MERGED: Should heating / water be left on?

Cagey

Robert, I find that as it gets colder outside I have to set my thermostatic valves higher to compensate.

Robert5988

Cagey wrote:

Robert, I find that as it gets colder outside I have to set my thermostatic valves higher to compensate.

Cagey,
I can’t think why that would be. The position you set the valve to should correspond to a set temperature – say 70F – and the principle is that the thermostatic valve(like any thermostat) should maintain the room to that 70F.

Could it be psychological? When its cold outside and we often want to be warmer inside.

clive_the_gas

You could do your own comparison.
Take your gas meter reading, then leave your heating set on 24hrs low/med setting for a week.
Then take the meter reading and for the next week put your heating on 6hrs twice per day.
At he end of the fortnight take the final reading & you will have a rough comparison.

Woby_Tide

a drastically different topic than yesterdays "is it cheaper to have heating on low all day or higher a couple of times a day".....:)

sunflower_2

i worked out it was cheaper in my old house to have it on low constantly (large house, quite open plan)

in my current house its loads cheaper to put it on high just when i need it (2 bed ickle dinky house).

however, on GMTV thismorning it said that to prevent your boiler from breaking down in the winter when it gets whacked - it should be kept on constant at a low temperature
apparently better for your heating system?

may use more gas but save on heating repair bills? and frosty toes

bbbrrrr

wish the snow would go away - its like a blizzard out there

[Deleted User]

Robert5988 wrote:

Exactly 17p a year - not per night?
Could you please give us your figures to prove that statement.

About 17p per year. This is based on the following assumptions:
Water temp 50C
Surrounding temp 20C
Tank size 36"x18" (= 150L, surface area 1.64sqm)
Lagging thickness 50mm
Immersion Heater 3kW (substantially irrelevant)
The reason I calculated this is because it is a bit counter intuitive. Are you sure you want the calcluation, it's quite long winded?
The cost of providng hot water is made up of the cost of heating the water you use plus the cost of heat leaking out through the lagging. You only save on the former by using less water, this is a consideration of the latter.
The heat loss from the tank (and hence the cost) is proportional to the difference between the temperature of the water and the temperature surrounding the tank. At the point you switch the heater off, the tank is just as hot as with it on, and here's the rub, it remains so for many hours after. If the tank is well lagged, by the time the heat is back on the temperature has only dropped by a degree or two, so the heat loss has remained largely constant throughout the off period, hence the wasted heat has also remained the same. The fact that the immersion heater replenishes the lost heat during a 16 hour period rather than the full 24 hours is irrelevant.

[Deleted User]

Cagey wrote:

Robert, I find that as it gets colder outside I have to set my thermostatic valves higher to compensate.

This is because the body loses heat through radiation to the wall as well as convection to the air. When weather gets colder the rad valves keep the air temp constant, but the walls get colder and your body loses more heat.

Robert5988

jack_pott wrote:

About 17p per year. This is based on the following assumptions:
Water temp 50C
Surrounding temp 20C
Tank size 36"x18" (= 150L, surface area 1.64sqm)
Lagging thickness 50mm
Immersion Heater 3kW (substantially irrelevant)
The reason I calculated this is because it is a bit counter intuitive. Are you sure you want the calcluation, it's quite long winded?
The cost of providng hot water is made up of the cost of heating the water you use plus the cost of heat leaking out through the lagging. You only save on the former by using less water, this is a consideration of the latter.
The heat loss from the tank (and hence the cost) is proportional to the difference between the temperature of the water and the temperature surrounding the tank. At the point you switch the heater off, the tank is just as hot as with it on, and here's the rub, it remains so for many hours after. If the tank is well lagged, by the time the heat is back on the temperature has only dropped by a degree or two, so the heat loss has remained largely constant throughout the off period, hence the wasted heat has also remained the same. The fact that the immersion heater replenishes the lost heat during a 16 hour period rather than the full 24 hours is irrelevant.

I understand what you are saying about it being counter intuitive. However it boils down to(no pun intended) comparing heat losses between a tank maintained at 50C and a tank allowed to cool down to(using your figures) 48/49C.

Firstly although I have never checked it I would have thought that a tank would drop by more than “a degree or two” over 8 hours.

Secondly you have obviously worked out the differing Btu losses for 8 hours per night and costed that energy at 17p per year. 17p equates to less than 3kW. Thus a 3kW immersion heater would only need to operate for less than 10 seconds each night to replace the differing heat losses

Therefore switching off for 16 hours would save less than 34p a year – heat losses would be lower in the second 8 hours as “The heat loss from the tank (and hence the cost) is proportional to the difference between the temperature of the water and the temperature surrounding the tank.”

Using that logic could you calculate how much you would save in a year by switching the immersion heater off completely rather than leaving it on permanently to maintain the temperature at 50C. Obviously we are assuming the no hot water to be used – the house vacated but leaving the immersion heater on.

[Deleted User]

Robert:
10 seconds a night is about right, remember that's not the heat lost, but the difference between heat lost when off, and heat lost if left on.

Switching off for 16 hours saves about 67p, because when you increase the time the temperature difference increases too, and the saving is proportional to the product.

When you say the heat losses are lower in the second 8 hours you are confusing the heat losses with the saving in heat losses. As the water cools, the heat losses go down, but the saving goes up.

The saving over a year in a vacated house is £18.88 (see below), so if it weren't for the slow cooling of the water you might expect to save about £6 pa by swiching off for 8 hours in 24. If you switch off for 6 days in 18 the saving is about £2.20 pa. If you switch off for 1 month in 3, the saving is £4.80 pa.

I've whittled the calculation down to an approximation, which is very accurate, but only if the heater is switched off for a short time relative to the time that the tank takes to cool. This is the case for an 8 hour period, if the tank is lagged.

Data
U=0.04 W/m/K: Thermal conductivity of lagging.
z=4.18 J/K/g: Specific heat of water.
r=0.12 Ksqm/W: Thermal resistance of a surface in contact with convecting air.

Parameters
a=1.64sqm: Surface area of a 36"x18" tank.
m=150kg: Mass of 150l of water.
d=50mm: Lagging thickness.
W=50C: Water temp.
A=20C: Airing cupboard temp.
h=28800s: Time heater is switched off for (8hrs).


The total thermal resistance of the lagging and convecting air

R =d/(Ua)+r/a =0.835 K/W

The thermal capacity of the water

C =mz =627 kJ/K

Temperature difference across the lagging

D =W-A =30K

Power loss

P =D/R =36W

This is a loss of £18.88 per annum @ 6p per unit, (and compares with about £130 pa if the tank were unlagged, and R=0.073)

Over a period h, the lost energy if the heater remains on

E =Ph =Dh/R

If the heater is switched off for period h, the temperature change

c =Ph/C =Dh/(CR) =1.65K

The mean temp during an off period

M =D-c/2

Energy lost during period h, if the heater is off

E =Mh/R =Dh/R-ch/(2R)

The difference between the lost energy with the heater staying on and the lost energy with the heater off:

Difference =Dh/R-Mh/R =ch/(2R)

=Dh^2/(2CR^2)

=28.5kJ/day

=2.9kWh pa

=17p pa.

This calculation makes the approximation that the rate of heat loss from the tank is constant. The reason that this is an accurate approximation is that the temperature only drops by 1.65 degrees over the eight hours, so the rate of temperature change only changes in the ratio 28.35/30.
Were the tank unlagged, this approximation would be alot less accurate because the tank cools much more. With an unlagged tank the annual saving is about £8, but this method overestimates it at about £18.

rcman

We have a lagged immersion tank/heater. We are a family with two young children. We have a dishwasher and washing machine running from both hot and cold feed - use hot water to wash out hands and for a bath about once every 3-4 days (shower runs from cold mains).

My question is: Is it more efficient to leave the heater on 24/7, retaining the heat and working the thermostat - or would it be better to run it from a timer, just heating up a tank full once or twice a day and then switching off?