Any experience of Willis Economiser Immersion Heater?

[RichardG]

I have spare power output from a PV solar array which is diverted to heat the HW via a standard top inserted 27 inch immersion heater. The problem is that this only heats the top 1/3 to 1/2 of the HW cylinder before the thermostat cuts out and I would like to heat the full cylinder when power is available.

The solution I'm familiar with is a HW cylinder with 2 immersion elements, one at the top and one at the bottom. If you want to heat the whole cylinder you use the bottom element otherwise the top one. Trouble is this requires a new HW cylinder.

My research has thrown up something called a Willis Economiser. This is an external heating element meaning the longer it is switched on the more of the HW cylinder is heated by circulation. This seems the perfect solution but wanted to get some first hand feedback on how well they really work? Can anyone sing their praises?

 

[Bod]

I cannot see that working for you, plumbed as shown.
Can you fit a longer immersion heater?
Does the HW tank have a spare internal heating coil?
It is possible to fit a lower heater in a standard HW tank, using an "Essex flange"(think thats the right name, but does exist.)

Bod

 

[RichardG]

I cannot see that working for you, plumbed as shown.
Can you fit a longer immersion heater?
Does the HW tank have a spare internal heating coil?
It is possible to fit a lower heater in a standard HW tank, using an "Essex flange"(think thats the right name, but does exist.)

Bod

The plumbing shown is from the manufacturer so I hope it's correct.....I think the way it suppose to work is that the longer the heater is powered the more of the HW is heated as a circulation is created by taking CW out the bottom of the tank??

I can't remember the exact length of the immersion when I fitted it 10 years ago but the thermostat is 18 inches long which from my experience means it's a 27 inch element. I may be able to fit a longer one but it may well foul on the indirect heating coil which also stops an additional side element being added.

I know the real answer is to fit a new cylinder but I'm trying to avoid doing that.

 

[Bod]

The Willis heater is designed to provide a small amount of hot water, for showers, hand washing etc.
To heat the full tank a pump would be needed.
Check the lenght of the thermostat fitted, it may be a short one, in a long tube, is there a "Bath/Basin" switch fitted?
If you have enough "spare" PV, then fitting a circulation pump, triggered by the thermostat could move hot water from the top of the tank, to the bottom, triggering the immersion heater to come on.
Fit the pump, in place of the Willis in the above diagram, to pull hot water from the top, pushing it into the bottom, this will need No return valves fitting to prevent heated water entering the cold water system.

Bod

 

[user 35906]

I have been in heating all my working life and have never heard of them but I think it's a brilliant idea, if you have the space. It would work as when the water in the economiser heated it would rise being replaced with cool/cold water from the bottom of the cylinder. As the warm/hot water rises it will circulate down into the cylinder (because the expansion pipe is just looped over the cold water storage tank there can be no circulation on that circuit) and it would heat the whole cylinder. It's not much different than the old gas circulators that we used to fit in the seventies, on the side of existing hot water cylinders

 

[Noel]

We’ve used Willis heaters for years.
On for 4/5 minutes and enough hot water for a sink full of dishes.
On for an hour, enough for a full hot bath.
Get one, well worth it, especially if you have spare electricity lying around.
Works much as Hunter27 describes and works well.
PS- never known anyone needing a pump of any kind.

 

[gmgmgm]

Never heard of Willis heaters, very simple, very clever.

RichardG, are you moderating the power of the immersion heater to use the available PV, or is it just a dumb 3kw load? (Immersun etc. moderate the power so it's more efficient).

If it's just a dumb load, then could you replace the immersion by a longer & lower power immersion, to heat more of the tank, and to heat it more slowly? I believe you can also get double-immersion heaters (two circuits on one mount) so one could be 1kw PV and one for high-power override.

 

[RichardG]

RichardG, are you moderating the power of the immersion heater to use the available PV, or is it just a dumb 3kw load? (Immersun etc. moderate the power so it's more efficient).

If it's just a dumb load, then could you replace the immersion by a longer & lower power immersion, to heat more of the tank, and to heat it more slowly? I believe you can also get double-immersion heaters (two circuits on one mount) so one could be 1kw PV and one for high-power override.

I'm in the process of building my own modulating controller and this is part of the design process to decide what features I need. None of the commercial products have the flexibility or control I want, mainly because I also have a battery installed so I have a lot of flexibility.

For instance, if the next day looks promising for solar gen then I can heat the water at the full 3kW using energy stored in the battery before showers in the morning if there's not enough HW left in the tank. The battery can be recharged later in the day, just need to leave enough to run the house until the PV kicks in.

So my controller will be able to modulate the output power but also be able to read the temperature of the tank by having several sensors down the side of the tank. It may also need to control a stratification pump depending on the solution I go for to heat the whole tank. By having multiple sensors I can calculate how much energy is needed to raise the tank temperature by 'n' degrees. The controller also needs to be able to take input from my HEMS.....

 

[RichardG]

So after more reading and the views here, thanks everyone, my first job is to remove the current immersion element and see exactly what I have, I assume it's a 27inch but I'm basing a lot on an assumption, I need to be sure but need to get a new gasket first...

I contacted the tank manufacturer and they said I could fit a 900mm element, however, I later find a rating label on the back saying the maximum is 850mm. I'm waiting for clarification on exactly what it is as it's the difference between a 30 inch common, 33 inch, not common or a 36 inch length element which could make a big difference. I'll try and confirm when I remove the element.

I've also seen that the Willis style heater is not a good match as I'm in a very hard water area and they get clogged with limescale very quickly which is a shame as it looked like a good option.

So I think my options are either a replacement tank, a de-stratification pump to stir the existing tank (Apollo 13 comes to mind here!), a longer element, or possibly a second economy 7 tank for summer use....

 

[Pedronicus]

The plumbing shown is from the manufacturer so I hope it's correct.....I think the way it suppose to work is that the longer the heater is powered the more of the HW is heated as a circulation is created by taking CW out the bottom of the tank??

All well and good until someone wants to draw off hot water. There will be a pretty good chance that the cold water from the feed tank will bypass the cylinder and flow up through the unit to the hot tap (but will deliver cold water therefrom). Remember water likes to follow the line of least resistance.

I know the real answer is to fit a new cylinder but I'm trying to avoid doing that.

Fitting a new cylinder would be a better option and one can specify exactly what tappings are requires for additional immersion heaters. Also the latest cylinders have better insulation that ones that may be elderly.

Pete (plumber & htg bod for 50+ years!)

 

[user 35906]

All well and good until someone wants to draw off hot water. There will be a pretty good chance that the cold water from the feed tank will bypass the cylinder and flow up through the unit to the hot tap (but will deliver cold water therefrom). Remember water likes to follow the line of least resistance.



Fitting a new cylinder would be a better option and one can specify exactly what tappings are requires for additional immersion heaters. Also the latest cylinders have better insulation that ones that may be elderly.

Pete (plumber & htg bod for 50+ years!)

We never had that problem but it was thought you could maybe get an initial surge of hotter water from the heater to the taps to start with but I never saw that happen. Most gas circulators were connected via flanges / bosses we fitted in the side of the cylinder one around 4" up and one a couple of inches down from the top seam but if that was not possible they were connected as per diagram above.