Electrical supply for 9kW Electric flow boiler

[ccam108]

Just to note, my figures are for a 1950's flat with insulated cavity walls and double glazing, but with an open chimney with exceptional draw to pull the hot air out!

 

[Steve]

Heres a breakdown of my energy over the last 12 months. I live alone, in a 2 bedroom semi with good loft insulation but solid walls. I have an 11kw gas boiler, 20 years old, and use it to heat a cylinder of hot water every day, as well as heating the radiators.

Gas: 18000 units. First 2600 @ 7p, 3p thereafter.
£182+£462 = £644.

Electric costs, for me, 22p first 900, then 11p.

Based on the fact that a unit of electric is equal to a unit of gas, energy wise, and a gas wet system will have similar losses in the pipework as an electric wet system, here are the heating costs for electric:

900x0.22 = £198
17100x0.11 = £1881.

Over £2000.

Of course, your figures, being for a flat, will be lower, but a modified/custom gas boiler installation would pay for itself in a year or 2 based on this. Get it done right.

If you do have the electric system, do enquire with your supplier about electric heat tarrifs, or economy 7. I guess it would be possible to run the electric boiler at night, maybe using a thermal store. Again, all this will be disruptive, maybe erring towards a gas installation.

My cousin lives in a modern flat, never runs his heating because his flat is between 3 others and they must have theirs on very hot.

 

[Jack155Q4]

Of course, your figures, being for a flat, will be lower, but a modified/custom gas boiler installation would pay for itself in a year or 2 based on this. Get it done right.

If you do have the electric system, do enquire with your supplier about electric heat tarrifs, or economy 7. I guess it would be possible to run the electric boiler at night, maybe using a thermal store. Again, all this will be disruptive, maybe erring towards a gas installation.

My cousin lives in a modern flat, never runs his heating because his flat is between 3 others and they must have theirs on very hot.

Our flat is between 2 others so that helps which is why we don't use the heating much.
At the moment we get the boiler to do one cycle ever two days to give us 2 days of hot water. Going to call the electric company tomorrow to see what options there are.

 

[Stoday]

Based on the fact that a unit of electric is equal to a unit of gas, energy wise, and a gas wet system will have similar losses in the pipework as an electric wet system, here are the heating costs for electric:

Get it done right.

Indeed. Getting it done right means making the comparison fair and not missing out costs to make the electric choice seem so much more expensive.

The comparison should be between an optimum electrical and gas systems. For a small one-bed modern (post 2005) flat the electric system would comprise panel heaters and an immersion heater in a tank. The gas option would comprise a combi boiler supplying radiators and hot water directly.

So to the comparison. First, a kWh of gas is not the same as a kWh of electricity.

You are charged per kWh of gas, but it’s impossible to get all this heat from the gas even if you had a boiler with 100% combustion efficiency. The “tariff” calorific value is the sum of the CVs of each component of the gas. The CV of each is obtained from a perfect mixture of the component and oxygen with start and finish temperatures of 25°C. That’s greater than the gross CV which is based on a 60°C finish temperature, dumping the warmed nitrogen in the air and recovering all the latent heat of vaporisation of the water produced by combustion.

So seasonally, you’d be lucky to get 90% of the kWh you’ve paid for in the gas even with a boiler of 95% combustion efficiency measured by flue gas analysis.

A forgotten gas cost is the electricity needed to run the water pump and the boiler controls, say 150W. At 30% duty factor that will amount to 394 kWh/year, £43 @ 11p/kWh.

The electric system does not need an annual service. There are no wearing parts and there’s little to go wrong after it’s commissioned. On the other hand gas needs at least an annual inspection, costing (say) £80 (or a lot more is you use BG )

The capital cost of the gas installation is far greater than the electric option. Let’s say £2000 extra (including a credit for the water tank that’s not needed). With an 8% mortgage that’ll cost £160.

The electric system will last; the gas boiler will probably need to be replaced in 10 year’s time. If the boiler costs £2000 installed, that’s another £200 a year to put towards the boiler replacement cost.

The first block of high-priced kWh is tantamount to a standing charge. You’ll always use the 900 kWh in a year on lights TVs etc. Electric space heating won’t increase the number of high cost kWh anymore than they would affect a standing charge.

Finally, if you don’t need gas for space heating and you cook with electricity, you can save the gas standing charge. Another £104 saved.

So let’s see a less biased version of Steve’s calculation:

Gas space heating cost:

Gas: 18000 kWh. First 2600 @ 7p, 3p thereafter. £182+£462 = £644.
Plus cost of electricity for pump etc. 394 @ 11p = £43
Plus cost of servicing = £80
Plus extra mortgage cost = £160
Plus contribution to boiler replacement cost = £200
Plus extra cost of 1st block of kWh of gas 2600 X (0.07 – 0.03) = £104
Total gas cost: = £1231.

Now the extra cost for electric heating:

18000 X 90% X 0.11 = £1782

Electricity costs £551 more than gas.

That’s for a semi built to the thermal insulation standards two or more decades ago. You’d expect a modern flat to need (say) a third of Steve’s energy requirement. The comparison then becomes:

Gas 6000 kWh. First 2600 @ 7p, 3p thereafter. £182+£102 = £284.
Other gas costs as before

Total gas cost: = £871.

Now the extra cost for electric heating:

6000 X 90% X 0.11 = £594

Electricity costs £277 LESS than gas.

.

 

[Jack155Q4]

Many thanks for the calculations. The real cost is very interesting.

The quotes for a new gas boiler is £3000 + all the work to get the flue through the bedroom plus moving the gas supply if it went on the outside wall and finding a drain somewhere for the condensate so maybe £4000 (a lot of the work would be one off though.

 

[ban-all-sheds]

Have you also got an electric shower?

 

[Jack155Q4]

No.