Efficiency of heating DHW by gas

Sterlingtimes

Some one on this forum wrote a detailed and excellent posting about the above topic. Despite spending some time searching, I cannot find it. Any pointers please?

HappyMJ

What's the query? Yes heating water by gas is less efficient than heating water by electricity but despite that it's much cheaper even if you were to heat the water overnight on a cheap E7 tariff. You would also need to calculate the higher cost of daytime electricity as well and the occasional boost that may be required in the evening. A electric only system generally has a bigger cylinder so more water is heated at the overnight cheap rate so a boost isn't required. There are a lot of variables it's very hard to calculate the savings.

To figure out the exact efficiency of heating water by gas you would need to look at the manual of your boiler which should give you an idea. If it's a heat only boiler heating the water stored in a cylinder then you'll need to know how much heat is being lost from the cylinder too.

Sterlingtimes

HappyMJ wrote: »

What's the query?

Than you, HappyMJ.

The boiler is as follows:

The Ideal Classic FF270 is an 'E' rated boiler with an efficiency of 77.2%.The Classic FF270 is a Non-Condensing Conventional boiler with a maximum power output of 20.5kw that uses Gas as its fuel source. Being a 'E' rated boiler it would be worthwhile replacing it with a higher efficiency 'A' rated boiler.

But is 77.2% a good indication of the real efficiency for heating my hot water by gas? I rather presume that there are losses beyond the periphery of the boiler.

Above 62.5% efficiency it is cheaper for me to heat by gas and take a solar export income of 4.85 p per kWh.

Below 62.5% efficiency it is cheaper for me to heat by solar generated electricity.

Hence if my efficiency is less than more 62.5% I should switch off the immersion diverter (iBoot).

So what is my real efficiency likely to be?

michaels

Most solar systems don't measure how much of the electricity produced is actaully used, just pay an export tariff on 50%. This means that effectively any amount used has no cost to you so put the iBoost on all the time - effectively that is what an iBoost is designed for.

WE will put our gas to 50 degrees (keeping the morning and evening schedule) and the iBoost immersion to 70 degrees so hopefully using as little gas as possible but still always having hot water when we need it. Note that when calculating the returns of the solar install electricity units that relace gas ones should only be valued at the cost of the gas unit replaced - we have a high efficiency boiler so I am estimating about 85% of each gas kwh goes into the hot water.

Sterlingtimes

michaels wrote: »

Most solar systems don't measure how much of the electricity produced is actually used, just pay an export tariff on 50%.

Thank you, but Ovo has moved me from deemed export to measured export so the considerations have changed.

HappyMJ

Sterlingtimes wrote: »

Than you, HappyMJ.

The boiler is as follows:

The Ideal Classic FF270 is an 'E' rated boiler with an efficiency of 77.2%.The Classic FF270 is a Non-Condensing Conventional boiler with a maximum power output of 20.5kw that uses Gas as its fuel source. Being a 'E' rated boiler it would be worthwhile replacing it with a higher efficiency 'A' rated boiler.

But is 77.2% a good indication of the real efficiency for heating my hot water by gas? I rather presume that there are losses beyond the periphery of the boiler.

Above 62.5% efficiency it is cheaper for me to heat by gas and take a solar export income of 4.85 p per kWh.

Below 62.5% efficiency it is cheaper for me to heat by solar generated electricity.

Hence if my efficiency is less than more 62.5% I should switch off the immersion diverter (iBoot).

So what is my real efficiency likely to be?

You will have losses from the cylinder by both methods so I wouldn't take that into account in your calculations. You will have some losses in the pipework leading from the boiler to the cylinder but after a few minutes once all the water in the pipework is up to temperature the losses are small. I wouldn't calculate that either.

It'll be close..I wouldn't bother trying to save a few fractions of a penny I'd just keep the gas heating the water. You should have it running a little bit anyway so the boiler is used. A boiler not used for 6 months has a higher likelihood of breaking/seizing up.

michaels

Sterlingtimes wrote: »

Thank you, but Ovo has moved me from deemed export to measured export so the considerations have changed.

That is interesting, I didn't even know that metering supported that. Is this something I should bear in mind when adding an iBoost to my new system and factoring savings from it?

Andy_WSM

Sterlingtimes wrote: »

Thank you, but Ovo has moved me from deemed export to measured export so the considerations have changed.

Have you been smart meterised?

That puts a spanner in the works for all these immersion diverters then!

Sterlingtimes

michaels wrote: »

That is interesting, I didn't even know that metering supported that. Is this something I should bear in mind when adding an iBoost to my new system and factoring savings from it?

I had the iBoost installed supposedly "free of charge" although the reality is that it is built into the price. The iBoost saving in gas i seven months has been 437 kWh >>> 437/0.77 * 0.0303 = £17. That's almost inconsequential.

But with the installation of my Smart Meter my export is measured accurately and I move to actual export payments. The iBoost is probably now of dubious value.

Once export is measured, there is no financial case for buying an iBoost.

Sterlingtimes

Andy_WSM wrote: »

Have you been smart meterised?

That puts a spanner in the works for all these immersion diverters then!

Exactly!

The rather odd thing is that Ovo thinks (but doesn't know for sure) that I will need to provide manual readings for export from my Smart Meter. The "smart" is a little dumb.

Sterlingtimes

HappyMJ wrote: »

You will have losses from the cylinder by both methods so I wouldn't take that into account in your calculations. You will have some losses in the pipework leading from the boiler to the cylinder but after a few minutes once all the water in the pipework is up to temperature the losses are small. I wouldn't calculate that either.

It'll be close..I wouldn't bother trying to save a few fractions of a penny I'd just keep the gas heating the water. You should have it running a little bit anyway so the boiler is used. A boiler not used for 6 months has a higher likelihood of breaking/seizing up.

Thank you. This was my own thought. If the efficiency for hot water heating is the same as radiator heating then my efficiency may be about been my threshold of 62% and 70%.

In practice, it makes no more that a few pounds difference over the year. I think we still use enough gas to keep the 20 year old boiler (not one fault ever) ticking over.