Immersion heater running costs?

[Colourstar]

Hello all,

Long story, but despite my best efforts, my Rayburn Regent solid fuel range is proving too expensive to feed on my limited budget, therefore I need to look at alternative ways of supplying my 3 bed semi (contents one person ) with hot water.

I already have a 2.7kw immersion heater installed (but never used) and I'm wondering what the running costs of this would be if it were used for, say, an hour a day first thing in the morning. To be honest I'm not sure how to do the maths to work it out! I'm not on Economy 7 and wonder if it would be worthwhile taking that option for this level of electricity usage.

An alternative is 'on-demand' gas water heater, of which the most popular seems to be this Main model:

http://www.mainheating.co.uk/docs/Main_W...ochure.pdf

I'd value any opinions on either option. I see from my last energy statement that electricity was charged at 24.07 p/kWh and gas at 8.165 p/kWh, so presumably on that basis it's a victory for Mr Therm.

I should say that the house has no central heating as I have open fires.

Steve J

[Mark Hennessy]

Hi Steve,

When our boiler broke, we used our electric immersion heater for about 6-8 weeks. It added around £200 to our electricity bill! Clearly, we weren't paying much for gas during this period, so the gas bill was around £200 less than normal, but that £200 would have heated the whole house as well as the water!

Pretty early on, we realised that we were wasting our money by switching it on and off, so left it on continuously. The heat loss through the well-insulated cylinder is quite low.

We don't have economy 7, and there are four of us here. Between us, each day we have two showers and bath the kids after tea, and the shower is a pumped power shower. I guess you use a lot less water than us...

We had a gas-fired instantaneous hot water heater in our last house, and it wasn't bad. Pretty economical to run, despite having a pilot, but it took a while to fill a hot bath, especially in winter - and that would have been annoying if we'd had the kids back then.

Looking at the link, they look quite good for the money. I'm interested to see that one of them doesn't require electricity (just like our old one), and this has certain advantages - I remember one time when we were without electricity for 3 days, and our house was one of the few on our estate that could have heat, hot water and cook, thanks to our low-tech hot water heater, gas cooker and gas fire. Everyone else had modern boilers and cookers that wouldn't operate in the absence of mains.

They are both quite powerful at ~23kw, so should be good for a shower. Will definitely be cheaper to run than the electric immersion heater - although personally, I like the reassurance that the immersion heater is there to cover boiler breakdowns.

Because these things are simple non-condensing burners, I'd wonder if they are as energy-efficient as a modern condensing boiler? One of them even has a pilot light!

Of course, these will free up the space that your hot water cylinder takes, and you'll be able to remove the cold water tank in the loft if nothing else uses it (though connecting bathroom taps, etc, to the rising main instead should be easy enough - though you might get condensation on the outside of the cistern and perhaps you'll need to change a few washers if they are seeing mains pressure for the first time). But care in siting them is required because of the flue - especially so in the non-fan version. And what are you using for a shower? If it's fed from the existing hot water cylinder, perhaps via a pump, then you might need to change the mixer valve and shower head for one that is suitable for mains pressure (low flow rate, high pressure, as opposed to high flow rate at low pressure).

An outside thought; if you're going to the effort of installing something like this, is it worth considering getting a gas-fired heating system installed? If there's only you, and you're willing to deal with the cold, then it might be tempting to concentrate on just heating the water, but it might be better to think about the bigger picture. Personally, I think that it's a mistake to avoid heating a house that's occupied because the lower temperatures will encourage condensation, damaging the decoration and potentially causing a health risk. I know you have open fires, and these will provide good ventilation that will help with this issue, but open fires have a negative overall efficiency rating when you consider the times they aren't used.

Of course, this is going far beyond your original question, but these things do tend to spiral when you think them through. But most of a CH install can be a DIY job, which is a good way to keep the costs down.

Hope this helps,

Mark

[AlanBeckett]

At our old house, for quite a long time, we had a Rayburn which did Cooking, DHW and CH. It worked splendidly in the Autumn, Winter and Spring, but in the Summer, when we had one, it was a pain. So during the Summer we ran an Immersion Heater and I installed a gas hob. We had an Immersion heater which we ran on a timer, generally in sympathy with the Economy 7 but with the ability to override it when required. We had two children at home at the time. The tank was well insulated and I don't remember any horrifying bills. That would be my option provided you don't want to go the whole hog as Mark suggests.

There is one other thing to remember; it's axiomatic that the conversion efficiency from Electricity to Heat (hot water) is 100%. That is never true with gas. Further, the supply of hot water from a heated tank is almost instant whereas there's a lot of waste, which generally doesn't get counted, while the gas heater gets going.

Alan

[Mark Hennessy]

Further, the supply of hot water from a heated tank is almost instant whereas there's a lot of waste, which generally doesn't get counted, while the gas heater gets going.

Yes, this is especially important to consider if you are on a water meter

Mark

[Yorkie]

The type of 'Main' gas water heater you refer to Steve is known as an 'instantaneous multipoint', meaning that it serves all the hot water points in the house - eg, bath/shower, wash basin and sink, and supplies water instantaneously. Its advantage is that it doesn't store any water -the burner only comes on when you open a hot water tap, and it will continue to provide hot water for as long as you wish (to fill a bath for example), then when you turn off the tap, the heater shuts down. The disadvantage is that the flow rate isn't as fast as it would be from a tank-fed hot water storage cylinder. This is especially so in the winter months because these instantaneous gas heaters work on the basis of a thermostatically controlled output temperature, whereas the inlet water temperature from the water main in the street is much colder in winter. The constant outlet temperature is achieved by the heater automatically reducing the flow rate of water through the heat exchanger when the inlet temperature is colder.

This is the same shortcoming that combi boilers have as they too don't store any hot water (albeit there are combi systems that do). However, if you have showers rather than baths, this isn't a problem as the water delivery rate through a shower is so much less than through a bath tap.

You'll notice that the 'Main' gas water heater illustrates that the fluing arrangements are very flexible, especially the fan-assisted one, to suit the layout of the house. The big problem of course is that though the running cost would be far lower than that of an immersion heater, the high cost of installation would have a very long payback period.

As Alan says, 100% of the electrical energy from an immersion heater goes into heating the water (though there are heat losses from a lagged cylinder). The efficiency losses with electricity occur at the generation stage at the power station when the primary fuel (gas, oil, coal) is used as the feedstock. That's why domestic electricty is more expensive than gas - you have to pay for what goes up the power station chimney and cooling towers, and for distribution losses. (An instantaneous gas water heater is about 80% efficient - the other 20% goes up the flue).

To be frank, I think you're between a rock and a hard place Steve. Either you stand the high capital cost of an instantaneous gas water heater with a long payback peiod, or use your immersion heater as sparingly as you can, at a higher running cost, but saving on the capital outlay.

Hope that helps a bit, rather than confuses!

David

[Colourstar]

Thanks for the wisdom. This forum is proving such a good resource for informed opinion and I really value it!

Mark, there is no shower here, just an old fashioned enamel bath. I can certainly see how running the immersion constantly could be more economical in the long term. After all it has a built in thermostat, so once it's up to temperature it should switch on and off to top the heat up, rather than having to rev it up from almost cold every day. The cylinder is new with a sealed jacket so it should be efficient. Somehow thought it's still a slightly scary prospect running the immersion constantly. As soon as I flick the switch I have this mental image of an old-fashioned electricity meter whirling around!

Sadly available funds mean that a full c.h. install is probably not going to happen anytime soon, hence looking at a simple water boiler if only as a halfway-house measure.

It is blooming cold in the unheated bits of the house, such as the hall and bedrooms, where you can see your breath. It's actually 6 degrees in the hall as I write. I must say I am looking forward to summer!

I don't know about a rock and a hard place David, more like an iceberg and a snowdrift!

Steve

[Joe]

Actual cost is hard to predict but if your electric is 15p per kWh than the heater would cost 40½p an hour. That's the maximum as it wouldn't be on constantly due to the thermostat.

You could always clip a light bulb across the element so you could see what proportion of the time it's on for.

- Joe

[Colourstar]

if your electric is 15p per kWh than the heater would cost 40½p an hour

Joe, I must be being dense! What formula are you using to work that out?

Steve

[Joe]

What are you doing to me, Steve? I don't do maths.

A kilowatt-hour is the unit of energy for which you pay, back in the day they were called Board of Trade Units. If you run a one-bar fire (a 1kW one) it will burn a unit in an hour. A 100W lamp will take ten hours to burn the same.

So, if you run your 2.7kW heater for an hour it will burn 2.7kWh. All I did was multiply the arbitrary 15p by 2.7.

I know this isn't very clear but I am a bear of very little brain and numbers bother me.

- Joe

[Colourstar]

Doh! Thanks Joe, must be the cold weather making my brain run slow...

EON are charging me 24.07 pence per kilowatt hour so the immersion will cost 65p an hour to run, although as you point out, it will be a little less then this allowing for the thermostat cutting in and out.

It actually doesn't sound too ruinous put like that....

Steve