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

gadgetmind

malc_b wrote: »

The most efficient way to run a condensing boiler is with a low water temperature, less than 55C on the return is the usual advice.

Yes, so maximum time in condensing mode is when heating a cold room using large radiators as the return temperature is lower.

victor2

malc_b wrote: »

Again to draw an analogy, heating a house is like filling a leaky bucket. If you let the level drop then the leak slows but then when you fill it up to the top you have to put in water a lot faster than if you just matched the leak.

That's a nice analogy, but I'd argue with the term "faster". You could put the water in at the same rate as when it was full and it would eventually fill the bucket, as the leak slows with a lesser amount of water in the bucket. Of course it would take some time to fill, but it will fill it at that rate. Boilers don't work that way anyway although it will heat the water more if it returns cooler...

lstar337

r2015 wrote: »

I have found that as well.

Since I retired I have the heating on from 8 am to 11 pm and if it is very cold outside sometimes 24 hours and I do not seem to use much more gas than when the heating was on from 7 am to 10 am then from 3 pm to midnight.

From when I bought my house in 1991 to when I retired in 2005 my average yearly gas consumption was 20721 kWh per year and from 2005 to present it has been 18058 kWh per year.

A difference of 2663 kWh per year.

Let the laws of physics explain that.

Different weather. Done.

Unless your house is in a laboratory under controlled conditions?

lstar337

malc_b wrote: »

Fact: The laws of physics say turning off heating gives lower losses because as the house temperature falls the losses decrease.

Agreed, but this is lower losses,that is NOT the same as lower fuel bills. It is a well know FACT that condensing boilers are less efficient when running hard, i.e. high water temperature. The most efficient way to run a condensing boiler is with a low water temperature, less than 55C on the return is the usual advice.

So maximum fuel efficiency is to heat for as short a time as possible with 55C return, AND, when the house fails to reach the desired temperatures, increase the heating TIME.

The increase in efficiency through condensing is actually very small (maybe a few %), and no where near enough to overcome the cost of heating a house (due to losses) when nobody is home.

Now, I do keep my return below 55c (In fact my feed isn't much higher) to keep my boiler condensing and at it's highest efficiency, but I have a very well insulated house. Last winter (admittedly was pretty mild) I only ran my heating for 1hour at 5am, and 1hour at 7pm to maintain 21c+ virtually all the time.

lstar337

malc_b wrote: »

Again to draw an analogy, heating a house is like filling a leaky bucket. If you let the level drop then the leak slows but then when you fill it up to the top you have to put in water a lot faster than if you just matched the leak.

And if you fill it constantly then the leak will increase, meaning more losses over a greater time.

I fail to understand why people are bothering to post examples that they think are able to violate the basic laws of physics?

Cardew

malc_b wrote: »

It is a well know FACT that condensing boilers are less efficient when running hard, i.e. high water temperature. The most efficient way to run a condensing boiler is with a low water temperature, less than 55C on the return is the usual advice.

So maximum fuel efficiency is to heat for as short a time as possible with 55C return, AND, when the house fails to reach the desired temperatures, increase the heating TIME.
.

Malc
Your post gives the impression that the boiler water temperature varies automatically.

As you say higher efficiency is achieved(i.e. condensing mode) when the RETURN water temperature is low. To achieve this it modulates(turns down) the output of the boiler.

As gadgetmind states:

Yes, so maximum time in condensing mode is when heating a cold room using large radiators as the return temperature is lower.

The latter part of your leaking bucket analogy also is incorrect; and defeats your argument. What matters is the total amount of water(heat in a house) that leaks. We agree that the rate of leakage slows as the house gets cooler(and eventually there will be no leak when inside and outside temperatures are the same) however all we need to do is replace the heat that is lost.

So using your example, as you state, the maximum rate of leak(of water or heat) is when the bucket is full or the house is warm. Thus to keep it full/warm you will need to put in water/heat at a faster rate.

Your theory still doesn't explain the question I posed above:

You say 'away from the house 12 hours or more' So let us take the situation where someone was away from the house for 5 years*. Would you maintain it was cheaper to leave the heating on for those 5 years?

How about 1 year? 1 month? 1 week? 1 day? at what point do you feel the laws of physics do not apply?

Cardew

Fitzmichael wrote: »

I haven't read this thread right from the beginning, so maybe there's an answer I've missed. I'm asking via this Cardew's post because he raises another question that has puzzled me, saying: The problem with a combi is that in winter they struggle to produce a good flow for a shower even if that is the only HW demand.
I have a combi and from the shower there's always a v good flow, and far too hot for me well below max on the temp control setting, but my wife turns it right up and stays under it for a good quarter hour.
The problem is that the temp setting (at my daughter's, too) is only approximate and depends on the ambient temp (presumably of the intake). It, too, of course, can suddenly go colder/hotter if somebody runs water elsewhere. The only thermo-controlled domestic shower I have used which adjusted v quickly and reasonably precisely was electric, but it gave poorer flow the higher the temp; this was not the case in a couple of hotels I've used. Is this, too, dependent on the combi/hot tank question? Assuming the inflow is sufficient, is it not possible to have the outflows from the boiler to the feeds to the bath/shower(s) and kitchen, for that matter, controlled from the outlets? Would it be prohibitively expensive?
It's particularly puzzling as my grandparents' house, long in the family and having housed different combinations of relatives, had the cold and hot water tanks on the top (2nd) floor and 2 bathrooms and a shower-room on that and the 1st floor. On the ground floor were a toilet next to a 'wash-house' upgraded to 'utility room' and two kitchens at different extremities of the house. These were a long way from the hot tank and seemed never to have been connected to it, but each had a small gas geyser above the sink, which supplied instant hot water.
This last has made me wonder, over the last 20 years of increasing emphasis on minimising waste of fuel and water, why we still have water heated far from where it is wanted, so that yards of cold water run to waste until the warm/hot water gets into the, colder, pipes, which consume some of its heat, in a repeated cycle a dozen or more times a day. Is a more efficient system not possible (why?) and if it is, why have our, supposedly concerned, governments not altered Building Regs to impose it?
Now to my intended question about efficient regulation of house temperature. Posts seem to concentrate on adjusting the TRVs according to room usage but surely what's needed is to vary the temperature according to time, and I'd never heard of valves to do that until, in the course of this post, I spotted Weston Dave's "there is such a thing (widely available) as a programmable thermostat which combines timer and stat", but I'm not sure he's talking of something to control a radiator. Can he, or anyone, enlighten me? Thanks.

To answer your last question first, there are radio controlled TRVs(thermostatic radiator valves) that can be set for time and temperature:


http://www.honeywelluk.com/products/Valves/Thermostatic-Radiator-Valve/Electronic-TRVs/


On the question of 'gas geysers' above sinks, these were quite common in old houses mainly in urban environments; and would have had a pilot light burning 24/7. However do we want gas pipes all over our houses? That, with the cost of the appliances and annual servicing would surely make them impractical.


Given the normal maximum rated domestic electric shower is 10.5kW - with many 8.5kW or 9.5kW - it is not surprising that the flow rate is not powerful. Gas combis are typically 18kW to 35kW.


Are you sure that the 'electric showers' in a hotel with a high flow rate were not 'power showers'? Power showers use hot water from a tank which is pumped to the shower head.

orrery

malc_b wrote: »

So maximum fuel efficiency is to heat for as short a time as possible with 55C return, AND, when the house fails to reach the desired temperatures, increase the heating TIME.

I can set my boiler to regulate on the return temperature to ensure maximum efficiency, but I've tried this a couple of times and it really doesn't effectively heat the house or the water.

Most modern intelligent thermostats will switch to 'proportional' control when close to the target temperature. My old Honeywell did this (badly) and I now have a Tado system which does it very well indeed. The effect is to run the boiler infrequently so that the return temperature is always low.

gadgetmind

Cardew wrote: »

Are you sure that the 'electric showers' in a hotel with a high flow rate were not 'power showers'? Power showers use hot water from a tank which is pumped to the shower head.

Hotels typically use direct fired storage water heaters with unvented hot water, so at mains pressure.

We've got similar at home, but it's driven from a very efficient condensing boiler. The showers are great, and it's well insulated and gives the condensing boiler a very cool return.

http://www.acv.com/gb-en/03_04/441/app.rvb

SiHa_2

Re: Painting radiators black:
The reason this is pointless is because radiators are misnamed. They are, in fact, convectors. They heat the room air, and rely on setting up a convection current with cold air coming in the bottom and warm air rising out of the top. Painting the radiator black would increase the radiation from the surface, and you'd feel the extra heat on your face if you stood in front of it. It would do nothing to heat the air in the room other than by the fact that any surfaces that are warmed by radiation will, in turn, warm the air slightly.