Economy 7 Immersion Heater Switches - which is which?

Sally14a

My daughter has just moved into a flat with Economy 7 heating and we are confused about how the immersion heater works. There are 2 switches. The one at the top is labelled o/p and the one at the bottom is labelled 24 hr (see photos). When we switched on the 24 hr switch in the afternoon for the first time, we got hot water after about an hour, so it's not just heating the water during off peak hours. Can anyone explain what the switches do and what is the most economical way to heat the water? 

Talldave

The o/p one is the one to leave switched on as (in theory, but it would be good to confirm) it'll be connected to a circuit that is only live during the 7 off-peak hours (ie: overnight).  You would expect to see the wire from this switch connected to a heating element in the middle or bottom part of the tank (it heats a full tank up).

The 24hr one is for emergency top-ups only and should only be used when all the hot water's been used.  This is because if you use it during the day it's using expensive day-rate electricity.  Ideally, you would replace this with an immersion "booster" switch that gives you 30,60 or 90 minutes of power, before turning off again - thus preventing an expensive mistake of leaving it switched on 24/7.  You would expect to see the wire from this switch connected to a heating element at the top of the tank.

tim_p

I’d look at that picture and think they were labelled wrong or that the cables cross over out of sight. The top immersion (physically top on the side of the tank) should be the boost and live 24hrs, the bottom one (lower on side of tank) should be only live during E7 / off peak hours.  I’d assume the label “o/p” refers to off peak. Needs a bit more detective work although sounds like the 24hr switch is working as expected, so just need to check the o/p comes on overnight but also check the physical locations on the tank where each wire goes to. 

Jules2054

Hi, one way to find out which is which, is to change the socket encasements around the switches, to ones with an LED light in them. You can then switch them on just before the E7 rate period begins. Your booster switch light will come on at any time day or night, but only your E7 switch will light up over night.  The E7 should be working off a different wired circuit. 
I also had storage heaters, and changed those switches, so I could always make sure they were working properly  on the E7. 
😊

Gerry1

Jules2054 said:

Hi, one way to find out which is which, is to change the socket encasements around the switches, to ones with an LED light in them. You can then switch them on just before the E7 rate period begins. Your booster switch light will come on at any time day or night, but only your E7 switch will light up over night.  The E7 should be working off a different wired circuit. 
I also had storage heaters, and changed those switches, so I could always make sure they were working properly  on the E7. 
😊

It's a good idea to have switches with indicator lights, but why would you want to switch anything on just before the cheap rate E7 period begins?  The meter will switch on the off-peak circuit, so you can just leave it on permanently.

If you also switch on the boost heater the tank will warm up twice as fast (which is a bit pointless) but it will also top up during the day at expensive rates.  Leave the 24h switch permanently OFF except for short periods in emergency.  Better still, fit a local timer than limits it to just a couple of hours.

Jules2054

Gerry1 said:

Jules2054 said:

Hi, one way to find out which is which, is to change the socket encasements around the switches, to ones with an LED light in them. You can then switch them on just before the E7 rate period begins. Your booster switch light will come on at any time day or night, but only your E7 switch will light up over night.  The E7 should be working off a different wired circuit. 
I also had storage heaters, and changed those switches, so I could always make sure they were working properly  on the E7. 
😊

It's a good idea to have switches with indicator lights, but why would you want to switch anything on just before the cheap rate E7 period begins?  The meter will switch on the off-peak circuit, so you can just leave it on permanently.

If you also switch on the boost heater the tank will warm up twice as fast (which is a bit pointless) but it will also top up during the day at expensive rates.  Leave the 24h switch permanently OFF except for short periods in emergency.  Better still, fit a local timer than limits it to just a couple of hours.

I know! I thought that was self explanatory!
Its obvious that they're not going to leave the booster on.....thats why they're asking which is which....so they can leave the E7 switch on, and not worry about getting high bills! 

Sally14a

Talldave said:

The o/p one is the one to leave switched on as (in theory, but it would be good to confirm) it'll be connected to a circuit that is only live during the 7 off-peak hours (ie: overnight).  You would expect to see the wire from this switch connected to a heating element in the middle or bottom part of the tank (it heats a full tank up).

The 24hr one is for emergency top-ups only and should only be used when all the hot water's been used.  This is because if you use it during the day it's using expensive day-rate electricity.  Ideally, you would replace this with an immersion "booster" switch that gives you 30,60 or 90 minutes of power, before turning off again - thus preventing an expensive mistake of leaving it switched on 24/7.  You would expect to see the wire from this switch connected to a heating element at the top of the tank.

Thank you! That is really helpful and makes sense.

Sally14a

tim_p said:

I’d look at that picture and think they were labelled wrong or that the cables cross over out of sight. The top immersion (physically top on the side of the tank) should be the boost and live 24hrs, the bottom one (lower on side of tank) should be only live during E7 / off peak hours.  I’d assume the label “o/p” refers to off peak. Needs a bit more detective work although sounds like the 24hr switch is working as expected, so just need to check the o/p comes on overnight but also check the physical locations on the tank where each wire goes to. 

I will check where the cables go and test the o/p switch to ensure it just heats the water overnight. Thank you!

Sally14a

Jules2054 said:

Hi, one way to find out which is which, is to change the socket encasements around the switches, to ones with an LED light in them. You can then switch them on just before the E7 rate period begins. Your booster switch light will come on at any time day or night, but only your E7 switch will light up over night.  The E7 should be working off a different wired circuit. 
I also had storage heaters, and changed those switches, so I could always make sure they were working properly  on the E7. 
😊

Good idea. I'll try that thank you!

dirtyrottenscoundrel

My economy 7 water heater must be reversed.  During the daytime the top switch turns on the heating element instantly I can hear it heating the water straight away. I never thought about the bottom switch, I guess this is the one I should leave on all the time for cheap rate electricity, I will test it tonight. I've lived here years and didn't know about this so thanks for the info.

harrys66

dirtyrottenscoundrel said:

My economy 7 water heater must be reversed.  During the daytime the top switch turns on the heating element instantly I can hear it heating the water straight away. I never thought about the bottom switch, I guess this is the one I should leave on all the time for cheap rate electricity, I will test it tonight. I've lived here years and didn't know about this so thanks for the info.

its a waste of electricity leaving it on alll night during economy 7 hours, your wasting money keeping it at the temp set on the heater element in the bottom tank all night whilst your asleep/not using it

better option is to buy a timer like this and wire it up between the bottom switch and the bottom tank and leave the bottom switch permanently on... https://www.screwfix.com/p/lap-7-day-digital-immersion-timer/1804r

you then find out when your economy 7 time finishes and set the timer to come on for an hour before (normally enough time to bring the water up to the set temperature on the element in the bottom tank)

ps the timer has battery backup, so should keep the time during daytime when the economy 7 feed switches off to the timer/bottom tank