Immersion hot water system, what schedule is most optimal?

Arfa__

Moved into a new(ish) house a few months back, heating is done by a gas combi boiler, but hot water is via electric immersion heaters in a large Megaflo tank. Both are controlled via a Hive system. Tank is 300ltr with 3kW and 2.7kW heaters in it (according to the label) and has a separate Varem expansion/pressure tank. I suspect this system was chosen as it's a large house with several bathrooms, so perhaps a Combi boiler wouldn't be able to supply multiple showers concurrently.

I wasn't sure about the best schedule for optimal energy usage whilst supplying sufficient water, so I've just gone for 2hrs at 6am and 1hr at 6pm. Since our energy usage is pretty high (454.0 kWh for July), my hunch is this system will likely be one of the larger electric consumers (not got a smart meter so tricky be sure). So, are there any recommendations on schedules for the most efficient energy use? I understand the immersion heaters should cut out once the water is hot anyway, so is it a bit moot? I read somewhere someone would schedule the water to be on just a bit each day, then longer once a week - is this better? Any tips?

FWIW there are 4 of us in the house (5 in near future). Baths/showers tend to be in the mornings and late evenings. Dishwasher/washing machines are all cold feed, so little other hot water usages.

Thanks!

Apodemus

Are the two immersion elements separately controllable or are you running both every time it is set to come on?  5.7kW for two hours would seem a lot for a family of four's needs in the morning!  

...and is there definitely no pipe from your combi boiler also available to heat the Megaflo?

Ebe_Scrooge

Arfa__ said:

 I understand the immersion heaters should cut out once the water is hot anyway, so is it a bit moot?

Pretty much yes, for all practical purposes.  Assuming the tank is well-insulated, then it'll stay hot for many hours once it's been heated.  Of course there will be a small heat loss, but this should be minimal.  Probably the most practical timing would be to set it to come on at say 6:00 in the morning for an hour or so, meaning you've got plenty of hot water for morning showers.  Then another hour in the evening for baths or whatever.  Obviously change the timings based on your personal schedule.

Unless you have Economy 7 electricity available, in which case it makes sense to make use of that.

The only other thing to consider is that, although electricity is 100% efficient, it would still be cheaper to use gas.  If the combi boiler would give you sufficient flow of hot water for your needs, it may be worth seeing if you can get it altered to supply hot water as well.  You'd obviously have to factor in the upfront cost of this versus the savings in fuel, but it's something to think about.  Also, do you need to heat the whole tank, or would heating only half of it give you enough each day for your requirements?

Bendy_House

Hi Arfa.

Yes, the pressurised cylinder would have been chosen to deliver a good flow to multiple outlets. And, yes, if you plumb your combi to deliver hot water instead, then expect only one good hot outlet at a time.

But, are you sure the boiler is a 'combi' type? Ie, it could deliver instant hot water if plumber correctly? Make and model, please, plus a photo of the pipes exiting its underside.

Oh, do you have any way to cheaply heat the cylinder using electricity - do you have a dual meter, with a 'cheap rate' for, say, overnight use?

If it's a combi, then you have more options, including - I think - the ability for it to heat up your hot cylinder using gas instead of electricity.

Currently, gas is a lot cheaper than electricity, and I suspect it'll continue to be that way as prices for both increase (ie soar...)

Ok, assuming you wish to stick with what you have - a hot cylinder heated via an immersion heater at max energy cost - then the best heating procedure is to heat the tank simply to match demand. So that means, heat it up just enough to provide what you need, and no more. So, if you find that the upper immersion element heats enough water, then use only that one, and don't heat up the whole tank.

You obviously will need a good supply of hot for the mornings, so set the timer to provide this. Some experimentation will show just how much time the tank needs to be hot, but I'd have thought that less than an hour should do this, and certainly under an hour if only heating the top half. If you find that this IS enough, then set your Hive accordingly, but perhaps with some overlap so it continues to heat the tank for part of the time folk are still showering.

Eg: if the showering period for all folk takes 45 minutes, then I'd try heating the tank up beforehand (obviously...), and then for, say, 30 minutes into that 45. That means that the last 15 minutes will be someone consuming the last of the hot water, and it'll then run out (serves them right for being last in the shower...)

The tank will then be 'cool' for the duration of the day. By being 'cool', it'll lose very little heat to the air.

If you need to wash lunchtime dishes, boil a kettle - that's 100% efficient (tho' still electricity-expensive).

Then, you repeat this in the even - have it timed to come on to just provide what's needed for evening showers, and no more. Again, the tank should be 'used up' - cool - as you go to bed, ready for it to be heated again the next morn.

That all changes IF you have cheap-rate leccy overnight. If you do, then use the bottom - full-tank - element, and get the tank piping, and hopefully good for the morn and whole day, and only needing a small (expensive-rate) top-up before evening showers.

Does that make sense?

Best of all would be to try and employ that boiler, since gas will almost certainly do this cheaper for you.

Make and model... And a photo of its pipes. And a photo of the Megaflo, please - we need to see what pipes are going to it. Ta :-)

Section62

Arfa__ said:

Moved into a new(ish) house a few months back, heating is done by a gas combi boiler, but hot water is via electric immersion heaters in a large Megaflo tank.

Are you sure it is a combi?

Have you got (or would you consider) Economy7 electricity?

Arfa__

Thanks for the replies. First up, I misdescribed the boiler - I'm pretty certain it only does heating, so a 'condensing boiler' and not combi boiler (correct?). It's a Greenstar 40CDi Classic Regular, installed 2014, and it only has controls for heating. See photos below.

The house is large (3 en-suites and a family bathroom), so unlikely a combi boiler could supply enough hot water directly for all of those. Using a Combi boiler to fill the tank could be an option, but as above, I suspect I'd need to swap the boiler and add more pipes.

We're not on Economy 7, have contemplated it, but I work from home and kids are heavy computer users after school and in the holidays. So not sure if it will be best for us.

I'm aware of a method to control the two immersion heaters independently. Maybe you can spot something in the photos below.

Apodemus

That bottom pic looks like a motorised valve on a loop from the central heating?

With the central heating on, move the lever on the silver box across and see if the pipe warms up.

Section62

Apodemus said:

That bottom pic looks like a motorised valve on a loop from the central heating?

With the central heating on, move the lever on the silver box across and see if the pipe warms up.

The manufacturers generally instruct that the manual lever shouldn't be used when the electrical supply to the system is turned on - there is risk of damage to the valve.

Bendy_House

Hi Arfa.

Many thanks for the series of photos, many of which are just soooo out of focus that I cannot read the labels, and the rest are just soooo sideways that they miss the bit of pipework I'd like to see. Nice job...

The good news is that your boiler can - and probably does - do everything. Ie, heats the DHW and the CH.

As you say, a combi wouldn't suit your house, so you do actually have the best system already - hurrah! You just need to learn how to use it - boooo.

Could you show us, please, photos of all your control systems:

Your wireless room stat (how you control your CH).

A close-up, and in focus... - shot of that wireless receiver.

A wider photo showing the boiler and the pipes leading off from it.

A wider shot showing the hot tank and all the pipes ditto.

Bendy_House

Your current boiler is a 'system' type, or 'heat-only' or 'conventional'. Ie, all it does is heat the water.

The heated water is circulated by that red pump (top-LHS of the boiler) and, if you follow the other side of that pump's pipe, you should find that it splits and has a motorised valve on each split pipe.

One of these valves - the one for your DHW - is where Apod pointed out - on the pipe supplying the bottom of your hot tank. The other one - for the CH (rads) is where?

Somewhere there will be controls for both the DHW and the CH. Again, track 'em down, and please post photos.

Arfa__

Hmm, intriguing. I'll get you more/better photos and post back later. Many thanks!