Central Heating Efficiency Question

[Danny]

Hi all,

I've a couple of questions that I was hoping someone could help me out with.

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 temp.

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 gas also!

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

[Set Square]

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 heat.

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.

[Michael Chare]

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 not have 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.

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!.

Michael Chare

[Dave Plowman]

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.

[Owain]

"Danny" wrote | 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 regular lifestyle).

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 on.

Owain

[Harry Bloomfield]

On 13/01/2004 Danny opined:-

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 frost.

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.

[IMM]

Timer/Thermostat..

>

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.

[IMM]

The system may require balancing, also a timer to delay cycling is a good thing. Many thermostat/programmers have an anti-cycle function incorporated.

[Set Square]

>> One advantage of the TP9 is that it only supplies power to the boiler

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 a C-plan.

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.

[Andy Hall]

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.

.andy

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[Peter Taylor]

Michael Chare wrote

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.

Peter

[Christian McArdle]

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.

Christian.

[Danny]

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!)

[The Natural Philosopher]

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.

[Christian McArdle]

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.

Christian.

[Andy Hall]

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.

.andy

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[Andy Hall]

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.

.andy

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[Michael Chare]

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.)

Michael Chare

[Danny]

Excellent. Thanks for all of your help (again!)