sadie water heater & E7 : too much hot water

[jives]

Hi,

My mum lives in a bungalow built in the 1960's and designed for Economy

7 heating and how wtaer.

Storage heaters are wired into a dedicated ring main which is switched on at the fuse box to exploit the cheaper rate. Thats fine and my Mum likes it.

She also has a 7' Sadie water heater/tank which is also on the same circuit. It has a stat in the bottom (!?) , and is well insulated, only getting warm to the touch even when the water inside is piping hot.

The problem is that my mum also has a power shower which heats on demand and is used the most. As nothing else uses much hot water from the sadie (the odd bath), she is heating water every night which is mostly not used.

As the water temp stays how for several days (you can switch it off - all the E7 captive connection points are switchable and fused) I wondered if it would be possible to have the tank heat every other night ?

The problem I can forsee is that while a 7 day emersion timer would do the job, these assume they are the primary switch. In my mums case the switch is at the fuse box in the E7 unit. This would mean that any timer 'downstream' would be unpowered during the day. I assume that these timers require 24hour power in order to function and switch.

Has anyone solved this or a similar problem ? I think there is also a normal 13amp socket near the tank. I wonder if timers exist that can be powered by one supply while switching another ? That way I could power from normal 24hour mains, and switch the E7 supply out every other night.

The other thought I have is that perhaps this is not really helping. Is it better to heat the tank for a few hours (it has a stat) every night, or for a few more hours every other night ?

many thanks

[Dave Liquorice]

On E7 the immersion controller is not normally wired to the switched (space heating) circuit but to the unswitched supply. This is to allow the user to manually "boost" the hot water during the day if they use it all.

The only real way would be to monitor the actual energy used now for say 2 weeks. Make the chnage and measure again. I will say that when the time switch on my gas CH/HW system broke the gas bill went down noticeably even during the summer. This was me living alone, heating water on demand for baths (it'd go from cold to hot in 20mins), residual heat would be enough for washing for at least a day.

It's not quite so simple when on E7 as the energy costs between the peak and off peak rates are so great. Making the (rash?) assumption that the heater would be on for say 2 hrs every night to top up the heat in the tank at E7 rates, thats roughly the same cost as 2hrs every third day at peak rates. Can the immersion heat the tank from cold to hot in two hours?

[Mike Dodd]

If it has a thermostat then, provided that the cylinder is well insulated (as you described) and water use is minimal (as you describe), then although it will be available to heat the water each night, the immersion heater would only be on whilst the thermostat presents a demand for heat. In principle, ignoring losses in the system, switching on every other night would be no more efficient, since the amount of cold water requiring heating for minimal use would be doubled, requiring double the energy to heat it to temperature.

I know there's issues related to losses here, however, you don't seem that concerned about those, so I fail to see the benefit that you're obviously hoping for.

Regards

Mike

[Christian McArdle]

It is unlikely to provide significant efficiency benefits, if the insulation is good. Basically, it would have to be switched off long enough so that the temperature of the cylinder is much reduced before there is a reduction in heat loss. Keeping it hot once it is already there will not cost much.

Christian.