Water change heater project.


Global Moderator
Staff member
25 Jan 2012
One issue I have my high tech tank is the length of time required for me to do a 50% water change (90litres in my case). This is made worse in, my house, due to use of a water softener (incoming water is 22° Clark) meaning I can't use hot house water in water changes. I have been mixing tap water & boiled kettle & boiled pan water in 40 litre containers in the kitchen and pumping across the house to the tank in the lounge. Not exactly quick and involves a lot of trips tank to kitchen, kitchen to tank ensuring all is OK and not freezing the fish and/or flooding the lounge

So what was needed was:-
  • Some way to heat approximately 90litres of water to say 25°C unattended.
  • Must be quick. Time to heat water is given by:
    • Time = Kg water * heat capacity * change in temp / power
    • A 300W aquarium heater dunked in 90litres of water to raise it by 10°C will take 3.5hours !!!
  • A more respectable 3KW will heat 90litres by 10°C in about 20minutes, which is acceptable.
    • Time = 90Kg * 4200 * 10 / 3000Time = 1260 seconds = 21 minutes.
  • Must be unattended, so can be doing tank cleaning & planting whilst water is filling & heating.
  • Must be safe, so if one gets distracted ie major plant trim, child exploding etc nothing untoward will happen.
  • This includes
    • no flooding
    • no boiling water
    • no heating without water
    • Finally must be electrically safe as water and electricity are present.


So what we have here is
  • 160litre keg.
  • Hose pipe inlet connector to connect hose pipe from sink.
  • Manual valve to turn water off
  • Ball float valve so keg fills to level and stops.
  • 3KW anti-corrosion immersion heater. Have to use this type as normal types are copper based.
  • Thermostatic control.
  • Float switch so we can't accidentally turn heater on when no water is present.
  • Water proof heater on/off switch.
  • Indicators for power, heat and heating.
  • 2nd switch to control pump.
  • Pump used to circulate water to speed up heating as well as emptying tank.
Below is a partslist.

You will also need two short lengths of 15mm pipe to connect the elbows to the valve. You can use either 15mm copper pipe (about 7cm long each) or I had plastic plumbing pipe (Hep2O) left over (remember the reinforcing inserts as well).

Use Fernox LS-X thread sealer to seal all the threads.

I used M4 nuts, bolts, washers and spacers to bolt things together.

For water pump make sure it has enough delivery head so that it is capable of pumping from the keg into the tank. The Eheim Compact 1000 pump has a delivery head of 2 metres, unlike lots of cheap pumps that are less than 1 metre.

You will also need a selection of tools, pipe cutter (copper of plastic), hole saw (65mm), drills, files etc. I used crimps to connect to the relay, you will need a suitable crimp tool eg tlc-direct DV DHCR15.

I soldered the wires to the neon indicators and covered the connections in heat shrink tubing. I also placed red sleeving over the “live” blue connection to the orange neon to indicate it is not neutral (see photo).

I also sleeved (6mm size) the wires from the float switch.

I used hose pipe "stop" connectors on both ends of the hose pipe, so that if the hose pipe did come off in the lounge there would be no flood.

Build notes.

  1. Calibrate your keg.
    1. You need to record the approximate level of water you require for your fish tank as you only need to fill for 50% of your tank volume.
    2. On my water change day I emptied my 50% tank water into the keg (I have a line on my tank).
    3. Shine a torch into keg and mark the level on the outside.
    4. This is so you know where to attach the float valve. It was slightly above ½ way for my 90litres.
  2. Fit immersion flange.
    1. Cut a 65mm hole near the bottom of the keg to take the immersion flange (check yours is 65mm).
    2. I screwed the immersion into the flange before tightening the flange up, so that the cable entry to the immersion heater was facing downwards so as to not to catch any water drips. Also ensures that the heating elements are not directly below the thermostat.
    3. Ensure all threads & gaskets & mating surfaces are liberally coated with Fernox LS-X so as to seal water tight.
  3. Work out what height to place the float valve.
    1. It has to be above the highest point of the heater element to turn off the heater if water level is low.
    2. From inside the keg, using a spirit level work out the highest point of the heater and mark on inside of the keg.
    3. Shine a torch inside and mark this level on outside.
    4. I placed my float valve about 4cm above this and located not above the heater.
    5. I used 16mm drill and filed to fit (16.5mm).
    6. Insert float valve and gasket and nut and tighten. Again ensure all surfaces are coated with LS-X.
  4. Fit float valve.
    1. Cut a suitable hole for the float valve. Mine was about 3-4cm above the level you determined in stage 1.
    2. Do not finally attach until you have made and affixed the hose connector and manual valve.
  5. Fit remainder of plumbing
    1. Assemble the full bore valve, compression elbow (hose connector on one end), and female elbow (see picture).
    2. I lightly coated the compression olives & threads with LS-X.
  6. Attach the above to the keg using the pipe clips.
    1. You will need to invent (piece of wood/plastic) spacers about 5-10mm. I used item 99189 from Screwfix, a 6mm and 5&3mm cut to size.
    2. I used 4mm nuts and bolts to bolt the clips. Again LS-X the bolts on the inside to protect them.
  7. Attach the float valve to the keg and valve assembly. Did I mention use LS-X.
  8. At this point you might want to test the water tightness of the setup, outside is nice, I didn’t and got kitchen wet, whoops.
    1. Check that the incoming water cuts off at the right level. Move ball float to adjust the level.
  1. Calibrate the immersion thermostat.
    1. Remove the thermostat from immersion and take to fish tank.
    2. Dangle the end in the tank. Leave for 5 minutes, then turn the thermostat backwards and forwards so that you can hear it switching (or use a resistance meter).
    3. This is to give you a rough idea where the thermostat should be set for your tank temperature.
    4. Mark the dial with permanent marker as the required temperature. Clean and put back when done.
  2. I attached the moulded box and switch to the keg using M4 nuts and bolts.
    1. I use M4 nuts as spacers behind the boxes, to cope with the curve of the keg.
    2. I used 4 off for each to provide maximum strength.
    3. Remember the LS-X....
  3. I attached the relay to the bottom of the box using M4 nuts & bolts, with a plastic washer to lift it up slightly so the lip of the box lid seals correctly.
  4. Wire up as per diagram, ensuring crimps are well made as we are dealing with 12A currents here.
  5. I used the 0.75mm cable to wire up the heater to the neon indicators, the neon indicators and relay coil, as this is easier to work with than 1.5mm and is not carrying the full power of the heater current.
  6. I used a rubber grommet on underside of relay box, rather than cable gland for the float switch cable.
  7. Ensure glands are tightened to provide water tightness.
  8. To electrically test with no water, disconnect neutral from the heater and tape up so there is no chance of the heater coming on with no water in.
    1. Verify the relay switches when the float valve is lifted and that the relevant indicator lights.
  9. Reconnect the neutral and test with water.
    1. Verify heater turns off when up to temperature.
    2. When adjusting the thermostat disconnect the incoming mains before touching.
  1. Hike keg out of garage to fish tank.
  2. Ensure heater & pump are switched off. Plug into mains.
  3. Plug in hose pipe, drag other end to kitchen, connect to cold tap and turn on.
  4. Add dechlorinator.
  5. Turn on incoming water & circulation pump.
  6. Check lights are on and heating.
  7. Clean tank, chop plants, cup of tea etc.
  8. Turn off incoming water.
  9. Transfer outlet of circulation pump to tank and fill tank.
  10. Job done.
  11. Put back in garage until next week.

Was if all worth it.....well probably yes. Water changes are now a lot quicker for me, can be done in under an hour, even quicker if not too much plant fiddling.

Pictures below


Mains wiring

Actual pipework

Wiring in relay box.

Immersion wiring

Plug and socket for pump.

Water filling.

Water circulating.

Not heating as water not high enough.
09 Power Applied.jpg

Water now heating.
10 Power Heating.jpg

Water heated and ready.
11 Power Its Ready.jpg


Happy & warm fish.

Updated 18/02/2013 - Updated parts list with 0.75mm wire and instructions where to use the wire.


Global Moderator
Staff member
25 Jan 2012
I have used it twice since building, first time (and in first picture) I left it in the kitchen in case there was an emergency water issue. I had a slight weeping leak in immersion heater, which is why it is on a towel. Easily fixed with yet more LS-X.

Second time I used it next to the tank. Fills and heats water to 25C in about 30minutes, whilst I plant fiddle and pump dirty water onto my front lawn. Needs a bit of nerve to cope with running and recirculation water in ones lounge !!!

Managed 50% water change, tank clean, filter floss change, wash coarse filter foam in external, quick plant trim all in under an hour.:D
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Reactions: tim


25 Apr 2008
Great write up. Nice to see all the right precautions being taken with the electrics.


Staff member
21 Feb 2008
due to use of a water softener (incoming water is 22° Clark) meaning I can't use hot house water in water changes.
Did not understand this statement, why can't you use hot water from the tap?


Global Moderator
Staff member
25 Jan 2012
Did not understand this statement, why can't you use hot water from the tap?
My hot water has been ion-exchange water softened, swapping the 320ppm (22° Clark) calcium for sodium, in form of sodium carbonate. I thus get no scum on the bath, soap & shampoo last for ages, scale free taps and showers, but I think it has a GH of 0 and is a no no for fish. See here.
Water softening | The Skeptical Aquarist

Years ago I did use hot water mixed with soft cold water from the bath, but whilst fish didn't seem to mind the plants just melted.


15 Feb 2013
One ofmy other hobbies is brewing real ale. This reminds me of one of my hot liqueur tuns that I made from a mango chutney barrel. We build these using the elements removed from the Tesco value kettles.

Here's a pic


8 Aug 2009
Chester, Cheshire
super write up :)

another way to heat water from a stored barrel is to pump through an electric heater. And if you want to keep cost down use an old electric shower for the job:)


Global Moderator
Staff member
25 Jan 2012
..another way to heat water from a stored barrel is to pump through an electric heater. And if you want to keep cost down use an old electric shower for the job:)
Did think of this, but electric showers are not cheap, are generally 6KW, so need to be hard wired and have very specific earthing requirements if you don't want to kill yourself !!! There have been deaths where someone has botched and electric shower on a long extension lead and promptly electrocuted themselves.

Looked at using a paddling pool heater, handy 3KW so 13A plug OK, but generally don't have a thermostat and rely on large volumes of water and considerable heat loss to limit temperature, so no good for 90lites odd. Also cost £60 odd, but could be used.

Also looked at heating tape wrapped around copper pipe, but 3KW of heating tape would cost a fortune.

Also considered, buying 6 off 500W metal aquarium heaters on Ebay for £10 each and dunking them in the barrel of water. This would work, but some of the 500W heaters are very long and would not lay horizontal in the drum.

Even also considered, getting say 10metres if 6mm copper pipe to make a heat exchanger. Coil the pipe in a bucket, fill the bucket in the sink with running hot water and pass cold water through the pipe and straight to the tank (or water keg). Might work, not too sure what temperature water would be. Wouldn't work for me as I have a mixer tap in the kitchen only.

My solution was about minimising the risk and getting a solution that would work first time by design, with out too much "messing around".


31 Mar 2008
Planet Earth
Wow!:wideyed::wideyed: That is some set up mate, you have obviously gone to great lengths to solve your problem and it must have taken you ages to that kind of write up, absolutely detailed..hopefully others with better DIY skills than mine will benefit from it.

Unfortunately my DIY skills are severely lacking especially with the risks surrounding the use of water and electricity:eek: And I have flooded the living room once too many times to be able to stand the grief I would get from the wife if I ever did it again...:arghh:

I really appreciate the advice but the question I was really asking was it safe to use the hot and cold mixer tap in the kitchen to fill the tank up with 200 litres of fresh water. I think I know the answer now so will be trying it this weekend.

Thanks again,



Global Moderator
Staff member
25 Jan 2012
Yes most likely will be fine with mixer tap, just watch temperature.

Could always fill 200 litre drum with cold water, the night before, add airstone ( to dechlorinate) and 500W aquarium heater and leave overnight. In morning pump into tank.


29 Mar 2011
Excellent!! :clap:
Nicely put together, definitely worth setting up once I have the space.
Thanks for sharing :thumbup:.