Water heater booster v Economy 7

lohr500

Is your hot water tank really well insulated? This will help reduce the hot water heat loss once you have heated it up using the off peak electricity.

Swipe

Enigmaman said:

The £137 O quoted for 33 days' use is a bit misleading as I was only there about 20 days...equating to £212 if I'd been there every day. Is thst still high for a winter month?

Sounds OK to me. 

QrizB

Enigmaman said:

Rate 1: 37.61p and Rate 2: 15.32p.

Thank you, that's useful.

A typical E7 hot water tank like this one has two separate immersion heaters. The lower one should be connected to the E7 supply (Rate 2) and be used normally, the upper one connects to the regular supply (Rate 1) and is used for boost. Because it's higher in the tank, the upper heater will only heat the top half of the tank.

To heat a typical 150-litre hot water tank from 20C to 65C will take about 9kWh, You can think of those 9kWh being stred in your tank.

A typical modern hot water tank will lose about one kWh a day (this does vary considerably; older tanks could be much worse). So, if you leave your Rate 1 heater switched on, even without using any hot water it's going to be topping up 1kWh a day. Over three days, that's 3kWh or ~46p.

If you leave it switched off, the tank will lose 3kWh (a bit less, in prctice, as the water cools). When you return after three days and use the Rate 2 heater, you then heat the top half half of the tank back up. That will take 1.5kWh and cost 56p.

So, based on the assumptions Ive made, it seems that leaving your overnight heater switched on might save you a few pence. However this all depends on your tank, where the two heaters are and what the heat loss is.

It's also possible that someone has wired the heaters back-to-front and you're running the top heater overnight on Rate 2 and the lower one on boost on Rate 1. you need to check this too.

Rodders53

Info on the exact tank make model / size... will help to discover the actual heat loss per 24 hours.

My unvented loses 2kWh per day (when water kept at the design 60C I expect).  IF that was true loss per weekend away then it'll take 6 kWh to reheat --- exactly the same energy as required if left on while away!

Ideally you need a (smart) switch timer to turn the E7 element off when away on holiday, but set to reheat in the early morning before your return.  Suitably  rated ones (electric current wise) are a bit more difficult to find that for gas/oil heating systems.

DO however first see if the HW is sufficiently hot still on your return, without boost heating at peak rate.  My 210 litre HW cylinder is heated, by oil, once per day between 1030 and 1130 as that's when we use HW to shower (lazy retirees ).   It is still hot enough to shower earlier in the morning or in the evening if we want and for doing the dishes at any time.  
EDIT: I've not tried it after a weekend away... as it reheats so quick on oil and costs not a lot to do so.

Scot_39

General Billing

A winter bill of £137 is not very high for an all electric home - even a small one modestly heated - can hit that level and more.
You switch off HW - what do you do with room heating for the 2-3 days - on / off / lowered thermostats ?

I guess your knocking off heating as your clearly escalating for a full 31 days.

But in terms of hot water suspect your not saving on £/p on hot water at all with a decent modern HW cylinder.

Hot Water Costs

If you are not using any hot water - it is likely that you are simply only replacing heat lost from tank to say airing cupboard or boiler store losses - but in winter that is likely to be heating part of the house - if a tank - e.g. in an airing cupboard - so reducing the need to heat it from another source - so not really lost per se - summer a different issue.

It's not going to lose all of it's stored heat right away - in fact it could well take several days - to drop to house ambient.  As it cools - its rate of loss will slow - as a function of tank vs room ambient temp difference

Tank losses

A modern well insulated hot water tank - might have losses - in the 1-1.5kWh for small to medium volume - older and uninsulated tanks (if they still exist - we had one at parents decades ago - needed 2 lagging jackets - most these days are lagged with foam jacket or internall to outer skins)

Taking this current model cylinder spec as an example -

https://mediacdn.heatraesadia.com/-/media/themes/heatrae/literature/technical-data-sheets/dept_348519_megaflo_ecodirect_datasheet_issue5.pdf?v=1&d=20230503T134602Z


Towards the bottom size end - 125l capacity - losses at 60C - are rated at just 1.05kWh / 24 hr period - at the high end 300l - 1.69 kWh.  Note the losses are are not linear with volume.

Taking the 125l figures

Keep the rate 2 heater on every night - it will need effecitively to supply just 1.05kWh to water to combat those losses  - to maintain the water at nominal 60C target.

If heat to replace at off peak

Over 3 nights - 1.05x3 = 3.15 kWh to water.

(There is a risk the thermostat hysterisis might push the water and so energy cost a bit higher of course)

If dont heat it for 3 days/nights - your going to need to replace the energy lost on return - so how much ?

1st day starting at 1.05kWh, but less 2nd night, less again third - as temp drops.
1.05kWh - would be energy equavelent of c7.2C temperature change in that mass of water.

But at say 53C on 2nd day - the losses would be lower - so lose less than 1.05kWh etc

You'd need someone who understands the physics of loss better from an insulated short tank  - but you should definitely lose less than the 3.15kWh as water begins to cool, losses will start to drop.
But as a fag packet guess taking online equations for a long pipe conductive losses as a guide - and say it varied linearly with temp diff - at 18C in house

 - 60-18C = 42C -> 1.05kWh, 

2nd day - 53-18=35->1.05*35/42=c0.88kWh, 6C equiv, 

47-18=29C ->0.72, 5C equiv

So total losses c2.7kWh

(in reality continuous - just steped daily for simplicity - for fag packet calc)

On return - at rate 1 - the water in tank would then in theory need heating by 18C - using that 2.7kWh energy consumption.

As a sanity check though - you say you need the water on for a minimum of an hour - a typical hot water cylciner immersion heater would be rated 3kW - so for 1 hour would be 3kWh. (What make, model and size of tank ?)

So then would need your rate 1 vs rate 2 pricing.

If those above guestimates - I am no mechanical / thermal calc expert  - and a long time since I did even simple physics - and using a rough average of EDF rate card for typical E7 rates / costings

3.15 kWh at say 16p/kWh rate 2 (off-peak) = 50p
2.7kWh at say 35p/kWh rate 1 (peak) = 94p

It is likely that recovering the losses at peak rate is going to remain far more expensive - even if away for several days.

But there would be savings to be had - but smaller than you might imagine cf normal HW heating costs - so didn't have to revert to boost on return.

Possible Solution

A smart switch or 7 day timer (if your off peak rate applies to whole house) on boost heater - so could have it all off 2 nights then automatically boost/heat late AM in off peak slot - the morning before return.

(Other configs on off peak element maybe possible e.g. if digital with battery - to ride across 7 on / 17 off  power daily)

Scot_39

QrizB said:

Enigmaman said:

Rate 1: 37.61p and Rate 2: 15.32p.

Thank you, that's useful.

A typical E7 hot water tank like this one has two separate immersion heaters. The lower one should be connected to the E7 supply (Rate 2) and be used normally, the upper one connects to the regular supply (Rate 1) and is used for boost. Because it's higher in the tank, the upper heater will only heat the top half of the tank.

To heat a typical 150-litre hot water tank from 20C to 65C will take about 9kWh, You can think of those 9kWh being stred in your tank.

A typical modern hot water tank will lose about one kWh a day (this does vary considerably; older tanks could be much worse). So, if you leave your Rate 1 heater switched on, even without using any hot water it's going to be topping up 1kWh a day. Over three days, that's 3kWh or ~46p.

If you leave it switched off, the tank will lose 3kWh (a bit less, in prctice, as the water cools). When you return after three days and use the Rate 2 heater, you then heat the top half half of the tank back up. That will take 1.5kWh and cost 56p.

So, based on the assumptions Ive made, it seems that leaving your overnight heater switched on might save you a few pence. However this all depends on your tank, where the two heaters are and what the heat loss is.

It's also possible that someone has wired the heaters back-to-front and you're running the top heater overnight on Rate 2 and the lower one on boost on Rate 1. you need to check this too.

Doesn't that 56p comparison cost - miss the added cost of heating the lower half of the tank - replacing the other 1.5kWh of losses as it where - at his off peak rate over next night or two - so adding another c23p?

And also since the OP is putting his boost on for an hour - which would be 3kWh typically - if didn't cut out on thermostat (he doesn't actualy say it is actively on for 1 hour - just switched on).

Gerry1

This might be the answer, switch on the E7 immersion heater supply remotely the night before you return home.

https://www.amazon.co.uk/dp/B07B7NXV4R

QrizB

Scot_39 said:

QrizB said:

Enigmaman said:

Rate 1: 37.61p and Rate 2: 15.32p.

Thank you, that's useful.

A typical E7 hot water tank like this one has two separate immersion heaters. The lower one should be connected to the E7 supply (Rate 2) and be used normally, the upper one connects to the regular supply (Rate 1) and is used for boost. Because it's higher in the tank, the upper heater will only heat the top half of the tank.

To heat a typical 150-litre hot water tank from 20C to 65C will take about 9kWh, You can think of those 9kWh being stred in your tank.

A typical modern hot water tank will lose about one kWh a day (this does vary considerably; older tanks could be much worse). So, if you leave your Rate 1 heater switched on, even without using any hot water it's going to be topping up 1kWh a day. Over three days, that's 3kWh or ~46p.

If you leave it switched off, the tank will lose 3kWh (a bit less, in prctice, as the water cools). When you return after three days and use the Rate 2 heater, you then heat the top half half of the tank back up. That will take 1.5kWh and cost 56p.

So, based on the assumptions Ive made, it seems that leaving your overnight heater switched on might save you a few pence. However this all depends on your tank, where the two heaters are and what the heat loss is.

It's also possible that someone has wired the heaters back-to-front and you're running the top heater overnight on Rate 2 and the lower one on boost on Rate 1. you need to check this too.

Doesn't that 56p comparison cost - miss the added cost of heating the lower half of the tank - replacing the other 1.5kWh of losses as it where - at his off peak rate over next night or two - so adding another c23p?

Yes, it does. I thought about including that but then decided not to; reheating the tank the following night might be thought of as business-as-usual.

But yes, you could add another 23p.

Scot_39 said:

And also since the OP is putting his boost on for an hour - which would be 3kWh typically - if didn't cut out on thermostat (he doesn't actualy say it is actively on for 1 hour - just switched on).

Yes, we don't know exactly how long he needs to run the boost for. He might only need 30 minutes, or (if the tank losses are higher / tnk volume is larger than I've assumed) he might need longer.

Enigmaman

lohr500 said:

Is your hot water tank really well insulated? This will help reduce the hot water heat loss once you have heated it up using the off peak electricity.

Not especially - something to think about 

Enigmaman

Scot_39 said:

General Billing

A winter bill of £137 is not very high for an all electric home - even a small one modestly heated - can hit that level and more.
You switch off HW - what do you do with room heating for the 2-3 days - on / off / lowered thermostats ?

I guess your knocking off heating as your clearly escalating for a full 31 days.

But in terms of hot water suspect your not saving on £/p on hot water at all with a decent modern HW cylinder.

Hot Water Costs

If you are not using any hot water - it is likely that you are simply only replacing heat lost from tank to say airing cupboard or boiler store losses - but in winter that is likely to be heating part of the house - if a tank - e.g. in an airing cupboard - so reducing the need to heat it from another source - so not really lost per se - summer a different issue.

It's not going to lose all of it's stored heat right away - in fact it could well take several days - to drop to house ambient.  As it cools - its rate of loss will slow - as a function of tank vs room ambient temp difference

Tank losses

A modern well insulated hot water tank - might have losses - in the 1-1.5kWh for small to medium volume - older and uninsulated tanks (if they still exist - we had one at parents decades ago - needed 2 lagging jackets - most these days are lagged with foam jacket or internall to outer skins)

Taking this current model cylinder spec as an example -

https://mediacdn.heatraesadia.com/-/media/themes/heatrae/literature/technical-data-sheets/dept_348519_megaflo_ecodirect_datasheet_issue5.pdf?v=1&d=20230503T134602Z


Towards the bottom size end - 125l capacity - losses at 60C - are rated at just 1.05kWh / 24 hr period - at the high end 300l - 1.69 kWh.  Note the losses are are not linear with volume.

Taking the 125l figures

Keep the rate 2 heater on every night - it will need effecitively to supply just 1.05kWh to water to combat those losses  - to maintain the water at nominal 60C target.

If heat to replace at off peak

Over 3 nights - 1.05x3 = 3.15 kWh to water.

(There is a risk the thermostat hysterisis might push the water and so energy cost a bit higher of course)

If dont heat it for 3 days/nights - your going to need to replace the energy lost on return - so how much ?

1st day starting at 1.05kWh, but less 2nd night, less again third - as temp drops.
1.05kWh - would be energy equavelent of c7.2C temperature change in that mass of water.

But at say 53C on 2nd day - the losses would be lower - so lose less than 1.05kWh etc

You'd need someone who understands the physics of loss better from an insulated short tank  - but you should definitely lose less than the 3.15kWh as water begins to cool, losses will start to drop.
But as a fag packet guess taking online equations for a long pipe conductive losses as a guide - and say it varied linearly with temp diff - at 18C in house

 - 60-18C = 42C -> 1.05kWh, 

2nd day - 53-18=35->1.05*35/42=c0.88kWh, 6C equiv, 

47-18=29C ->0.72, 5C equiv

So total losses c2.7kWh

(in reality continuous - just steped daily for simplicity - for fag packet calc)

On return - at rate 1 - the water in tank would then in theory need heating by 18C - using that 2.7kWh energy consumption.

As a sanity check though - you say you need the water on for a minimum of an hour - a typical hot water cylciner immersion heater would be rated 3kW - so for 1 hour would be 3kWh. (What make, model and size of tank ?)

So then would need your rate 1 vs rate 2 pricing.

If those above guestimates - I am no mechanical / thermal calc expert  - and a long time since I did even simple physics - and using a rough average of EDF rate card for typical E7 rates / costings

3.15 kWh at say 16p/kWh rate 2 (off-peak) = 50p
2.7kWh at say 35p/kWh rate 1 (peak) = 94p

It is likely that recovering the losses at peak rate is going to remain far more expensive - even if away for several days.

But there would be savings to be had - but smaller than you might imagine cf normal HW heating costs - so didn't have to revert to boost on return.

Possible Solution

A smart switch or 7 day timer (if your off peak rate applies to whole house) on boost heater - so could have it all off 2 nights then automatically boost/heat late AM in off peak slot - the morning before return.

(Other configs on off peak element maybe possible e.g. if digital with battery - to ride across 7 on / 17 off  power daily)

Thanks for your very full answer! I will look at a smart switch but you have confirmed what I suspected about being better off just leaving Rate 2 on.