Inefficient water heater

glocal

I moved into a flat a purchased and I can't believe the water heater can be so inefficient. It was recently serviced, so I am trying to figure out what I am doing wrong or how to use it more efficiently.

It is an OSO Direct 20 RD, unvented, 250 litres (see photo). It is operated through an Economy 7 timer. According to the meter, between 18:00 and 15:00 with heating and water heater off I consumed 4kWh. On another occasion, with the water heater on and the heating always off, between 00:00 and 10:00 consumption went up to an astonishing 17kWh out of a total 22kWh for the whole day! This is way too much for a couple of showers.

Does this sound normal? I thought I would switch the timer off and use boost to heat water on demand but I looked around on the net and apparently it can take up to 3 hours at 3kW x 2 to heat the water. If this is true it sounds like an environmental and financial disaster.

I am wondering if I am doing something wrong. To make things worse, the previous owner had the meter replaced with a single rate one, so there is no Economy 7 here, which might be a good idea as we don't use electricity for heating.

Among other things, in addition to the timer, there are three switches marked by hand as 'top', 'bottom' and 'fan' (I think), currently all switched off. I am not sure what they do. Please tell me each shower won't cost me £2.50!

Richie-from-the-Boro

glocal wrote: »

I moved into a flat a purchased and I can't believe the water heater can be so inefficient. It was recently serviced, so I am trying to figure out what I am doing wrong or how to use it more efficiently.

It is an OSO Direct 20 RD, unvented, 250 litres (see photo). It is operated through an Economy 7 timer. According to the meter, between 18:00 and 15:00 with heating and water heater off I consumed 4kWh. On another occasion, with the water heater on and the heating always off, between 00:00 and 10:00 consumption went up to an astonishing 17kWh out of a total 22kWh for the whole day! This is way too much for a couple of showers.

Does this sound normal? I thought I would switch the timer off and use boost to heat water on demand but I looked around on the net and apparently it can take up to 3 hours at 3kW x 2 to heat the water. If this is true it sounds like an environmental and financial disaster.

I am wondering if I am doing something wrong. To make things worse, the previous owner had the meter replaced with a single rate one, so there is no Economy 7 here, which might be a good idea as we don't use electricity for heating.

Among other things, in addition to the timer, there are three switches marked by hand as 'top', 'bottom' and 'fan' (I think), currently all switched off. I am not sure what they do. Please tell me each shower won't cost me £2.50!

E7, your tank has two heating elements top & bottom, the bottom one comes on all night and gives a FULL tank of cheap water for half price or less.

Non-E7, your tank has two heating elements top & bottom, the top one should come on with the 'boost mode' and will take about 60 minutes to give about 10% ONLY of the volume of the tank of hot water at full price.

- a non-E7 system shouldn't have a timer other than a 60 minute boost mode'
- you would usually have a Horstmann controller like the bottom one on this page
- going to this controller are 4 cables, 2 of them, the night rate cables should have been dissociated when E7 was discontinued

glocal

A controller similar to the Horstmann controller is build into my E7 timer. Do you mean that having the E7 timer switched off and using the boost controller only means I will only be using 10% of the capacity? In other words, do I really have to get the night rate cables dissociated, or can I achieve the same thing by switching the E7 controller off and using only the boost controller? Presumably, I will have to have the three switches (top, bottom, fan) on. (Not sure that that 'fan' is though).

The alternative of course would be to get the supplier to install a dual rate meter, but not using electricity for general heating means this may not be a good idea overall. I still find it incredible that the water heater consumes 17kWh per day as standard. If I understand this correctly, there isn't very much I can do about this. I can only try to get cheaper kWhs, right?

Fire_Fox

Have shorter showers, if it has a high pressure you can get through a serious amount of hot water in a very short period, as much or more than a bath.

Pincher

It takes 1 kCal to raise 1kg (~1 litre) of water by 1 degree C.

1000 kCal = 1.163 KWh

Assuming you are raising 250 litres from 15 degrees to 65 degrees C, i.e. by 50 degrees C.

250 x 50 = 115 x 45 = 12,500 kCal = 14.5375 kWh

At 15p per kWh, £0.15 x 14.5375 = £2.18

Assuming the bottom element is 3kW, it will need 5 hours to heat the hot water cylinder from cold.

The simplest thing to do is to fit a manual countdown boost button for the top element.

http://www.tlc-direct.co.uk/Products/SMTGBT4.html

Hit the button about 15 minutes before the shower, and you should have enough for a shower. The water lower down is not heated.

glocal

Thanks, @Pincher. There is a manual boost countdown dial built into the E7 controller, so what you are suggesting can be done already. If it takes about 15 minutes to heat enough water for a shower, it's not too bad. Given my pattern of consumption, I don't see a better alternative, unless I am missing something.

If what @Richie-from-the-Boro says applies to this water heater, boost will turn on the top element to heat 10% or 25 litres. According to your calculations, that should cost £2.18/10 = £0.21 per shower which is not bad. I will test these theories today.

With the E7 timer off, do I need to turn the mains switch marked 'BOTTOM' to off as well, leaving only the 'TOP' switch on permanently?

In the meanwhile, can someone guess what that third switch marked 'FAN' (or is it 'MAN') does? All I know is that when on, I can hear a fan-like sound coming from inside the cylinder.

Thanks everyone for the replies.

Richie-from-the-Boro

Assuming the water cylinder was not changed when the dwelling was made non-E7, and at the price it would almost certainly not have been changed but left in situ, the bottom line is the plumber / sparky knows changing to a single bottom port non E7 standard non part L cylinder is going to cost about £200 plus fitting costs. The benefit of the direct [heated by immersion heaters directly - hence why the cylinder is called “direct”.] PartL cylinder is its much improved thermal performance The problem with it as described is its inability to heat other than the top of the water.

- element in the bottom and you will get a tank full of boiling water
- element in the top and you will get a 1/4 tank full of angry bubbling boiling water if you are lucky

- confusion pictures now to indicate convection.



Liquid, such as water, is heated from the bottom, the layer of water closer to the heat source expands and hence becomes less dense compared to the water layer above it. Expanded water is less dense than the surrounding water and therefore it rises. The cooler regions of the water in the upper part of the flask, being denser, sink. This movement of liquid due to a difference in density sets up a convection current.

So the heated [ hot ] water pushes up and forces the cooler water down over the thermostat which in turn is heated until the whole volume of the tank reaches your pre determined setting then the stat switches the leccy off. Its a bit atomic science and the random motion of atoms and all that, but sufficient to say that's why the top down element is rubbish at heating a full tank of water.

Last comment, if put simply, the sparky has swapped the top & bottom supply cables, and used a standard on / off controller with an 'override' boost you would have a PartL insulated tank able to use the bottom only immersion and stat giving a full charge bottom up of heated water.


Hope this helps my friend.

glocal

Thanks, Richie for a very useful explanation. After some testing, it took one hour with the boost on to get water that was just about warm enough for a shower. The meter says that consumed 5 kWh (almost everything else in the flat was switched off).

Unless I am missing something, it seems getting the energy supplier to change my meter to dual rate and running the water heater on the E7 timer will be cheaper. It will consume 17 kWh every night, but each kWh will cost around a third and at least I will have a full tank for the whole day. With boost it consumes 5 expensive kWh and the water isn't even sufficiently warm or enough.

The alternative would be to change the heater to one like the one I had in my previous flat which was super-efficient (the type with the separate tank containing very hot water). I have the vague impression the cost is prohibitive.