I'd like to see the temperature of the hot water in my cylinder, and be able to modify it easily.
Is there an immersion heater unit one can use for this?
Basically, I would like a gauge inside my airing cupboard, with a temperature control I can turn.
On Mon, 22 Oct 2007 14:55:10 +0100 someone who may be Timothy Murphy wrote this:-
Never seen one with a built-in temperature gauge.
I suggest that you either get a pipe thermometer and connect it to the vent pipe coming out of the top of the cylinder, or a thermometer which will fit against the side of the cylinder under the insulation (I take it there is proper insulation). ISTR places like BES do both. The former is less work but less accurate.
For a really accurate reading you would need a cylinder with a pocket for a temperature sensor. These allow the sensor to be placed in the middle of the cylinder. I wouldn't want to fit one to an existing cylinder.
There shouldn't be any need to fiddle with the setting of the thermostat built into the immersion heater. Why do you want to do this?
however, you would need a relay to control the current.
What about getting an infra-red thermometer and painting a target area on a part of bare metal of the tank?
It happens that Timothy Murphy formulated :
Ready made to suite you spec., I would doubt it.
Monitoring the temperature is not too difficult, you can get various probes with a temperature display battery powered - Maplin would likely be able to help with this.
The control part is much more difficult...
A UK company called Morgan Hope used to sell a precise temperature control for immersion heaters, water heaters and boilers. They turn full on then gradually modulate the power on and off as it nears the set temperature. They were sold as a means to economise on fuel, but I won't go into that argument.
Basically they used a temperature probe feeding a comparator circuit which in turn fed a Triac to do the switching. Alter the reference input to the comparator and it would change the temperature it would aim for. Use switches to change the reference inputs and you could mark them up 50 C, 60 C, 80 C or what ever temperatures you needed. The temperature probe fitted in place of (in the pocket) designed for a standard mechanical thermostat.
Modern mechanical stats contain two stats in the one package. The normal stat plus an over temperature trip for safety. Replacing this with something like the above might reduce the safety.
This document tells you all you need to know:
To save energy by choosing an appropriate temperature at different times.
There's a variety of different ways that this could be done depending on what you want to spend and how sophisticated you want this to be.....
A simple solution would be to use a strap on type of thermostat as used in central heating systems. These only have low current capacity contacts so would require some kind of relay, solid state relay or contactor in a box to do the immersion heater control. The basic ones with bimetal strip are very inexpensive, but don't offer very good accuracy and have several degrees of hysteresis - in other words for a nominal set temperature of 60 degrees you might get a +/- 5 degree range between off and on.
There are more sophisticated electronic versions of this. For example, the Danfoss Randall WP75H has a temperature sensor with lead and then a control unit with temperature setting and indication and even 7 day time settings with different temperatures at different times of day. That model even has 16A contacts so directly usable with the immersion heater. Sounds ideal for what you describe. Unfortunately, it appears from its absence from their web site and anywhere to buy that it has been discontinued. There is an RF version WP75RF, but this is really intended for boiler control since it has 3A contacts on the receiver. It could be used with a relay. However, pricing seems to be about £75 for the thermostat, another £45 for the receiver, so by the time you had the other bits needed at least £130 implementation cost.
Another avenue to look at would be industrial temperature controllers. RS sell a range of these with different capabilities, although most are low contact current. Search
If you want to go to that level of sophistication and expense, it would be worth looking at programmable logic controllers (e.g. Crouzet Millenium and others). These allow you to build your own control systems by creating logic diagrams which hook the internal elements together. Many of these types of controller have PID blocks plus analogue inputs for temperature sensors and so on, so it would be possible to make your own controller to your requirements with the addition of sensor plus relay. Once you are getting to a cost of over £100 with the other solutions, these are worth considering.
Bear in mind that with any of these solutions, the results are only as good as the sensor. Strap on ones are OK, but if you are looking for accurate control, you really ned to measure inside the cylinder. That really means a pocket installed in the cylinder. These are like a copper tube with blind end, normally fitted in the side as an option when the cylinder is made.
As a cost saving exercise, I am not sure of the ROI of a project like this. You would probably find that insulating th cylinder with an additional jacket would be a better bet, or overall to do something with the walls of the house. As a science project that saves some money as well, it could be interesting.
On Tue, 23 Oct 2007 03:14:20 +0100 someone who may be Timothy Murphy wrote this:-
Is the cylinder insulated properly? Are the hot water pipes insulated properly? Both of these are important first steps to saving energy.
The next question is what is an appropriate storage temperature and why would it vary? For sink use a relatively small volume of heated water is necessary. Saving energy is probably best achieved by turning the heater on half an hour or so before hot water may be wanted and turning it off about an hour before the want for hot water ends. There are time clocks to do this. In this situation a short immersion heater is ideal. The water temperature should not be low as this will encourage rapid multiplication of legionella.
Such a scheme may not cope with a bath as there may not be enough hot water. A way to solve this is with a dual immersion heater arrangement so that a bathful of water can be heated up when wanted.
Thanks for your suggestions, which I'll look into. I do actually have quite good insulation on my hot water cylinders - more than the recommended 60mm - and have insulated the hot water pipes as far as I can without pulling up carpets and floor boards. But I was quite surprised when I did an experiment how quickly the temperature of the water fell when the heat was switched off (and no water was used). I had expected it to keep at more or less the same temperature for a day or so, but the temperature started to fall pretty fast (according to Newton's Law, I assume) and was close to the ambient temperature after 4 hours or som.
It seems to me that the scope for energy-saving at this point is far higher than the touted solutions of turning off TVs on standby, etc, and even using energy-saving bulbs.
Also, it seems illogical that one has room thermostats with adjustable temperature but nothing similar for hot-water cylinders.
It is actually quite awkward to change the temperature setting on my immersion heaters.
It's surprising that the temperature is falling by 35 degrees or so as quickly as that. Is the insulation a jacket on a bare copper cylinder or a spray foam?
The spray foam should yield a better result
Generally, maintaining HW at 60 degrees is recommended because it kills Legionella. How big a risk there is in practice from not doing that is open to debate. As you have realised, there is a lot of politically motivated bullshit around the whole energy saving discussion but it's not that likely for recommendations to be made which would have an apparent health risk.
I can't help thinking that you might be better off changing the cylinder to one with thick foam insulation - you can 100mm to order if you want - and two immersion heaters. That way you could heat a small amount for hand washing use and then heat the whole cylinder when needed.
Hi,
I'd fit a capilliary thermostat _in series_ with the immersion thermostat
For an example go to and search for item #
9780.That said, I use the timer period on my water heater to control the temperature, and make it longer in winter.
cheers, Pete.
On Tue, 23 Oct 2007 12:29:22 +0100 someone who may be Timothy Murphy wrote this:-
How many do you have? How are they heated and how many heaters does each one have.
Then the cylinders are not well insulated and fiddling with heat input will not change that. I would suggest new cylinders with double or triple the minimum level specified by the (British) standards. These stay hot for a long time. One supplier is
I have 2 hot-water cylinders, in 2 bathrooms. Each has a single simple immersion heater "stick".
around the cylinders - in both cases they are almost up to the bath on one side, and to a wall on the other.
I'll have to see if a change of cylinder could be worth-while.
But what surprises me is the lack of simple energy-saving devices for use in connection with an immersion heater. I couldn't even find a simple timer for use with the heater, unless I do a little wiring, which I'm reluctant to do in a bathroom.
Is there a timer which can simply replace the on-off switch in a standard socket-box?
To answer your question, but not your problem - yes.
Jim A
On Thu, 25 Oct 2007 01:41:35 +0100 someone who may be Timothy Murphy wrote this:-
Unless you have heavy peak hot water demands; for example in a guest house, foster home and so on, that may well be one too many. It may be possible to gain some space by only having one and perhaps providing some local heaters.
At the extremes a replacement single cylinder with two immersion heaters to keep up with demand may be worth considering.
While many cylinders are 450-500mm in diameter they can be made in other sizes, as the people I mentioned will do. One of 400 or 350mm diameter would allow plenty of room for double or triple insulation and pipework. It would need to be taller to have the same volume, but this is often not a great problem. A taller narrow cylinder improves stratification of the water to wring as much heat as possible out.
If I was fitting a new cylinder I would consider at the very least providing connections for solar heating, especially if electricity was my only form of heating
As a reference the top of my double insulated thermal store will be at around 62C at 23:00 and after no overnight input or draw off it will be at around 57-58C at 08:00 the next morning.
Immersion heaters are simple devices and it is probably best if they remain that way. Adding fancy controls can hide the fact that there are other things which can be done to minimise heat losses and these are best done first.
I don't know why you say that. I have insulated the tank as far as is possible given that there is very little space round it. I have also insulated the pipes as far as I can.
As far as I can see, any major improvement would require a new cylinder, and it is not clear that this would be worth while. However, I do intend to look into the price of installing this, and see how long it would take to pay back.
I do find it surprising in this electronic age that there is no simple way of monitoring the temperature of a hot-water cylinder. The immersion heater rods I have - which seem to be standard - do not seem to have seen any development over the last 50 years.
A thermometer?
cheers, Pete.
Well, I meant the temperature of the water in the cylinder. But even if I wanted to measure the temperature of the cylinder itself, is there a simple way of attaching a thermometer to it, and seeing the temperature on a dial?
As I said, it surprises me that there is no standard fitting on a cylinder or the immersion unit to give the temperature. I get 3 or 4 temperatures on my PC for nothing ...
Some kind of a thermocouple or thermistor probe device attached to the cylinder would give you the temperature of the copper which will be reasonably close to the temperature of the water at that height.
Oregon Scientific make various products with remote sensors and a wire connecting to a display device or even wireless ones. Not all go above 60 degrees though.
Probably because most people don't care.
The thermostat in an immersion heater is in a blind tube which positions it in the body of water.
You can get probe tubes fitted to copper cylinders but I think that that is at manufacturing time.
There's a cost in there somewhere, however small it might be. Do many people care about that unless they are into these water cooling systems using fluorescent dye?
In message , Timothy Murphy wrote
But only after you have paid many hundred quid for your PC.
The probable reason that the temperature sensor for an immersion heater hasn't been updated is because the existing mechanism is cheap to produce and it works.
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