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IOWJJBTM2025

Hello everyone. Many thanks for your comments / responses. I really do appreciate and can now see some of my errors.
I am currently at home recovering after having a bleed on the brain 10 weeks ago. It affected my eye sight and also zapped a lot of my brain power. Life has changed massively and I am spending a lot of time sleeping plus hospital visits. The doctors say I need to rest and doing puzzles is a good way to check my progress.
With not working for a while (self employed) and cost of living rising, over the last couple of weeks I have been looking at any easy win savings. As part of the exercise I have been trying to do sums that I could do in my sleep previously, is all good brain training for me.
4 weeks ago I would not have been able to write this so I have already made giant leaps forwards. Please excuse mistakes and !!!!!! grammar.

I did some more sums again this morning after doing some tests with a 1 litre jug as follows.
On the shower on maximum flow (setting 1) it takes 6 seconds to fill the jug. We cannot use high power setting on max flow in the summer as the water is too hot and have to use low setting. This equates to a max flow rate of10 Litres per min which is just what Southern Water supplies to the house and I expect the restriction from the shower head.
To get a good hot shower today on high power setting the flow is set to 6.5 and it takes 12 seconds to fill the jug. This equates to 5 Litres per min.
To get a hot shower on Low power setting the flow is set to 8 and it takes 19 seconds to fill the jug. This equates to 3.16 Litres per min and is in my opinion to the point where there is not enough flow to have a decent shower.
We have an energy Meter and can confirm that on high Power the shower is using 8700 W and is using 4900 W on low setting.
Running the shower for an hour @ 21.59p/kWh is £1.88 on high power and £1.06 on low power.
The cost for water over an hour is on high power - 300 litres @ 0.0062625p/ltr = £1.88
The cost for water over an hour is on low power - 189.6 litres @ 0.0062625p/ltr = £1.19.
Therefore total shower cost for an hour a day at the moment is £3.76 on high power @ 5 litres per minute.
Therefore total shower cost for an hour a day at the moment is £2.25 on low power @ 3.16 litres per minute.

Of course we can save money by showering less etc but the hour gives a good benchmark. We have 4x adults in the house plus an extra when my yougest sons girlfriend stays over.
As previously stated we have a big differential between the incoming cold water temperature in the summer and in the winter. I have not measured the temperature but would guess at least 20 degrees Celsius between the seasons.

Using the Gas boiler is still racking my brains so I had a bath and ran the hot tap fully open. It was filling the 1 Litre jug in 5 seconds (12 litres / minute). I did a video of the gas meter and the boiler was permanently on. It was using a cubic foot every 35 seconds. I ran enough water for a bath. When testing the water in the bath it was too hot so had to add some cold water.
What I can do is open the hot tap less and the water coming out the tap is even hotter. If I restrict the tap too much then the water gets very hot and then cold water is supplied for a few seconds because the boiler cuts off and then hot water is supplied again when the boiler cuts in again.
I have no way to adjust the boiler water heating - it is either on full or off.
I already know that at 12 litres per minute the water from the boiler is too hot for a shower.
If I get a shower mixer tap to add cold to the hot I can only assume that I have to have a fast flow rate of approx 12 litres per minute = 720 litres over an hour @ 0.0062625p/ltr = £4.51 of water.
To run the boiler for the hour @ 5.48p/kWh costs £1.79
Therefore total shower cost for an hour at the moment would be £6.30 using gas boiler @ 12 litres per minute.

I do not see a way to get a cheaper shower using the gas boiler - unless I have totally missed something?

QrizB

IOWJJBTM2025 said:

I have no way to adjust the boiler water heating - it is either on full or off.

How ancient is your boiler?

All gas boilers for the past 20+ years will modulate to reduce their burner power.

The Worcester Bosch Greenstar 32CDI, for example, has a minimum hot water power of about 7kW. See:

https://media.plumbarena.co.uk/uploads/files/2016/02/17/Worcester_CDI_Technical_Specification.pdf

IOWJJBTM2025 said:

I already know that at 12 litres per minute the water from the boiler is too hot for a shower.

If your boiler reduced it's output to 8kW, though, it would give 3lpm of too-hot water that your mixer would blend with 3lpm of cold water to give a 6lpm, 8kW shower. Which is similar to the "high power, flow 6.5" shower you're enjoying today.

victor2

IOWJJBTM2025 said:

...

I do not see a way to get a cheaper shower using the gas boiler - unless I have totally missed something?

Hope you are recovering well.

As @QrizB said, gas boilers should modulate their burners, so won't be running flat out whenever you have a shower. That's partly why I did a real time experiment with my new gas boiler and a shower, so that I could see from the meter exactly how much gas was used. Of course, there are other factors to consider too, but it gave me an idea.

My conclusion was that a gas powered shower was just slightly cheaper than an electric one, but not by enough to justify the expense of the conversion. Your simplest way to cut costs would be to reduce the amount of time people are in the shower! 

IOWJJBTM2025

How ancient is your boiler?

All gas boilers for the past 20+ years will modulate to reduce their burner power.

My boiler is approx 12 years old.
It is a Vaillant EcoTEC Plus 831.

I had a look at the manual and I cannot find anything.
It does have the ability to set the output temperature of the water but I do remember the plumber telling me not to go lower than 60 degrees.

QrizB

IOWJJBTM2025 said:

My boiler is approx 12 years old.
It is a Vaillant EcoTEC Plus 831.
I had a look at the manual and I cannot find anything.

I'v found the Vaillant brochure from 2010. It shows that the EcoTEC Plus 831 will modulate down to 8.7kW.

https://www.gotogasdocs.co.uk/f/m/Vaillant/Brochures/EcoTEC Brochure 2010.pdf

IOWJJBTM2025 said:

It does have the ability to set the output temperature of the water but I do remember the plumber telling me not to go lower than 60 degrees.

You can definitely turn the hot water temperature down below 60 degrees. I think there are folk on this board with similar combi boilers who are running a 50C temperature, for example.

You probably have a separate control over the central heating water temperature? If that's also set to 60C you might want to turn this down too, it'll improve the efficiency of your boiler and reduce your gas bills.

WiserMiser

QrizB said:

IOWJJBTM2025 said:

My boiler is approx 12 years old.
It is a Vaillant EcoTEC Plus 831.
I had a look at the manual and I cannot find anything.

I'v found the Vaillant brochure from 2010. It shows that the EcoTEC Plus 831 will modulate down to 8.7kW.

https://www.gotogasdocs.co.uk/f/m/Vaillant/Brochures/EcoTEC Brochure 2010.pdf

IOWJJBTM2025 said:

It does have the ability to set the output temperature of the water but I do remember the plumber telling me not to go lower than 60 degrees.

You can definitely turn the hot water temperature down below 60 degrees. I think there are folk on this board with similar combi boilers who are running a 50C temperature, for example.

IIRC that's correct only for combi boilers.  However if you have a traditional boiler with a hot tank then 50°C would not be a good idea.

https://www.hse.gov.uk/healthservices/legionella.htm#:~:text=Using temperature control,Keeping the system clean

QrizB

WiserMiser said:

IOWJJBTM2025 said:

It does have the ability to set the output temperature of the water but I do remember the plumber telling me not to go lower than 60 degrees.

You can definitely turn the hot water temperature down below 60 degrees. I think there are folk on this board with similar combi boilers who are running a 50C temperature, for example.

IIRC that's correct only for combi boilers.  However if you have a traditional boiler with a hot tank then 50°C would not be a good idea.

https://www.hse.gov.uk/healthservices/legionella.htm#:~:text=Using temperature control,Keeping the system clean

Opinions vary, as they say, and my home is not Health Service premises (the target of that article). But let's not sidetrack this thread which concerns a combi boiler.

IOWJJBTM2025

Thanks all for the advice.
I will look to reduce the temperature to 50 degrees.
I have a thought that it does not make sense to heat water and then cool it down with cold water.
Also it is good news that my boiler can feed directly into a mixer tap.
It would be even better if the boiler could output at 40 degrees so that it would be more energy efficient so no having the mixer tap to add cold.
I did some reading and there is a warning that every 24 hours the boiler will heat water up to 90 degrees for a very short period to kill any legionella.

QrizB

IOWJJBTM2025 said:

I did some reading and there is a warning that every 24 hours the boiler will heat water up to 90 degrees for a very short period to kill any legionella.

Legionella is only really a concern with stored water; it can't grow in the mains supply (it's too cold and chlorinated), and you don't usually have a storage tank with a combi boiler.

And 50C will kill it; higher temperatures will kill it faster.

People with open vented hot water systems with loft tanks and HW cylinders are the folk who need to be most careful. But that's not what you've got.

Eldi_Dos

@IOWJJBTM2025
            If it is of any interest I have a combi boiler and run the hot water at 46 deg, warm enough for shaving and doing dishes without the need to add cold water. My thinking is the water is never going to be sitting in the pipework for any length of time while house is occupied.Now the weather is getting a bit colder I might up that slightly if the cold spell is prolonged.

I also use a electric shower and although more expensive to run if you take the cost of changing to gas it may be a long time to break even.Plus it is reassuring to have electric shower if boiler stops working.