Economy 7

No, it makes pretty well no difference long term. it is a common misconception that if you have a tank with (say) 100l of hot water, and draw off (say) 20l of it, then it takes the same energy as heating 100l again from cold. it doesn't, it takes the same energy as heating 20l from cold.
If you need only 20l of hot water in 100l cylinder and only heat 20l, then you are not paying to heat 100l. Simple. If the grandkids come around then you can heat say 80l of it. Your chose by a controller
Free lecky ? Hands up anyone here who has such a tariff :whistle:
It would be better to look at the video I posted, and understand its operation, and take note of what I wrote. Yes FREE electricity, or for next to nothing. The grid pays companies to use electricity when too much is produced. They want to dump this surplus into 100,000's or millions of hot water cylinders.

BTW, gas is cheaper than E7 tariff but gas is approx 80% efficient while about 98% of the electricity use in an immersion goes to heating water. So taking in account the inefficiency of gas, gas in comparison is not as cheap as first appears.
 
Last edited:
Sponsored Links
Yes, the stored water (hence thermal store) is heated and then usually high pressure cold mains water is run through an internal coil and instantly heated. The stored water (the heat medium) is static and can be low pressure, then no problems with explosions and the complexity of high pressure stored water. They tend to have a blending valve on the DHW outlet which mixes with cold to give the set temperature.
More and more hints of snake oil are showing themselves.

Why do you particularly want to want to heat 'high pressure water'? We've been quite content with the pressure provided by header tanks in conventional vented systems for a very long time, even in bungalows.
They are very simple indeed. Some do not need overflow or discharge pipes to outside, so they are ideal for fitting way inside premises.
Is that 'marketing copy'?

Kind Regards, John
 
Why do you particularly want to want to heat 'high pressure water'? We've been quite content with the pressure provided by header tanks in conventional vented systems for a very long time, even in bungalows.
Is that 'marketing copy'?

Kind Regards, John
You clearly have little knowledge of water systems and poor comprehension. High pressure hot water gives high pressure showers and balanced mixer taps. Snake oil? Many highly insulated modern flats are all electric and many use electric thermal storage for DHW (I showed you piccies from two makers) to provide high pressure water. Many manufacturers make them. Duh!

No one puts in loft tanks any more if the cold mains pressure is good enough. That is something of the embarrassing era of British plumbing, of which the country was a joke.
 
It looks like this https://www.mcdonald-engineers.com/products/thermal-store is what hard_work is referring to, that is where the diagrams he included come from.

From that page I think the main advantage they are pushing is that you can use sources such as solar thermal, heat pumps, wood stoves or wind power to heat the water in the heat store and 'only' use gas/electricity when those are not sufficient.

Of course, the more methods of heating the water in the heat store there are, then the more complex the system is to install & maintain.

If one wants to use hot water that has been (ultimately) heated by some prior heating, is it not going to be inevitably more efficient to initially heat the water (and then store it in a well-insulated store) than to heat something else (maybe even 'other water') and then use that to heat cold water when hot hot water is needed?

I meant to say that a two-stage water heating process will probably be less efficient. I'm not sure how much though.
- In a normal system you would heat water in a (hopefully well insulated) tank and store it until it is needed. When some hot water is drawn off it is replaced by cold water and the temperature of the water drops.
- In a two-stage system you would you would heat water in a (hopefully well insulated) tank and store it until it hot water is needed. When some hot water is produced by passing cold water through the heat exchanger then the temperature of the water drops.
 
Sponsored Links
BTW, gas is cheaper than E7 tariff but gas is approx 80% efficient while about 98% of the electricity use in an immersion goes to heating water. So taking in account the inefficiency of gas, gas in comparison is not as cheap as first appears.
Try the arithmetic. Piped gas is less than half the cost of off-peak E7 electricity. Hence, taking efficiency into account (using your figures), for the same amount of heating, the cost of using off-peak E7 electricity would be at least 63% greater than would be the cost with piped gas.

Kind Regards, John
 
StephenOak, you are getting it, unlike some.

Here is the Gledhill, electric thermal store:
https://www.gledhill.net/products/thermal-storage/pulsacoil-eco-stainless/

PCoil.jpg


Thermal stores are very useful when multiple fuels are used. But very simple electric only thermal stores are extensively used in electric only flats. Safe low pressure water storage requiring no annual services (saving a lot just there of the lifetime of cylinder) as with unvented cylinders, with zero risk of explosion, service free and mains pressure hot water. Legionella risks near zero.
 
You don't appear to provide provide a link - so perhaps you can you tell us ... does that use electricity to heat something (maybe water) and then later use that 'heated something' to heat cold water? If so, what is the point of that 2-stage process?
There are a number of reasons for using a thermal store.

You can, as already suggested, heat the water directly and store it. Such systems are common - lookup "unvented cylinder". These have the advantage (over the old vented hot water cylinder) of providing mains pressure* hot water. However, there are dangers with storing large volumes of hot water under pressure - hence they can only be installed as a complete package which includes multiple safety valves, and (IIRC) must be installed by a registered installer or something. They also are not suitable for use with uncontrolled heat sources such as wood burners - this creates a risk of producing superheated water, and a flash boiling explosion if the cylinder fails.

With a thermal store or heat bank** - I've never ever heard them referred to as instantaneous water heaters - the stored hot water is not under pressure. Unless you block off the vents then it's impossible to create such an "explosion" risk, you could boil the water in the store but you could not create pressurised water or steam. By having the second stage, you can get mains pressure* DHW without all the dangers (and mitigation requirements) of storing it under pressure.
One of those mitigation measures is a temperature & pressure relief valve that will dump hot water if the primary temperature or pressure controls fail. The rules on that safety vent pipe mean that many locations are not suitable to install an unvented cylinder.

Where thermal stores really win is when you want to combine multiple energy sources and sinks - especially if one of them is something like wood burner. It provides a neutral pressure point where flows in different circuits don't interfere with each other - so (for example) the (variable) flow round the central heating is decoupled from the thermo-syphon through the wood burner, and both are decoupled from the DHW.
Even without such multiple sources, they are good for providing "copious" DHW flow rates for DHW while also decoupling the variable flow rate central heating from the 20th century*** gas boilers we are still stuck with. Don't recall specific numbers, but DHW capacities of 100kW are not uncommon for heat banks.

I put one in the flat specifically for that reason, AND to allow the use of an immersion heater as a backup for when the boiler breaks down. It allows the immersion heater to provide both DHW and heating as long as you don't go too overboard on usage.

EDIT: You may well, correctly, assume that I rather like thermal stores. However I can't help thinking that hard-work has been at the cool aid as our friends across the pond like to say.


* Well they actually have to have a pressure control valve to limit the working pressure, so in high pressure areas it's not actually "mains" pressure.

** Historically, "thermal store" tended to mean a passive system with a heat exchanger coil for heating the DHW, while "heat bank" tended to mean an active system where the hot water is pumped round one side of a Plate Heat Exchanger (PHE) while the DHW passes through the other side of the PHE. Over time, the influence of marketing types has "somewhat blurred" those already "soft" definitions.

*** Manufacturers still don't allow variable flow rates through the boiler - especially not arbitrarily low flow rates - which are required to interface with an "all TRV" heating system and not have a bypass nobble the system efficiency.
 
Try the arithmetic. Piped gas is less than half the cost of off-peak E7 electricity. Hence, taking efficiency into account (using your figures), for the same amount of heating, the cost of using off-peak E7 electricity would be at least 63% greater than would be the cost with piped gas.

Kind Regards, John
You have comprehension problems : "So taking in account the inefficiency of gas, gas in comparison is not as cheap as first appears".
 
You clearly have little knowledge of water systems and poor comprehension.
If you say so.
High pressure hot water gives high pressure showers and balanced mixer taps. ...
If one is dissatisfied with the pressure from a gravity-fed system, a 'shower pump' can achieve what you are talking about at dramatically less cost.
No one puts in loft tanks any more if the cold mains pressure is good enough. That is something of the embarrassing era of British plumbing, of which the country was a joke.
The main advantages of combi boilers are the avoidance of need for space-occupying header tanks and HW storage cylinders and the inevitable losses associated with storing heat - the system you are talking about involves a pretty large storage vessel and the inevitable heat losses. As Stephen has written, it might make sense if there are substantial amounts of 'off grid' sources of energy feeding into the system - but I cannot see the point otherwise.

Kind Regards, John
 
Why do you particularly want to want to heat 'high pressure water'? We've been quite content with the pressure provided by header tanks in conventional vented systems for a very long time, even in bungalows.
Actually, fewer and fewer people are happy with that - hence why (noisy) shower pumps are so common.
 
You have comprehension problems : "So taking in account the inefficiency of gas, gas in comparison is not as cheap as first appears".
No comprehension problem - and I totally agree that gas is not "as cheap as first appears" (to someone who does not consider relative efficiencies) - with the figures I used (which under-estimate the true difference), gas is only around 40% cheaper than off-peak electricity, rather than the 50% cheaper which you might expect (from a 2:1 price difference) 'at first sight'.

However, that's before one considers that gas-heating of water (or an intermediary) does not have to happen many hours before one wants to use the hot water (with the attendant heat losses in the latter case).

Kind Regards, John
 
It would be better to look at the video I posted, and understand its operation, and take note of what I wrote. Yes FREE electricity, or for next to nothing.

It is not clear which video you are referring to as you have posted five but none were in the comment (#90) where you said "how much the grid gives you for free, or rock bottom priced electricity".

However in your previous post you said "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." (emphasis added).

Electricity supply is varied to meet demand so there is rarely any surplus. When there is a surplus (e.g. from baseload nuclear plants that are hard to 'turn down' at night-time) this is used to charge pumped storage. If there is surplus beyond that, I can imagine companies having arrangements* to increase usage to absorb that. And you refer to companies doing that. I have never heard of electricity companies simply giving power to domestic customers.

How would they choose who to give it to? How would they even do it? If you consume X kWh overnight then your meter will go up by X.

* Like some companies have arrangements to lose their power when there is a shortage in exchange for lower a tariff.
 
If one is dissatisfied with the pressure from a gravity-fed system, a 'shower pump' can achieve what you are talking about at dramatically less cost.
As long as you don't mind waking the household up when you shower. A friend's parents had their bathroom redone and he made of point of instructing the bathroom fitters about there not being a noisy pump. There was a very noisy pump such that everyone anywhere in the house knows when the shower is in use.
With mains pressure DHW, you get mains pressure to all the outlets - which makes selection of (eg) mixer taps a lot easier.
The main advantages of combi boilers are the avoidance of need for space-occupying header tanks and HW storage cylinders and the inevitable losses associated with storing heat - the system you are talking about involves a pretty large storage vessel and the inevitable heat losses.
Actually I have done measurements and found a combi to have double the losses of the thermal store I put in the flat. It does depend on the combi and it's settings, but a combi will either have the "wait for ages for hot water" problem, or it will have high (higher than a storage tank ?) losses keeping itself warm. Measured standing losses on the thermal store in the flat were about 80W (measured over several days using the lecky meter), while the combi in the house next door had losses of around twice that with eco mode off.

Neither a thermal store nor an unvented cylinder need a header tank. A small F&E tank is needed for a thermal store - but it isn't the "coffin in the loft" that a header tank is. I agree that needing room for the tank is an issue - more so if you are relying on cheap rate lecky for charging. If using a gas boiler, then the cylinder doesn't need to be huge as the very worst case you can get to is use up all the hot water and be restricted to the heat that the boiler can provide. In practice, the boiler will be reheating the store as you draw off, so you don't use as much stored heat from the store as you draw from the tap (if you see what I mean).

EDIT: Of course, one of the reasons developers love combi boilers is because they can reduce the size of the rabbit hutches by a square yard by not having the storage tank.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top