Pre heat hot water

r2015

I normally do the dishes but hurt my hand so my wife had to take over.

My wife got fed up running all that lukewarm water down the drain until the water was hot enough and told me that we were wasting gas because of it (we don't have water meters in Scotland) so I decided to change the setting on the combi from economy to preheat to prove her wrong.


I did.


For the past 3 weeks the gas consumption has been on average 56 kWh of gas a day.


Today when I read the meter it was 2 m3 (22 kWh) more than usual.

A whole 78 kWh from yesterday :eek:

The economy button got pushed back in again and she was told to put up with it.

And it didn't really make all that much difference anyway.

At least I didn't think so.

Cardew

Presumably pre-heat involves keeping the small internal hot water tank in the Combi permanently heated.

However 22kWh extra in one day is a huge amount. As its been cold in Scotland recently, could it be that heating the house was responsible for a large element?

[Deleted User]

I hate the "pre-heat" mode as it starts the boiler every hour for 2 minutes which in the middle of the night is noisy!
We just boil a kettle for washing up hot water.

r2015

However 22kWh extra in one day is a huge amount. As its been cold in Scotland recently, could it be that heating the house was responsible for a large element?

It could have been a rounding error, meter 12345.99 yesterday then 12352.01 today which would give 6 m3 rather than 7 m3 but my average daily consumption has not varied from 56 kWh for the past 2 weeks.

So it could have been 11 kWh.

But it was still noticeable on my spreadsheet.

and as gsmlnx says

I hate the "pre-heat" mode as it starts the boiler every hour for 2 minutes which in the middle of the night is noisy!

I noticed that as well.

Perhaps I should start entering the 2 decimal numbers into my spreadsheet as well.


Also the most gas I have used in a day since my new heating was installed 20 months ago was 90 kWh

st999

However 22kWh extra in one day is a huge amount.

But perfectly believable.

it starts the boiler every hour for 2 minutes

equals 48 minutes.

For a 30 kWh boiler that is 24 kWh, so OP's usage is almost spot on.

Gizmo247

Many boilers (including mine), let you control the preheat via a timer.

lisa110rry

Gosh, I'm astonished! We had a new boiler in June. The old one (29 years old) took forever to heat water but the new one takes no time at all, and no "pre-heat" going on. Is your boiler working properly do you think?

r2015

After turning off the pre heat yesterday my usage has went back to 56 kWh a day from 78 kWh.


Also last night was one of the coldest nights this winter, colder than the night before, so using 78 kWh is not due to the weather.


Which makes st999's explanation the most likely reason for the 22 kWh increase.

Cardew

However 22kWh extra in one day is a huge amount

st999 wrote: »

But perfectly believable.

it starts the boiler every hour for 2 minutes

equals 48 minutes.

For a 30 kWh boiler that is 24 kWh, so OP's usage is almost spot on.

I really cannot see that it is 'perfectly believable'.

I have a large(180 litres) modern Hot Water tank. Like all modern HW tanks the heat loss is tested to a British Standard(BS) with the water @ 65C and the room temperature at 20C.

The heat loss from my tank is 54 watts so in 24 hours it will lose 1.3kWh. This is a typical loss for a modern well insulated HW tank and you can often see such a figure stamped on a HW tank.

We don't know the size of the internal tank in the OP's Combi boiler, but surely it will not be bigger than 50 litres??

As you are accepting that the boiler will be outputting 30kW for 2 minutes every hour(through the night when no usage) it follows that this internal tank will be losing heat at a rate of 1kW(and it is probably not heated to 65C)

So to justify heat loss of 24kW a day this small internal HW tank will be losing heat at over 18 times as much as my 180 litre tank.

I also have a 1 year old combi fitted in an annex. The internal HW tank size is 46 litres. I spoke to their technical department and they told me the heat loss was 'minimal'. They are attempting to find out actual figures and will contact me.

Cardew

r2015 wrote: »

After turning off the pre heat yesterday my usage has went back to 56 kWh a day from 78 kWh.


Also last night was one of the coldest nights this winter, colder than the night before, so using 78 kWh is not due to the weather.


Which makes st999's explanation the most likely reason for the 22 kWh increase.

For st999's explanation to hold true, it means that the your boiler is losing heat at the rate of 1kW. It is the same as having a 1 bar electrical heater on permanently inside your boiler. If you had that sort of heat loss, the boiler casing would be too hot to touch.

r2015

it means that the your boiler is losing heat at the rate of 1kW

Can you offer an other explanation then?