Economy 7

[JohnW2]

Combi or instantaneous water heater (on demand) - Heats incoming cold water in real time. No legionella threat. Will operate infinitely as long as the fuel supply is available.

No argument with that. That is my understanding of what "instantaneous" water heating is, but ....

Electric (on demand) instant water heater - Heats incoming cold water in real time. No legionella threat. Will only operate as long as a hot water cylinder is charged up.

This is my problem. With such systems (like 'under-sink' ones etc.), if there is a hot water cylinder which contains a finite amount of hot water, in what sense in the water being heated 'in real time'? If it merely means that 'the incoming cold water is heated when it's heated', that's plain daft!

We are accustomed to think that instantaneous means no stored water.

We are, and I still regard that as the meaning of 'instantaneous hot water'.

As I have asked before, with no answer from anyone yet, what sort of water heating would not qualify as 'instantaneous' by your definition? My (very common) conventional (extremely well lagged) hot water cylinder heated by immersions would certainly seem to qualify - but I would never have dreamed of describing it as 'instantaneous water heating'.

Kind Regards, John

 

[JohnW2]

With a thermal store, the DHW is "instantly" heated as it passes through the best exchanger. The store needs heating, and with electric that will be at a power level that won't support continuous use without some time to recharge.

As I've just written, if the "instantaneous" in hard-work's definition merely means that "cold water is heated when it is heated" (rather than at the time it is needed), I think that's daft.

Can you answer my question - i.e., by this definition, what sort of water heating is not "instantaneous"? Would you really call my conventional system of heating water in a lagged HW cylinder by immersion "instantaneous water heating"?

Kind Regards, John

 

[ericmark]

You can use a calorifier which is a heat exchanger to heat water yet keep it isolated, used a lot in narrow boats to heat domestic water from the engines cooling water. You can build what ever you want.

However what has the price of gas to do with Economy 7? I am sure if the poster has gas, he would not use Economy 7.

 

[hard-work]

An instantaneous water heater heats incoming cold water instantly - it is that simple.

Storing hot water for later use is quite different.

 

[ericmark]

Instantaneous hot water is where you have a storage tank and a pump to circulate the water so on opening tap you have no wait for hot water, or were the water heater is at point of use.

The combi boiler does not in the main give instantaneous hot water, both the internal heat exchanger has to warm up, and the water needs to travel to the tap. Plus in my mother combi boiler there is a small reservoir of hot water inside the boiler, switch-able, how ever that reservoir means turn on shower and water is cold, as expected it clearly is not instantaneous, then it gets warm as the water in reservoir gets to shower, at which point those unaware get under the shower, then it goes cold again as reservoir runs out, but boiler has still not heated up the heat exchanger quick enough i.e. not instantaneous.

 

[hard-work]

Eric, An instantaneous water heater heats incoming cold water instantly - it is that simple.

 

[JohnW2]

An instantaneous water heater heats incoming cold water instantly - it is that simple. Storing hot water for later use is quite different.

I agree that it should be that simple (and I have always assumed that it was), but you have been telling us that a system which heats (a finite amount of) water and stores it for future use can be described as an 'instantaneous water heater' - which makes absolutely no sense to me.

No-one has yet been able to tell me what sort of water heating would, by this definition, not be called 'instantaneous'.

Kind Regards, John

 

[JohnW2]

Instantaneous hot water is where you have a storage tank and a pump to circulate the water so on opening tap you have no wait for hot water, or were the water heater is at point of use.

There doesn't even have to be a pump to get 'instantaneously available' (which is different from 'instantaneously heated') hot water. When I moved into my present house, the DHW system involved a very wasteful (of energy) Victorian system of lots of loops of (large calibre steel) pipes, with hot water constantly circulating through them by convection, so that hot ('stored') water was almost instantly available, even on the top floor.

However, as above, 'instantly available' and 'instantaneously heated' are two totally different things.

Kind Regards, John

 

[hard-work]

I agree that it should be that simple (and I have always assumed that it was), but you have been telling us that a system which heats (a finite amount of) water and stores it for future use can be described as an 'instantaneous water heater'

have written no such thing.

 

[JohnW2]

have written no such thing.

Maybe I've misunderstood you, but I have to say that I can think of no other way of interpreting ...

Electric thermal stores, sometimes called instantaneous electric DHW heaters ....

To my simple mind, one EITHER stores (previously) heated water OR heats water 'instantaneously' ('on demand' / 'in real time') - those two options being mutually exclusive.

Kind Regards, John

 

[hard-work]

The water coming out of the taps is heated instantly. It is not heated then stored for later use, as in say an unvented cylinder. Very simple.

 

[JohnW2]

The waster coming out of the taps is heated instantly. It is not heated then stored for later use, as in say an unvented cylinder. Very simple.

I'm still lost. They may (I think some do, particularly those producing 'boiling water') heat the stored hot water 'a little bit more' as it flows to an open tap, but if the water had not previous been "heated and then stored for later use", why would it be the case (as I agree it is) that ...

... Will only operate as long as a hot water cylinder is charged up.

(which is another way of saying "so long as the stored hot water has not all been used up)



Kind Regards, John

 

[hard-work]

I'm still lost.

There is nothing more I can do then.

 

[JohnW2]

There is nothing more I can do then.

Seemingly not. I do wish that some others would get involved, since I find it hard to believe that I am alone in having this difficulty.

Kind Regards, John

 

[EFLImpudence]

There has been reference to instantaneous and instant. I'm not sure if these are official definitions.

I think everyone is agreed that instantaneous water heating is a combi or electric shower, etc. - heating the water when required.

There are (under sink) heaters which maintain a small volume of heated water which is instantly available - indeed quicker than a combi supply may reach the sink.
Some of these purport to keep water near to boiling point to instantly make tea.