Smaller immersion heater

travisonline

In my small (50sqm) flat, there is no gas supply (large block of flats). So to heat my water I have a very standard electric immersion heater with 180L capacity. It's only me and my wife and we have very quick showers (3 mins each) per day and there is no much need for hot water beyond that. We are retired so no chance of increased family going forward so we can sefely assume the usage of hot water will not increase.  The Immersion electic heater (having two 3kw heating elements) was put in in 2016 (Glendhill) so it's not old by any means. For some reason it shows rating D efficiency whilst I can see lots of electic immersion heaters show Erp of B. 

Assuming 90L immersion heater is ok for (as mentioned above we are very low users of hot water), does it make sense to replace my 180L (which is clearly oversized) with 90L immersion heater? I found the 90L model that is B rated and have only 0.7Kw of daily heating loss per day (i.e. "standing" loss) which is very impressive. My daily heating loss would be c. 1.7kw due to slighly worse insulation of the bank (30mm insulation on the current tank vs 50mm for the new tank) and larger capacity.

What's the general rule about savings from let's say 180L capacity to 90L capacity immersion heater. Is it just a saving of 1kw per day difference due to higher standing loss or there is another savings as well simply because I have a larger tank. If it's only 1kw difference, I may not decide to switch as I may never recoup the money. But if there is another savings (let's say 1kw from standing loss and 1kw difference due to the fact that larger tank is less efficient as it's D rather than B rated) than I can make my money back in 3 years. Buying a new tank is £500 plus £500 installation so it's £1000 investment.

For some reason my current boilder shows "Annual usage of 6000kw" and rating is D and the new boilder says 827kw and rating is B. I am confused about such huge disparity. I obviusly do not use 6000kw for hot water.

getmore4less

If you have 2 emersion heaters in the tank then back in the day when we were electric one was at the bottom and one at the top.

For low water use you just use the one at the top.

Often it was bottom element on E7 and top for top up on demand.

[Deleted User]

I have a 210L hot water cylinder and we average less than 3kWh a day (over the year as cold water fill in summer is a lot warmer than now) to re-heat the cylinder to 65C with a 3kW bottom-fitted immersion heater. We are also a household of 2 retired people.

Replacing a hot water cylinder to save a few pence each day is unlikely to be cost effective.



 

FreeBear

If the bulk of your hot water usage is for showering, an electric shower may be a more cost effective solution.

Have a HW tank here heated by gas - For things like washing up, I'll boil a kettle and use an electric shower. The HW tank only gets heated if/when I want a bath.

Gerry1

Not having a go at you, but you need to distinguish between kW and kWh, otherwise it gets very confusing !

Alnat1

There's no reason to have the whole tank heated constantly.

Why not heat the water for a short time just before you take your showers? That way, you'll only be heating what you need. Try 45 minutes and change to longer or shorter time as needed. Will probably need longer in winter than summer as incoming water is colder. 

Reed_Richards

The water in your cylinder will be hottest at the top and coldest at the bottom.  Typically it will stay that way without mixing.  Hot water is always drawn from the top of the cylinder.  You probably have two immersion heater elements, one near the top of the cylinder and one near the bottom.  If you only use the one at the top that will be pretty similar to having only a half-size cylinder, 90 l instead of 180 l.

markin

If you are on E7 just using the top aliment on demand could cost more in day rate elec, Some people are happy to only heat every other day and have cooler showers.

 The simple answer here is to buy an extra jacket, And/Or a pack of insulation to stuff around it.

Gerry1

markin said:

If you are on E7 just using the top element on demand could cost more in day rate elec.

The answer would be to install a local timer for the upper immersion heater so that it comes on an hour or two before the end of the cheap rate period but can very occasionally be topped up during the day for an hour or two (at the expensive rate) if necessary.

Very important to use a non-motorised timer with battery backup otherwise power cuts can result in day rate electricity being used.  Also very important to make sure the settings are mimicking the meter's actual E7 switching times, not merely what they're supposed to be.

JohnPo

If you are after a timer for your immersion heater - I installed one of these which is wifi enabled and controlled via an app. it works well for me.

https://www.heatingcontrolsonline.co.uk/optimium-wifi-immersion-heater-timer.html 

I have a top mounted 28 inch immersion and I use it with my Octopus Go tariff cheap rate to heat the top part of my large tank to 60 Celsius each night, the flow temperature on the gas boiler is set to 45 (to keep it running really efficiently) unless really cold outside so that way we get enough pipping hot water without any risk of legionaries.  In summer we turn the boiler off and rely on the solar thermal setup.

Edited 20.04 on 2nd Jan - Noticed the OP has not been back on this thread, hope all our replies have not been overwhelming and apologies if that has been the case, please feel free to come back and ask any further questions.

travisonline

The replies in this thread are extremely helpful thanks all! I will summerise below with a few follow up questions if possible:

1. The consensus is that it's not viable to install smaller tank as its unlikely to pay off in the long run.
2. I don't have E7 and not planning to have it since my heating is via normal radiators (not storage heaters). Given that, should I still invest is WiFi controlled switch suggested above? I am really interested in that as it only costs £40 so it should pay for itself very quickly. If I use it I can make it on for an hour in mornjng before showers. Is it sensible? 
3. People say about temperature 65, 60 or lower for hot tank but I don't have this controller at all. My hot water (Glendhill 180 L slimline tank) does not have any controls. All I can do is switch off/on two of its heaters. How I can reduce the temperature then? Should I reduce it to 55 to have some savings but avoid leginela?
4. I can see some people know their usage for boilers and even show screenshots - how on Earth you are doing that? My tank is very basic.
5. Is it indeed a consensus that I should onmy use a top heater (rather than bottom) if I don't have E7 and I am a light user who does not mind waiting for este rto be heated (it's never urgent for me). 
6. What's the best way of checking what my temperature is in the boiler. 
7. If I have two heaters on with no E7 how this is different from only top heater being on? 

Thanks all!