Tank thermostat length and how it works?

[ericmark]

The tank thermostat would trip the over temperature trip if set higher, so have renewed it, there was what looked like sealing compound on the new one fixing setting to around 60ºC it only goes to 70ºC, but I left it as it came. Water now only just warm enough, and clearly not enough to fill a bath, but don't really need that much hot water, but seems no where near 60ºC at the taps, so am I doing something wrong?

It has not been in a week yet, so the report as to how much energy used, is not that accurate, but it shows 4.38 kWh used over last week, and before the change was showing around 2.50 kWh per week, so it does seem it is using more, and it said "Tank Full" so the thermostat has clearly turned off. But when switched on, it tends to used a lot less than the 3 kW the element could used, typical around 350 watt, as by time the solar can give 3 kW the tank is already hot.

Winter the DHW is stinking hot, when the oil heater is running, in fact too hot, but there is no valve to turn it down or off. Running C Plan, but I read about stratified water in the tank, and wonder if the low watts is causing less water movement, so is it the iboost+ which is causing a problem? Maybe some one can explain what is going on, and why DHW so cool?

May seem strange, but the DHW has got warmer since writing this. Clearly some thing to do with the thermals inside the tank, but would love to know more about what is going on.

Now shows 4.48 kWh for last 7 days, so it seems power use is going up slightly.

 

[CountryFan]

That’s looks like it’s set to about 45 degrees, need it hotter? Turn it up for legionella purposes too.

 

[Harry Bloomfield]

The tank thermostat would trip the over temperature trip if set higher, so have renewed it, there was what looked like sealing compound on the new one fixing setting to around 60ºC it only goes to 70ºC, but I left it as it came. Water now only just warm enough, and clearly not enough to fill a bath, but don't really need that much hot water, but seems no where near 60ºC at the taps, so am I doing something wrong?

As above, it looks like maybe set at 45C, if that is the new one. Is it the correct length?

The sealed part will be the trip setting calibration. At the tap temperature will be lower, due to heat loss, along the pipe run.

 

[ericmark]

Sorry that is the old one, new one was at 60ºC and there was a little sealant on it, so left as set. From the weeks read out, clearly using more energy, but both same length, which is rather short, and the power used is a lot lower than the immersion is designed to use, so does the low power cause the water to circulate less?

Moving from 45ºC to 60ºC I expected a massive change, but this has not been the case, hence the question.

 

[Harry Bloomfield]

From the weeks read out, clearly using more energy, but both same length, which is rather short, and the power used is a lot lower than the immersion is designed to use, so does the low power cause the water to circulate less?

Poke a bit of wire down the thermostat pocket, to measure how deep it is - it wouldn't be the first time, a too short 'stat had been fitted.

 

[jj4091]

Poke a bit of wire down the thermostat pocket, to measure how deep it is - it wouldn't be the first time, a too short 'stat had been fitted.

Wouldn't that have the oposite effect?

 

[Harry Bloomfield]

Wouldn't that have the oposite effect?

Eh?

The OP seems unable to get a cylinder full of hot water, despite fitting a new thermostat, and setting it to 60C. That suggests to me, that his element is only heating part of the cylinder, that also seems to be confirmed, by the much less than I would expect, kWh consumed in heating the cylinder of water.

 

[jj4091]

Yes I understood that but I thought the further it was from the heat source then the more it would stuggle to conrtrol the temp. and then the water would get hotter not colder because the stat would be closed longer.

 

[ericmark]

I will monitor power used, it seems I am getting hotter water, maybe just have not run the taps for long enough?

For hand washing don't really need a full tank of hot water, but the whole system seems a bit hit and miss. With no circulation the top will get hotter than the bottom of the tank, but it is top 4" compared with top 8" etc, which seems more of a problem?

If the immersion heats the water 12" below the top, then to monitor anywhere between 12" down and top will show the same. But if it heats water 1" to 12" then there will be a gradient of temperature, and since water drawn from the top, then it would be the top which is important, not 7" down.

I can find explanations of how the Willis exterior immersion heater works, and I note the shape of internal types,

seems to put more heat towards the bottom, there are diagrams for combined heating and I remember the school experiment of rapping a block of ice in solder to keep it at bottom of test tube, and boiling the water at the top, theory is water at bottom will be 4°C as that is when it is most dense.

However the heating of the water must produce some thermals, so although the bottom of tank is colder than top of tank, there must be a mixture layer where water is part heated due to thermals moving the water around.

I would assume if using 3000 watts, the movement will be more violent than if using 300 watts? Most the sites show but that does not really show what happens when so little power is used, hind sight the iboost+ was not a good idea, a simple time switch would have been better, as paid 15p/kWh for export, and charged 8.5p/kWh for import, so would save money using a time switch instead of the iboost+ but now fitted, is it really worth changing?

So at 5 kWh per week, around 25 weeks of the year, as winter DHW heated with oil, 15p - 8.5p = 6.5p/kWh = £8.12 per year if the tariffs stay as they are, forget that, just found I can set it to work as timer, so set to heat in off-peak times. Although that does not seem to stop it heating when there is solar. But it means that £8.12 is reduced, so just not worth messing with.

 

[Harry Bloomfield]

Yes I understood that but I thought the further it was from the heat source then the more it would stuggle to conrtrol the temp. and then the water would get hotter not colder because the stat would be closed longer.

Heated water rises, and will collect at the top of the cylinder - if the stat is short, then it will only operate based upon that upper level of water. Water lower down, can remain unheated.

 

[foxhole]

Do you have a single element or twin element unit? Was the replacement stat the same length as previous? The stat must match the length of the element.

 

[ericmark]

Single element, and replaced thermostat like for like. Not removed element so don't know what shape. Now I realise it can be powered during off-peak I will wait and see what if any difference it makes. More time when it can run may change things, so will need to wait and see.

I have come to the conclusion some thing odd going on. After setting the iboost+ to heat over night, water barely warm this morning, so some thing wrong. Maybe I have switched it off? It is a get the instructions out again job.

Walked past the airing cupboard and can hear the iboost+ singing so it is working, so what went wrong?

 

[Johntheo5]

Single element, and replaced thermostat like for like. Not removed element so don't know what shape. Now I realise it can be powered during off-peak I will wait and see what if any difference it makes. More time when it can run may change things, so will need to wait and see.

I have come to the conclusion some thing odd going on. After setting the iboost+ to heat over night, water barely warm this morning, so some thing wrong. Maybe I have switched it off? It is a get the instructions out again job.

Walked past the airing cupboard and can hear the iboost+ singing so it is working, so what went wrong?

Is the heating element mounted vertically in the cylinder top or mounted horizontally maybe say 1/2 way in the cylinder, maybe just above the heating coil??.
A single element top mounted immersion will generally have the same length stat as the element length, dual element top mounted immersions have a single stat of the same length as shorter element and switches the common neutral.

 

[ericmark]

Mounted from the top, well not changing the immersion until it fails, so it will likely stay as it is.

 

[Johntheo5]

Its easy to just pull the stat out and measure its length and as suggested, stick a bit of wire into the pocket until it bottoms out and also measure this.