immersion heater versus boiler

[Stewart]

Now that the warmer weather is here we are not using the central heating, just the hot water. I am heating the domestic hot water cylinder from the gas boiler, controlled by a cylinder stat and timed for 2 hours in the morning and 2 hours in the evening. The cylinder also has an immersion heater and I am wondering if that would be a more economical method of getting hot water. As well as the comparative cost of gas versus electricity the immersion would be on all the time (no clock) but of course controlled by a thermostat. Has anyone done a comparison of the relative costs? Thank you.

[Roger Mills]

To do a *real* comparison, you'd have to do it both ways and measure the actual electricity and gas consumption over (say) a week, and then work out the costs. Measuring the gas is easy if that's *all* you're using gas for - more difficult if you also cook by gas. Measuring the electrical consumption is harder, because immersion heaters are usually hard-wired. You'd probably have to temporarily rewire it to a 13A plug and use one of these plug-in meters.

Having said all that, bearing in mind that the unit price of on-peak electricity is about three times that of gas, I very must doubt whether electricity would work out cheaper. I always use gas throughout the summer - with the immersion heater being just for emergencies.

[A.Clews]

Thus spake Roger Mills ( [e-mail]) unto the assembled multitudes:

I have exactly the same setup and timings as the OP, and I only ever use my immersion heater in emergency like you, or to augment the boiler if I've left the system switched off for a few days, e.g. during holidays etc or if I've used lots of hot water in one go and want the water reheated quickly.

[David Hansen]

On Wed, 21 May 2008 09:40:11 +0100 someone who may be "Stewart" wrote this:-

Just the time of year for solar water heating. DIY kits at

Electricity is 3-4 times the price of gas. Your boiler and pipework would have to be very inefficient to make electricity cheaper than gas. Are the pipes between boiler and cylinder insulated?

How well insulated is the cylinder and hot water pipework? Leaving an immersion heater on all day and night with poor insulation is throwing money away. Far better to fit a time clock and only have it on when hot water is likely to be required.

If you have cheap electricity overnight then the comparison is more difficult. In some circumstances it can be cheaper to use an immersion heater overnight, as long as the water stays hot so little boosting is necessary.

[meow2222]

Gas is a fraction the price. There are inefficiencies but not that large. The only time elec would work out cheaper is if you had an old cast iron exchanger boiler, low efficiency plus cycling that would waste a whole lot of heat.

NT

[meow2222]

fwiw with cast iron you can greatly improve the efficiency by modulating it manually. Turn it down to the needed heat output for the summer, which isnt very high for hw only. But I gather thats frowned on now.

NT

[dennis]

It would be as most boilers are less efficient run like that.

[Stewart]

Thanks all, from the comments it appears that we are doing the right thing. I have 2 jackets on the hot water cylinder so any heat is retained.

[Tim Downie]

If you're talking about loose jackets on a bare cylinder, I think you'll find that "any heat" is a bit of an over estimation. Foam insulated cylinders are a *lot* better at retaining the heat. I dunno what the payback period is but you might want to think about changing your cylinder.

Tim

[Mary Fisher]

Pinched my thunder!

Whatever the source of heating everything should be more insulated than you'd think necessary.

Mary

[David Hansen]

On Wed, 21 May 2008 15:07:52 +0100 someone who may be "Mary Fisher" wrote this:-

Sorry. I will go and hide now:-)

[David Hansen]

On Wed, 21 May 2008 14:38:37 +0100 someone who may be "Tim Downie" wrote this:-

Indeed. Even standard thickness sprayed on insulation is a lot better than jackets. Double or triple thickness insulation is even better.

In addition the pipes should be properly insulated.

[Doctor Drivel]

And rust up the burners as when the boiler stat is set far too low, condensing occurs.

[Doctor Drivel]

They are not. The lower the temperature and the wider the delta T the more efficient.

[Doctor Drivel]

Yep. Get a stainless steel ACV tank-in-tank cyinder.

[meow2222]

How would that be so? You're running less power through the same exchanger, thus using more exchanger area per kilowatt, which strikes me as a recipe for greater efficiency. AIUI this is one of the advantages of modulating boilers.

And experience with doing it on space heaters has given impressive results. At full power there's stacks of heat coming out the back, whereas at at low power hardly anything comes out exteriorly, whereas internal heat output is still good.

Re rusting, bear in mind iron exchangers rust in normal use, but if set _too_ low it could condense and thus rust faster. Even if it did though it would take many yrs to eat through an iron exchanger - and you'd soon notice condensate running out where it shouldn't.

Not recommending it, but it seems to add up.

NT

[Doctor Drivel]

Uh! ???

Liker rusting the burner box and burner. Which was quite common, as stupid manufacturers allowed the temperatures to go too low on the boiler stats. I think deliberately so.

If you had a blending valve on the return pipe to the boiler set to 58C, any lower and condensation occurs, and the boiler stat set to max, the boiler will run very efficiently and a 80% non-condensing boiler will return near condensing efficiencies.

[dennis]

Wrong. You are assuming that by running with a lower flame more heat is transferred to the water, this depends on boiler design, not some idea you dreamed up.

[dennis]

The boiler will have an optimum set of operating conditions, the designers will have decided where that optimum point should be and it probably isn't at the low end. Reducing the flame will effect all sorts of things, like the turbulence across the heat exchanger, flow rates, etc. all of which will have an effect on efficiency and may not be best at low outputs.

[dennis]

You don't even need blending valves, just zone everything and run it at full output all the time. The boiler will only fire if it needs to unlike these cra@py TRV controlled systems. My non condensing boiler system is easily more efficient than next doors condensing boiler and mine is more than 20 years old (and no cast iron to worry about either).