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Author Topic: Immersions Heater Efficiency  (Read 1191 times)
M0g
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« on: March 31, 2019, 11:32:36 AM »

Hello all,

I'm looking for ideas to make PV water heating more efficient. I'm in the process of to adding more insulation to my hot water cylinder and have done some research but have drawn a blank when it comes to the following:

1) Immersion heaters that can be covered. Not sure such an animal exists. Would be interested to hear from anyone who has found such a unit as it would be good to be able to insulate over the top.

2) Immersion heater convection tubes.  Has anyone fitted these to their existing immersion and if so how did you do it?

Cheers,

Stephen
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oliver90owner
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« Reply #1 on: March 31, 2019, 11:48:03 AM »

1) is an easy one to answer.  Why is that a requirement in operation?  To protect the operation of the thermostat is my only answer, so removing said thermostat from the area would rid the requirement for said heat leak.  Assuming here that one would use wiring suitable for the temperatures expected.

A simple alternative would be to use bottom immersion heaters?
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M0g
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« Reply #2 on: March 31, 2019, 12:01:14 PM »

I'd either have to modify the existing cylinder 0r change it in order to fit a bottom immersion heater. I very much doubt it, but am hoping there might be a simpler way!
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Stig
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« Reply #3 on: March 31, 2019, 12:57:10 PM »

I've insulated over the top of my immersion heater,  i.e. I've added another insulating jacket and that covers any gaps.
True that section of the cable gets a bit warm but as I've only got 2kW of PV I'm not running at full rated power unless it needs an early morning boost in winter, but that'll be because the tank's not very hot anyway.


Do you know what length immersion you've currently got?  If it was fitted as just a backup for gas heating etc. it's likely to be a short one to give a small amount of hot water quickly rather than to heat the whole tank.
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M0g
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« Reply #4 on: March 31, 2019, 01:00:29 PM »

Yes, it's 33 inches long. Bit of an odd length judging by today's offerings but it's probably 30+ years old!

The thermostat is an 18" affair. I've retired it due to heat damage (brittle housing) and it's replacement has bitten the bullet in less than a day. They don't make 'em like they used to banghead
« Last Edit: March 31, 2019, 01:20:37 PM by M0g » Logged

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Philip R
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« Reply #5 on: March 31, 2019, 01:24:40 PM »

Given up after one day!! probably tripped on the overheat stat that is now a safety feature built into modern stats.

Turn down the temperature, let the tank cool down and reset the stat.

Philip R
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M0g
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« Reply #6 on: March 31, 2019, 01:38:58 PM »

It's been out a few hours, been reset a few times and stiil open circuit, so off to change it.
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« Reply #7 on: March 31, 2019, 01:51:54 PM »

I'd either have to modify the existing cylinder 0r change it in order to fit a bottom immersion heater. I very much doubt it, but am hoping there might be a simpler way!

Clearly you ignored my initial suggestion?  That, too, is fairly simple.  K type sensors and a remote on/off controller is easy and cheap.  Control could be done for little over a tenner, but a 2kW immersion element might be rather more expensive...
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Philip R
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« Reply #8 on: March 31, 2019, 02:00:21 PM »

These rod type thermostats. If they get seriosly overheated, the expansion medium bursts out and the device becomes u/s.
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M0g
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« Reply #9 on: March 31, 2019, 02:32:19 PM »

I'd either have to modify the existing cylinder 0r change it in order to fit a bottom immersion heater. I very much doubt it, but am hoping there might be a simpler way!

Clearly you ignored my initial sugy
gestion?  That, too, is fairly simple.  K type sensors and a remote on/off controller is easy and cheap.  Control could be done for little over a tenner, but a 2kW immersion element might be rather more expensive...

Your initial suggestion was to remove the thermostat. You didn't mention K type sensors. I could do that as I have a temperature probe connected to a controller which regulates the cyclinder temperature but I wanted to retain the termostat as backup.

Thanks for the tip. Will investigate.
 
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« Reply #10 on: March 31, 2019, 05:58:41 PM »

I didn’t mentiom the minutiae in my first post.  You should be able to work something out for yourself.  Where would you expect to use the original type of thermostat once removed?  I suppose you could thread it through the insulation to impinge on the cylinder - but I wouldn’t!

Thinking even more laterally, you could retain the original 3kW immersion (if that is what it is) and operate a second, higher power rated, relay (powered from the really cheap STC1000 units, available from many sources, for example).

How about using yet another K type as back up?  Belt and braces, but not always a bad thing.
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M0g
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« Reply #11 on: April 01, 2019, 09:13:05 AM »

I didn’t mentiom the minutiae in my first post.  You should be able to work something out for yourself.  Where would you expect to use the original type of thermostat once removed?  I suppose you could thread it through the insulation to impinge on the cylinder - but I wouldn’t!

Thinking even more laterally, you could retain the original 3kW immersion (if that is what it is) and operate a second, higher power rated, relay (powered from the really cheap STC1000 units, available from many sources, for example).

How about using yet another K type as back up?  Belt and braces, but not always a bad thing.

I'm going to contact the manufacturer of the controller to find out what happens should the temperature probe fail. If it fails gracefully and cuts power to the immersion then I probably won't fit a backup thermostat. I've removed the replacement immersion thermostat and inserted the controller temperature probe into it's pocket. This should hopefully give a more accurate centre-cylinder reading. I'll get a refund on the replacement immersion thermostat.

I'm pondering the idea of fitting a bottom immersion heater, but as I have an indirect cylinder I have to be mindful of it's placement. I'll be running some Fernox DS-3 through the hot water system soon, so after I've flushed the system I might drain-down, fit an Essex flange at the bottom and fit an appropriately sized immersion heater. My controller has two variable outputs (also be configurable in threshold mode) so I could run both immersions from it but I'd like to reserve the second output for an electric fire in the lounge.

I could potentially use an STC1000 to drive a higher power relay to enable me to switch 3kW between the two immersions, remove the top immersion and run from a bottom immersion only or retain the top immersion, not fit a bottom immersion and fabricate a convection tube.

Whatever I decide, I will complete the project by boxing the cylinder in with 50mm of Xtrathem, adding mineral wool to the voids and then wrap it with Airtech.  
« Last Edit: April 01, 2019, 10:25:30 AM by M0g » Logged

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« Reply #12 on: April 01, 2019, 10:19:29 AM »

Don’t forget the heat leak on that top hot water outlet - or are you intending taking the hot water off from the bottom of the cylinder? Smiley
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M0g
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« Reply #13 on: April 03, 2019, 12:20:32 PM »

Don’t forget the heat leak on that top hot water outlet - or are you intending taking the hot water off from the bottom of the cylinder? Smiley

All hot pipes have been lagged and covered with Airtech. I won't be taking the hot water off from the bottom of the cylinder.

I've been advised by the manufacturer of my controller that it will cut power should the temperature sensor probe fail. I've ordered an STC-3000 (30A) to act as a backup temperature controller. 

I'm looking for a slightly larger cylinder at 'the right price' to which I'll fit an immersion heater at the bottom. My current cylinder is backed up against a wall and I cannot easily insulate behind it, so I might as well fit a larger capacity cylinder, insulate behind and modify the pipework accordingly.

I've managed to source a few 100mm 2.4 x 1.2 insulation boards for a price I couldn't refuse. Fortunately, I've just enough room to fit this instead of the planned 50mm.

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« Reply #14 on: April 03, 2019, 04:40:44 PM »

Yea, the bottom hot water outlet suggestion was dated appropriately. (April first).

The last lot of 100mm insulation I bought was over 20m^2 (7 1/2 full sheet equivalent) and cost me just over £50.  I donated some to another buyer as I could only fit 7 sheets in, and on, my car.  All in 1200mm square or 600mm x1200mm.  I thought that was a reasonably good buy.

There were probably over a hundred ‘equivalent’ full sheets on sale that day.  Price depends on demand on the day.  Sometimes better than that.
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