Energy myth-busting: Is it cheaper to have heating on all day?

malc_b

I would imagine that the water circulation driven by the heat element mixes the water so that the whole tank is heated, except maybe for the bottom few inches. In hard water area you'd get a build up of scale in the bottom of the cylinder anyway which I guess is a good reason for have a dead space at the bottom.

The element is a thin line running at an angle in the cylinder. Heating the water will cause it rise all alone the element creating a circulating current which will mix up the water. So the process is that when the thermostat, say at the top at tank, drops below X deg C then the element turns on. This will start mixing the water and so drop the water temperature at the thermostat. The heating will continue until the thermostat hits X+d deg C (+d because there will be a hysteresis). But this will take some while, by which time the water circulation will have mixed it up so just about the whole of the cylinder will be at X+d deg C, except for any dead space at the bottom.

Also, I would have thought that if you don't like a short thermostat the easiest solution is to fix an external thermostat such as the ones used for CH, maybe with an secondary relay to up the power handling if they aren't man enough for 13A. I'd leave the element thermostat in series as well though, just set rather high, as a safe feature to stop boiling the water. In fact maybe that is why the thermostats are near the top anyway. It is perhaps more important to ensure the hot water does not come out scalding rather than worrying about heating the whole tank full.

orrery

malc_b wrote: »

I would imagine that the water circulation driven by the heat element mixes the water so that the whole tank is heated....

I think with that opening statement you've hit the nail on the head.

It does, if you run the element at 3kW. If you bleed in low currents (as a device such as SolarImmersion etc will do) then the circulation currents don't seem to be set up. I'm guessing, based on how my system is behaving, that you get a film of very hot water adjacent to the element which then slowly rises to sit at the top of the tank, tripping the over temperature cut-out, whilst the thermostat some inches below, still hasn't reached cut-off.

bestyman

orrery wrote: »

Well, I'm trying to get a 30" one at the moment.

My current top mounted immersion is 21". The cylinder manufacturer tells me that the thermostat is only 5" long. I get only about enough hot water for a shower or a shallow bath, if we're lucky and the water is super-hot. This is only about 1/3 of a tank and is fully heated by about 11am on a sunny day. The supplier tells me that these are fitted to secondary cylinders (gas CH) and are intended for emergency use only.

I've spoken to the immersion manufacturer this morning and they can supply a 30" (the longest that will go in the tank) but the longest thermostat available is only 18".

I'm not sure what format this will be - some have a folded element at the bottom to get most heating effect as low down as possible.

I've tried using a de-stratification pump (which works brilliantly in terms of heating the whole cylinder, right down to the cold inlet), but having tried several pipe configurations, we can't stop it eventually filling with air, so I'm reluctantly giving up on that.

I assume you are talking about an electrical 240v immersion heater?
Before ordering anything expensive, I would try a standard 27" immersion heater with 18" stat. I know from experience that this cylinder fitted in a 36 x 18 cylinder will provide easily enough for a bath. This configuration was really popular before combi boilers came along about 20 years ago. I'm sure that the extra length of the stat will make all the difference.
I've had problems with the cheaper ones failing after a year or so now use long life, have a look at www.toolstation.com.


Edit: Just remembered the time a lad who we worked with didn't have a 18" stat so used a 7" stat instead. The customer complained of a half full bath and he had to go back.

Richie-from-the-Boro

orrery wrote: »

My 6 month old Stainless Steel cyclinder, made by Copperform, has a 21" as standard and the 27" 'option' has been discontinued.

Does anyone have a recommendation for one of those 'custom' immersion makers?

Two element immersion heating - If :

- you put an element in the top you will get a 1/4 dustbin full of angry bubbling boiling water if you are lucky
- you put an element in the bottom and you will get a dustbin full of angry boiling water

Liquid, such as water, is heated from the bottom, the layer of water closer to the heat source expands and hence becomes less dense compared to the water layer above it. Expanded water is less dense than the surrounding water and therefore it rises. The cooler regions of the water in the upper part of the flask, being denser, sink. This movement of liquid due to a difference in density sets up a convection current.

So the heated [ hot ] water pushes up and forces the cooler water down over the thermostat which in turn is heated until the whole volume of the tank reaches your pre determined setting then the stat switches the leccy off. Its a bit atomic science and the random motion of atoms and all that, but sufficient to say that's why the top down element is rubbish at heating a full tank of water


Hope this helps my friends

Smiley_Dan

Well you'd heat the top one first, trip over to the bottom one when heated and so on. That's how ideal PV hot water systems work. Makes sure you have usable hot water on bad solar days.

orrery

Smiley_Dan wrote: »

Well you'd heat the top one first, trip over to the bottom one when heated and so on. That's how ideal PV hot water systems work. Makes sure you have usable hot water on bad solar days.

Indeed - but the tank doesn't have a fitting for a bottom immersion and my plumber is advising against adding one in a stainless tank. So, I've ordered a 30". I need x2 baths/showers and the 21" will do one comfortably. I see no point in swapping to a 27" when I can get a 30" in the tank.

bestyman

orrery wrote: »

Indeed - but the tank doesn't have a fitting for a bottom immersion and my plumber is advising against adding one in a stainless tank. So, I've ordered a 30". I need x2 baths/showers and the 21" will do one comfortably. I see no point in swapping to a 27" when I can get a 30" in the tank.

1.Cos 27" ones are £19.87 and available from stock?
2. Everyone else manages perfectly Ok with a 27" ?


Isn't that a bit like saying there is no point buying a 1.6L focus when there is room for a 2.0 lump? ;o)

Smiley_Dan

bestyman wrote: »

1.Cos 27" ones are £19.87 and available from stock?
2. Everyone else manages perfectly Ok with a 27" ?


Isn't that a bit like saying there is no point buying a 1.6L focus when there is room for a 2.0 lump? ;o)

If his petrol was free, kinda.

orerry: Shame you can't get the destrat pump working. The only other thing is a new tank...

orrery

bestyman wrote: »

Isn't that a bit like saying there is no point buying a 1.6L focus when there is room for a 2.0 lump? ;o)

No, no comparison. I've got a 21" element now, 27" extends the heating 6" further down the tank and 30" extends it 9" further down the tank (that's a 50% better increase).

Extracting the maximum out of the solar PV is critical, as some days are good and some days are bad - having the immersion turn 'off' when there is still cold water in the tank and still surplus solar pv available is plain daft. Having as much left over from the previous good day helps ride out the bad days. Also, getting the element down below the gas c/h stat is good as it may help stop it turning on to add a trivial amount to the tank - obviously running the boiler at all has an overhead in wasted heat, which I want to avoid, if possible.

The benefit is obvious - using a de-strat pump (which I'm giving up on, as it seems impossible to keep the air out) to heat the whole tank means I've used only 2 units of gas in August.

I've even contemplated adding a second tank in the loft, below the cold tank, to pre-heat the water with a second immersion.

archiebabes

Hi,

My less than 5 year old boiler is dead and whilst I'm waiting to get a new one I need some advice.

I have an electric wall heater and I have been lent an electric radiator. But I have always been told using them is like burning £5 notes!

Please can you tell me how I can calculate how much they cost so I can see whether I can afford to put them on or if it would be cheaper to leave the gas cooker on with the oven door open. Or whether I should go and stay with a friend!

And also please is it cheaper to use a microwave to warm a heat pad or boil a kettle for a hot water bottle. Thank you very much.