Immersion heater temperature problems

[RickyTheCat]

Hi all

I moved into a flat 3 weeks ago and have been having problems with the temperature of my immersion heater.

Background
I am on Economy 7, so I have two elements - one half way up the tank for 'peak' times (this just heats the top half of the tank), and one at the bottom which comes on according to the economy 7 timer (non-adjustable).

When I moved in the previous resident told me that the bottom element didn't work so I needed to use the peak element. He also said that sometimes the ‘overfill’ trips so I’d need to bail water out of the brass expansion chamber in the top.

I called in an electrician who tested the connections and determined that the switch used for the off-peak element was low-rated, so had blown due to the load. He replaced it with a better switch and this seems to be working.

The problem
I am having issues with water temperature. I have a two-valve tap with a hose leading to the shower head. When I first get into the shower in the morning, I turn the hot on ‘full’ and then the cold about 20%. After 5 mins I have to reduce the amount of cold, until after 10 mins (max) I’m using water only from the hot tap.

However, when I increase the temperature on the element (it’s currently sitting at what appears to be 60) to 70 or higher, I find that the overfill switch is tripped during the night and I have to bail the water out of the expansion chamber before I can take a shower. Even then the same problem applies.

I seem to only have enough hot water to have about 20 mins worth of showering, and even then the last shower is lukewarm. Can anyone please advise me what I should do? See below for picture of my heater.

Untitled

• RickyTheCat
• 24 Mar 2014
• 1

Overfill is white box at top left. You can see the peak element half way up, and the off-peak at the bottom.

 

[picasso]

Do you have a gas boiler ?

 

[RickyTheCat]

No, my flat is fully electric.

 

[picasso]

Just wondered why there is a two port valve on the cylinder, I think you have a heat bank, they normally use a thermostatic mixing valve and this would give problems with the hot water but not the overfill tripping and having to bail the top tank out, can you give the make and model of your cylinder ?

 

[RickyTheCat]

If you mean the two pipes carrying water - the one with the 'dog tag' on it (exits the cylinder in the centre) is carrying hot water out. The other one is 'cold water in' - this is connected to the overfill switch so that the cylinder stops filling when it's tripped.

The make is 'Tristor'. I can check the model when I get back home.

 

[picasso]

Turn the bottom stat to 75c, if you look in the feed and expansion tank in the top of the cylinder you will see a fill level mark, dont over fill past this mark and disconnect the filling loop when finished, chances are the bottom thermostat or the mixing valve needs replacing if the above does not sort your problem.

 

[jeff the gasman]

20 minutes in a shower?
Who are you in there with?
Is the hot water okay at the taps when this fault appears?
If so you have a faulty thermostat cartridge on your shower.
Jeff

 

[picasso]

The op has a hose attached to his bath taps, what are the chances of it having a cartridge ?

 

[RickyTheCat]

Ha! That's 20 mins total...so can be spread between showers or just for one

@Jeff The shower is a two-tap design with a hose to the shower head - correct me if I'm wrong, but I don't think this can suffer from faulty thermostatic cartridge.

@Picasso - if I increase the temp any more then the overfill seems to trip when the element comes on overnight - could it be bubbling up too much and tripping the sensor? When you mention bailing out the top chamber and turning off the filling loop - won't this mean I run out of water to heat? I also can't see a fill level in the chamber...

 

[picasso]

You have a thermal store, basically its just a big tank of hot water with a coil of copper tube inside, the mains cold water goes through the coil picking up the heat and passes through a mixing valve (thats inside the top of the cylinder next to the filling loop)and out to the hot taps, the store is normally run about 75 to 80c, the mixing valve is their weak point, it also sounds like you are over filling it.

 

[RickyTheCat]

Thanks Picasso.

I've adjusted the temp up a bit to see what happens. I also removed the top from the cylinder and took the top off the chamber to take a look. I can see the sensor for the overfill switch but no fill line inside. Which part is the mixing valve? What I assume is the fill loop (blue tap to the right) was already turned off.

Untitled

• RickyTheCat
• 24 Mar 2014
• 1

 

[picasso]

View media item 75064
The red arrow is the feed to the cylinder, as long as the water in the expansion tank is a couple of inches higher than this pipe it will be fine, I have circled the mixing valve.

 

[Gasguru]

http://www.telford-group.com/content/34/how-a-thermal-store-works


http://www.telford-group.com/uploads/cms_documents/manual_tristor.pdf

 

[RickyTheCat]

Thanks picasso - that's really helpful!

So either the store is not hot enough to maintain its heat past a couple of showers, or the mixing valve is letting too much cold water in?

I notice from here that the mixing valve outputs water at 51 degrees. Is this standard?

 

[picasso]

The last one of these I did the thermostat was turning off the bottom element at about 50c (it was set at 75c) so there was never enough hot water, could be the same with yours, the mixing valves also tend to fail and run cooler.