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Oil fired water heaters – from 30 kW to 186 kW Output

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Oil fired hot water heaters / direct fired oil hot water heaters from Flexiheat UK

Oil fired water heaters or oil-fired storage water heaters from Flexiheat UK offer a regular and reliable supply of economical hot water for a variety of applications, such as domestic, commercial and industrial hot water heating applications.


Due to the revised part “L” of the Building Regulations, which defines the criteria for the energy performance and carbon emissions of new and existing buildings- which came into force on the 15/06/2022 – these units are no longer available – as basically condensing units are now required to achieve the efficiency levels required. Please see our webpage –  condensing water heater legislation 

Please now use an oil condensing boiler in conjunction with an indirect hot water cylinder. Or our commercial hot water cylinders range – which have high water recovery rates – meaning you can store less water, but still have good flow rates.

If you can convert to gas either natural gas or LPG (Liquefied Petroleum Gas) then take a look at this webpage – Gas hot water tank or cylinder

Or alternative if you have 3 phase electricity on site – then our Commercial electric water heaters are another option for water heating

When these oil fired water heaters are used in applications where there is a constant demand for hot water or where hot water usage peaks at certain times , for example in hotels , leisure centres, camping sites or in health related buildings such as nursing homes, health centres or hospitals, the fast hot water recovery rates of our storage oil water heaters , ensures you have the hot water when it is required.

The water heaters can be sized to accommodate your specific hot water requirements and also, in many instances, these domestic hot water heaters can be located close to the point of use, and thus eliminating heat losses due to long pipe runs, making your domestic hot water system more efficient and saving on your fuel costs.

Hot Water Heaters – Oil Fired – with Storage- Specifications & Performance

Water Storage Capacitylitres430680850135018002700
Thermal Power InputkW34.852.372.1104.7138.4209.3
Thermal Power OutputkW30.246.56493122.1186.1
Ø Flue Gas Connectionmm152152152152220220
Maximum Operating Water PressureBAR666666
(*) Peak output (ltrs / 10 mins)Litres5759031158179523833589
(*) 1st hour outputL/hr129720152685401753008035
Continuous output ∆t 35 ° CL/hr74211451575229030003500
Continuous output ∆t 30 ° CL/hr86713351835266735005335
Cold Water InletInches1.251.51.522.52.5
Hot Water OutletInches1.251.51.522.52.5
DHW recirculation connectionInches1.251.51.522.52.5
Magnesium Anode ConnectionInches1.
Electric Immersion Heater Connection (optional )Inches1.
Total Heightmm170018002050238024302830
Diameter with insulationmm820970970112012701420
Diameter without insulationmm65080080095011001250
Total Foot Print ( Excluding the oil burner )mm98011301130128014301580
Empty Heater Weight ( Excluding the oil burner )KG160215230310355440

(*): Data calculated with an accumulated water temperature of 60 ° C, with an incoming water temperature of 10° C, and an outlet temperature of 45 ° C

If you have a different water temperature rise, than the above chart, and need to calculate the water flow rate produced, then use the flowing formula –

m (kg/s) = P (kW) / the specific heat of water (Kj / Kg ºC) X ΔT of the water in Degrees C

Whereby – 
m = mass flow rate (kg/s) this is litres per second as one litre of water weighs one kilogram
P= Power in kW
Specific heat of water = 4.187 Kj/Kg ºC
ΔT = temperature increase in C


For our 1000 model oil fired water heater that has a thermal power output of 64 kW , with a temperature rise of 44 degrees C then it would be –
m = (64kW) / ((4.187 kJ/kg ºC) (44ºC))
thus – 64 / 4.187  x 44
64/184.22 = 0.3463 kg/s or 0.3474 Litres a second – times this by 60 seconds to get per minute then by 60 minutes to get the volume per hour – thus 1,250.67 Litres per hour for a 44 Degree C water temperature rise.

We also can provide a top to bottom recirculation pump kit, which has the benefit of boosting the domestic hot water performance by 20% for all models of our oil-fired water heaters range

The top to bottom recirculation kit can also be used to prevent stratification within the hot water heater. It is possible to control the recirculation pump, according to the control strategy deployed on site, or alternatively full-time operation of this kit may be considered should you require a uniform hot water temperature throughout the hot water cylinder.

Download the Oil fired water heaters / Indirect oil fired heaters – “Fire” Series Brochure

Oil Fired Water Heaters – “Fire” Series – Manual from Flexiheat UK- Download

Oil fired hot water heaters – Construction

These oil fired water heaters consists of a water storage tank, made from a top quality and very thick steel casing, which is electrically welded and enamel coated ( featuring a carefully applied, double cryolite glass, which ensure a higher resistance to scaling) to protect it from water induced corrosion. The tank is housed in a stippled metal casing containing the thermally insulating material. The water is heated by a forced draft, oil-fired burner mounted in the bottom section of the water heater which, in turn, is connected to the combustion camber. From the combustion chamber the flue gas venting pipes, which go across the entire width of the tank, and exits at the rear of the water heater. The oil burner is operated by the control thermostat located at the front of the water heater.

Sacrificial anode protection

Our oil-fired storage water heaters are all fitted with removable magnesium sacrificial anodes as standard which ensuring excellent protection against corrosion of the water heater. Clearance is required above the water heater for maintenance and replacement of the magnesium sacrificial anodes. Should you have insufficient clearance above the water heater that will prevent anode replacement using the standard magnesium anodes, then electrical anode protection may be fitted, instead of the magnesium anodes.

Electrical anode protection

Certain areas of the country have particularly soft water and therefore poor conductivity of the water with less than 200 micro-siemens per centimetre, such as Cornwall, Scotland and Devon for example. In these areas of soft water, the magnesium sacrificial anodes may not suffice in providing adequate protection against corrosion of the water heater. In these geographical areas, or other areas with particularly soft water, we insist that you use our electrical anode protection system, as it is more effective at providing protection in these circumstances. It is also to be noted that a constant and uninterrupted power supply is required to these anodes to ensure the full protection of the water heater.

Oil water heaters –  installation requirements

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Dimensions of our Oil Fired Water Heaters


These oil fired hot water heaters are fitted with burners that are designed to be fuelled by 35 second Class D oil only, which is more commonly referred to as gas oil or red diesel. This gas oil is the  most commonly used oil the commercial and agricultural industries. The oil burners we use are not designed to use Kerosene as a fuel supply, which is sometimes known as 28-second oil or home heating oil. Kerosene oil is primarily used for home heating


These oil fired water heaters can only be used inside permanently ventilated rooms; more specifically, the room must be ventilated with the quantity of air required by all the appliances installed in within the room. The air must enter through permanent openings made on the outside walls of the room to be ventilated. These openings must have a net free section of at least 6 cm2 for each kW installed on the appliance, with a minimum of 100 cm2. The openings must be made in such a way as to avoid obstructions both from inside and outside of the wall. Grilles, wired mesh etc. may be used to protect the openings, without reducing the free net section mentioned above. These openings must be positioned as close as possible to the ground and they must not interfere with the flue gas vents.


This range of oil fired water heaters has been designed to be fixed mounted to the floor. If the floor is made of wood or of a synthetic material, the water heater must be positioned on a fire retardant base. If the appliance is to be installed in an enclosed room without windows, adequate, fresh ventilation must be provided. An inadequate ventilation can cause fire, explosion or suffocation. Please refer to the national or local legislation applicable.

Maintenance of the water heater

The full range of these hot water oil heaters have a system which allows for the inspection and cleaning of the water tanks interior, which is required by the recommendations of the Health and Safety Commission for the control of Legionellosis, which can lead to Legionnaires disease.

The quantity of scale to be expected will depend on the frequency of use of the water heater and on the water temperature set on the thermostat. It is advisable to set the oil-fired water heater temperature to 60° C in order to reduce scale build-up and to comply with the anti-legionella regulations.

You extract the heat exchanger from the tank, and clean any limescale deposits on the heat exchanger pipes, and remove any limescale deposits from the bottom of the water tank.
From 1,500 litres capacity upwards, these indirect oil-fired water heater’s casing is fitted with an access door used for tank inspection and descaling. Drain the water heater before opening the internal flange.

Flue Pipe

The flue gas venting pipe must have the diameter shown in the technical table and must be resistant to acid corrosion caused by condensation. For these oil-fired water heaters, this flue pipe must be made of stainless steel,and preferably an insulated flue system.It is mandatory that the evacuation of combustible gases is carried out with a flue pipe system having a diameter no less than the flue connection on the water heater and that it is connected to a flue system suitable for the potential of the oil-fired water heater.


The standard burner mounting plate of water heater ensures the oil burner is an easy and quick installation. Also should you ever need or decide to change your fuel supply to the water heater e.g. natural gas or LPG, then you just need to install the relevant burner to the unit.

Frequently asked questions ?

1.Oil fired water heater vs electric ?

As at December 2019, the price of oil per kW hour, as compared to electricity per kW hour is around 60% cheaper, so the running costs for oil fired hot water heater will be less. Generally, though the purchase price for oil fired water heaters are more than their electrical equivalents. The other thing to note is that oil water heaters have a much better hot water performance with regards to heat up times of the water, as opposed to electric whereby you require normally a three-phase electrical supply and a serious amount of Amps to get the equivalent performance. Oil just wins hands down on this aspect.

2.Oil fired water heater life expectancy ?

Typically, the life expectancy of a good quality oil fired hot water heater should be at least 10 years especially when you have either sacrificial anodes or electrically powered Correx anodes fitted to the units. The better you protect your water heater especially from “soft water” the longer it will last. Obviously using more hot water increases the duty cycles of the water heater and thus exposes the water tank to the extra risk of corrosion if not protected correctly. The quantity of hot water used, determines how hard the water heater works. This is often referred to as the “duty cycle” or in plain English- the level of usage of the water heater – in essence the more “use” equates to a “shorter life”.

3.Oil fired water heater efficiency ?

Oil Fired Hot Water Heaters, from Flexiheat UK are a minimum of 84% efficient, and with today’s low NOx oil burners fitted, they emit less than 120 mg/kWh of nitrogen oxide emissions, thus complying with the European Union ErP directive 2018 for water heaters, which is designed to ensure these products more environmentally friendly.

Oil fired storage water heaters – Vented or Unvented

This range of oil-fired hot water heaters are suitable for unvented or vented installation. When used in an unvented installation, the heater must be equipped with an unvented hot water kit, which normally consist of an expansion vessel, pressure limiting valve, expansion relief valve, check valve, line strainer and a combined temperature and pressure release valve. The unvented kit allows you to feed these oil fired water heaters directly form the mains fed water, which has numerous advantages:

• Improved operating pressure of the domestic hot water system
• Elimination of the cold feed header tank and thus the risk of it freezing and bursting in cold weather and the associated pipework involved
• Reduction in the hot water system pipework diameter

The potable hot water expansion vessel should be sized no lower than 4% of the total capacity of the storage water heater , but you also need to account for the hot water volume in the system as well , as a general rule of thumb to account for this , people tend to use 8% of the capacity of the heater , as an allowance for the hot water system volume and the water heater volume. But if you’re in any doubt, please email us, as we have can send you a potable expansion vessel sizing calculation tool.

Also, to comply with the Part G (Sanitation, Hot Water Safety, and Water Efficiency) water regulations 2010, for unvented water storage heaters, you would have to install a combined temperature and pressure relief valve to the oil fired water heater

These unvented kits and temperature and pressure relief valaves are not supplied by us but are readily available from any plumbers’ merchants.

Efficiency and ErP of our Oil fired hot water heaters

The minimum efficiency of our oil water heaters is 86% for our smallest water heater and 88% for all other models .All of our water heaters are fitted with oil burners that emit less than 120 mg/kWh of nitrogen oxide emission levels which is commonly referred to as NOx levels. This is part of new European Union “ErP” directive which came into force on the 26th September 2018 which is designed to ensure that products are more energy-efficient and more environmentally friendly.