Gas use to heat water (hot water tank) always on?

pfpf

hi,
just musing options. boiler with hot water tank. currently we have it set to heat the water for one hour in a morning and one hour later. usually does the job but on occasions need a "boost".
if i were to just set it to be "always on" i am assuming it will only heat more water as we use it, maybe reheat if the tank cools?
i can't see how this would cost much more but until we turn the heating off fully i can't really test the theory. anyone out there who has done this?
thank you.

Spelunthus

This frequently comes up. We leave our hot water tank electric heater on permanently. We don't use much hot water from the tank, and our meter shows that there is a short (5mins) boost every three hours or so.

If you have the heater on the timer, then you are presumambly using electricity to heat up a lot of water, only for it to cool down again, then heat it again - that sounds wrong to me.

But it will depend on your tank, insulation, heater and hot water use. I suggest you try a test for two months and compare the costs. You probably need a smart meter to do the measurements.

pfpf

thanks but confused. what is the additional "electricity" use?

we do have a smart meter but with the heating still on not sure i can measure properly gas usage for my "test".

Scot_39

Heating the water more frequently is likely to increase heat losses (in winter perhaps useful in part to heat spaces - but often the pipes are hidden - and so less so) - but by how much - debatable.

Your boiler heats it's own water system that flows from boiler around a loop in the tank - and that will have it's own pipe run to heat and "loose" some energy from. And that will be a per metre loss - so how far is boiler from tank ?

Keeping the full tank fully warm all day - is likely to increase it's losses - but again - by how much - questionable without testing it.

As the tank will normally stay hotter at the top than the bottom - where any master thermostat likely to be positioned - and its a non trivial temperature gradient within the tank - which makes any maths difficult - and again a brief test could easily be lost in the noise of day to day use variation.

Theres some interesting experimental data in this post here - around some of the interactions - there are others on line.

Hot water heating in boilers appears more complex set of dynamics - to do with the boiler itself, its ability to regulate it's output, it's loop flow temperature setting vs hot water tank thermostat setting etc.

https://forum.ovoenergy.com/home-and-heating-138/optimise-your-hot-water-set-up-17178

But short of trying it with your own set-up and demands - and over several days to smooth out day to day variations - I am not sure their is a one size fits all answer.

Like many things - although in the end - you need a certain amount of energy to heat a given volume of water to the desired temperature - it comes down to the efficiency of doing so - and that includes the losses and boiler.

Can you spot the events that cause these rare shortages - visitors, car washing etc - and just boost after use ?

A new tank might only lose 1-2kWh - a badly lagged old one - likely much more.

It's about a half to third of my normal daily HW cost / use (tank doesnt feed my shower) - few dishes, rinsing hands etc - but I like the warm airing cupboard - and having HW on demand 24/7 - my tanks direct electric - and is het 3x per day - for 15-20min at a time most days - so left it as is (yellow spray on foam jacket, no lagged pipework)

WiserMiser

Won't make much difference. A modern foam insulated tank will only lose about 1.8 - 2kWh per day if always hot. However, for about seven months of the year the leakage will warm your home so the GCH won't have to work quite so hard.

Theoretically you might even save a small amount. You've probably set the tank thermostat so that it's still comfortable just before the timer switches on in the afternoon. If so, that means its maximum temperature will be higher than necessary. If you leave it on all day the water temperature will vary less so you can probably turn the tank thermostat down a degree or two.

However, depending on the type of boiler, beware that lowering the setting is associated with a higher risk of Legionnaires' disease; you may need to set it higher for an hour or two once per week to prevent the bacteria building up.