Oil-fired combi boilers – prehaps too much of a compromise!
Designed in the 1970’s with few technical advances since: are they really suitable for ‘modern day’ hot water supply requirements?
It’s essential homeowners are aware of the compromises they will have to make when living with an oil-fired combi boiler. Often overlooked are the significant technical/ engineering differences between gas and oil-fired combi boilers.
1. Modulating burner:
– Gas fired combis have modulating burners: when a hot tap is opened, a gas valve opens (modulates), passing more fuel into the burner / boiler to satisfy heating demand.
– Oil fired combis have fixed rate burners: they cope with additional heating demand in two ways:
a. An integral ‘heat store’ makes them physically big – often twice the size of a conventional heat-only boiler.
b. A larger boiler input (kWatts) than is actually required to satisfy central heating demand: typically a 30kW boiler where a 20kW heat-only boiler would surfice. It’s like running a Jaguar V12 when a Nissan Micra would do the job and be far more efficient!
2. Hot water ‘temperature rise’ and ‘water flow rates’:
– Temperature rise and flow rates quoted by manufacturers must be treated with caution. Flow rates of 10 – 13ltrs per minute @ 35°C degree temperature rise above mains water temperature is around the norm in proactice.
– Temperature rise will fall after the integral ‘heat-store’ in the boiler is exhausted.
– Temperature rise will be lower during winter months when the ‘mains in’ water temperature drops below the norm of say 10°C dregrees.
– To fill a bath, 25 gals / 112ltrs will take about 10 minutes, providing no other taps are opened.
– Showers, again temperature and flow rates will change when other taps in the system are opened.
3. Hard water problems:
– Hot water is produced via a small plate heat exchanger. These get scaled-up just like a kettle and often need replacing.
– A mechanical water softener is essential. They take up space. Magnetic / electric in-line scale reducers are available but there is no scientific evidence to prove they work.
4. Hot water back-up:
– If the boiler fails, for example if you run out of fuel, there’s no back up for hot water, as there would be with a conventional hot water cylinder with an immersion heater, this alone can lead to expensive emergency engineer call-outs.
5. Life time costs:
– Combi boilers are complex and have bespoke components: they are more expensive to maintain than a conventional system which uses industry standard components.
So what’s the answer?
Keep it simple! A heat-only boiler with even a small 60ltr pressurised hot water cylinder will provide nearly twice the hot water storage of a combi boiler.
There’s no need for an airing cupboard and at just 475mm x 600mm a 60 litre cylinder would work along side our Kitchen, External or Wall Mounted Boiler Models: a similar foot print to a combi boiler, a far better solution!
For more information see Sealed Heating System unvented in Heating Systems Explained. Download ‘Are oil-fired combi boilers a compromise‘.