Immersion heater running costs?

[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