100L or 15L storage heater to heat water?!

[morgan616]

Advice sorely needed on this. Could help save cash on bills!

I need to run taps for a kitchen sink, a basin and a shower/bath in my 2 bed flat.

I've been advised to go with a large 100L 2kw storage heater and want to know if that is the best option? It was this one here on amazon: http://tinyurl.com/qaw2kkw

Instinctively I don't like the idea of a large tank. Space is a premium and heating up 100 L for one/two people seems inefficient. I often have quick showers and don't come home for a few days at time.

I've seen an alternative Redring EW15 15L storage heater on amazon and from the few positive reviews wondered if it could be up to the job? http://tinyurl.com/lgmec9g

It left me with a number of questions:

- Could the Redring really heat a bath (one of the reviewers said it can)? IF so, as well as storing 15L is it also supplying "instant" hot water? If not how could a 15 L heater fill a bath? (Assuming it's not a very shallow one!)
- How would it handle a shower and the kitchen taps being run at the same time?
- Is the 100L or 15L likely to be cheaper to run for someone whose use of hot water is on the low side?
- Are there any other good electric options?

 

[Lectrician]

They are both storing hot water, so the 15l will not fill a bath.

Larger ones want to be 3kw for quick heat recovery, a couple 3kw elements preferred.

Unvented above 15l needs to be installed by G3 plumber. Even under 15l, they need installing by someone who is going to follow the installation requirements 100'%.

I would get a plumber to spec and supply.

 

[PrenticeBoyofDerry]

You are also going to require an expansion vessel and additional safety devices for storing 100l of water.
There is no way I can see 15l of storage dealing with your water loading units.
You will need to do a little more thinking in to loading units and flow rates to identify the best method of producing an efficient hot water supply.

 

[Lectrician]

Even the little 15l ones often need an expansion vessel. The install instructions always give how long the cold supply line must be from the last cold water draw off or stop valve, this section of pipe usually accomodates the expansion. Most are installed under sinks though, and teed from cold there, so this length is not met. An expansion vessel and possibly PRV is then required.

I also see them installed with shut off valves which is down right dangerous too!

Installers really need to read the requirements, rather than just looking at it and going "cold in hot out, simples".

There's a chain of sites we maintain, and if you open the cupboard under staff kitchen sink, there is meters of copper zigzagging back and forth

 

[bernardgreen]

Tanks without any expansion space will swell and possibly rupture long before the water gets anywhere near boiling point.

As mentioned expansion in a small tank the expansion will push water back down the cold feed pipe. Provided the pushed back water cannot reach a tap and the thermostat will prevent boiling this "expansion vessel" is OK.

Larger tanks need to be installed exactly as per the manufacturer's instructions.

Much as I dislike them I would opt for an electrically heated shower before an electrically heated tank of hot water.

 

[JohnD]

Could the Redring really heat a bath

No, of course not. A bath holds about 100 litres of water.

An immersion heater or similar water storage system hears very roughly one litre of water per minute

 

[ban-all-sheds]

You will need to do a little more thinking in to loading units and flow rates to identify the best method of producing an efficient hot water supply.

You also need to understand the physics of heating water. The specific heat of water is 4.19J/gK, and no water heater maker can do anything about that.

Every 1kW gets you 60,000 joules per minute into the water.

If we assume a required delta of 50°, the amount of water which can be raised by that by 60,000J is:

60000 ÷ 4.19 ÷ 50 = 286.4g.

So for every 1kW your heater has, it will cope with just under 300ml of water per minute.

 

[Taylortwocities]

Great maths, BAS
That confirms JohnD's rule of thumb for (I guess) a 3KW immersion

An immersion heater or similar water storage system heats very roughly one litre of water per minute

 

[ericmark]

In my old caravan we had a 7 litre under sink heater element changed to 1kW which was ample for all my needs never run out of hot water. The expansion was through the forcet the taps controlled water into the heater so cold water forced hot water out.

So only the bath is a problem. We have an instant water heater 27 kW gas of course and filling a bath is slow. Really slow. However for a shower it is more than ample even 8 kW can run an electric instant shower.

So if you really want to save then shower is the way to go. Even with a really large rose on our 27 kW instant water heater leave the bung in and only 2 inches of water in the bottom of the bath.

For a shower we likely use around 25 litres of water. So with a full tank yes 25 litres is enough but only the Irish seem to be able to fill the tank with hot water. The Willis system

heats the storage tank from the top down and so extends the time you can run a tap with hot water. So although holding 25 litres likely you can run 30 litres before it gets cold. All the other systems heat the water from the bottom and rarely heat a whole tank to start with. The Irish are really clever they just pretend to be tict.

Running a bath is similar with a 100 litre tank often only the top half is piping hot so you only get 25 litres of really hot water. Only with the Willis system will you get a full tank of hot water. So using the Willis system likely you will get a full bath with 50 litres tank.

Tanks are often designed not to be full of hot water. The example shown is designed to combine multi heating sources and the solar pre-warms and the wood burner boosts the temperature and it likely also has an immersion heater. In this example the domestic water is instant heated by the stored water I am told although I don't have this system as domestic water is drawn is goes gradually colder rather than running out as such. Also told works well with a shower as mains pressure hot water even though the tank is not pressurised. The guy I know with this system says it will fill a bath but when I looked in his cupboard to view them there were two 8 foot tall tanks and I would think more like a 1000 litres than 100 litres. It does however to be fair also run the central heating.

Again from what I am told in Ulster the Plumbers are use to the Willis system and it works well. But in mainland UK finding a plumber who knows how it works is hard. The instructions say you need 54 litres for a bath. But 9 - 14 litres for a shower. But do remember these are Irish litres where whole tank can be heated not English litres where only half the tank is heated.

As a Welsh man I can now stand back and watch!

 

[morgan616]

Great advice all around. Thanking you