Willis Heating System

[JohnW2]

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

I agree with you totally, but don't think it has much to do with what we're discussing - you are simply illustrating the desirability of avoiding an arrangement which encourages mixing of the entire contents of the cylinder. I would say the same if you were talking about installing a 'stirrer' in the cylinder, regardless of the method of water heating!

I agree that the Willis system certainly doesn't encourage such mixing but, as I see it, and given that real-world bottom inlets obviously don't use one of your hypothetical "90-degree, single small hole that, at mains pressure, would spray a jet of water at high speed towards the top of the tank", I don't think that my cylinder with a high-up immersion does, either [it's a vented system, so the cold water enters the bottom of the tank pretty 'gently' from the header tank (not 'at mains pressure')] , does it?

So, this idea of moving water slowly so very hot floats on top of very cold is a good thing

Indeed.

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.[

Indeed - but, as above, that's more-or-less as true with a 'top immersion' as with an external Willis heater, isn't it?

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.

Agreed - but I haven't been talking about the situation when I'm using my 'bottom immersion'. I'm talking about the situation when I use the top one, and still don't really see why the Willis system offers much, if any, advantage over that.

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.[/S]

Yep, although I very rarely have the need, that's exactly what I do with my internal top immersion. Why do you think Willis would be better?

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.

As above, I don't believe that there is appreciable 'stirring' with the 'gentle inflow' into my cylinder, so I would expect most of the 75 litres of hot water to remain essentially undiluted by colder water, at the top - and hence for most of the 75 litres to be available at a reasonable temp to be used.

Again as above, I do that very rarely, but it seems to work. More to the point (and the reason why I very rarely need to use the top immersion), 20 hours or so after the last nocturnal heating (hence 'nearly time for the next top-up') my very-well-insulated (140 L) cylinder still has plenty of hot water (heated the previous night by the 'bottom immersion') still available for use, despite the fact that it must be sitting on top of a fair bit of cold water that has been 'gently drawn in' to the cylinder during the course of the day. I therefore can but presume that 'stirring' is pretty minimal in my installation.

In view of the above, you may understand why, unless/until you can convince me otherwise, I remain of the view that a Willis system would offer me little, if anything, over what I already have - and that that may be the reason why one rarely sees the Willis system being used outside of Ireland?

Kind Regards, John

 

[JohnW2]

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

Fair enough, but as you imply, and presumably accept, few, if any, domestic installations are going to be that complicated?

 

[JohnW2]

I think you you understand the situation as you ask the questions which I've already given answers to - now emboldened.

Fair enough, but you obviously wrote all that after I posted mine!

As I said, I suspect that conduction is very slow and feeble, so probably isn't much of a factor. However, perhaps the most crucial thing is that, as I have alluded to and as you have written:

All of this can only happen if the heater is running continuously and not tripped by a water temp stat.

There must be some thermostatic control and, as I've said, the position (and setting) of it seems crucial. In the case of a 'top (internal) immersion' (like mine, the stat is invariably integral with the element, and will therefore switch it off as soon as water at the level of the immersion is 'up to temp'. In the case of the Willis system, I don't know where the stat is, but presume that it is probably again integral with the element. If the Willis's stat turns it off as soon as the water within the Willis heater is 'up to temp', then I think we may need to re-visit some of the exchanges we've had about 'how Willis works'

... and, as I've just written, I have some 'proof of the pudding' - 20+ hours after it was last heated, the water at the top of my cylinder is still hot (not much less hot than it was 20 hours previously), despite the fact that it must be sitting on top of a large volume of cold water that has been drawn into the bottom of the cylinder during the day. There therefore presumably cannot be any appreciable 'mixing' going on, nor appreciable transfer of heat from top to bottom of the cylinder by conduction or any other process?

The Willis system will provide 3KW worth of instant hot water (providing I understand the Willis system), which is the same as those feeble oversink hand wash instant heaters, an immersion heater in a large vessel cannot do that

True but, as I've said, a heater with a small under-sink hot water storage, a traditional cylinder with a top immersion and a Will system can all supply a modest amount of immediately-available hot water.