Boiler continious, timed or Immersion Heater?

grahamc2003

BAA1 wrote: »

OP :-


If you have all of the above, then it is almost certain that using the timed option for the gas boiler will be the cheapest. Especially for you as you are out at work, so you should only heat the water when you need it.

QUOTE]

To say that using the gas boiler to heat water only is cheaper than an immersion, you have to demonstrate that the boiler under those conditions ifs working at greater than about 33% efficiency (assuming electricity is 3 times the price of gas).

Even though figures state that modern condensing boilers are 95% efficient, that doesn't mean they always operate at that efficiency - far from it in fact. If conditions aren't correct for condensing the water in the exhaust gases, then you lose the latent heat recovery there, which accounts for about 10% of that efficiency, so you're already down to 85%. The main factor affecting the efficiency after thjat is, as has already =been noted, the return water temperature. If it's returning at about 25C (i.e. the water has lost much of its heat to the hot water tank and several radiators), then it can pick up a high temp gain from the burnt gas (the boiler would be designed to be at max efficiency for this input water temp, or thereabouts). If it's returning at say 50C (i.e. because it's only heated the hw tank and no radiators, as is the point in question), then it's capacity to absorb heat from the burnt gas is much less, hence the overall efficiency drops.

Whether it drops to 33% is the question. I can't see how people can say with certainty that it won't, so the question as to whether it's cheaper to heat just the hw tank with a ch gas boiler or an immersion is difficult to answer, and it's certainly not straightforward.

System

If a condensing boiler's efficiency were to drop that low in these conditions, then the manufacturer would be poor (in my opinion) non-condensing older boilers were mainly much better than 33% efficient, so the chance of a modern condensing being that bad should be non-existant - surely.

Also, since 2009, the efficiency rating for new condensing boilers already takes into account use for HW only - From Energy Saving Trust

Boilers are rated according to their efficiency in converting fuel to heat as a percentage. This rating is called SEDBUK - Seasonal Efficiency of Domestic Boilers in the UK.
There are two SEDBUK rating systems, SEDBUK 2005 and the newly updated SEDBUK 2009. SEDBUK 2005 uses an A to G rating, A-rated boilers being more than 90% efficient. SEDBUK 2009 no longer uses the A-G rating but only displays the percentage efficiency.Under the new system a SEDBUK 2005 90% efficient A-rated boiler will be regarded as 88% efficient, according to SEDBUK 2009. This goes for gas, oil and LGP boilers. The 2% difference between SEDBUK 2005 and 2009 is mainly down to differences in efficiencies between using a boiler for space heating in the winter months and only using it for water heating in the summer months.

BTW, I did write "almost certain"

Also remember that many immersion heaters are wired direct off a switch in the airing cupboard and hence have no facility to be controlled via a timer, so would probably remain on for longer than required, or you may find insufficient hot water at the times needed.
Also, what is the stated efficiency of an immersion heater, and do not forget the inherent inefficiency of the electric power generation at the power station which does not effect the cost directly for the home but does impact the environment.