Water heating efficiency

Louise3365

Hi, I’ve recently moved into a house with an old gravity fed boiler (downstairs) and an immersion heater upstairs in my wardrobe. 

At my old house we did not have gas and only electricity to the house so this is new to me. 

I’m shocked at how much gas costs especially when trying to keep the house warm and the need for hot water. 

I live alone and my electricity comes in at about 50.00 a month but the gas can be as much as 7.00 a day!! 

I have the heating in a timer usually 3 hours in the early hours of the morning and 4/5 hours in the afternoon/evening. 

I’m on the lowest rate of energy on a flexible rate if 26.76 per kilowatt hour and gas 6.76 per kWh. 

To change to a combo boiler I will need a full rip out off all the pipes etc and cannot afford that at the moment. 

Would it be cheaper for me to use the immersion heat to heat my water or stick with the gas?
any advice greatly appreciated. 

JSHarris

Gas is always cheaper than electricity for heating water.  I suspect you could reduce the cost of your hot water a fair bit by having a look at ways to improve the insulation of the tank and pipework (all usually pretty cheap stuff to sort out) and also by adjusting the hot water temperature down a bit to further reduce heat losses.

As the system is gravity fed there is also a possibility that the hot water is siphoning over into the cold water tank in the loft, which will waste a great deal of heat.  This happens when the water level in the cold tank gets too high (usually because of a leaking or poorly adjusted float valve) so that the overflow pipe that loops up and over into the cold tank can touch the surface of the water in the cold tank (there should be a gap of a couple of inches or more between the end of this pipe and the surface of the water).

The boiler is likely to be inefficient by modern standards, but it should still be cheaper than using the immersion, as long as the system is working properly.

If you could post some photos of the boiler controls, the hot water cylinder and also the cold tank in the loft (I appreciate the latter may be a challenge!) then I'm sure some here can give advice as to how you may be able to make some cost-effective improvements.

matelodave

It takes around 9kwh of energy to heat a hot water tank from cold. Electricity is 100% efficient but costs 27.76p/kwh = approx £2.50 to heat your tank

Whereas gas is a quarter of the cost of leccy at just 6.76p/kwh.

Assuming a manky old boiler with an efficiency of around 50% a kwh would cost you 13.52p therefore, to reheat your tank with 9kwh would cost you about £1.22

So even if you had an off peak electricity tariff at 16-17p/kwh you still be cheaper using gas.

FreeBear

JSHarris said: Gas is always cheaper than electricity for heating water.  I suspect you could reduce the cost of your hot water a fair bit by having a look at ways to improve the insulation of the tank and pipework (all usually pretty cheap stuff to sort out) and also by adjusting the hot water temperature down a bit to further reduce heat losses.

Had an old Baxi back boiler here with an uninsulated hot water cylinder. Fitted TRVs on all the radiators that didn't have one. Put two insulating jackets on the tank to reduce heat loss. Fitted a thermostat on the cylinder which made a noticeable difference in gas consumption - Setting the temperature to 45-50°C also helped to keep gas consumption down. Installed a fully programmable control system that allowed for different temperatures (for both central heating and hot water) throughout the day/week - That was worth a 10-15% saving, but took a while to fine tune. Final saving was to turn the gas off during the summer months except on days when hot water was required - The pilot light was using some 1500KWh annually.

Saw gas consumption drop from 8500KWh down to 3500KWh after doing all of the above. Also been improving the levels of insulation and draught proofing the place which has helped to keep the heat in. This year, I bit the bullet and ripped out the old heating system and fitted a combi boiler along with larger radiators in most of the rooms. Whilst it wasn't cheap, I did all the plumbing work myself which kept the cost down to a manageable level. Too early to say if I'm saving any more gas (probably not, as the heating seems to be on a lot more often).

QrizB

Louise3365 said:

... the gas can be as much as 7.00 a day!!

That would suggest that you're using 100kWh of gas in a day. How big is your new home, how onld and how well insulated?

matelodave said:

Assuming a manky old boiler with an efficiency of around 50% a kwh would cost you 13.52p therefore, to reheat your tank with 9kwh would cost you about £1.22

Which further underlines that the main gas cost is for space heating.

On the other hand, if you were still all-electric those 100kWh/day would be costing you £27, not £7.

Scot_39

matelodave said:

It takes around 9kwh of energy to heat a hot water tank from cold. Electricity is 100% efficient but costs 27.76p/kwh = approx £2.50 to heat your tank

Whereas gas is a quarter of the cost of leccy at just 6.76p/kwh.

Assuming a manky old boiler with an efficiency of around 50% a kwh would cost you 13.52p therefore, to reheat your tank with 9kwh would cost you about £1.22

So even if you had an off peak electricity tariff at 16-17p/kwh you still be cheaper using gas.

Probably wise to be wary of using the off-peak rates in isolation like that.

As to get that 16-17p on off peak electric rate - you then also probably have to pay a premium - e.g. c35-36p peak rate on all day time usage on a standard E7 type multirate deal "balance".

Which can drag the average price paid for electric units up considerably.

And for many without storage heating or home battery type time shifting or an EV or solar - or a combination of the above - it can be diffcult to even have that average E7 unit price beat the normal single rate deals over a full year - and we still see occasional splits like that posted here.  

The average split according to Ofgem for profile class 2  was 60% day - for period ending Nov 2022 - at 16p vs 36p split - thats an average unit price of 28p - 12p higher than off-peak rate - and similar to single rate.  

And the 28p or whatever is what really matters to many - as you cannot easily / cheaply get the 16p without paying the 36p.

Using 16p/kWh off peak vs 36p to simplify maths - not saying thats the only sort of E7 split avaiable

at 20% night average 32p - 16p higher than off-peak - so probably shouldn't be on E7 - but some still are

at 40% night average 28p - 12p higher than off-peak - Ofgem Nov 22 split - often close to breakeven

at 50% night average 26p - 10p higher than off-peak

at 75% night average 21p - 5p higher than off-peak

at 90% night average 18p - 2p higher than off-peak

Until your at a much higher night usage than average Ofgem use figure of 40% (or 42% still used by them for E7 pricing) - looking at night rate in isolation vs gas is therefore kind of misleading.

I suspect many if moved from modern gas to E7 with storage heating may well struggle to get average E7 electric to within 3x price of gas.  Edit - at that sort of off-peak price level - others are cheaper of course - but come with correspondingly higher peak rates - so again balance of use and rates - average price key

Caveats about age and pilot lights etc not withstanding. My parents old gas back boiler would have been over 40 years old by now - as it the combi that replaced it is now over 10 years old.

Gas users pay no such premium on their normal electric consumption to access the 7-7.5p / kWh rates (they do of course pay an additional SC - c30p/day DD - but that doesn't buy much electric at all these days).