Huge hot water cylinder heating more water than needed

Shedman

[Deleted User] said:

We have a 250L unvented cylinder with just two occupants in the home. We use on average about 3.5kWh of electricity a day to re-heat the cylinder. The timer is set for 2 hours a day. We never run out of hot water and my wife prefers to have a bath rather than shower. She is also not a great fan of dishwashers. I suspect that a dual immersion heater might save us a bit of money but it is not worth the cost of fitting a new HW cylinder.

Is yours a top mounted immersion heater element and, if so, does it heat the whole of the tank as I read that, as the elements are normally only about 27" long, they only heat the top section of the tank.  

Interested to know real life example as wondering whether to use the immersion heater on ours instead of gas  (esp during summer when GCH is off) as on a fixed 19.5p kWh electricity tariff for next 18 months vs 7.5p gas until end Jan (and god knows what thereafter) so thinking it might actually be cheaper to use electric esp for the evening boost

Benny2020

I have an iboost diverter that puts excess solar power into the hot water tank via a 28" immersion at the top of the tank.
This gives enough water for a hot bath and a couple of lots of dish washing and its still pretty warm after that.
It normally takes around 5 kwh a day to do that but all free and if the sun is out it keeps on reheating it but 5 - 6 kwh max.

doodling

Hi,

I strongly suspect that, unless your cylinder is sufficiently old that it is at risk of failing due to old age, then the most cost effective thing to do would be to spend money on further insulating your existing cylinder (and connecting pipes).

If your cylinder was perfectly insulated then its size would have no effect on the amount of energy you used to heat hot water - energy usage would only be affected by how much water you used.  The only reason a bigger cylinder uses more energy in reality is because it loses more heat to the airing cupboard than a smaller one. Doubling or even trippling the amount of insulation would solve that problem (and woukd almost certainly be cheaper than a new cylinder)

Sea_Shell

We're similar in that we're just 2 in a 4 bed.

We have a standard hot water cylinder off a system boiler.

We used to have it come on at 7am and again at 11am, to recharge it, but then realised there was really no need, so just have it on the once now, for about 30 mins (in isolation).  We don't use much after about 2pm.  So have enough.


However I can't quite get my head around the thermodynamics of it all.   How come the fresh cold doesn't mix more with the hot.

Obviously, I realise that the hot water rises, but at the boundary between the two "layers" is the bit that baffles me. 

 

[Deleted User]

I always wonder where the steam goes in an immersion tank as surely using some hot water and it topping itself up with cold water would create steam, a bit like topping up a boiled kettle. How does the tank not swell up and explode?

C_Mababejive

The first thing to remember is that you arent actually heating the whole cylinder unless you want to.

Hot water rises and stratifies at the top of the cylinder so if you only have your hot water on for a shorter time, it will only heat the top half of the cylinder.

The other thing is that heat loss from the cylinder is not necessarily heat lost. If the cylinder is within the living space then the heat loss from the cylinder is warming the home.

I honestly think that many homes would actually be better off with a hot water storage cylinder, preferably unvented if you can afford it. The idea that combi boilers are the automatic choice and are more efficient is simply not true.

My current boiler has an input of about 13Kw and i have a HW cylinder. It provides all the heating and HW i need . I have modern controls with a twin channel 7/2 day programmer with as many offs and ons per day as i could need.

Ectophile

[Deleted User] said:

I always wonder where the steam goes in an immersion tank as surely using some hot water and it topping itself up with cold water would create steam, a bit like topping up a boiled kettle. How does the tank not swell up and explode?

If there's steam in your hot water tank, you've got problems.  It should only be full of water, and that water should be heated to around 60C - nowhere near hot enough to boil.

Traditional vented systemd have an overflow pipe from the hot water tank to the cold tank.  Unvented systems will have an emergency pressure relief valve if the thing ever does boil.

Lemonjuiced

I used old bed quilts draped over the tank to cut the heat loss. Before this was done, the cupboard was always warm and now it is not and cost nothing.

Reed_Richards

Deleted_User said:

I always wonder where the steam goes in an immersion tank as surely using some hot water and it topping itself up with cold water would create steam, a bit like topping up a boiled kettle. How does the tank not swell up and explode?

There will not be any steam but hot water expands and this could pressurise the cylinder.  In a "vented" system the cylinder is topped up by a header tank (usually in the loft).  This is open to the air so any expansion merely causes the water level in the header tank to rise slightly.  This is the traditional system but it has the disadvantage that the hot water pressure is limited by the height of the header tank above the tap or shower head.

More recently we have  had "unvented" cylinders where the water pressure in the tank is regulated and set to nearly the same pressure as the mains pressure.  In combination with "mains pressure" cold water this makes the hot and cold water pressures much more even so it is much easier to set the desired temperature on a mixer tap or shower and you get better flow rates near the top of the house.  Expansion in the tank is taken up by a "pressure vessel" -which is essentially like a balloon in a metal case which is compressed when the water in the tank expands.  If this fails you will see water discharged through the tundish whenever the tank is heated.

doodling

Hi,

There are several different ways of accommodating the expansion of water due to heating in an unvented cylinder.

The simplest and most reliable (but with a maintenance burden) is to simply have a pocket of air of the top of the cylinder. This works well but eventually the air dissolves into the water and then any expansion causes the overpressure valve to operate. To keep this arrangement working you need to partially drain the tank every year or so to recreate that air pocket.

The next approach is very similar but to have a plastic board floating on top of the water, below the air pocket. This minimises the surface area of the air exposed to the water and therefore minimises the rate at which the air dissolves and the frequency with which the tank needs to be partially drained.

The other approach is to use an accumulator / "pressure vessel" as described by Reed_Richards. This shouldn't require any maintenance to ensure continued accommodation of expansion but the rubber membrane in them will eventually fail.

There is no perfect solution.