Economy 7

[JohnW2]

eric: it's time you persuaded your 'speech to text' application to recognise 'infrared' and not say 'inferred'. It gets quite difficult to read, and I assume is reasonably easily fixable.

Agreed, and he might consider addressing "collage" as well whilst he's at it.

Kind Regards, John

 

[hard-work]

The thing is, it would be really REALLY easy for them to make the boilers variable rate and interface with the variable flow rate heating system. All it would take would be to separate the main and condensing heat exchangers and use an internal pump to maintain the main heat exchanger flow rate while using an external modulating pump to put a variable flow rate through the condenser heat exchanger - hardly rocket science

What you are saying is that the boiler should operate in the most efficient hydronic environment. A thermal store will give you that. Find out what the optimum flow rate through a boiler is and what is overall temperature efficiency is, and set the pump to that speed and add blending valve(s) to the linking circuit to the thermal store to maintain the most efficient temperature. Then have the heating circuit off the thermal store which can be a variable speed pump. The boiler operates in its most efficient way isolated from the heating demand.

Electric thermal stores, sometimes called instantaneous electric DHW heaters, do not really have that problem if heated by resistance heating. The resistance elements do not care. They are around 98% efficient at all times.

With solar tiles becoming as cheap as normal tiles (Elon Musk), and the Tesla Powerwall battery, operating on Economy 7 and/or the solar roof, I see it when gas in domestic homes will be redundant and only used for industry and power generation.

 

[JohnW2]

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

I'm a bit confused - aren't those two terms incompatible (essentially opposites)?

Kind Regards, John

 

[hard-work]

[QUOTE="hard-work, post: 4196359, member: 239055]Electric thermal stores, sometimes called instantaneous electric DHW heaters ...

I'm a bit confused - aren't those two terms incompatible (essentially opposites)? [/QUOTE]They do heat incoming cold main fresh water instantly. Once the store of heat is exhausted then it does not work. In a gas combi boiler the delivery of hot water is infinite.

 

[JohnW2]

They do heat incoming cold main fresh water instantly. Once the store of heat is exhausted then it does not work. In a gas combi boiler the delivery of hot water is infinite.

I'm still a bit confused

I certainly agree that a gas combi boiler (or similar), or an electric shower (or similar) qualify as 'instantaneous water heaters' - since they heat the water in real time, as needed, and, as you say, can supply an 'infinite' amount of heated water (if one waits for an infinite amount of time!).

However, it is far less clear (to me) how/why a system involving stored hot water can be called an "instantaneous electric DHW heater" - and, if one is going to use that terminology, what (if anything) would qualify as non-instantaneous electric DHW heating? Can you help me understand?

Kind Regards, John

 

[hard-work]

• 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.
• 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.

We are accustomed to think that instantaneous means no stored water. A combi or instantaneous water heater maybe should have the word infinite in its title.

 

[SimonH2]

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.
The heat exchanger can be a passive coil immersed in the tank, or it can be a plate heat exchanger with the stored hot water pumped round.
However it is not correct that there is no legionella risk. There is still a large volume of stored hot water, some of which may spend time at a "dangerous" temperature, but this should not present a risk as that water is not what comes out of the tap or shower. It is a small risk to anyone working on the system and releasing any of the stored water.

 

[hard-work]

True legionella has a higher risk than a thermal store when a coil is used as it has a decent volume of water inside the coil. That risk is eliminated when using plate heat exchanger as the water volume in side the plate is very low.

 

[ericmark]

See

he shows what I was talking about, it may work, but does not pay, OK Australia, but shows the problem.

Oh as as to combi boiler they are not required to give instant hot water, I have a Main 7 in my old house for hot domestic water, and a completely separate boiler for central heating. OK was fitted around 30 years ago, but don't have to be combined.

 

[SimonH2]

What you are saying is that the boiler should operate in the most efficient hydronic environment. A thermal store will give you that. Find out what the optimum flow rate through a boiler is and what is overall temperature efficiency is, and set the pump to that speed and add blending valve(s) to the linking circuit to the thermal store to maintain the most efficient temperature. Then have the heating circuit off the thermal store which can be a variable speed pump. The boiler operates in its most efficient way isolated from the heating demand.

You mean like this ? It's not that simple if you want to really maximise efficiency. BTW, that setup was done primarily to prevent condensing in a non-condensing boiler
The return temperature from the store is variable - with no heating (eg summer) the bottom of the store (and hence return flow to the boiler) can be only a little above the incoming cold water temperature. But when heating demand is high, it can be some 20-30˚C higher due to the rad returns raising it significantly.
Assuming a desired charging temperature of (say) 70˚C then the required delta-T could be between as much as 60˚C and as little as 30˚C.
For a condensing boiler, I had in mind looking for a model where the condensing HE can be plumbed separately and run the pipe from the store to the boiler through the condenser HE before the loop through the boiler as shown in the link above (and slightly re-arrange the loop to use the valve as a diverter on the hot side rather than a blender on the cooler side). That way, the condenser HE has a variable flow rate and can cool the exhaust to as cool as the store temperature allows, while allowing full flow rate through the main HE to allow control stability (which is the primary constraint.)

True legionella has a higher risk than a thermal store when a coil is used as it has a decent volume of water inside the coil. That risk is eliminated when using plate heat exchanger as the water volume in side the plate is very low.

In either case, the real saving factor is that the DHW is "instantly" heated above the "kill everything" temperature before leaving the heat exchanger (whether coil or PHE). So in either case, the risk to users is very low.
There is still the risk when servicing the system as there may well be parts of the system harbouring the bacteria. Periodically I turn the cylinder stat right up and run the boiler until it cuts out at it's 82˚C over-limit. That should get the bottom of the store above the temperature required to sterilise it.

 

[ericmark]

Is there a legionella risk when you have inhibitors in the water? In my father-in-laws house he has a store of hot water heated from the solar panels on the roof, the solar panels water is not connected to the mains just like any central heating system and has inhibitor in the water. The water store in his case is a risk since it is direct to taps, so the taps can't mix water until the point where it comes out, so mixer taps are more expensive, and the shower has to take cold water from cold water tank.

However with the new storage tanks the water in the tank is not changed, so it has inhibitor in, and the water to tap is mains pressure, the heat exchanger is much more efficient than the old hot coil. So correctly fitted it is not a problem, only if the old type cistern is used is there a problem. And then only if water held below 60°C, this is a problem with solar panels as they don't get water that hot, but this thread is talking about electric heating which will be over 70°C so not really a problem.

 

[SimonH2]

Is there a legionella risk when you have inhibitors in the water?

Depends on the chemicals I guess - but I'd imagine that they are designed to inhibit corrosion rather than bug growth (unless specifically for bug growth). There are chemicals for treating water that could be a legionella risk - at a place we used to work, every quarter the maintenance manager would go to get a purchase order (for testing and treatment of the factory water system) signed, get the cost queried, mention "Forum 28"* and immediately get a signature
* Many of us still reckon there was a significant amount of scape-goating went on there.

 

[hard-work]

You mean like this ? It's not that simple if you want to really maximise efficiency. BTW, that setup was done primarily to prevent condensing in a non-condensing boiler

Makers will tell you the widest heat exchanger temp differential for the heat exchanger. Many are 25C. They can also give the flow through the boiler to maintain maximum temperature output. So if the charge temperature of the thermal store is say 75C, the return to the boiler can be 50C by setting the return blending valve to 50C. Getting the optimum flow through the thermal store would mean having a flow sensor that controls the flow via the pump - a lot of expense for a domestic system.

Electric thermal store just do not have this problem. A new cylinder that heats top down with a smart computer control. You decide how much of the cylinder you want heated. Also with a Smart app. Nation Grid are using them to dump surplus electricity into when they produce too much, or to balance the grid in a test. It appears they are using one immersion element at the top of the cylinder and a shunt pump to heat the lot or as much as you want.

Quite neat. One immersion element (can be a 12Kw) with a shunt pump with a clever computer control board. Quite an advance in that alone. But the grid will periodically charge it up for a little charge to dissipate surplus electricity. The grid pay companies to use surplus electricity, when too much is being produced. This must be cheaper than using gas for DHW as it now stands as you only heat what to heat, not full cylinders. But I see no reason why gas cannot be used with a high efficient coil at the top of the cylinder, or probably a plate heat exchanger using the shunt pump in dual mode (HSW plate and shunt). The savings are heating the cylinder top down and as far as you want the hot water down the cylinder. They must also have a pancake spreader for the cold inlet to the cylinder to disperse the cold inlet water to only the bottom strata of the cylinder. So stratification is not disturbed. That is applicable to any fuel used.

They have got Newark Copper cylinders to make the units.
https://mixergy.co.uk/about/#step1

 

[SimonH2]

This must be cheaper than using gas for DHW as it now stands as you only heat what to heat, not full cylinders.

With that one statement, anything else is completely discredited. For me, lecky is around 4 times the price of gas - so allowing for boiler efficiency etc there is no way that lecky can be cheaper. The "heat only what you need" bit is a red herring since what you don't use will sit there in the cylinder till you do want to use it - and as long as the cylinder is well lagged then it will not lose much heat.
Also, heating "what you want" takes us back to the days when you planned in advance and "put the immersion on" a few hours before going for your bath - how very 1960s
For a reference point, when I did the thermal store install I linked to above, I actually measured the standing losses - a mere 80W for a 170l cylinder. Incidentally this was around half the losses of the combi (160W equivalent in gas used when not in "eco" mode*) in my house next door which coincidentally was also empty at the time.

* In eco mode, you are back to the "turn on hot tap, turn it off, wait a while, then you can expect hot water" or "turn on hot tap, waste loads of water while the boiler heats up" modes that make combis so horrible (not the only reason I hate them).

EDIT: And when I see that "The Mixergy tank connects to the internet" then they can sod right off

 

[hard-work]

If you only heat what you need then far less electricity is used, also it must depend on how much the grid gives you for free, or rock bottom priced electricity, to heat it. Occasionally you will get out of bed to a full hot cylinder of water maybe for free.

Having a top mounted 12kW immersion only heats from the top down, and heats very quickly, in the Mixergy case, to get usable hot water. They say they have applied for a patent for the cold inlet spreader they use which does not de-stratify the water in the cylinder. OK most of what they are doing can be applied to gas system with a top mounted heat exchanger.

I think I read they will have a self adaptive system that heats the amount of water in the cylinder to your use. It learns your usage patterns.