Hot water tank not keeping heat

ForestBluebells

neilmcl said:

That's because the Octopus Go tariff offers cheaper electricity than the standard Economy 7 tariff. They reckon it's 50% cheaper.

Yes it’s a fantastic tariff, 5p cheap and 15.59p peak. I was paying more than that before I switched! 

Bendy_House

ForestBluebells said:

The economy timing is weird. I have a normal economy 7 smart meter which switches to the night meter for 7 hours overnight Summer 12.30-7.30 and Winter 11.30-6.30 however I’m on octopus go so my cheap electric is based on my smart meter sending half hourly readings to them and it’s only cheap between 12.30-4.30am, the night meter is irrelevant to cheap times but does switch on /off my storage heaters so some of the time they charge at peak prices… it still works out cheaper this way as the tariff is incredible value compared to anything on the market. 

There’s no red light it’s pretty basic setup 

Based on that, then, there is no actual electrical 'interruption' or 'switching' between the std and eco supplies that could be the cause; it's simply a case of the new smart meter recording the consumption at its chosen times - "between blah and blah is 'eco', after 4.30 it's 'full-fat'" type of thing.

So that would seem to rule out a 'timing' issue outwith the rotating 'peg' timer. Not to say it is the peg timer either, of course, but I'm just trying to discount as much as possible, as best as possible.

I'm getting to the 'stumped' point. Actually, I'm already there...

So could that leave the much colder incoming water as suggested by others? Seems unlikely, but I just don't know.

ForestBluebells

Ill try pushing different pegs and heat earlier tomorrow to see if that makes a difference, if no difference I’ll try heating longer as could be colder water, definitely want hot water next week when it’s freezing overnight as there’s no heating in my bathroom so a cold shower will be awful 😂

Grizzlebeard

This is the simplest and basic experiment.

Rotate the clock by hand slowly through the 24 hours. You should hear a noticeable click as it turns on, and again as it turns off at the set times. If you don't the timer or pegs aren't working/set properly. The click will be very comparable to an electric kettle being turned on or off.

Second experiment. Turn the tank on via the timer clock and check your Electricity Meter is registering some hefty consumption (LED flashing once a second or so).  Edit - p.s. Don't use any heating or the kettle or oven while you're checking this.

Section62

Bendy_House said:

I'm getting to the 'stumped' point. Actually, I'm already there...

So could that leave the much colder incoming water as suggested by others? Seems unlikely, but I just don't know.

ComicGeek covered it on page 1 of the thread.  In the Winter the incoming water temperature will be lower, so it takes more energy to heat the same volume of water to the desired temperature.

Coupled with setting the heating time back by ~1.5 hours, it would be expected that the temperature of the hot water later in the day will be lower (but not 'cold') compared to the previous set up.

The water in the cylinder is subject to something known as 'layering' or 'stratification' such that if the incoming cold water enters at the bottom of the cylinder in a non-turbulent manner, then the hot water at the top won't mix (excessively) with the cold at the bottom, and hence the output temperature remains 'hot'.

However, given enough time some of the heat energy in the top water will be transferred to the colder water at the bottom, eventually reaching a point where all the water in the cylinder is at the same temperature.  This isn't the same as heat-loss from the cylinder as the heat energy is still there, just redistributed.

Normally this doesn't matter, because if the heat source is applied for long enough the total volume of 'hot' water in the cylinder means the layering effect will allow 'hot' water to be provided (in the normal quantity) until the next heat input.

But if you only apply heat for a short period of time you will end up with a small volume of (initially very) hot water at the top of the cylinder which is insufficient to keep the water in the cylinder 'hot' several hours later.


TL;DR - so long as the immersion heater has a functional and correctly set thermostat, increasing the 'on' time will only use as much additional energy as required to heat a sufficient volume of water to the required temperature.

Grizzlebeard

Grizzlebeard said:

This is the simplest and basic experiment.

Rotate the clock by hand slowly through the 24 hours. You should hear a noticeable click as it turns on, and again as it turns off at the set times. If you don't the timer or pegs aren't working/set properly. The click will be very comparable to an electric kettle being turned on or off.

Second experiment. Turn the tank on via the timer clock and check your Electricity Meter is registering some hefty consumption (LED flashing once a second or so).  Edit - p.s. Don't use any heating or the kettle or oven while you're checking this.

After more deliberation....

Correction. The click will likely not be as loud as a kettle (what you hear is mainly the switch lever moving) but is hopefully still a detectable difference to the ratcheting sound on the timer mechanism.

ForestBluebells

Section62 said:

Bendy_House said:

I'm getting to the 'stumped' point. Actually, I'm already there...

So could that leave the much colder incoming water as suggested by others? Seems unlikely, but I just don't know.

ComicGeek covered it on page 1 of the thread.  In the Winter the incoming water temperature will be lower, so it takes more energy to heat the same volume of water to the desired temperature.

Coupled with setting the heating time back by ~1.5 hours, it would be expected that the temperature of the hot water later in the day will be lower (but not 'cold') compared to the previous set up.

The water in the cylinder is subject to something known as 'layering' or 'stratification' such that if the incoming cold water enters at the bottom of the cylinder in a non-turbulent manner, then the hot water at the top won't mix (excessively) with the cold at the bottom, and hence the output temperature remains 'hot'.

However, given enough time some of the heat energy in the top water will be transferred to the colder water at the bottom, eventually reaching a point where all the water in the cylinder is at the same temperature.  This isn't the same as heat-loss from the cylinder as the heat energy is still there, just redistributed.

Normally this doesn't matter, because if the heat source is applied for long enough the total volume of 'hot' water in the cylinder means the layering effect will allow 'hot' water to be provided (in the normal quantity) until the next heat input.

But if you only apply heat for a short period of time you will end up with a small volume of (initially very) hot water at the top of the cylinder which is insufficient to keep the water in the cylinder 'hot' several hours later.


TL;DR - so long as the immersion heater has a functional and correctly set thermostat, increasing the 'on' time will only use as much additional energy as required to heat a sufficient volume of water to the required temperature.

I think this is what is happening! Thank you 

getmore4less

I was on economy 7 heating my hot water for 45 minutes about 5.30 am about 1 hour before I had my shower and I could still have a shower later in the day if I’d gone for a run just fine. 

you were having a second shower later in the day.

Now the first shower is not hot enough

It's a heating up problem not insulation.

Incoming water is colder or the heating time is messed up.

NSG666

Out of curiosity I found a calculator for how long it takes to heat 70l water from 20degC to 65degC using a 2.7kW immersion and it came out at c. 1hr 20mins. I'd be tempted to have it coming on for 2 hrs - as others have said the thermostat will cut the power anyway once the water has reached the set temperature.

twopenny

Blimey no, I have mine on for 30mins and it's hot enough for a few jobs.

I have noticed though that there's been a difference between summer and winter which I didn't get with the old tank. No reason I can see except the pipes - a job I've been meaning to do.

A way to check if it's the thermostat/heating element is to put it on for a long time and see if it's not hot enough or scalding. That's what set off mental alarms for me. An accident of leaving it on and it was scalding not 65.