Heating hot water cylinder -gas or economy7?

QrizB

On that gas tariff, I'd be heating my water with electricity - provided the tank insulation is up to the job.

FreeBear

tired_dad said:  how do you know when your boiler is condensing? Ours a a relatively new Worcester Bosch condensing boiler

You can get some idea of how well your boiler is condensing by looking at the plume coming out of the exhaust on a cold day. Lots of white "smoke" means your boiler is not condensing. Just a faint whiff is a good sign. The return temperature will also give a good indication as will the exhaust temperature (below 50°C is good) - Not everyone has the ability to measure these though..

sheenas

As I posted flow temp. is 55 degree for water and heating. Good call on the steam test. I get plenty of steam at first but it soon drop down as the boiler warms up. I have shown my costs below and posting above shows an good electric tariff rate. The gas is approx. 12% cheaper, so based on my usage it really does not make any difference either way. To save 10% over gas you would need to be 22% more efficient and then you would be saving 57p a week.  

Scot_39

sheenas said:

As I posted flow temp. is 55 degree for water and heating. Good call on the steam test. I get plenty of steam at first but it soon drop down as the boiler warms up. I have shown my costs below and posting above shows an good electric tariff rate. The gas is approx. 12% cheaper, so based on my usage it really does not make any difference either way. To save 10% over gas you would need to be 22% more efficient and then you would be saving 57p a week.  
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Later in the Ovo thread - there's a measure by the author of the graph you posted  - a graph of average power for heat cycle - and of apparent system efficiency his rhs y axis key  - taking into account full system not just boiler.

The poster system overall capping at around 70% - so in their case electric would likely meat your 22%, 10% saving.

And shows the power transfer limitation of the indirect coil  - as does the decay in temperature rise rate mentioned in mh post above -  taking average much lower than likely boiler raw power - so c5kW.

But elsewhere uses 90% for the boiler efficiency itself as part of energy split calculations. Of course pipe losses in winter will heat property to some degree  - depending on tank location - some are installed in lofts etc.

But as you say 57p a week - is it worth it.

If you have a gas boiler sitting idle x months of the year - is it more or less likely to break down ?.

My friends pump - oops auto coorect seized not sized - over last summer - a mild spring and early winter meant it was idle a couple of months longer than normal.  The heat engineer managed to unfreeze it by suitable use of hammer and driver on shaft end. 


Edit 2  and do you then have to add some fraction of c30p gas SC to the electric hw cost anyway?

lohr500

This post is getting very technical and complex as do many other posts on the forum 

But in simple terms, to answer the OP's original question, as @tired_dad is paying 6.92p per kWh for gas and 7p per kWh off peak electricity, then surely he is better off using electricity? 

The electric immersion heater will be 100% efficient.

The gas boiler with associated heat loss through the connecting pipework would need to operate at +/- 99% efficiency to achieve an equivalent cost per kWh as the electricity.

So it is irrelevant if his boiler is running at 70%, 80%, 90% or even 95%.(And I will eat my hat if it can achieve 95% when heating only the water).

In the OP's scenario, electricity must be the winner.

QrizB

lohr500 said:

But in simple terms, to answer the OP's original question, as @tired_dad is paying 6.92p per kWh for gas and 7p per kWh off peak electricity, then surely he is better off using electricity?
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In the OP's scenario, electricity must be the winner.

Yes, I agree. For the OP it seems off-peak electricity is the best option.

Depending on the size of his tank and the daily HW demand he might still need to reheat the tank with gas in the evening.

tired_dad

QrizB said:

lohr500 said:

But in simple terms, to answer the OP's original question, as @tired_dad is paying 6.92p per kWh for gas and 7p per kWh off peak electricity, then surely he is better off using electricity?
...
In the OP's scenario, electricity must be the winner.

Yes, I agree. For the OP it seems off-peak electricity is the best option.

Depending on the size of his tank and the daily HW demand he might still need to reheat the tank with gas in the evening.

so this is where I was heading. Use off peak electricity to heat the water 0400-0530.
During the day, excess solar to go back to grid for FIT
After morning use gas to top up the HW through the day

matelodave

Do you really need to top up the tank all day. we can get away with once a day, or at a pinch once every two days and that gives us both a shower each a day. 

Personally I'd  try heating it during the off-peak period and see how you get on and, if necessary, just give it a short top up in the evening if you need more hot water for evening ablutions. 

Leaving it to keep the tank topped up just wastes more energy as the boiler ends up stopping a starting several times a day to maintain the temp. If we leaves ours on to keep the tank topped up, it nearly triples the cost for no real benefit.

Try to be a bit more frugal with your hot water. Take shorter showers (five minutes or less)  or shallower baths, fit flow restrictors or an eco shower head. Dont run the hot tap just to rinse stuff (including your hands) .

Remember, every time you run-off half a gallon to get the tap water hot, you leave half a gallon of expensive hot water sitting in the pipes to get cold.

FreeBear

matelodave said: Do you really need to top up the tank all day. we can get away with once a day, or at a pinch once every two days and that gives us both a shower each a day.

When I had a hot water tank, the boiler would only get turned on when I needed a bath. The rest of the time, an electric shower and a kettle supplied all the hot water needed.

matelodave

As we've got a heatpump it only uses about a kwh a day to heat our tank  to 45degree so it's really not a problem. Dishwashing gets done in the dishwasher and if we need really hot water for a manky saucepan etc then its the kettle.

If I leave my heatpump on to top up the tank, it will try to reheat possibly three times a day but it then uses around 3kwh a day