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

orrery

SiHa wrote: »

Re: Painting radiators black:
The reason this is pointless...

No, it isn't pointless. When I found out about black body radiation in physics I painted all the radiators matt black (well, all the ones my Mum would allow) and the effect was very noticeable. With the boiler set to a high temperature it became impossible to sit near a radiator.

Black radiators are used extensively in industrial premises, where they are located high on the wall.

malc_b

gadgetmind wrote: »

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

No. You have this the wrong way round. When you first fire up the boiler the water is cool, and the boiler is in condensing mode. But if the return water stays cool then you are not heating the house and you cold room stays cold.

The maths works like this.

1. Boiler puts 84kJ into 1l of water raising temperature 20C from say 60C to 80C. This flows into a radiator at 80C and out at 60C giving up the 94kJ added to the room. The radiator average temperature is 70C so if the room is at 20C the difference is 50C and the radiator is working at its rated d50 value, say 1026W (Kudox 600Hx1000W).

2. Boiler puts 42kJ into 0.5l of water raising temperature 20C from say 40C to 60C. This flows into a radiator at 60C and out at 40C giving up the 42kJ added to the room. The radiator average temperature is 50C so if the room is at 20C the difference is 30C and the radiator is working at its rated d30 value, which is now only 534W.

So the only way to say continuously in condensing mode is with a continuously low radiator temperature and hence low radiator output. How is that going to heat a cold room?

Smiley_Dan

gadgetmind wrote: »

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

Does mains pressure cope with 10+ bedrooms, a kitchen and everything else? My eyebrow is raised... ;-)

malc_b

Cardew wrote: »

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

It's a setting on the boiler, at least output water temperature is, return depends on the pump speed and radiator sizing but a well designed and set up system will aim to have about 20C difference between input and output once it has reached stable state. I think modern boilers sense return water temperature too.

Cardew wrote: »

As gadgetmind states:

(Incorrectly as I explain)

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.

Yes and no. The total amount of heat lost is important but also important is the efficiency of turning fuel into heat. Say I left the heating off in the morning until the last second then put the heating on a max temperature and turn on loads of electric fires. Would that be cheaper? Why not, I've cut the time down to the minimum, so what if my fuel cost per run minute is higher, I'm being told cut the on time is the only factor to consider.

Cardew wrote: »

Malc
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.

Yes, which is exactly what I say and if you look at the maths in the link I posted that is what the example says, 24hr = 24kWh, 16/24hrs = 22kWh.

The point is that the difference between 24/24 and 16/24 or other hours is quite small and can be less than the difference between condensing and non condensing.

Let me ask you a question. Middle of winter, say 0C outside, how cold is your house before the heating comes on after it has been off all night? Warmer than 10C I bet. So overnight when average temperature is 15C heat will be leaking from the house. Where does that heat come from? And the next morning it's the same, so where has the heat for the next morning come from? It has to be the house structure. Physics says energy can only be moved about. So if each night you take energy out of the house structure then each day you have to put it back. Which makes it harder to run in condensing mode because during the day you need more kW from the boiler.

And to answer your question, heat loss from a house stops when it reaches outside temperature. I'd guess a few days, but let's do the maths for a day.

Inside = 20C, loss = 1kW, outside = 0C, takes 24hrs for inside to reach 0C. Average temperature on day 1 is 10C, so 0.5kW loss. 24hrs x 0.5kW = 12kWh. And next we heat it back up to 20C for 24hrs, that will be 24kWh loss. Over the 2 days it is a total of 36kWh i.e. 18kWh/day. Compare that with leaving heating on, 24kWh, 75%. Is 25% > than condensing vs non-condensing, yes so it saves fuel to turn heating off. The maths work for all cases.

gadgetmind

malc_b wrote: »

So the only way to say continuously in condensing mode is with a continuously low radiator temperature and hence low radiator output. How is that going to heat a cold room?

Because even the "cool" return water is hotter than the room, so thermodynamics does its thing. You address the issue of low radiator output by derating them when designing the systems, so effectively fitting "oversized" radiators.

Trust me, my return temperature is around 40C max and we have a lovely warm house. (OK, one room struggles, but that's because I've got some draft issues to address.)

gadgetmind

Smiley_Dan wrote: »

Does mains pressure cope with 10+ bedrooms, a kitchen and everything else? My eyebrow is raised... ;-)

I'm guessing they use something larger than 22mm!

Smiley_Dan

Of course but what diameter? And what happens when water pressure drops? Seems unlikely to me but I don't know either way.

gadgetmind

There is some stored water, and they get to choose between storage calorifiers, semi-storage ones, or instantaneous. I guess only the latter relies 100% on mains pressure.

The hotel I used to work in had a beast that looked similar to this.

http://www.mccallumwaterheating.co.uk/maximisers.php

malc_b

lstar337 wrote: »

The increase in efficiency through condensing is actually very small (maybe a few %),

Maybe you want to try googling before making statements as to the efficiency of condensing mode.

lstar337 wrote: »

and no where near enough to overcome the cost of heating a house (due to losses) when nobody is home.

And support that with maths.

As an example cold water always freezes faster than hot right? The fact it doesn't cold water can take longer to freeze that hot (google it). Sometimes what we think is obvious is actually fact.

lstar337 wrote: »

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.

So in fact you follow what I said, keep the boiler in condensing mode and the hours as short as needed.

Most of us are not as blessed and need to heat longer. Following the advice to heat shorter would likely mean NOT running in condensing mode.

victor2

malc_b wrote: »

And to answer your question, heat loss from a house stops when it reaches outside temperature. I'd guess a few days, but let's do the maths for a day.

Inside = 20C, loss = 1kW, outside = 0C, takes 24hrs for inside to reach 0C. Average temperature on day 1 is 10C, so 0.5kW loss. 24hrs x 0.5kW = 12kWh. And next we heat it back up to 20C for 24hrs, that will be 24kWh loss. Over the 2 days it is a total of 36kWh i.e. 18kWh/day. Compare that with leaving heating on, 24kWh, 75%. Is 25% > than condensing vs non-condensing, yes so it saves fuel to turn heating off. The maths work for all cases.

Over-simplified maths?
The above assumes the heat loss is linear, which of course it isn't, just as the leaky bucket doesn't lose water at a steady rate.