Willis Heating System

[JohnW2]

The stratification, in a Willis arrangement, is simply much more effective than in the usual, near vertical element arrangement.

I could agree with that, BUT, as I just wrote my cylinder's 'bottom element' is horizontal, and the top one (which one would use for Willis-like functionality) is very short.

With the usual element, you get heated water rising, then falling again as it cools, around the outer edge of the cylinder. Hence, you get much more mixing of hot and cooler water. Compare that, with the Willis system, where the heated water, simply goes in the top of the cylinder, with no convection currents from an element to mix it.

True - but, again, not in the case of my short 'top element', which I still believe provides much the same functionality as does the Willis system.

Mind you, my top element does not appreciably heat the water below it - which is why I'm still pondering the matter of the lower parts of the cylinder with a Willis heater. With mine, the thermostat turns off the top element as soon as the top bit of water (down to bottom of element) reaches the target temp - which I'm tempted to think may be an advantage in comparison with Willis?

 

[ericmark]

Sorry, I didn't see your submission, before typing my own - but at least we agree on the control methods

Your English is better than mine, you put it spot on.

I don't understand. Warmer water will rise 'to the top', regardless of how (or where) it got warm.

But it will mix, so hot water at the bottom of the tank will warm water on its way up, so the whole tank warms together, but putting separate it is not mixing on the way up.

The method involving least kw energy is instant, local heat, which is electric. Better, would be an instant gas water heater, at every hot tap, but simply no practical.

I remember the gas geezers on the wall, they did work, but today we want the combustion products to be piped outside to reduce humidity, there is a trade-off, seven hot taps, in five locations, two kitchens, two bathrooms, and a shower room, so only really practical to fit in one location, the front kitchen, but with a dishwasher, we really do not use enough hot water to be worth the effort.

I could agree with that, BUT, as I just wrote my cylinder's 'bottom element' is horizontal, and the top one (which one would use for Willis-like functionality) is very short.

I only have one boss on the cylinder, so only one immersion heater, I did change this from 11 inch to 27 inch, I had not expected to use much more energy, OK, to heat it up to start with, but once hot thought the difference would be small, I got it wrong again dad, using over double the energy (kWh) to the short one. This may have been due to running out of DHW with the short one, or the short one having the temperature set lower.

If the Willis heats from the top down, even with just 200 watt, it would be far better, as soon as my battery is charged, it will start heating DHW, but if the lower wattage results in the water not being as warm, then it would be a waste of money, today 5.1 kWh exported so it would have done well, but as seen, 6 poor days, summer is better, but still can't be sure no more than two poor days in a run, so I use off-peak, and heat the whole tank, so in real terms my iboost+ is a bit useless, and if one has to guess how much one needs each night, then no real point using the Willis.

 

[Harry Bloomfield]

which I'm tempted to think may be an advantage in comparison with Willis?

Not really - the Willis stat, or stats, can be placed at any water level you desire.

 

[JohnW2]

But it will mix, so hot water at the bottom of the tank will warm water on its way up, so the whole tank warms together, but putting separate it is not mixing on the way up.

It would, but neither the Willis system nor my installation (when in 'Willis mode') should have any hot water at the bottom - since the only heating, or introduction of heated water, is at the top.

 

[JohnW2]

Not really - the Willis stat, or stats, can be placed at any water level you desire.

If you say so. Since the stats seem fairly crucial to the Willis system, I asked if anyone knew how/where they were sited, but have so far not had any answers.

Are you saying that the Willis system has a 'moveable' thermostat?

 

[SUNRAY]

Let's not forget that a heat source in a tank of water can and will heat the entire tank eventually but unless there is a reasonable circulation taking place within the tank the top will be hotter than the bottom. This is far more prevalent in a tall thin tank where circulation is more restricted. This is why destratification pumps are installed which basically pull hot water from the top of the tank and put it back into the bottom (some arguement exist on whether top to bottom or bottom to top is best - I've encountered both and both seem to work so sit on the fence) or mechanical moving paddles installed in the tank. Many heating coils are fitted with vanes arranged to start the heated water moving in a circular motion to form a helical/rotating current.

The Willis system introduces this circulating current by default.

 

[Harry Bloomfield]

Are you saying that the Willis system has a 'moveable' thermostat?

Not at all - what I said was the stat(s) could be located where ever desired, to suit the demand for hot water. A series of switches, could be used to select which stat, the element worked on. Alternatively, a timer could be used, but the advantage is - that the Willis system provides a quantity of water, at temperature, very quickly.

 

[robinbanks]

Let’s try something else.

Imagine you have a tank full of water, and the whole lot is at 50 degrees.

A pipe exits the top of the tank and goes to the shower head

A water inlet connected to the mains pressure is at the bottom of the tank.

Suppose the manufacturer had a choice of two different nozzles that could be fitted to the inlet.


One was a 90-degree, single small hole that, at mains pressure, would spray a jet of water at high speed towards the top of the tank.

The other is a large baffle with many parallel water paths that, at mains pressure, achieves a very gentle dribbling of water over its entire surface.


Which of these nozzles would you prefer fitted to your tank that supplies your shower?

I would like the baffle, because it means that the cold water from the mains comes into the tank very slowly and gently and stays at the bottom, like a bartender pouring cream onto a cocktail. All the 50-degree water would come out and I’d have a nice hot shower for, say, 30 minutes (300-litre tank, 10 litres a minute).

The hardcore guys might choose the other nozzle that instantly and powerfully starts agitating the tank, stirring the whole thing when the shower is turned on. Very rapidly, my shower wouldn’t be a toasty 50 degrees, as all the hot and cold water is blended by the fast stirring.

So, this idea of moving water slowly so very hot floats on top of very cold is a good thing. It’s like having a panel in the tank that can slide and stops cold and hot mixing. There isn’t a panel, because there doesn’t need to be if you introduce hot at the top and cold at the bottom slowly enough not to stir the tank.


Putting an immersion at the bottom doesn’t do this. The water next to the heater might be at 80 degrees when it’s heating, but it doesn’t float up to the top of the tank and stay there at 80. When you turn your tank immersion on, it stirs the tank quite a bit as it heats.

A Willis doesn’t do this.


If you want 20 litres of hot water at the top of the tank, turn it on and wait X minutes. If you want 40 litres, turn it on and wait Y minutes.


If you have an immersion 3/4 of the way up a 300-litre tank, it heats 75 litres of water. If it’s the same power as a Willis and runs for the same time as a Willis, then both it and the Willis have made 75 litres of hot water. But you can’t choose to have 40 litres of 80-degree water with your fixed-position immersion that is in-tank, because of the stirring. You have 75 litres at 30 degrees, and some time later you have 75 litres at 40 degrees, and then 50…

The Willis keeps the stratification well, so you can choose to have anywhere between 0 and 300 litres of hot water in your tank. You don’t get that level of flexibility with an in-tank immersion.

Ultimately, there are two factors here: volume and temperature. A Willis fixes the temperature and lets you vary the volume by run time. An in-tank immersion fixes the volume and lets you vary the temperature by run time.

Which is more useful to you?

 

[SUNRAY]

If you say so. Since the stats seem fairly crucial to the Willis system, I asked if anyone knew how/where they were sited, but have so far not had any answers.

Are you saying that the Willis system has a 'moveable' thermostat?

Again commercial systems are different to domestic with multiple sensors but basically the 'main' temperature of interest will be at the bottom of the heated water. Additional sensors will be on the HW draw off point and in the case of Willis style systems the cold draw point and hot return (the in and out 'pipes' of the heaters). There may very well be multiple sensors of each , such as for each heater (to monitor which are working etc), multiple heights of the main vessel may also be used to cater for shift patterns etc

 

[SUNRAY]

...Putting an immersion at the bottom doesn’t do this. The water next to the heater might be at 80 degrees when it’s heating, but it doesn’t float up to the top of the tank and stay there at 80. When you turn your tank immersion on, it stirs the tank quite a bit as it heats.

A Willis doesn’t do this.

It doesn't agitate the water but it very much creates a circulation of water, that is the whole point of the system

If you want 20 litres of hot water at the top of the tank, turn it on and wait X minutes. If you want 40 litres, turn it on and wait Y minutes.


If you have an immersion 3/4 of the way up a 300-litre tank, it heats 75 litres of water. If it’s the same power as a Willis and runs for the same time as a Willis, then both it and the Willis have made 75 litres of hot water. But you can’t choose to have 40 litres of 80-degree water with your fixed-position immersion that is in-tank, because of the stirring. You have 75 litres at 30 degrees, and some time later you have 75 litres at 40 degrees, and then 50…

I wouldn't have thought it's as clear cut as that

My gut feeling is it is going to be something more along this line where the vast majority of the convection will be limited to the section near and above the heating element. I've added some figures to give a bit of an idea rather than being definitive. There is also a limited amount of conduction downward

As the temperature rises the hot area increases in height and therefore lower position, eventually the top will stop heating further and convection will be occurring in lower and lower. Eventually convection at the heater will be surpassed by conduction and possibly most? downwards
conduction heating the water below takes over from convection.

All of this can only happen if the heater is running continuously and not tripped by a water temp stat. As I wrote before the bottom cannot match the top temperature unless there is some circulation provided and possibly no thermal losses.

This is only my thoughts and I may be p***ing totally down the wrong tube

The Willis keeps the stratification well, so you can choose to have anywhere between 0 and 300 litres of hot water in your tank. You don’t get that level of flexibility with an in-tank immersion.

That's a very broad statement, with the heat source at the bottom one can chose where to site the stat and therefore how much water to heat

Ultimately, there are two factors here: volume and temperature. A Willis fixes the temperature and lets you vary the volume by run time. An in-tank immersion fixes the volume and lets you vary the temperature by run time.

Again I'm not so sure it's as clear cut as that

Which is more useful to you?