Odd question perhaps - has anyone tried to improve how their immersion heater heats water?

[AliG]

As @Radianpointed out - A Megaflow cl210 has a 24.3kW heating coil connected to the boiler versus a 3kW electrical heating element.

 

A 3kW element would take 2 hours to heat just 100L of water from 8C to 60C.

 

So 1 hour on the boiler would put roughly the same amount of heat into it as 8 hours from the immersion.

 

On top of that the heating coil will be much larger than the heating element. Thus the heating element will tend to heat a more localised area within the tank before cutting off when thermostat is tripped.

 

I have a Eddi PV diverter and the same thing happens. It cuts off before the full tank is heated up as the water around the immersion has hit the thermostat temp.

 

One thing you could check is the immersion itself has its own thermostat. This is a little arrow on e Megaflow element. If this is not at the maximum level it will cut out early. The maximum it can be set to is 60C which may be another reason it cuts out earlier than you expect. I would set the main tank thermostat lower than this so that it tries to heat from the immersion first.

[MrTWales]

That box is about 40% up from the floor and has one screw holding it in place. I'm thinking of going for it and unscrewing to see what is behind it - I couldn't kill myself could I? Maybe I can change the temp!

[MrTWales]

2 minutes ago, AliG said:

As @Radianpointed out - A Megaflow cl210 has a 24.3kW heating coil connected to the boiler versus a 3kW electrical heating element.

 

A 3kW element would take 2 hours to heat just 100L of water from 8C to 60C.

 

So 1 hour on the boiler would put roughly the same amount of heat into it as 8 hours from the immersion.

 

On top of that the heating coil will be much larger than the heating element. Thus the heating element will tend to heat a more localised area within the tank before cutting off when thermostat is tripped.

 

I have a Eddi PV diverter and the same thing happens. It cuts off before the full tank is heated up as the water around the immersion has hit the thermostat temp.

 

One thing you could check is the immersion itself has its own thermostat. This is a little arrow on e Megaflow element. If this is not at the maximum level it will cut out early. The maximum it can be set to is 60C which may be another reason it cuts out earlier than you expect. I would set the main tank thermostat lower than this so that it tries to heat from the immersion first.

 

Thanks, this does make sense. What I stuggled with is why the immersion stops after about 90 mins max (so 4.5kwh maybe) - unless it just does that even if the temp isn't at the set level (as I'm assuming that the middle being hot would mean that most is).

[AliG]

Manual here -

 

https://mediacdn.heatraesadia.com/-/media/themes/heatrae/literature/discontinued-literature/megaflo-cl-installation-manual.pdf?v=1&d=20170629T112101Z

 

The heating element is at the bottom behind the cover.

[AliG]

6 hours ago, MrTWales said:

I've had panels since the winter, and now many days are providing lots of excess energy I'm using some of this to heat the water in the tank. This does work but I'm noticing that the amount of water heated seems far less than when gas is used, even if the immersion heater is switched to on all day. It's OK mostly, but very noticeable if a couple of people have showers.

 

I'm assuming that this is because the gas heating involves passing water through the boiler so you get water swirling around a bit, whereas the immersion heater just heats water at the top of the tank and not below the depth of the heating rod goes. 

 

My question is, is there anything anything I can do to heat more water in the day (when I'm passing loads back to the grid for nothing)? 

 

I can't think of any way to get the water swirling around, so that only leaves the length of the heating rod? I've got no idea how long my rod is (ahem) but I'm guessing that it's not super long.

 

This is an indirect tank. The boiler does not send hot water into it. It sends it into a coil that heats up the water in the tank. However, the coil is around half the height of the tank so will heat up most of the tank. The warmer water will take a while to rise inside the tank so the immersion will cut out when only some of the tank is hot.

 

 

A 210L tank is not very large. It could conceivably be cooled considerably by two showers. 2 showers could be 150ish litres of hot water and the tank would be filling up with cold water and cooling down.

 

What you may be noticing is that the immersion can only heat up 50l of water an hour, but the boiler can heat up 400l. If you spent 10 minutes in the shower and used 75l of hot water, the boiler could almost heat the water as fast as you were using it. Thus the water will not cool down anywhere near as much when two people have a shower and the boiler is on versus the immersion.

 

 

 

 

[MrTWales]

Thanks. The boiler is amazing really, esp when combined with the heat retention of the tank. I doubt we've averaged more than 2.2 hours a week of gas for heating the water, and that's a family of four. But when I'm getting 24Khw from the panels and using 10Kwh I really want to heat up what I can via that method!

 

I undid the box thing and see this. It feels like there are two dials - the red one on the blue bit and the small black one at the top right - and one may be setting the temperature before the immersion cuts out?

 

[MrTWales]

OMG I just have the turn the dial a bit to the left to help things a bit!

 

[MrTWales]

13 minutes ago, AliG said:

 

This is an indirect tank. The boiler does not send hot water into it. It sends it into a coil that heats up the water in the tank. However, the coil is around half the height of the tank so will heat up most of the tank. The warmer water will take a while to rise inside the tank so the immersion will cut out when only some of the tank is hot.

 

 

A 210L tank is not very large. It could conceivably be cooled considerably by two showers. 2 showers could be 150ish litres of hot water and the tank would be filling up with cold water and cooling down.

 

What you may be noticing is that the immersion can only heat up 50l of water an hour, but the boiler can heat up 400l. If you spent 10 minutes in the shower and used 75l of hot water, the boiler could almost heat the water as fast as you were using it. Thus the water will not cool down anywhere near as much when two people have a shower and the boiler is on versus the immersion.

 

 

 

 

Damn I really had no idea. I assumed that the water just flowed through a copper pipe in the boiler and this was sent into the tank via a loop!

[AliG]

Instructions for your newer model-

https://irp-cdn.multiscreensite.com/7d2b88c7/files/uploaded/megaflo he manual.pdf

 

The red arrow is the main thermostat and the black one is the immersion.

I would set the immersion to maximum and the red arrow to 55.

 

Was the red arrow really set to the position in the pic you posted? That would be less than 40C.

 

[AliG]

Thinking about what you actually want to do.

 

You want to divert PV electricity to the immersion.

 

I bought an Eddi which does this automatically. Other similar PV diverters are available.

 

Doing this manually will not work well.

 

The immersion draws 3kW. Depending on the size of your solar array and other use of electricity in the house it is unlikely you have enough excess PV to fully run the immersion constantly. It varies a lot with clouds etc.

 

As electricity costs 3.4x as much as gas if you end up purchasing electricity when the PV fluctuates it will offset the savings. Including higher efficiency electricity is roughly 3x the cost of gas for heating water. Also if you simply forget about it and leave it on it would be quite expensive.

 

PV diverters vary the electricity sent to the immersion to only the excess so this does not happen. But they cost a few hundred quid.

 

[MrTWales]

53 minutes ago, AliG said:

Instructions for your newer model-

https://irp-cdn.multiscreensite.com/7d2b88c7/files/uploaded/megaflo he manual.pdf

 

The red arrow is the main thermostat and the black one is the immersion.

I would set the immersion to maximum and the red arrow to 55.

 

Was the red arrow really set to the position in the pic you posted? That would be less than 40C.

 

I've not touched anything and it does seem low. I'll turn it to 55 but I only ever put the gas water heating on for 30 mins max, and not even every day, so it may not make much difference. 

 

I'll turn the other know the final bit more to the right!

[TonyT]

Remember the electric immersion is only 3kW.

 

your boiler ( depending on the size of it) could be putting 10 times that in the same time frame.

 

even preheating the immersion with PV to reduce the gas demand is a worthwhile exercise.  
 

[MrTWales]

45 minutes ago, AliG said:

Thinking about what you actually want to do.

 

You want to divert PV electricity to the immersion.

 

I bought an Eddi which does this automatically. Other similar PV diverters are available.

 

Doing this manually will not work well.

 

The immersion draws 3kW. Depending on the size of your solar array and other use of electricity in the house it is unlikely you have enough excess PV to fully run the immersion constantly. It varies a lot with clouds etc.

 

As electricity costs 3.4x as much as gas if you end up purchasing electricity when the PV fluctuates it will offset the savings. Including higher efficiency electricity is roughly 3x the cost of gas for heating water. Also if you simply forget about it and leave it on it would be quite expensive.

 

PV diverters vary the electricity sent to the immersion to only the excess so this does not happen. But they cost a few hundred quid.

 

 

Totally agree. I have a diverter unit that just needs to be installed (the installers ran out of time so will have to come back to install). It's been months but I didn't chase much over the winter as it wouldn't have been used. Though, to be fair, chasing hardly seems to help. 

 

I had the immersion turned on for a while today yet still managed to feed in over 9Kwh...sigh.

 

 

[AliG]

Ah good on the diverter.

 

The issue you have is that you need to set it up so it is more likely to heat from the diverter than the boiler.

 

If your boiler is set to provide hot water in the morning for example, then the tank will already be hot by the time the sun comes out.

 

There are two things. As discussed you want to set the immersion thermostat higher than the boiler thermostat. This way there should be some room to heat the water even if the boiler has been on. If you are going to get consistent hot water from the immersion during summer, you might set the boiler thermostat to only 48C as a back up if there has been no sun.

 

The other thing you probably want to do is to set your boiler hot water to not be on during the day. It will depend a bit on when you have showers/baths. Assuming showers in the morning, you want to give the PV a chance to heat the cooled down water before the boiler kicks in. 

 

It might be a bit difficult to manage due to the small tank.

 

Say for example you have the tank heated to 48C overnight by the boiler.

 

Then two people have showers in the morning and the boiler is turned off. The water will be cool before you finish your showers.

 

Basically due to the size of the tank, the boiler has to be on at any point two people might have a shower.

 

But if you heat up the water with the boiler then you won't be able to use your PV.

 

It only takes 2.8kWh to heat 200L from 48C to 60C.

 

You cannot really set your boiler much lower as you won't have enough hot water.

 

It is one of those, "I wouldn't start here" problems. Unfortunately if this was the plan you should have had a bigger tank.

[MrTWales]

These are all great points.

 

To be honest, I keep the boiler water heating to a minimum and I even stopped the automatic scheduling. Now, for example, with me on my own (the wife and kids are away) I won't use the gas at all at this time of year and even in winter it would be on for 30 mins in a week if it's just me - otherwise maybe 4 or 5 (at the most) 30 mins heating bursts per week. I get the impression that this is very low useage, but I don't care if the water isn't always hot. Worst case is having to wait 15/20 mins for a shower, but it's usually OK and managable. The tank keeps the water hot for days if it's not used.

 

Having a decent battery helps also as it's usually nowhere near depleted in the morning for this month, so if I know there is sun the next day I've probably got 4-5Kwh spare so I could stick the immersion on in the evening for a hour or so without worrying, provided that we don't have the oven + some other things on at the same time I guess.

 

It would be much simpler with the diverter. I've chased again today! Honestly, all they have to do is install the thing (which I have) and give the warranties and they get the final £1k. 

 

 

 

 

 

 

[jamieled]

We have a slightly similar setup and play about a bit with trying to maximise our PV use. We have a 210l oso tank with two 3kw immersions and an eddi connected to 3.6kw of PV. Family of 4. We rarely run out of hot water.

 

In the middle of winter we tend to assume the PV does nothing and just use the immersion (using the eddi boost timer to programme on/off).

 

At this time of year we'll have it set to boost for a couple of hours later on after the sun goes down. Soon we'll switch off the boost altogether and just let the PV diversion do its work. On the odd occasion of prolonged grey weather in summer it might need a manual boost, but I think it's a small price to pay for the saving.

[MrTWales]

All, thanks for the responses.

 

My conclusion is that, without doing anything fundamental, I can't do much except for setting the thermometer to the max for the immersion (not gas). It was turned almost all the way before, but now it certainly is hotter (it would be too hot without a mixer) and when I turned the immersion on today it stayed on for longer than it ever has before so that's am improvement. It still seems slightly odd that, after a shower at 2pm, the immersion only flicked on briefly many times. I guess the thermoneter is in a place where the water is still hot, and it certainly is! I probably got over 11Kwh into the water today.

 

Also, 28.8 Kwh today - woot!

 

 

Edited April 15, 2023 by MrTWales

[JohnMo]

I put my immersion on to an external thermostat, as I could never get the cylinder hot enough, even replaced the immersion and integrated thermostat, but it made little or no difference.  Max temp I could get to was about 60, even the immersion thermostat set at 75.

[MrTWales]

7 hours ago, JohnMo said:

I put my immersion on to an external thermostat, as I could never get the cylinder hot enough, even replaced the immersion and integrated thermostat, but it made little or no difference.  Max temp I could get to was about 60, even the immersion thermostat set at 75.

I didn't realise how complicated it can be to get the most out of water heating and solar panels.

 

In my case the thermostat tweak, barely half a tick, made a big difference. The water was then too hot to use on it's own (ie without a mixer tap). Maybe next time we have loads of solar I'll use a thermometer to see hot hot is it once it's passed through a tap.

[Radian]

On 15/04/2023 at 22:24, MrTWales said:

All, thanks for the responses.

 

My conclusion is that, without doing anything fundamental, I can't do much except for setting the thermometer to the max for the immersion (not gas). It was turned almost all the way before, but now it certainly is hotter (it would be too hot without a mixer) and when I turned the immersion on today it stayed on for longer than it ever has before so that's am improvement. It still seems slightly odd that, after a shower at 2pm, the immersion only flicked on briefly many times. I guess the thermoneter is in a place where the water is still hot, and it certainly is! I probably got over 11Kwh into the water today.

 

Also, 28.8 Kwh today - woot!

 

 

So I can see the immersion cycling on and off but what's the setpoint - did you wind it all the way to 70^oC? Seems to by tripping lower than that.

Oh, and what took around 6kW shortly after you switched off the immersion? The battery max'd out at 3KW and you imported another 3kW from the grid by the look of it. 😕

[MrTWales]

I did wind up to 70, and I think it did make a big difference.

 

Just checked and I've imported 4.0kWh for the whole month, and hardly any of it on the day of the chart. I was on my own that day and the immersion was the only thing with a large draw, so consumption was never high at any point.

[Radian]

8 minutes ago, MrTWales said:

I was on my own that day and the immersion was the only thing with a large draw, so consumption was never high at any point.

Just wondered about the import spike at 5PMish. Nosey ain't I 🤣

Maybe an induction hob?

[MrTWales]

19 minutes ago, Radian said:

Just wondered about the import spike at 5PMish. Nosey ain't I 🤣

Maybe an induction hob?

I can't see the spike on the chart but I do have an induction hob! I thought it was only 1kw though. It's something I never ever use in the winter but it may be worth using now, so long as the import spike is only small.

[Radian]

14 minutes ago, MrTWales said:

I can't see the spike on the chart but I do have an induction hob! I thought it was only 1kw though. It's something I never ever use in the winter but it may be worth using now, so long as the import spike is only small.

 

You had cropped off the time axis legends so it may have been earlier than my guess of 5PM

 

[MrTWales]

If it was consumption of 6kw I think the orange line would be at 6kw, with the solar/battery/grid making this up. That spike looks like those random (to me!) spikes that I see sometimes. What you say about the induction hob seems right in that this does cause some spikes, but I seem to get them anyway. I think the import as per the app slightly overstates the actual draw though, when I look at the actual meter readings.

 

All I did that day was heat up my take away in the microwave!