Energy myth-busting: Is it cheaper to have heating on all day?

malc_b

oldskoo1 wrote: »

I'm a bit confused about boiler stat / water temp. In your previous post you said you just wanted hand hot radiators not mad hot, but in this reply you said set the boiler stat to max (which would create mad hot water for the rads).

I find the only way to stop cycling in my system is to put the pump on 3 regardless of stat temp setting on boiler.

It would be good for you to clarify the correct boiler temp setting as what you posted seems to contradict unless I understood it wrong.

Btw my boiler is non condensing, non modulating afaik. I'm looking at what setting would potentially use less gas once wall stat is up to temp

The temperature the radiators get to depends on what the hall stat calls for. The boiler runs and heats up the water in the radiators. Then stops. Then the heat in water gets given up to the house and the radiators and water cool down. The more heat the house needs the longer the boiler runs, the hotter the water/rads become.

Note this assumes either an intelligent PID controller or a small difference stat. If the stat only clicks in when there is 5C to make up then the boiler always works hard. PID controllers are better because they keep the temperature to within 0.5C.

So for the above the water dial on the boiler does not come into it. The peak water temperature is just governed by the house need. All the water dial does is cause short cycling.

I suppose if the hall stat is rough and ready then using the water dial could limit the max water temperature and so by limiting the heating input to the house smooth the temperature ranging. But the correct fix for that is a better hall stat.

The pump speed should be set for 20F, 12C across the boiler (condensing is larger I think). Think of this way, the boiler dumps say 10kW into the CH water. You need a certain flow to get that 10kW away. Too slow and the water gets too hot, too fast and the system is noisy. Hence measuring the temperature drop across the boiler (i.e. outlet temp - inlet temp) tells you the water flow. If you don't have enough flow then you can short cycle.

grahamc2003

malc_b wrote: »

This is exactly the natural thinking but TOTALLY wrong. 16 on, 8 off feels like a saving of 1/3 but look back at the calculations I've done, they gave a saving of 8%.


It's only you - and no one else - who keeps repeating the straw man of '8 hours off is not a 33% saving' as if correcting someone who is saying that.


This is because the house stays warm overnight so the losses stay high. The better insulated the house the less the saving because the higher the overnight temperature. Counter-intuitive isn't it.

What a cracker! Everyone rip out their insulation because it's keeping your house warmer and costing you more money!

If you're being serious with that - and I hope you're not - then yes insulation keeps the house warmer (and I'll add the Pope is catholic), but no, it doesn't cost you more The house is warmer precisely because heat losses are less, not more. That is sort of the idea of insulation.

I'm just curious. Did you get your C.Eng via the BCS? If so, you have me to partly thank for that, and you're now confirming a growing unease I have of some of my passed actions. If not, mind me asking which professional body you got your C.Eng through?




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malc_b

I've mention PID temperature controllers so I should explain that jargon. I've the CM67 and my son has the CM907 (67 was no longer made). There are certainly others around too. PID is proportional–integral–derivative controller. Rather than the on/off of stat it measures the temperature integral and so keeps the temperature very stable and the boiler burns very regular.

The above units run on a fixed cycle, 15min for oil, 5min for gas, and then give say a 50% burn (7.5 or 2.5min) if it reckons the house needs 50% power. It also works out a temperature rise time for the house so you set say 20C at 7am. At some time it starts ramping up the house heat to reach 20C at 7am. When it starts the heating it decides, cold nights it starts earlier than warm nights.

These replace a standard wall thermostat and become both time clock and thermostat. If you want to move the thermostat then you can also get 2 part units where the controller display and sensor is wifi'ed to the wall mounted unit.

Cardew

malc_b

This was the question!

So a related question whilst we have assembled all this
knowledge.

Is it more economical to have the gas central heating boiler set at a high temperature and hence getting the house warmer more quickly but maybe be losing some radiant heat from the radiators when they get cold quicker

Or a low boiler setting giving any radiant heat for longer.

This was your reply!

By this I assume you mean the water temperature dial on the boiler and not the hall thermostat?

On any modern system the water max temperature dial on the boiler should be set to max and left there.

Do you stand by that statement?

P.S

Not what you do with your non-modulating non-condensing oil boiler.

wantanswers

Q I note you recommend running the boiler on the “24-hour” setting at low temperature during cold spells (December 5). Why is this? And would this not imply that boilers are conventionally run too hot? JN, Epsom

A The point of the exercise is to try to prevent condensation, because when this occurs, every time the heating system starts up, it first has to use energy to evaporate the condensed water, before the energy can go towards heating the house.
Remember that condensation is not something that occurs only on window panes — it forms chiefly within the fabric of the walls, thus lowering their thermal insulation value. That’s why I suggest aiming to keep the radiators constantly warm, rather than cycling them from cold to very hot and back again.
Think of the motoring analogy, where everyone agrees it is more fuel-efficient to drive at a steady speed, rather than continually stopping and starting.

http://www.telegraph.co.uk/property/propertyadvice/jeffhowell/8214378/Home-improvements-how-to-heat-the-house-this-winter.html

wantanswers

Q You mentioned turning down the boiler and turning the radiator thermostats fully open. We have a condensing boiler and British Gas advised the opposite: turn up the boiler so that we get the fuel-economy of a condensing boiler and turn down the radiator thermostats. They say condensing boilers don’t work efficiently on low heat. Who is right? AT, by email

A I’m right and they’re wrong. Condensing boilers only really condense properly (thus extracting the waste heat from the flue gases) at a water return temperature of 54C to 56C. This is usually achieved by having a flow temperature (that is, the water flowing out of the boiler) of around 70C.
As the water flows through the radiators it gives up its heat to the rooms, thus returning to the boiler at a lower temperature. TRVs (thermostatic radiator valves) cause no end of problems, so I advise leaving them fully open.

wantanswers

Q We have a couple of bedrooms that are seldom used where we keep the radiator valves turned down to one. Is this economical? ES, by email

A In my opinion, no. A common thread of the government propaganda is that money can be saved by only heating some rooms. However, this is bound to create condensation problems in the unheated rooms.
This will not only result in boiler inefficiency – as described above – but the consequent mould damage and redecorating costs will soon use up any minor savings on fuel. Keep your home warm and dry throughout and it will save you money in the long run.

malc_b

Originally Posted by malc_b
This is exactly the natural thinking but TOTALLY wrong. 16 on, 8 off feels like a saving of 1/3 but look back at the calculations I've done, they gave a saving of 8%.

It's only you - and no one else - who keeps repeating the straw man of '8 hours off is not a 33% saving' as if correcting someone who is saying that.

You said "Remeber the situation is 8 hours off - that is one hell of a time". I was just pointing out that this hell of a time only amounted to 8%

This is because the house stays warm overnight so the losses stay high. The better insulated the house the less the saving because the higher the overnight temperature. Counter-intuitive isn't it.

What a cracker! Everyone rip out their insulation because it's keeping your house warmer and costing you more money!

For heaven's sake read what I post. The "less the saving" not the less absolute heat loss.

Take a well insulated house that is 20C inside to outside during the day and falls just 2C over a 8hr switch off. The night loss rate is ~19/20 of the day rate. Now consider a house that falls 10C overnight. The night loss is ~15/20 of the day. 19/20 > 15/20 so with more insulation the night heat loss rate is closer to the day heat loss rate.

In absolute terms the heat loss is less with more insulation but also the change between day/night is less so the saving switching off over night is LESS (relative to the day or total house loss).

When the losses are close to variation in boiler efficiency under load variation you can't make a blanket statement that it always saves fuel to run shorter times.

Just so we are clear I'll do the maths again on my pretend house with a 1kW loss at 20C. 24hr -> 24kWh. With 8hrs off at an end temperature of 10C using a linear (i.e. rough) model then the average is 15C so night loss is 750W x 8 = 6kWh rather than 8kWh. A fuel saving of 2kWh or 8% (2/24).

If the house is better insulated so losses are halved to 500W then 24hrs -> 12kW. At the end of the night the temperature is 15C not 10C so average is 17.5C i.e. loss is 500 x 17.5/20 = 437.5W, over 8hrs this is 3.5kWh vs 4kWh (8hrs x 500W). That is a 0.5kWh saving or 4% (0.5/12).

So the better insulated the less the saving for running shorter, QED.

malc_b

wantanswers wrote: »

Q We have a couple of bedrooms that are seldom used where we keep the radiator valves turned down to one. Is this economical? ES, by email

A In my opinion, no. A common thread of the government propaganda is that money can be saved by only heating some rooms. However, this is bound to create condensation problems in the unheated rooms.
This will not only result in boiler inefficiency – as described above – but the consequent mould damage and redecorating costs will soon use up any minor savings on fuel. Keep your home warm and dry throughout and it will save you money in the long run.

The other point to think about is the insulation between rooms. All of the house insulation is in the outside walls etc.. Between rooms the U value is quite high so if you turn off a radiator in a room all that happens is that it is heated by the rooms around it. Again I come back to a point I've made, if the room is above the outside temperature it is leaking heat out. That heat has to come from somewhere, in this case the surrounding rooms whose radiators now have to work harder requiring a higher water temperature and possibly meaning the boiler no longer condenses. And with less radiators the more likely the boiler is to short cycle.

wantanswers

malc_b wrote: »

The other point to think about is the insulation between rooms. All of the house insulation is in the outside walls etc.. Between rooms the U value is quite high so if you turn off a radiator in a room all that happens is that it is heated by the rooms around it. Again I come back to a point I've made, if the room is above the outside temperature it is leaking heat out. That heat has to come from somewhere, in this case the surrounding rooms whose radiators now have to work harder requiring a higher water temperature and possibly meaning the boiler no longer condenses. And with less radiators the more likely the boiler is to short cycle.

I've just corrected this situation......thanks.