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Energy Efficiency: Combi Boiler

Energy Efficiency: Combi Boiler

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Profile Iain Inglis
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Message 46899 - Posted: 27 Aug 2013, 23:54:46 UTC
Last modified: 6 May 2014, 23:02:13 UTC

My quite efficient gas boiler was replaced in February this year by a combi boiler, which means there is no longer a hot-water tank or a cold-water tank.

The effect can be seen in this chart of gas consumption ...

... in which the summer minimum is brought effectively to zero after a fortnight's holiday.

I conclude the following from this:

1. The boiler functions as advertised.

2. Air heating dominates water heating by a large margin: I had expected a seasonal variation but not by that amount. Perhaps we need new houses rather than new gadgets, including new boilers or solar-powered water heating.

3a. British weather is very variable.

3b. The British climate is even worse than I had thought - i.e. the period during which water heating dominates is appallingly short.

PS The roof was insulated courtesy of a council grant just before the weekly spot measurements began in 2008.
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Profile Greg van Paassen

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Message 46922 - Posted: 1 Sep 2013, 2:22:34 UTC - in response to Message 46899.  

...Air heating dominates water heating by a large margin
...
PS The roof was insulated courtesy of a council grant just before the weekly spot measurements began in 2008.

Do you have downlights, Iain?

Recessed-into-the-ceiling downlights work hard against ceiling insulation, unless they are the sealed type (which are getting quite affordable, btw). Ventilated downlight fittings act as very effective chimneys, drawing hot air into the roof space. In nearly all houses, the roof space is neither well sealed nor insulated.

(Insulation must be kept 100 - 150 mm away from ventilated downlights, too. Although it seems a small fraction of the total roof area, that has a surprisingly large effect.)

Nearly all houses built before this century leak air like tents, downlights aside. Four to five changes of air per hour was the standard.

An infrared imaging heat loss survey or blower door test could help you decide what to invest in next, if you wish to go further.
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Profile Dave Jackson
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Message 46923 - Posted: 1 Sep 2013, 7:31:01 UTC - in response to Message 46899.  

We are working up to replacing our gas boiler. However our results would differ dramatically. Since we put in a wood burning stove that also heats the hot water. (2KW to the room and 7.1KW to the hot water) our gas bill is considerably less in the winter than the summer! This despite having solar hot water heating. The thing that might change that is if we were to get one of the gadgets that diverts unused electricity from our pv to heat the hot water.
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Profile Iain Inglis
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Message 46927 - Posted: 1 Sep 2013, 10:54:25 UTC - in response to Message 46922.  
Last modified: 1 Sep 2013, 11:04:26 UTC

[Greg van Paassen wrote:]
...Air heating dominates water heating by a large margin
...
PS The roof was insulated courtesy of a council grant just before the weekly spot measurements began in 2008.

Do you have downlights, Iain?

Recessed-into-the-ceiling downlights work hard against ceiling insulation, unless they are the sealed type (which are getting quite affordable, btw). Ventilated downlight fittings act as very effective chimneys, drawing hot air into the roof space. In nearly all houses, the roof space is neither well sealed nor insulated.

(Insulation must be kept 100 - 150 mm away from ventilated downlights, too. Although it seems a small fraction of the total roof area, that has a surprisingly large effect.)
We have a small bathroom upstairs with ceiling lights that are installed as you describe, with no insulation above. Fortunately, the room isn't in constant use. Following the disconnection of the loft cold water tank, I need to clear the loft space again and re-insulate: there was no insulation beneath the tank, for example, to avoid it freezing.

My brother, who lives in the US, finds the British habit of putting cold water tanks in the roof space very strange: all his tanks are inside. However, we do have the perennial lack of space in comparison. After the roof space was insulated, I installed a temperature monitor to see how likely the tank was to freeze. Out of that came an unexpected appreciation of how building styles match climate: after a year or two of monitoring I realised that the only weather that would freeze the tank/pipes would be a sustained cold and cloudy spell. But those just don't happen in London. For it to be really cold we have to have blocked high-pressure, which is actually sunny, which in turn heats the roof space. The flip-side of this observation is, of course, that a substantial change in climate might invalidate an entire building tradition. When I lived for a time in Germany, for example, it was striking how many buildings had shutters for the continental-climate summers: that would be a decided improvement in British habits during hotter periods: I despair of the number of Brits who believe you can cool a house by letting in hot air ...
Nearly all houses built before this century leak air like tents, downlights aside. Four to five changes of air per hour was the standard.
We're in double trouble here as the house is early 1900's and end-of-terrace. Fitted cupboards against the external wall, sealed chimneys, insulating carpet underlay and replacement (or renovated) windows have transformed the feel of the house in winter. When my wife first bought the property the carpets would lift off the floor when it was windy and one of the back doors would blow open!

An infrared imaging heat loss survey or blower door test could help you decide what to invest in next, if you wish to go further.
I suspect the blower test would be truly shocking but something is needed to get that winter peak down.
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Profile Iain Inglis
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Message 46928 - Posted: 1 Sep 2013, 11:11:05 UTC - in response to Message 46923.  
Last modified: 1 Sep 2013, 11:11:35 UTC

[Dave Jackson wrote:]
... our gas bill is considerably less in the winter than the summer! This despite having solar hot water heating. ...
I'm having trouble unpicking this, unless you live pretty much exclusively in the room with the stove!!
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Profile MikeMarsUK
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Message 46940 - Posted: 2 Sep 2013, 7:52:54 UTC - in response to Message 46928.  

... I'm having trouble unpicking this, unless you live pretty much exclusively in the room with the stove!!


Presumably due to the 7.5kw hot water heating which comes from his wood-burner. That's quite a lot of heat!!


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Profile Dave Jackson
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Message 46942 - Posted: 2 Sep 2013, 8:27:50 UTC

Yep, it is the water heating that makes up most of our gas usage, followed by cooking. We only heat the bedroom occasionally, Kitchen is heated mostly by waste heat from the cooker. Getting the doors properly draughtproofed has made a big difference to the amount of space heating required along with increased insulation in loft.

If our roof was facing due South, we would probably get nearly all our hot water
from the solar heater in summer. Not quite sure how the bills would pan out then.

I have played this year with cooking bread in a home made solar cooker which works quite well though it takes a bit longer to cook. Next model will have an increased mirror area. If I had got going with this earlier in the year it would have made a small difference to bills. Of course last year it would have been a complete failure!
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Message 47691 - Posted: 29 Nov 2013, 18:23:22 UTC
Last modified: 6 May 2014, 23:01:34 UTC

Here's what the boiler looks like now:
.
That image is depressing in so many ways ...

The 'Green Deal' is in the news again (e.g. here). Perhaps that chart is one of the reasons it must inevitably be a flop - and the poor returns mirror my experience of energy saving generally. It's like the joke in which a country bumpkin is asked the way to London: he thinks for a while and then replies, "Well, I wouldn't start from here."
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Message 47692 - Posted: 30 Nov 2013, 7:57:36 UTC

We got a free green deal assessment done. When I looked at the figures, even allowing for a higher than predicted increase in energy prices, replacing our boiler would take over 20 years to pay for itself. Next year I will enjoy not having to buy any wood in which will make it a really cheap winter as I have about a ton and a half of wood to cut up over this winter so it will be dry for next.
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Message 47693 - Posted: 30 Nov 2013, 16:05:52 UTC - in response to Message 47692.  

We got a free green deal assessment done. When I looked at the figures, even allowing for a higher than predicted increase in energy prices, replacing our boiler would take over 20 years to pay for itself.

Depending on your circumstances, that may well be true. However, improved controls (e.g. electronic room thermostat with timed temperatures) can also improve both comfort and energy consumption. That said, their direct energy consumption effects can be hard to estimate.

For example, our latest bills shows that the wet heating & hot water system with a condensing boiler & up-to-date heating control, that replaced the warm-air heating and an open-flued gas water heater, saves over 50% on gas consumption. I estimated the ROI as 10-15 years when it was installed nearly three years ago. The actual ROI is 7.5 years on current prices.

The kitchen was refurbished at the same time, it now has A-rated appliances (oven, induction hob, extractor, fridge etc) and LED lights. The actual saving is 40% on overall electricity consumption. Appliance ROI of about two years.
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Message 47694 - Posted: 30 Nov 2013, 16:51:32 UTC - in response to Message 47693.  

I guess our case is not usual - we rarely heat bedrooms and bathroom only for short periods. Living room is kept warm by wood burner in winter which also heats all our hot water when we use it. We have just been given enough wood that we won't have to buy any at all for next winter - I just need to cut it up so it can dry out before then. Kitchen mostly gets heated by waste heat from cooker. - That may change when we get A rated oven etc. Our boiler is old enough that it is likely we will need to replace it in the next 5 years or so.
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