Immersion heater v. Gas heating

Ciprico

I'm on the octopus Go tariff as we have an EV.

I am wondering if it would be cheaper to heat the immersion heater or use the gas boiler

Either way one full tank of hot water generated between 12:30 and 04:00 will last us all day...

There are too many unknowns for me to calulate if there is much of a cost difference.

For example I suppose an immersion heater is 100% efficient whereas our old gas boiler isn't, so the sums are not as simple as comparing unit prices....

Are there any rules of thumb out there...

Netexporter

If it isn't a condensing boiler, so perhaps 60% efficient, then I don't think there is much in it. You won't be emitting as much carbon by using electricity, as the grid is generally pretty green during the Go hours.

Rodders53

Without more information one cannot compute.

What is the unit rate per kWh for leccy at that time?  What is the gas kWh rate?  

Divide gas kWh rate by 0.9 and that would be a rough equivalent to the leccy to compare for a modern condensing boiler.
Choose another figure depending on how inefficient your gas boiler is... e.g 0.8 for 80%.

60% is extremely pessimistic IMHO - even my late 1980s-installed Potterton Netaheat was nearer to 90% than 60%!

Many would advise keeping the boiler to heat water over the summer as they have moving parts that can cause issues come winter if unused.  Reheat time will typically be much shorter, too.

coffeehound

Rodders53 said:

60% is extremely pessimistic IMHO - even my late 1980s-installed Potterton Netaheat was nearer to 90% than 60%!

Reminds me of this old thread where a poster measured his tank temperature changes and compared with gas consumption.  It was a very back-of-the-envelope calculation, and he was only measuring temperature at the top of the tank which doesn't account for the temperature below, but seemed to be 8 kWh of gas for 1.5 kWh of hot water

Veteransaver

Condensing boilers don't really condense whilst heating the hot water, that and the large run of pipework to and from the cylinder means the efficiency isn't great. 60% isn't unreasonably low imo.
Now the benefit of an immersion heater if you are only a low user is that it only heats the top of the tank. 
So yes it can be cheaper to use immersion on octopus go Vs gas.

Rodders53

coffeehound said: but seemed to be 8 kWh of gas for 1.5 kWh of hot water

But no mention of how much electric was needed for the same amount of water via an immersion element.  It won't be just the 1.5kW of hot water!  All HW cylinders lose heat. 

My 210 litre unvented has a maker figure of 81 Watts = 1.95 kWh every day.  That's 2kWh of energy just to make up for the standing losses.

Similarly there's no need to heat water to 65C.  Legionella is killed at 55C it just takes longer.  (Plus Legionella risk in sealed domestic water systems is low).  That may allow a boiler to be in condensing mode / be operated at lower temperature anyway?

My tank reheat time is quoted as 23 minutes via the coil or 175 minutes via the 3kW immersion at the bottom of the tank.  So a part-tank of water will probably have a low enough return temperature to be condensing for at least some of that short time?

Anyhoo.... a smart gas and electric meter should be able to settle the argument once and for all with some careful timing and measurements via the 30 minute monitoring apps?  (As HW will be pretty much the only gas use now that summer is almost here).

FreeBear

Rodders53 said: Similarly there's no need to heat water to 65C.  Legionella is killed at 55C it just takes longer.  (Plus Legionella risk in sealed domestic water systems is low).  That may allow a boiler to be in condensing mode / be operated at lower temperature anyway?

Even 50°C will kill the bacteria - May take a few hours rather than the minutes it takes at 65°C. But with just some 400 cases reported annually in the UK, the risk is extremely low.

I have my DHW set for ~45°C, and don't subscribe to the notion that it has to be 60°C or more. Only used ~4kWh of gas for 100l for a bath last night (combi boiler). Boiler was running at 60°C, so should have been returning around 85% efficiency.

Scot_39

Homeowners can take the risk.

And of course for a combi - their is no real storage constraint - so even some manufactures sites even suggest mid-high 40s safe.

I have probably the worst case domestic house set-up - loft cold water tank - hot enough in summer for breeding - gravity fed vented tank - vent expansion pipe flow / drop to that cold water tank etc.  So do stick rigidily to the 60C min on immersion thermostat.  

Tennants and landlords cannot - they have IIRC to stick to HSE 60C min.

Any scares and press / HSE incident reports I have read have almost always been related to industrial condensors / cooling towers and ventillation systems - not domestic. The only press report I can recollect - was the case of the elederly man who was thought to have contracted it from his undrained garden hose - but again IIRC testing of the hose wasn't conclusive.

coffeehound

I suspect 60 degrees was adopted as a 'very safe' figure for taking account of the large switching differential with both DHW cylinder- and immerision heater thermostats