Fitting a Willis Immersion heater

tnegun · 21-05-2022 09:18PM

I think it's about 150l there are no markings on it to say for sure.

The Continental Op · 21-05-2022 09:23PM

Height and rough diameter and I'm sure we can work it out.

John.G · 21-05-2022 09:33PM

So ~ 9.2 kwh to heat a full tank, so the diverted 5kwh (assuming little or no HW draw off) should have been fairly close to rising the middle tank temp steeply, be interesting to see when it does.

tnegun · 21-05-2022 10:15PM

https://www.boards.ie/discussion/comment/119095187#Comment_119095187

Its approx 900mm x 500mm.

https://www.boards.ie/discussion/comment/119095226#Comment_119095226

That's not far off what I was hoping to get into it with the willis. I did some calculations before I bought it and thought if I got 8kwh into it from cold I'd be doing well! I submitted the gas reading this evening too I've only used 1 unit in a month vs 29 last May. I did the electric too 250kwh this May vs 611kwh last year.

The Continental Op · 21-05-2022 10:38PM

Its approx 900mm x 500mm.

900mm x 400mm is approx 95l

900mm x 450mm is approx 120l

Assuming an inch of foam on the outside of the tank I'd guess its approx 450 mm actual diameter of the tank so probably around 120 liters.

John.G · 21-05-2022 11:17PM

22/28X.45^2X0.9X1000 is 143 litres.

John.G · 22-05-2022 01:52PM

Just looking at pressure drops in orifices, a few interesting numbers showed up. The circulating head at a flowrate of 0.72LPM with flow/return temps of 15C/75C is 0.00758M and if one considers the ball valve bore as a orifice then it would have to be greater than 8MM. The circulating head at a flowrate of 1.23LPM with flow/return temps of 15C/50C is 0.003M the ball valve bore would (theoretically anyway) have to be 12MM or greater.

You have a spare, can you measure the bore with drill or a calipers?.

tnegun · 22-05-2022 06:14PM

Great idea! A 7mm drill bit fits through with a little to spare the 8mm won't. The CT readings are all over the place and not accurate I guess due to the diverter varying voltage, current etc as it adapts to the available excess but it's done a great job today soaking up excess with 3 showers taken and another 6kwh gone into the tank which is about half full now going by the temp readings.

John.G · 22-05-2022 07:10PM

It can't do any harm to change that sometime to a full bore ball or a simple gate valve, every bit helps.

tnegun · 22-05-2022 09:02PM

I'm going to go for it. Up to 2kw diverted there is enough flow through the willis not to open the stat any higher and it opens after 1-2 mins then closes again after about the same not bad but it would be nice to have it run at the full 3kw.

tnegun · 24-05-2022 02:57PM

Picked up the replacement valve today so will try fit it when I've time in the mean time the impact of the willis can be seen here nicely. The tank immersion has priority so it heats first with the red(willis) temp tracking the yellow tank temp. The tank stat opens and the diverter detects this so starts sending surplus to the willis and the red line now climbs. The stat is opening and closing along with less available excess giving the peaks and troughs but the tank temp continues to climb and most importantly the bottom of the tank - blue line is climbing too so the whole tank is heating.

John.G · 24-05-2022 04:38PM

The blue (bottom) will never rise with the tank immersion/stat as the immersion is above that (and the reason for installing the willis) so what is the yellow trend? if its the middle of the tank temp then that should rise uniformly on the tank immersion assuming long (bath) element in service and no HW draw off?, and is the tank immersion stat set to 60C?. or is it only set to 38C?.

tnegun · 24-05-2022 07:08PM

Yeah I understand that I was highlighting that the willis was having an effect as warmer water was making its way down the tank as hot water entered from the top. The yellow trend is the middle of the tank and the blue line the bottom. The in-tank immersion is set to 65c on bath but could never heat more than the top 1/3 of the tank sufficiently the yellow line would rise to about 40c and that was it which is what sent me down this rabbit hole 😀 . The tank was almost completely cold this morning and took 7kwh of excess today before the showers started.

John.G · 24-05-2022 07:56PM

The dual immersion brings its own challenges/problems, If the cylinder is stone cold then it should all (IMO) heat up uniformly with a fully heated volume from the bottom of the bath element to the cylinder top, if hot water is then drawn off (after tank stat switches off) until the layer of hot water just extends to the bottom of the sink element then the stat will not switch back in because the stat rod only extends the same length as the sink element and will only switch back in when nearly all the HW is drawn off?. Now, the willis was tracking the cylinder top temperature presumably and it coincided with the middle temperature because the whole cylinder was cold as you say and at ~ 25/26C when the cylinder immersion came on so if my theory of the whole tank heating uniformly under these conditions then why did the cylinder stat cut out at only ~ 38C??, if it was reaching 65C or so then shouldn't the willis reflect this or close to it with reverse circulation until the diverter cuts in. It maybe begs the question, perhaps the sink element is being utilized and not the bath element?, or maybe faulty stat, if no faults then it should heat ~ 70% of the cylinder say 100L from 25C to 65C, 4.9kwh, if its only heating 100L to 38C then only 1.5kwh, if heating 1/3rd or 50L from 25C to 65C then 2.3kwh. Your ESB meter should tell a lot re immersion consumption if monitored.

tnegun · 24-05-2022 08:22PM

Its a top entry 24" Shel immersion with an 11" thermostat if it makes any difference. I tested the resistance and confirmed that the bath element is the one wired. The top of the willis is positioned about the same height as the middle of the tank so it tracking the temp sensor at the middle of the tank makes sense to me as with the reverse circulation the temp at the same height in a closed system should be similar. The 38c at the middle is well below the bottom of the tank thermostat and the top of the tank is almost too hot to touch when the tank stat cuts out. The cylinder immersion takes 3-4kwh from cold before cutting out I can see this from the diverter.

John.G · 24-05-2022 11:08PM

I would have thought that the willis temp with reverse circulation would be nearer the cylinder top temp than the cylinder middle but you could well be correct.

The 24" (610mm) bath element should/might heat 68% of the cylinder vol to 65C, so 101L heated from 25C to 65C = 4.7kwh. Immersion was on for ~ 2.25 hrs & assuming a 2.5kw element = 5.6kwh but you will know its power by the measured resistance. The bath element bottom is ~ 160mm below the cyl middle (temp) so would have expected the middle to be higher than 38C at the end of the heating immersion period. You say it takes 3 to 4 kwh before immersion cut out so at say 3.5kwh = cyl temp of 55C but again assumes that the whole 101L is being heated to at least this temperature but we it isn't as the center temp is only 38C. The 11" stat definitely has a bearing but is a compromise length to satisfy different length elements, one 24" element only with a 24" stat might achieve a greater volume of hot water but debatable whether this is a advantage or not now that the willis is installed. All very interesting.

tnegun · 25-05-2022 11:23AM

I think its the water density settling rather than full reverse circulation, also adding the insulation to the feed off the top of the willis seems to have helped here too. I'll recheck the bath element of tank immersion is the one wired as I did have to rewire that to replace the stat and when adding the willis. A neighbour has the PV same system and diverter and has similar performance to mine in that he can only get 3.5kwh into the tank but doesn't have any temperature monitoring.

John.G · 25-05-2022 11:59AM

Something not adding up there in both you and neighbors, if the cylinder immersion is only inputting 3.5kwh into a cold tank (say 25C) then that will only heat 75L to 65C. and a 24" immersion should heat close to 68% of a 900mm tall cylinder which means the cylinder capacity is only 110L, highly unlikely. Also, it would only leave 35L to be heated by the willis which would only require another 1.62kwh, no way obviously. If a 150L cylinder then 3.5kwh will heat 101L to 55C which looks more likely but then (assuming no HW draw off) would only need a additional ~ 3.45kwh to get the whole cyl to 65C. (101L from 55C to 65C & 49L from 25C to 65C).

What bath element resistance did you get and can you also measure the resistance across the live & neutral upstream of the immersion, where they are connected to the diverter or whatever.

The Continental Op · 25-05-2022 01:34PM

If you can measure around the cylinder and give an indication of the depth of the foam we can be a bit more certain of the size of the tank.

John.G · 25-05-2022 04:05PM

Standard insulation is 35mm so vol could be ~ 130L and if 5% is allowed for the domed top, as low as ~ 120L??.

tnegun · 25-05-2022 04:49PM

I'll measure again when I get home from a pic on my phone the tank immersion is a Shel C24DT2 the first google result returns a 24" Shel immersion but the part no isn't displayed so maybe mine isn't that long?

The Continental Op · 25-05-2022 05:11PM

Its always possible the the plumber tried a longer immersion and found it fouled on the coil so put in a shorter one?

John.G · 25-05-2022 05:36PM

For the moment, if a cylinder size of 120L is assumed then a 24" 2.5 kw (21 ohm) element that heats 68% or 82L from 25C to 65C will consume 3.8kwh, not too far away from the stated 3.5kwh.

tnegun · 25-05-2022 07:55PM

I cleared enough junk from the hot press to measure the circumference and its 1600mm with 30mm of insulation also the resistance for the immersion is about 20 ohms it wanders about a bit.

John.G · 25-05-2022 08:50PM

The above measurements give a cylinder capacity of 135L allowing 5L for the cylinder dished top, the bath element is 2.65kw based on a 20 ohm resistance, so you have a 24 ins, 2.65kw element heating (135*0.68), 92L of water. It requires 4.28kwh to heat 92L from 25C to 65C and will take 1.62 hrs ( 1hr & 37min) to achieve this. OR, if the measured energy consumption is/was 3.5kwh then the final temperature of the HW will be 58C.

The Continental Op · 25-05-2022 09:37PM

I'd go far more for it being a fairly standard 900mm x 450mm which is often sold as being 120 liters but is normally a tad shy of that at 117 liters.

Look up that size online and you'll find it fairly universally stated to be 117 liters.

As an aside - My guess is that like a surprising amount of manufactured items the machines its made on were set up for imperial measurements making that a 36 inch by 18 inch cylinder.

John.G · 25-05-2022 10:18PM

Yes, definitely looks like a 900X450MM, 117L does seem to be the universal capacity but even allowing for a wall thickness of 2MM it still comes to ~ 141L NOT allowing for dished top so the manufacturers seem to calculate a reduction of 24L for this which seems a bit high but they would scarcely undersize their cylinders. SO to edit post # 144........

For the moment, if a cylinder size of 117L is assumed then a 24" 2.65 kw (20 ohm) element that heats 68% or 80L from 25C to 65C will consume 3.72kwh, not too far away from the stated 3.5kwh.

HOWEVER, if we subtract the 24L for the dished end then the volume heated is only ~ 56L and should only require ~ 2.6kwh???, time for the bed I think.

The Continental Op · 25-05-2022 10:24PM

Would the reverse dome for strength in the base account for the extra in the dome at the top?

John.G · 25-05-2022 10:49PM

The dome in the top reduces the capacity IMO, but I would have thought by only say5 to 10L, , there's definitely a calculation for it, the dome in the bottom further reduces the vol but I suppose the sum of the two dome losses come to that 24L so maybe allow 12L for top dome capacity loss which means the heated vol by the bath immersion is, 80-12, 68L, requiring 3.2kwh, that definitely sounds the more likely case?.

Milsey · 31-05-2022 04:15AM

A lads I've read all 5 pages and no conclusion on the willis install. I have to commend your tenacity to make it work optimally. I can't wait to hear more.

I've never seen such a productive string. Fair play all that have contributed.

Fitting a Willis Immersion heater

Comments