Boiler or leccy shower. Which is more efficient?

[Kent]

another graph -

This needs viewing as a rough guide, pellets are mostly over stated and a lot still depends on the efficiency / state of service and stored moisture when burnt (not when fresh off the production). Oil of course is plummeting and nat gas on the up at present and LPG has many hidden costs. If you look at the earlier list you will see batch burning coal is actually very cost effective to heat your home and water ( now why have the Chinese just built a massive amount of coal fired power stns to generate electric? LOL)

 

[gerrymoore]

I've just completely re-done our bathroom and also fitted a Worcester combi to replace an old boiler that was cr*p. Live in a bungalow and the bathroom is the opposite end of the house from the kitchen/boiler. If I had used the instant hot water from the boiler it would have to travel through at least 15 metres of pipe up into the loft, across, and down again so, apart from the waste of water (on meter) there would also be a loss of heat unless very well insulated.

I put in a top of range Mira 9.5K - retails at over £400 but got it 3 months old off EBAY for £70. It is very efficient and also gives a fantastic shower. Don't have a bath. For the little sink I fitted a 5.5Kw instant water heater...£60 of the Bay.....water for hand washing and shaving is hot instantly, no waste water and no time wasted waiting for hot water. Our electric bill is not particularly cheap at around £60 month, but I do have a couple of welders and a plasma cutter working quite often!!

 

[rtcosic]

another one that hasnt got or had a decent 10.5kw shower then cos not many people have by the sounds of it

Balderdash!!

I normally use my mains pressure shower at 15 litres per minute, but it can deliver >20 litres per minute.

So assuming:

15 litres/min = 15 kg/min = 0.25 kg/sec
inlet water temperature = 15 Deg C, shower temperature 45 deg C = delta T 30 deg
specific heat capacity of water = 4.187 kJ/kgK

then 0.25*30*4.187 = 31.4 kJ/sec

three times the puny 10.5kW a piddly little kettle on the wall puts out.

a lot of houses cant have a mixer shower on there heating system for one reason or another , another point is some houses have a couple of showers even 3 and couldnt cope with demand thats where electric scores .one on the boiler other of mains electric. each case has to be thought out for best result .

Now there you are right, if you have an HMO or very large family house, where more that one shower may be in use.

 

[normspanners]

a massive insulated hot tank as a heat store ?? rather than dump it ?

This heat store is not for domestic HW
I am not sure on the maths on this but they work on the fact of having massive amount of water heated by the boiler when running to 'excess' and stored in an insulated tank, then, that stored hot water, warms central heating through heat exchangers and acts like a 'battery'.

 

[Fintray]

Yes (i guess i must have mentioned it, but i dont remember when!)

On Navitron forum?

 

[normspanners]

Just found this hope it explains a little better.
My idea was to use several IBC interconnected and make a large insulated shed type store , back off transit freezer truck?? wrap each IBC with rockwool,
There is plenty of space through lid for heat exchanger coils etc.


Thermal stores
Thermal stores are becoming increasingly common in domestic renewables heating technologies as a means of storing excess heat generated. They can be used with an individual renewable heating technology or to combine different renewable heating technologies. Thermal stores can also be used as a renewables technology with a conventional boiler or immersion heater.

Thermal stores have proved to work particularly well with wood-fuelled biomass boilers, heat pumps, wind energy and solar water heating systems.

What is a thermal store?
A thermal store is a way of storing and managing renewable heat until it is needed. In a domestic setting, heated water is usually stored in a large well-insulated cylinder often called a buffer or accumulator tank. A thermal store may contain one or more heat exchangers, usually in the form of internal coiled pipes or external flat-plate heat exchangers. It may also include an electrical heating element, such as an immersion heater.

A purpose-built thermal store can take inputs from a number of different technologies, provided it has been designed and sized to work with them all. It might store heat from a wood-fuelled boiler, solar water heating or a heat pump.

A thermal store can provide:

• Space heating and mains pressure hot water.
• Space heating only (which may be the case with a heat pump system).
• Hot water only (common in the case of a solar water heating system).

The full potential of a thermal store is realised when it is used to store and manage a number of different heat inputs and outputs. For example, it lets you use your excess solar heat for space heating or to act as a heat pump pre-heat when this is used to supply hot water.

Find out more about how a thermal store works.

The benefits of a thermal store

• Allows management of the difference in time between when heat is available and when it is needed. For example, hot water produced by a solar water heating system during the day can be stored for use when little or no solar energy is available.
• Enables warm water to be heated up by a secondary heating source such as a conventional boiler or electric immersion heater.
• Enables a renewable heating system to work more efficiently. This is particularly relevant to wood-fuelled heating systems such as log boilers that operate much more efficiently if they are used at maximum output rather than kept ticking over.
• Lets you use a wood burning boiler stove or a stove with a back boiler at maximum efficiency without overheating the room.
• Reduces the need to buy expensive fossil fuels to meet on-demand hot water or space heating.

Types of thermal store
There are a number of different thermal stores on the market designed with different technologies in mind. Some of the most common are highlighted below.

Thermal stores for wood-fuelled heating
Thermal stores are very important for the efficiency of biomass heating systems, particularly log boilers, which are designed to burn batches of logs at high levels of efficiency rather than in small quantities throughout the day. A log boiler linked to a large thermal store can be used in this way. A thermal store can also reduce the time lag (which could be at least an hour) between lighting the boiler or stove and the demand for hot water by storing water from the last time the stove or boiler was last lit.

As thermal stores used with wood-fuelled heating systems are usually designed to provide hot water for space heating as well as domestic hot water, they tend to be fairly large. Thermal stores linked to wood fuel heating systems are commonly referred to as accumulators or buffer tanks. Typically they will hold between 500 to 5000 litres of water and can store hot water for days if properly insulated.

Smaller thermal stores (300litres) can also work well with boiler stoves and stoves with back boilers. These stoves tend to be in living spaces and are fed with fuel throughout the day. Boiler stoves and stoves with back boilers differ in the proportion of heat they put into the room or water. Boiler stoves will put around 65 per cent of their output into water, whereas stoves with back boilers may only put 20 per cent into water.

The sizing of a thermal store for a wood-fuelled heating system will depend on many factors, in particular the type of wood fuel being used. A pellet boiler will need a relatively small thermal store as it can cope quite quickly with changes in heat demand. A log boiler designed to burn logs in batches will on the other hand need a large thermal store to take all the heat from the batch of logs in one go.

A thermal store working in conjunction with a log burning batch boiler will need to be fairly large, probably no less than 25 litres/kWth and preferably 50 litres/kWth. The sizing of the thermal store connected to a wood fuel boiler is something for the installer to ascertain as part of the total system design.

Thermal stores for solar water heating
Thermal stores work very well with solar water heating systems as they allow solar thermal heat to be used for space heating as well as heating water. On a sunny spring or autumn day, a solar thermal array may harvest far more heat than would be needed for hot tap water alone. Combined with a thermal store also supplying space heating, this harvest can be put to good work.

With a simple design and control strategy a thermal store can also be designed to prioritise solar thermal heat above all other sources. This will mean that if solar heat is available, no other heat source will come on.

Thermal stores for heat pumps
An air source or ground source heat pump will work more efficiently with less wear on the pump and compressor if it does not have to continually cycle on and off (short cycling) when the demand for heat is low. This is more likely to happen if your heat pump is relatively large and less likely if it is relatively small and running continually to meet demand. It is also less likely if you have an air source heat pump with a motor that can modulate its output.

One of the ways to avoid the short cycling of a heat pump is for it to be linked to a thermal store (usually referred in this instance as a buffer tank). However, there are other ways (such as leaving a part of the heating system permanently open). Your installer, in conjuction with the recommendations of the manufacturer, will decide whether a buffer tank linked to your heat pump is appropriate, as well as what size buffer tank you should use.

Combining technologies
A thermal store allows you to link up a number of different heating systems, for example a wood burning boiler stove and a solar water heating system. This is a particularly beneficial combination as it means that you can have hot water in the summer without having to light the stove.

You can also connect the following to a thermal store:

• One or more renewable energy technologies (solar, biomass, heat pump).
• A conventional boiler.
• An Aga-type range with a boiler.
• Electrical heating elements such as an immersion heater.

 

[brightspark]

Why heat only...?

system boiler does both heating and hot water, just need a decent sized hot water storage tank, mine is flipping huge (surprised the floor joists aren't complaining)...from ice cold to filled with 65C hot takes sub 35 minutes

a system boiler is a a single output boiler and is known as heat only. my system is a honewell y plan with a 3 port valve fully pumped system with hotwater storage indirect tank or u could make it an s plan using two twin port valves one for water and one for heating

 

[Kent]

Heat stores (buffers) are the future for one thing they allow for multi heat sources - way too big to go inside a normal house and the size needs calculating by the supplier generally but 80L per KW if I remember right without checking. Too small defeats the object too big is just like trying to tow a 5 birth caravan with a mini, I went to one guy who was trying to run a Big high ceiling Barn Conversion of a fairly std size log burning fire to a heat store and get this underfloor heating on a twice a day cycle - So there are some right old numpties in this marketplace

I will put one in here some time soon linked to multi sources of heat.

 

[normspanners]

too big is just like trying to tow a 5 birth caravan with a mini

This guy has a heat store that works so he tried your idea with the caravan

 

[julianf]

Could you not use/make , a massive insulated hot tank as a heat store ?? rather than dump it ? I had thought about this for my waste oil burner, you can shut boiler off at night and run off the heat store.

Certainly. However, when i say "heat dump" i dont mean wastefully - i mean another radiator in the house.


Our hot water tank (thermal store) is 300lts ish. I was going to build a 900ltr one, but then an off-the-shelf 300ltr one came up. (i had been collecting coils from scrap tanks for ages for the 900ltr one - so maybe at the next house) A massive store would be great, but we only have a small terrace house so footprint is important (300lts still leaves space in the utility room for other things, whilst 900lts woulnt really!)

Our thermal store is vented, with a mains pressure domestic hot water coil running through it.

Size wise, cooking generally produces about enough hot water for DHW demands, but, if you use the rayburn for room heating also, that's when you need to start dumping heat from the water.

This wouldnt really be an issue, if i had a central heating circuit connected to the store, but, at present, the only drain on it is DHW, hence having to dump heat. In general you would only want to use the rayburn for room heating at the same time as running the CH circuit, so it would never need to dump heat, however, a heat dump is a requirement under section J of the buildings control.

So, i will fit one, but, as i say, when the CH circuit is sorted it probably wont be required anyhow (its just a saftey thing in case of power outage - but with a vented tank, its not like its the end of the world if it boils anyhow - but its still a document J requirement anyhow)


In case its not clear - our setup is currenty incomplete. With the CH circuit, and the heat dump it should not really require much thought.

As mentioned i also have an oil boiler to connect to it... And a stack of solar panels... If i ever get arround to it!

 

[Farside]

Any electric heater is 100% efficient, if that's what you're asking.
However, as has been said, it's less effective than a proper stored-hot-water power shower and only really much use as a stand-by or in a guest bedroom en-suite. Having said that, I used one for years and just got used to not being a hot water hog. Otoh, when I was able to luxuriate in a decent shower, game on.

Just found this hope it explains a little better.
My idea was to use several IBC interconnected and make a large insulated shed type store , back off transit freezer truck?? wrap each IBC with rockwool,
There is plenty of space through lid for heat exchanger coils etc.

Exactly as I plan to. Got an IBC sitting there, ready to install some time soon-ish. I reckon 8" of rockwool all around will be adequate. However, there's one big unknown - the integrity of HDPE at stored HW temps - it's ok at 60C, still fine at 80C, but you don't want it much above that and if it reaches 90C, it starts getting a bit floppy, iirc.
IBC containers are made in SS, but I've never found one.
Another container that I should have snapped up when it was going, was an old milk bulk tank off a neighbouring farm - would have cost 100 quid or so, but would have been perfect.

 

[dannyp]

put in a biomass woodchip burner, if you can claim RHI it seems to make sense to be as inefficent as possible

 

[normspanners]

Exactly as I plan to. Got an IBC sitting there, ready to install some time soon-ish. I reckon 8" of rockwool all around will be adequate. However, there's one big unknown - the integrity of HDPE at stored HW temps - it's ok at 60C, still fine at 80C, but you don't want it much above that and if it reaches 90C, it starts getting a bit floppy, iirc.
IBC containers are made in SS, but I've never found one.

There was a guy that was putting hot lard in standard IBC's at 130*C and they stood that ok .
So water at 90 should be no problem.
Another guy used a roll of blue water pipe as his heat exchanger coil, I suppose you could feed it in through the neck and would work, you would just have to fab up a bracket that kept it at the top, where the hottest water would be.?
OK not the best at transfering heat but its very cheap for a massive roll and easy to connect to.

 

[dannyp]

i was thinking at one point of washing out a 1500 ltr oil tank and cladding it with 300mm or so of kingspan or simalar for a massive heat store, then a local dairy got ripped out so i aquired a tank from there. still not got round to makeing the heat store tho

 

[Farside]

There was a guy that was putting hot lard in standard IBC's at 130*C and they stood that ok .
So water at 90 should be no problem.
Another guy used a roll of blue water pipe as his heat exchanger coil, I suppose you could feed it in through the neck and would work, you would just have to fab up a bracket that kept it at the top, where the hottest water would be.?
OK not the best at transfering heat but its very cheap for a massive roll and easy to connect to.

I was collecting lard from chippies and leaving them HDPE barrels to fill - more than one was partially melted and deformed, but not actually collapsed, so it's probably ok with hot water. Just that the specs I found for HDPE as used in IBCs gave me a bit of a concern.
I thought of cutting a larger square hatch in the top, but re-sealing it adquately is problematic. Not impossible though. This thing will be sitting in a corner of the workshop, so I don't want it heavily humidifying the space.

 

[Farside]

i was thinking at one point of washing out a 1500 ltr oil tank and cladding it with 300mm or so of kingspan or simalar for a massive heat store, then a local dairy got ripped out so i aquired a tank from there. still not got round to makeing the heat store tho

Yep, great minds, etc
I have an old 1000L steel oil tank, but rust would be a problem, so SS is the perfect solution.
I'm keeping an eye out for bulk milk tanks locally.

 

[julianf]

(straying from the original topic now...)

In line with my previous post, i was going to use 3' of a 4x2x6 1200ltr kerosene tank (ie 900ltrs) and the plan was to stand it on its end, for a 2x3 footprint.

I got as far as chopping it down to 3' size, and am still confident that it would have worked, but a smaller, off-the-shelf thermal store came by...

Anyhow, my point -

Youre not dealing with portable water (at least not directly) so, i figured, mild would be fine. All the radiators in your house are mild, but in a semi-sealed (ie vented, but no turnover) system, there is only so much disolved oxygen to make iron oxide, and then, when its gone, providing youre not adding more all the time, thats pretty much it.

As another example, oil boilers (i only know about oil, not gas) only have stainless parts in the condensing sections, and thats for corrosion from the outside, not protection from the inside (ie acidic distillate in the secondary exchanger). Or the back boiler in my rayburn - all just mild steel.


Id been collecting coils from scrap hot water cylinders for ages, and had also got a few of the finned copper coils that are used in thermal stores for the mains pressure DHW coils, but a much simpler way to do it (all be it relying on electricity) would just be to cycle the water from the store through a flat plate heat exchanger, to heat up the portable water on the other side of the exchanger. But, i did not want to be reliant on electricity for hot water, give that we live in rural Devon. (a small computer UPS would last for quite a while though)


If you keep an eye out on ebay for long enough, you can get indirect hot water tanks for anything up to £30. I think my cheapest was probably a tenner, and anything more than £30, and there's a danger it will cost you money, as, up to that sort of point, you can safely take out the coil, weigh in the rest, and probably still get yourself some drinks money.

Enough non-finned coils, in series, would do for DHW, but you would need more than one. I did the calculations once, but the numbers in my head would need checking - what i remember is that each coil is between 0.5 and 0.7 m2 in area, and you need about 3.5 m2 (obviously dependant on other factors, but thats what the commercial stores use) for the DHW coil.

I was collecting them all up, and then got hold of a large finned coil anyhow. If youre not in a rush, you can get it all (basically) for free off ebay...

 

[Farside]

Exactly so: I have about 3 gash copper cylinders, 2 of which are scrap singles with good coils and one is a double coil ( I thought I'd lucked-in with that one, but the bottom was like a sieve). So, that gives me 3 coils and one good tank for a spare for the main one, iyswim, it's too good to scrap. Of course, if the main tank pops its clogs, that releases another coil...
So, with 3 (or four) coils, I might be good to start off with it.
I nearly lost the lot a couple of years ago when a pair of robbin' barstewards tried to break in - I'd made the mistake of leaving them in the middle of the floor and they could be seen through a crack in the door.

Good point about the static nature of the thermal store water - I can dose it with inhibitor, as a one-off cost, which shouldn't break the bank.

 

[Kent]

Can I make a few points about Heat sores
1. Its best thought of as a massive battery you change up from the mains power, then you run all your electrics off it. OK its just that heated water replaces electric.
2. if too small a battery is used it wont hold all that change as hot water expands it becomes a bomb! Vented to an expansion tank makes it a std big hot water cylinder not a store! This is an out dated system effective but not very efficient
3. if the battery (store) is too big it will never reach a high enough charge from the supply. Effectively you have warm not hot water and just like you can have part charge in a battery when it will no longer give you a bright enough light in your torch correctly the bulb might glow. Luke warm water in a radiator is of course near useless

4. for reasons 2+3 you cant really make your own and sizing needs to be spot on, even those who know involve the manufacturer to do the final calcs (its good to have someone else to blame and sue / pass the liability to)

5. most heat generators work most efficiently when they are in mid run (like an old car the choke needs to be off and the engine warm to get the best power and economy with THE LEAST STAIN on the engine). Constant on off cycling of a boiler shortens its life, drops economy and leads to more servicing, this is why you shouldn't fit too big a KW rating for your home. With heat source and store in perfect balance cycling is cut to bear minimum

6, additional source of heat can be added but these must be under control and should be appropriate (not much good putting say solar PV to an immersion inside the store as its better into the home electric / grid) There are lots of ways of controlling multi sources from a simple manual shut off valve to clever and complex units.

7. safety blow off valves are included in stores but are never intended to be used (like an ejector seat in a jet fighter you want it to work but hope it never gets used). Its still a pressure system and needs a person qualified to work on it

 

[octo0072000]

so this one must be a self contained heat store then