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M

Matt0029

Gas Engineer
Is an immersion heater on a splan usually just used for back up. Incase the boiler stops working thus no flow through the coil. Wentto a house where the customer keeps the immersion on all year around. So his basically using it as a direct cylinder?
 
C

Chuck

Is an immersion heater on a splan usually just used for back up.
Yes. If the system is sized and setup correctly you should only need to use the electric heater if the boiler is faulty.

Electricity costs ca 5 times as much as gas per kW hr and a typical 3kW immersion heater takes eight times as long to heat the cylinder as a 24kW coil and boiler.
 
Last Plumber

Last Plumber

Esteemed
Plumber
Gas Engineer
If they are heating an older type standard copper cylinder - say 36" x 18" OV- then all that was usually allowed for the coil was around 8000 Btu's. Which is roughly 2.3Kw. It does heat more water than an immersion so there may not be much in it running cost wise. We have a number of customers who find it much of a muchness when using DHW only, in the warmer months. Performance and running cost depends on the use, the length of immersion and where it is sited V the cylinder coil. It's really up to the customer how they use it.
 
J

John.g

For many years, before I installed a modest solar thermal system, I only used a 11" immersion element (summer time) in the top of the HW cylinder which heated ~ 30 litres of hot water every morning, quite sufficient for our needs with a dish washer and electric shower. I know one could heat the whole cylinder with oil/gas every few days but if one took cylinder losses & system losses etc then I don't think there would be a whole lot in it. I just had a 60 min boost on the immersion timed to come in every morning as a 2.5 kw element will heat 30 litres from 10C to 60C in ~ 40 mins at a cost of ~ 30 cents.
 
M

Murdoch

Res.Electrician
The electric immersion should be the back up for the gas ...
 
R

Ric2013

Plumber
Have to agree with Last Plumber. I find my system (with a standard, though Part L compliant coil and 72% SAP seasonal efficiency boiler) can only take the 8kW boiler output for around 20 minutes, then the boiler starts cycling. I estimate the total system efficiency to be around the 25-30% mark (useful heat in cylinder vs chemical energy in gas supplied to house).

I used to think that the inefficiency of the national grid (around 35% of chemical energy in coal etc at power station reching my immersion heater) made gas the more environmentally-friendly option, but I think I was mistaken - plus the decarbonisation of electricity in sunny weather due to solar photovoltaics probably makes the immersion cylinder a better option, environmentally as well as in terms of cost.

To get back to the OP's point, the immersion is usually a backup, though it can also be used in the summer months due to historically low efficiencies of boilers running at part-load. It can also be used at the same time as the boiler, for when you want a bath and don't want to wait (2.3kW from the coil + 3kW from the immersion = 5.3W = less time to wait).
 
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J

John.g

Yes, I would like to be able to monitor meter readings under the above conditions but as I have a oil fired boiler I am unable to do so.
 
Best

Best

Esteemed
Plumber
The Willis external immersion heater is basically a small copper vessel outside the hot cylinder which gives almost instant hot water to top of cylinder.
It uses a 3 kw 11” immersion element.
It has the advantages of rapid heat up and for whatever amount of hot water required - small or large
 
J

John.g

I looked a few times for any info re the design and actual heat rise through it initially...it might appear that it heats the water to ~ 40C which is fine for normal duties but if you require a full cylinder of water at say 60C then it might take a long time to achieve this because once the bottom of the cylinder reaches ~ 30C then the thermostat in the external part will cut off the power (if set to 60C) and slow down the process, it may just need the thermostat set somewhat higher to achieve a cylinder full of water at around 60C.
 

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