiently
during the warm up. So, with a larger modulating condensing boiler, it is
more efficient to keep the heating off, or at a reduced room temp of 15C.
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I admit I was assuming that the OP had some control over hot water
temperature - either a Y-plan or S-plan. If he's got gravity hot water, it's
BAD NEWS unless he also has a motorised valve in the circuit to make it into
I think that what you are remembering from your schooldays is "Newton's law
of cooling" which, as you say, states that the *rate* of heat loss is
proportional to the excess temperature. But the rate is *also* dependent on
the level of insulation.
The secret of stopping the house cooling too much when the heating is off is
to ensure that the insulation is good. It's then not too hard a job to
re-establish comfortable temperatures after a period of no heating.
I agree that a programmable room stat is a good idea in that it enables
different temperatures to be programmed at different times of the day to
suit your lifestyle. It also enables the heating to be turned OFF when the
house is not occupied - and this facility should be used, particularly if it
also has a feature to re-establish a target temperature by a specified time,
as some do.
This is not true, certainly not in every case. With my Honeywell S Plan system
with conventional programmer, the power to the boiler is controlled by
microswitches in the motorised valves which only operate when the stats are
calling for heat. Only the frost stat overrides this.
IMO the question of whether to have the heating on constantly has more to do
with personal preferences and the thermal capacity of the building rather than
with efficiency. It will always be marginally more efficient in itself to turn
the heating off when not required, but if the building cools down quickly (or
warms up slowly) to the extent that you need another heat source (eg a gas fire
or fan heater) to reach the comfort zone when you come home, this instantly
wipes out any benefit of switching off the heating. Like Danny, I leave mine on
constantly at a lower temp.
As Tony Balir would say "You have to consider the totality of what I said". :-)
My post was about gravity systems without H/W tank Thermostats, and would also
apply to gravity systems with H/W Thermostats which control a zone valve on the
gravity side which does not have the additonal contacts for controlling the
boiler. (As installed by some else in my house.)
You'd save yourself some effort by getting a programmable thermostat -
they divide the day up into bands and allow different temperatures for
each one - and for different days of the week. They are very easy to fit
as they are battery operated, so will replace near any room stat.
*Consciousness: That annoying time between naps.
Dave Plowman email@example.com London SW 12
| I currently leave my central heating system on - constantly (during
| winter). When I'm out of the house I turn the room thermostat down to
| 20 degrees, when I return I pop it back up to 22-23 degrees, at night
| I put the system back down to 20 degrees. ...
| This question may seem silly but if my central heating system is
| switched off all day the house temp drops to approx 15-16 degrees,
| this takes quite some time to get to 22-23 degrees and I guess alot of
| gas also!
You could get a programmable thermostat, which will do this automatically
for you, so you could drop the 'out' temp down to about 17deg and have it
start warming up to 'in' temp before you return (assuming you have a fairly
Your temperatures seem a little high; unless you have a need for such
temperatures (eg elderly or ill person in the house) you could try dropping
your 'in' temp a few degrees - my flat is usually 16-17C with the heating
It is never more efficient to leave the heating on for a period than to
turn it off. Having said that, we do leave our heating on, but set to
retain a low temperature. Even when the house will not be occupied for
several days, we leave the heating on set at 5 deg C to protect against
If your boiler cycling on and off for 30 seconds at a time, when there
is no obvious call for room or water heating, then there would appear
to be something wrong with it.
On Tue, 13 Jan 2004 23:00:11 GMT, Harry Bloomfield
It can be, depending on the thermal behaviour of the house and the
control system. Systems with setback, optimum start and outside
temperature compensation can result in little or no temperature
overshoot at the wrong times and more accurate temperature control
than where a simple thermostat and timer is used.
That is typically a lack of interlock on a non-modulating boiler.
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That is what I am finding - not that I have the gubbins.- but where huge
thermal masses are involved..
See if this makes sense.
You have a well insulated house but with a big slab of concrete floor.
In order to feel warm in this house not only does the air in the house
have to be warm, but the floor has to get warm too. In the meantime to
compensate for the cold floor the air in the house has to be warmer than
it otherwise needs to be. During this period it loses MORE heat per unit
time than it does once everything is up to temperature.
If this extra heat loss exceeds the savings by having it overall cooler
at might/during the day etc, then you have a net loss.
In my case, it seems to make very little difference which way I play it
to fuel consumption.
This was my point. You have a complex system in the house itself and
then external influences are added to it such as weather change. In
most systems it is then controlled by a very simple control loop
arrangement with large hysteresis (bimetal thermostat). Then you add
in the vagaries of the boiler.
Added to all of this you put in humans and there are a whole ton of
factors in feeling warm or not there. As you say, if the human then
does something to the heating arrangement in order to feel comfortable
like winding up the thermostat or adding other heat sources, then the
simple control system is going to do a poor job anyway.
Given all of that, it is way too simplistic to say that turning the
heating off always leads to greater efficiency. Even on a simple
control basis and ignoring the humans, it may well not.
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No, only in pathological lab setups will you find a system that is more
efficient if left on. It is far more efficient to turn it massively down
when not around. Any potential inefficiency will be due to overshoot as the
house warms back up. By choosing a sophisticated programmable room
thermostat, this can be avoided. This is because it will learn the response
of your house to heating demands and turn the heating back on just the right
amount of time before you are due to come home. This timing may vary
depending on the temperature.
Your system should always be pumping hot water through the cylinder. The
cylinder should have a thermostat that only asks for heat when the cylinder
is cold. This should cause a boiler interlock that prevents the boiler
firing when no heat is required. Keeping the primary circuit hot all the
time is very wasteful.
If you have a Y or S plan system, it is easy to add the thermostat. If you
have gravity circulated hot water, then you need to add a zone valve into
the pipework and control this with the thermostat after checking that it
won't damage the boiler. Some back boilers, for example, need to have the
gravity circuit open to avoid boiling.
If you have gravity circulated water, you usually have four water pipes from
the boiler. 2 will be 22mm for the radiators. 2 will be 28mm for the
cylinder. Obviously, combis will also have four water pipes, but usually
22mm for the radiators and 15mm for the water.
So to answer my question, using the system that I have. Is it best to
switch off the central heating completely during the day, switching on
when I'm in the house, and also to have the immersion heater
(permanently isolating the CH circuit from the HW storage) coming on
for 1/2 hour in the morning and 1/2 at night.
Or should I leave my immesion completely off, turn down my system to
say 17 degrees when out of the house, and back up when in the house
(except at night!)
d firstname.lastname@example.org (Danny) wrote in message
No, you should heat your hot water from the central heating system. It is
much more efficient. However, you MUST ensure that you have a cylinder
thermostat with effective zoning and interlock. This is not just an energy
efficiency issue, but a safety one. Without them, you'll find that the hot
water gets scaldingly hot, up to the output of the boiler, which is likely
to be 82C, rather than the more normal 55-60C.
On 14 Jan 2004 02:29:08 -0800, d email@example.com (Danny) wrote:
It really depends on the house and the controls in use.
It also depends on the hours of occupancy.
If you have a simple bimetal thermostat, for example, you can easily
get into a situation where the temperature massively overshoots if you
are warming from a lower starting temperature. If that is happening
as you are going out in the morning then you've wasted the heat. If
it happens during the evening, then you may well overheat the house
and make it uncomfortable.
The best, reasonably economic solution is to fit a
thermostat/programmer with optimised starting, different setback
temperatures at different times of the day.
You will then be able to set appropriate temperatures for the
different periods and the controller will take care of starting the
system at the right time to achieve them without significantly
overshooting and wasting energy.
You should certainly leave the immersion heater off except for
emergencies because the cost of energy is 3-4 times that of gas, so
even if you heat the water with gas non-optimally, it will cost less.
Whether you want to turn the gas heating of the hot water on and off
is up to you. If the cylinder is really well insulated, then it
doesn't make a great deal of difference as to whether gas heating of
it is enabled 24/7 if your usage is relatively high and at arbitrary
times during the 24 hour period. For example, in my case, hot water
could be used by somebody in the house more or less any time and the
cylinder is highly insulated. However, if you are a creature of
habit, then you may save energy by timing the gas heating of the water
for shorter periods to match your pattern of use.
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