Seasonality of kWh used for hot water heating

Sterlingtimes

During the months when my radiator heating tends to be zero, my hot water and cooking usage are as follows in kWh: May 635, June 473, July 359, August 381 and September 639.

I presume that in the colder months, the hot water will take more kWh to heat. Is there a rule of thumb that I can use to estimate my hot water usage during the months when I also use central heating? 

sandy700

I use, on average, 11 kWh a day for hot water and cooking and have not noticed any noticeable difference in consumption between the colder months and the warmer months but then I can't separate hot water only from heating radiators.

Heating and hot water is from a combi.

wild666

My monthly usage of gas is around £4.20 with £8.33 SC. My combi boiler doesn't seem to have a function for stopping the preheating of water it's a Valiant ecotech pro 28.
If you can turn off a function to preheat water do it and lower temperatures to 50 degrees for water and 55 degrees for heating.

[Deleted User]

Hi,

remember that over the winter months the water that you are heating is colder, so that's another consideration.

getmore4less

Sterlingtimes said:

During the months when my radiator heating tends to be zero, my hot water and cooking usage are as follows in kWh: May 635, June 473, July 359, August 381 and September 639.

I presume that in the colder months, the hot water will take more kWh to heat. Is there a rule of thumb that I can use to estimate my hot water usage during the months when I also use central heating? 

I have been doing some analysis of electric,  the most significant reductions in use are going on holiday.

if your heating was definitely off for those 5 months can you identify why the 2 higher months are over 75% higher than July.

Xbigman

Do you cook less in the summer, or maybe go out more? Less cooking means less hot water for washing up. 


Darren

k_man

frugalmacdugal said:

Hi,

remember that over the winter months the water that you are heating is colder, so that's another consideration.

Mains water temperature only varies by a few degrees during the year (there was a similar thread last winter),  so this will only make a few % difference.
(Corrected)

There will be more pipe heat loss between boiler and taps/tank if the ambient temperature is colder, but it may even be warmer if the heating is on.

There also may actually be a reduction in hot water energy use, if the water is heated while the boiler is already heating for CH, so already up to temp.

I think the biggest factor is change in hot water use in the winter (longer hotter showers, more baths etc).

BUFF

k_man said:

frugalmacdugal said:

Hi,

remember that over the winter months the water that you are heating is colder, so that's another consideration.

Mains water temperature only varies by a few degrees during the year (there was a similar thread last winter),  so this will only make a few % difference.

Depending upon where you live it can be up to 15C variation in the mains.

QrizB

Sterlingtimes said:

During the months when my radiator heating tends to be zero, my hot water and cooking usage are as follows in kWh: May 635, June 473, July 359, August 381 and September 639.

I presume that in the colder months, the hot water will take more kWh to heat. Is there a rule of thumb that I can use to estimate my hot water usage during the months when I also use central heating? 

I don't know a rule of thumb, exactly, but if:

• You heat your water to 60C;
• In the summer months the incoming water is at 15C;
  
• In the winter months the incoming water is at 5C; and
• All other factors remain unchanged, then
  

Your winter energy use to heat water will be (60-5)/(60-15) ie. 55/45ths or 11/9ths of your summer energy use. That's about 22% more.

k_man

BUFF said:

k_man said:

frugalmacdugal said:

Hi,

remember that over the winter months the water that you are heating is colder, so that's another consideration.

Mains water temperature only varies by a few degrees during the year (there was a similar thread last winter),  so this will only make a few % difference.

Depending upon where you live it can be up to 15C variation in the mains.

Thanks, I must have misremembered. Maybe it was daily/weather related variations that were over estimated.

Sterlingtimes

QrizB said:

Sterlingtimes said:

During the months when my radiator heating tends to be zero, my hot water and cooking usage are as follows in kWh: May 635, June 473, July 359, August 381 and September 639.

I presume that in the colder months, the hot water will take more kWh to heat. Is there a rule of thumb that I can use to estimate my hot water usage during the months when I also use central heating? 

I don't know a rule of thumb, exactly, but if:

• You heat your water to 60C;
• In the summer months the incoming water is at 15C;
  
• In the winter months the incoming water is at 5C; and
• All other factors remain unchanged, then
  

Your winter energy use to heat water will be (60-5)/(60-15) ie. 55/45ths or 11/9ths of your summer energy use. That's about 22% more.

Thank you. This is very helpful. 

But I have realised an error in my calculations. I have forgotten to add in the 444 kWh annual solar diversion, which may explain some of the lower summer readings.

Nonetheless, I think that your logic is correct and that I need to add more kWh to the winter forecast for hot water heating.