DHW: Which is the cheapest to run?

[bludger]

An economy seven direct hot water cylinder or an electric shower and a couple of instantaneous sink heaters? I find this very hard to work out.

Our hot water cylinder holds about 140 litres of water and the night time rate for electricity is 5.099p per kwh. It takes about 3 hours to heat the tank up to its thermostat setting.

A decent shower is rated at about 10kw. Two of us take 6-7 minutes each to have our showers with the water running at about 5litres per minute. The trouble is that there are two rates for daytime electricity: 18.097p for the first 225kwh in each quarter and then the rated drops to 10.572pkwh.

I'm not even sure what a kilowatt hour is and I don't even know how to start doing the calculations. If anyone could help I'd be very grateful.

We are all electric in our place.

 

[JohnD]

the economy 7 electricity costs you half the daytime rate, so that is the cheaper way to do it

however, the pitiful trickle from an instant electric shower is nothing like what you can get from a good gravity or a pumped shower, so you will doubtless use more hot water if you get a proper shower fed from the cylinder.

A Kilowatt Hour is the number of Kilowatts you use multiplied by the number of hours you use it for.

An immersion heater uses about 3kW, so on your figures, running it for 3 hours costs (3kW x 3 hours) = 9KwH @ 5.1p = 46p

If you use a 10kW shower for 6 minutes (one tenth of an hour) that is 1kW each. So two of them cost you about 2kWh at 10.6p or therabouts, say 21p but this is a lot less water than the cylinder holds.

You would probably use twice as much water if you had a pumped shower, unless you were very mean. The generous supply from a pumped shower can drain a cylinder if you have a long shower.

If the cylinder and all the hot pipes are well insulated, it will stay hot for 24 hours or until you use it.

 

[bludger]

JohnD,

I like the way you figure. That way of working at one tenth of an hour is clever. So, realistically, the costs could break down like this:

A cylinder of hot water at 5.1p = 46p x 365 = £167.90 pa
two showers using 2kwh at 10.572p = 21p x 365 = £ 76.65 pa
or
two showers using 2kw at 18.097 = 36p x 365 = £ 131.40 pa

i imagine the cost of washing hands and crockery under instantaneous sink heaters wouldn't amount to much so the combined costs could be roughly equal to, or perhaps somewhat below, the cost of the cylinder of hot water. Things are looking up.

However, you say that elec showers aren't much good. I didn't know that, never having used one. We would want a reasonable heat in the shower water and my wife would not like less than the flow of 5 litres per minute we get now from our gravity system. Do you think we wouldn't get this?

I'm grateful for your help here.

 

[JohnD]

I don't use an electric shower myself, but I have done, and they are noted for their weedy flow. They have special small shower heads to squirt the water out of pin-holes. They are much worse in winter when the incoming mains water is colder so you have to get less flow or less heat.

with luck someone will be along in a minute with accurate flow figures

BTW it will also be limited by the cold water flow into your house from the mains. Run the cold kitchen tap into a bucket and see how many litres you get.

I have a gravity shower but with 10metres head (1 bar). If I feel up to it I might put a bucket under that and see what it delivers - it is quite satisfactory but not up to American standards.

BTW an instant sink heater will probably be 3kW or less, so it will deliver less than a third of what you get from an electric shower - you will find it very annoying for washing up as it will take an age to fill the bowl.

Have you no possibility of getting a gas boiler?

 

[bludger]

I see what you mean about weedy flows. I'm rapidly going off the idea of an elec shower....

Do you mean getting an calor gas boiler just to heat hot water? I didn't know they existed. If they do, wouldn't that be more expensive than electricity?

 

[JohnD]

if you don't have mains gas, I don't know, it might be rather expensive.

If I were in your position, I'd get a good big cylinder, with two elements, one at the bottom for off-peak heating and one near the top in case I ran out during the day, and would carry on with gravity showers. If you haven't got much gravity pressure, add a power shower pump, but I'd try to find one that wasn't so powerful it would drain all my hot water in ten minutes.

I am a householder not a plumber, I don't know how to choose one like that.

You will need a good big cold tank as well, that can deliver all the water the pump needs without running dry. My cold tank is on a raised platform in the upper loft to give it the most gravity pressure it can get without needing a pump, but the shower cubicle is on the ground floor, 10 metres below. The shower in the upstairs bathroom is adequate for washing hair, but not strong enough for a satisfying shower.

 

[Agile]

Your 5 li/min is the same rate you would get from an electric shower. Its a pretty miserly shower!

In simple terms if you can heat your cylinder on off peak and use it up every day then it will cost about 50p per day.

These are really very low costs and insignificant to most people in the UK.

Far higher costs are involved in heatin and thats where you should be looking for savings ( unless you dont use heating! ).

Tony

 

[bludger]

This didn't start out as an exercise in saving money. We've had a door blocked off from the kitchen which gives us the opportunity for more cupboard space in the hall. This is where the dhw cylinder is sited..

So I thought if I could move the cylinder as well I'd have even more storage space. So I looked into the methods of putting the cylinder in the loft: unvented, mains pressure, cylinders; vented mains pressure cylinders; and the heat exchanger jobbies. I concluded that these would only work well if you had a boiler to keep heating up the water. Mains pressure would empty an economy seven cylinder very quickly.

This led me to the idea of an electric shower and sink heaters, as posted. I've now gone off that. But I've spent most of the night looking up lpg gas fired, instantaneous water heaters, a new idea to me. There's a make called Rinnai which seems ideal. It can be sited externally and gives a flow of about 14 litres a minute at a very respectable temperature. No cylinder involved, just mains water being heated up on the way through.

Of course they aren't cheap, about 500 quid for the gizmo alone. And I'm sure that the running costs would be formidable for a household. However, there are only the two of us so it might be okay. They seem to measure the output in kw's, (35) which I find confusing.

Our existing dhw cylinder is 50 years old, only has one immersion heater from the top down, and is just wrapped in a jacket. There are signs, too, that it's getting past it.

So that's the state of play at the moment. A Rinnai or a modern new cylinder. I'd welcome your thoughts on these.

Yes, we have electric storage rads for heating and I'm having the loft insulation topped up under the old fogies carbon benefit or something.

 

[JohnD]

This didn't start out as an exercise in saving money.

I could have sworn...

Which is the cheapest to run?

Speaking as a simple householder, I reckon a large, well-insulated cylinder with the two elements will be the simplest upgrade, without much in the way of ongoing maintenance and repair costs. The top element will only come into play if you run out of hot water during the day (you can have a timeswitch, and the thermostat set a trifle lower than the Economy one) and you will presumably only need an occasional sinkful or so if you have a cylinder of 140 litres or more. You might consider putting it in an upstairs airing cupboard if you want to save room downstairs. Short pipe runs to the bathroom and kitchen are to be preferred. If the cylinder and the pipes are well insulated, you will be surpised how little heat it wastes into the cupboard

 

[bludger]

Yes, JohnD, that's what I wrote all right. But as the investigation went on it became clearer and clearer that some options could work out pretty dear. Servicing an unvented cylinder annually for instance. And I just got interested in the whole thing and needed a focus to base a post around. Hope you don't think I've misled you.

The instantaneous gas heaters tend, apparently, to fur up their water ways in hard water areas: another example of an unseen handicap (cost)

 

[bludger]

Whoops, I hit the submit button by mistake. To bring the post to an end let me say, again, that you've been most lucid and helpful. Economy seven is the way to go for ease of installation, ease of maintenance, and ease of the mind when it comes to payment. I daren't mention the c word again.

Many, many thanks. Hope all goes well with you and yours.

bludger