Opinion request on hot water demand

[Chris_Other]

Looking at something like a megaflo unvented, typically you heat this to 65 degrees.
Most of the day, I have hot water sitting in the tank not being used, and if I do not draw any off, the insulation is very good and keeps it at 65 degrees.

If I draw hot water from the tank it gets colder as cold water feeds in to replace the hot drawn off.(depending on whether the boiler is on or not changes the rate in which it gets colder)

Common sense says do not go above 65 degrees, or you stand more chance of burning yourself but not risk legionella.

I believe regulations now put bath and showers in requiring a mixer that stops it at 45 degrees.

Sorry for too many known comments, here is the question...

If I put a mixer on the hot water output of the cylinder that puts it to 65 degrees, and keep it heated to 65 degrees, then predicting just before high usage times heat it to 90 degrees and mix back down to 65 degrees for that period of time only, would it reduce the draw on the hot water by approx 20%.

Could this in turn mean I can reduce the cylinder size and decrease the heat up time?

 

[Last Plumber]

You won't get it to 90.
The MAX you'll get out of a Boiler is 82 plus or minus 3 degrees.

I wouldn't recommend you try either.

Even if you could it''s all swings and roundabouts in my opinion. You would spend more money on fuel to get the tank hotter anyway.

 

[townfanjon]

Should be 60 degees mate ,as LP suggests dont mess with it . Todays cylinders have excellent standing heat losses, please use one as its designed .

 

[Old Account]

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Don't get your point in regards the hot water demand

 

[Chris_Other]

Don't get your point in regards the hot water demand

Most the time, your hot water demand is very low, a basin of water for washing up, or a quick run off the hot tap. Once or twice a day you may hit high demand with showers and baths.

 

[cr0ft]

Just to be clear, are you on about heating an unvented cylinder to 90 degrees then blending it down to 60 using a mixer valve on the hot water outlet? I definitely wouldn't recommend this. Even if you can find a boiler that would heat the water to 90C (most stop at around 80C), the cylinder and safety controls are designed to work with the cylinder at 65C. At 90C on an unvented cylinder the Temperature Relief Valve will open and vent the water.

If you are on about doing this with an unvented cylinder, a word of warning, when these cylinders go wrong they can blow up your house. They should only be worked on by a person holding an unvented qualification.

 

[Chris_Other]

As I pointed out, this is for discussion only as a thought, putting a system in that did not meet regulations or manufacture usage guidelines would be foolhardy.
Some basic observations that heat loss does differ as the vesel gets larger resulting in a run cost saving, about £4 a year difference between 70l and 125l. (not going to set the world alight), other benefit is heat up time is faster and flow rate increased. Since heat out = heat in, no gains there on efficiency, so only other savings are space and purchase cost.

 

[cr0ft]

I hear you! I don't think there would be any saving to be honest. You're still storing the same amount of heat energy your way, just in a smaller cylinder. The smaller cylinder only has less energy losses if you heat both to 65C. If you heat the smaller cylinder to 90C the energy loss of the smaller cylinder will be higher. I would hazard a guess as high as the larger one heated to 65C.

You're correct that larger cylinders have higher heat losses but as you've identified, the cost difference is minimal to be honest.

Don't forget the pipework heat losses as well when you are pumping higher temperature water round from the boiler. If pipes are uninsulated from the boiler to the airing cupboard (as is typical) you will lose a lot more energy when running them at a higher temperature.

 

[Chris_Other]

Just to be clear, are you on about heating an unvented cylinder to 90 degrees then blending it down to 60 using a mixer valve on the hot water outlet? I definitely wouldn't recommend this. Even if you can find a boiler that would heat the water to 90C (most stop at around 80C), the cylinder and safety controls are designed to work with the cylinder at 65C. At 90C on an unvented cylinder the Temperature Relief Valve will open and vent the water.

If you are on about doing this with an unvented cylinder, a word of warning, when these cylinders go wrong they can blow up your house. They should only be worked on by a person holding an unvented qualification.

As below, agree with your points entirely, but 90 degrees is probably about protection from scolding, not from pressure. Water has to be nearer 137 to flash to steam and start to produce pressure of 3 bar, you need to be at 190 degrees. Either way, I would not want to take my heat intolerant body into my airing cupboard to find a 3 bar mains water pressure leak of 90 degrees in my face, not sure I would like 65, but it will take a few seconds to majorly burn me at 65 whilst 90.... Ouch!

 

[Chris_Other]

Thanks to Cr0ft, I appreciate your comments, I did not think about the temperature safety implication and disregarded the pressure, Also, 90 degree pipes would cause issue, so mixing would have to be at point of boiler exit and well shrouded as pipes at 90 degrees would be hazardous.