condensing mode vs non-condensing mode

spinningsheep

Hi people. In this extreme cold snap I have been leaving my heating on all day while out at work, as someone is in the house all day long asleep as on night shift, so house is always "aired", running thermostat in hall at 17.5 and the boiler on 57 degrees (baxi duotec 28kw). I usually turn the stat back up to 19 and boiler to 65 when I get in about 6pm and the heating then goes off at 10pm. Now, as much as I understand the fact that if I turn the boiler up the radiators get warmer, faster, if I was to turn the boiler up even further, would the decrease in efficiency mean a big increase in gas usage money wise ie say 10% or 20% less efficient? If the increase was say an extra £1 a day, while it is sub zero even by day it may be worth it. I know the burner modulates down to a gentle simmer but the water would be still flowing at say 70 degrees and would most likely be returning at over 55 degrees. I know most people wouldn't be bothered but I am keen to understand the cost implications vs comfort. I called Baxi & they just said it was down to personal preference, which I fully understand. I want to understand in real terms what the increase in gas would mean.

HappyMJ

It won't be anywhere near that much. The savings will be pennies. You would save much more by turning the heating off whilst whoever is in the house is asleep. If they are cold in bed get an electric blanket.

macman

Turning up the boiler CH output temp wil simply increase the temp of the circulating water, and so bring the house up to temp sooner. The amount of energy required to raise the temp from A t B is the same whehter you do it quickly or slowly.
Leaving the timer on 24/67 will massively increase your bills, as the boiler is on 24/7 rather than about 16/7. If the house is warm enough overnight without the CH on, it'll be warmer in the daytime.

spinningsheep

Hi folks cheers for the responses, I know it would be better to use a leccy blanket but it isn't just for his benefit, the house is way more comfortable, and when I was on my day off I noticed that the boiler fired for about 15 mins every hour or 2 to "top up" the temperature so the heating certainly isn't firing all the time even though it is on constant. Thanks for the clarification on the difference in costs, I was worried it would be way higher.

macman

It will be way higher!

spinningsheep

Sorry macman I am confused, I thought you said that the difference in running boiler non condensing as apposed to condesning would be small? ie less than £1 a day difference?

If you mean the cost of leaving the heating on all day on low, yes of course it will be more than if it was off but I disagree with WAY more, if the boiler only fires up say 4 or 5 times during the daytime for about 15 mins, thats only about an hour of heating would would be about £1 max when the boiler is running at low output. I just did my readings and I have spent £302 on gas since Nov 8th, and the heating has been on constant since last Thursday on low in the day, and by my calcs we have spent £32 on gas, whereas a normal week ie when it is not -2 by day, we spend about £25 ish a week on gas in the winter time. Anyway my credit balances amount to just under £840 so plenty to cover the quarterly bill due in about 1 week.

Cardew

Excellent question about condensing/non-condensing mode.

I have raised this matter several times on this forum - including a separate thread.

The principle of getting the boiler to operate in condensing mode is one thing, but to quantify the savings is quite another. I too have tried to get information from manufacturers without success.

It is commonly accepted that the theoretical efficiencies of boilers shown in SEDBUK are much like theoretical MPG figures for cars - rarely achieved in practice.

Unless on MSE we have heating engineers who work on the design of boilers I doubt we are going to get any answers. I doubt if even the most experienced RGI is any better placed to give an answer either.

Another aspect is what happens about condensing mode if you have a different heating load. For example using your 57C example.

A. You have every(say 12) radiator on in a cold house.

B. Just one small radiator on.

The difference in the return temperature of water will be considerable. In case B with very little heat being dissipated the return temperature will not be much lower than output temperature.

I just don't know how we are supposed to operate boilers to achieve, and stay in, condensing mode.

HappyMJ

spinningsheep wrote: »

Sorry macman I am confused, I thought you said that the difference in running boiler non condensing as apposed to condesning would be small? ie less than £1 a day difference?

If you mean the cost of leaving the heating on all day on low, yes of course it will be more than if it was off but I disagree with WAY more, if the boiler only fires up say 4 or 5 times during the daytime for about 15 mins, thats only about an hour of heating would would be about £1 max when the boiler is running at low output. I just did my readings and I have spent £302 on gas since Nov 8th, and the heating has been on constant since last Thursday on low in the day, and by my calcs we have spent £32 on gas, whereas a normal week ie when it is not -2 by day, we spend about £25 ish a week on gas in the winter time. Anyway my credit balances amount to just under £840 so plenty to cover the quarterly bill due in about 1 week.

It is WAY more...You spend £3.50 per day using your way. By your own admission this is £1 more than timing it to be on/off. That's 40% more expensive than it needs to be. If you did not have the boiler active for that 1 hour during the day then your daily bill would average £2.50 per day. I would rather not have a bill which is 40% higher that I need so I turn my heating down to low at night (about 7-12C depending on how I feel) and turn it off completely whenever I leave the house. Worrying about a few percentage points difference in efficiency is pointless when you could easily save £1 per day.

albyota

A good amount of information from CAT on condensing boilers here

spinningsheep

Thanks happymj, I honestly am not worrying about a few pence, I just didn't understand how much it may impact our gas usage, I am very well aware we are spending more by having the heating on more but we are more comfortable for it, and we can also afford it, I just didn't want to be going off using gas at huge rates only to get a massive catch up bill, and it seems that we are safe in that respect.

HappyMJ

Read your meter weekly and keep an eye on your bills to make sure it is in line with your estimates over a quarter and you won't get a huge catch up bill. Well you might but at least you will be expecting it....and it's a few pence per hour that the boiler is running but not enough to worry about. If you look at the manual it's output on low is 8.7kW in non condensing mode and 9.5kW in condensing mode. A difference of 0.8kW per hour at a unit rate of 3.5p/kWh that'll be about 2.8p per hour. If you boiler comes on for 15 minutes every 90 minutes that would cost you an extra 11.2p per day and an extra £10 over the quarter. However, it will still condense for most of that time as it is heating the house so nowhere near that figure.

I also don't understand your figures. Coming on for 1/6th of the time means it is keeping your house at a reasonable temperature for an output of 1.6kWh. My lounge or bedroom each lose heat at about 1kWh and that's only one room. I would think it comes on more often than 15 minutes every hour or two. It also doesn't tally with your spend of £25 a week which would indicate a consumption of 4.25kW per hour for 24 hours @3.5p/kWh which is a much more reasonable figure for heat loss for a house.