If you’re replacing an old boiler it’s a good opportunity to review and upgrade your existing heating system, here’s an overview of the pros and cons of typical heating systems.
SEALED SYSTEM, UNVENTED HOT WATER DHW
A sealed heating system with an unvented domestic hot water tank is the optimum system for a modern installation.
PROS:
A pressurised hot water cylinder such as the Heatrae Sadia Megaflow will provide up to 72 litres per minute of hot water at mains pressure.
This is the best engineered solution with low life-time maintenance costs.
There are no tanks in the loft to protect from frost, cylinders can be located at ground floor level, for example alongside the boiler.
SEALED SYSTEM, VENTED HOT WATER DHW
A sealed heating system incorporates an expansion vessel, this replaces the feed and expansion tank in the loft. PROS:
This system is an increasingly popular choice due to the simplified design, and there’s one less tank in the loft to protect from frost.
A ‘vented’ domestic hot water system can be used where mains water pressure is too low for the installation of a pressurised hot water cylinder.
FULLY PUMPED OPEN VENTED SYSTEM
In a fully pumped open vented system, water is ‘pumped’ around the hot water and central heating circuits. A motorised valve controls the flow of water, either to the ‘hot water’ circuit or the ‘central heating’ circuit. Water tanks in the loft replenish water in the system, with vent pipes above, a safety feature which allows for overflow or expansion of water in the system.
GRAVITY DOMESTIC HOT WATER
In gravity hot water systems, heating of domestic hot water relies on convective action to circulate hot water from the boiler to the cylinder. A gravity system is easily identified, there will be four pipes connected to the boiler.
CONS: Although simple it’s not efficient and typically will suffer from the following problems:
In a conventional boiler, fuel is burned and the hot gases produced are passed through a heat exchanger where much of their heat is transferred to water, thus raising the water’s temperature. One of the hot gases produced in the combustion process is water vapour (steam), which arises from burning the hydrogen content of the fuel.
Condensing boilers extracts additional heat from the waste gases by condensing the water vapour to liquid water, thus recovering its latent heat. The effectiveness of this condensing process varies, it depends upon the temperature of the water returning to the boiler, but for the same conditions, it is always at least as efficient as a non-condensing boiler.
It is normal for a condensing boiler to produce a plume of water vapour from the flue terminal; it demonstrates the boiler is working extremely efficiently and may be more prevalent when the boiler starts up. Dependent on the heating system, about 1 litre of condensate per hour is produced. Condensate will have a pH value in the range of 3.5-5, which is about the same acidity as tomato juice.
A non-condensing boiler is similar to condensing boilers except they do not have a stainless steel secondary heat exchanger.
A modern non-condensing boiler running on oil will be around 4% less efficient than an equivalent condensing boiler model.
When installing non-condensing boilers, a Building Regulations exemption is required, please refer to our installation guide documents.
Combination boilers, commonly called combi boilers, are similar to conventional boilers with a second heating circuit which instantaneously produces hot water. Often overlooked is the fact that the operating / engineering differences between gas and oil fired combi boilers is significant.
Discover why Hounsfield Boilers do not manufacture combi boilers
Designed in the 1970’s, with few technical advances since, are they really suitable for “modern-day” hot water supply requirements?
It’s essential that homeowners are fully 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.
Consider:
1. High life-time costs:
– Unlike a gas combi, water is stored in the boiler at circa 80°C; components subjected to this constant residual temperature will fail.
– Oil-fired combi boilers are complex, have bespoke components; they will be more expensive to maintain than a conventional system.
– Good business for boiler manufacturers, not necessarily the best option for the homeowner!
2. 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 increased heating demand.
– Oil-fired combis have fixed output burners; they cope with additional heating demand in two ways:
An integral “heat store” making them physically big – often twice the size of a conventional heat-only boiler.
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 suffice. It’s like running a Jaguar V12 when a Nissan Micra would do the job just as well and be far more efficient!
3. Hot water “temperature rise” and “water flow rates”:
– Temperature rise and flow rates quoted by manufacturers must be treated with caution. Flow rates of 10-13 litres per minute at a @ 35oC degree temperature rise above mains water temperature are typical in practice.
The temperature rise will fall once the integral “heat-store” in the boiler is exhausted.
“Temperature rise” will be lower during the winter months when the “mains in” water temperature drops below the norm of say 10oC.
To fill a bath, 25 gals / 112 litres will take about 10 minutes, provided no other taps are open.
– Showers, again, temperature and flow rates will change when other taps in the system are opened.
4. 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.
The photo below is of a plate heat exchanger, cross-sectioned, to show the cold water channels blocked with limescale.
At just 80 x 80 x 180 mm (3.5” x 3.5” x 7” inches), a combi boiler relies on this component to produce all hot water requirements.
Plate heat exchanger blocked with limescale
5. 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 that uses industry standard components.
So, what’s the solution?
Keep it simple! A heat-only boiler, even with a small 60-litre 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 alongside our Kitchen, External or Wall Mounted Models; a similar foot print to a combi boiler, a far better solution!
Some customers have built external housings to accommodate a boiler and hot water cylinder or located the cylinder in a garage.
Pressurised cylinders such as the Megaflow or Stelflow will provide many years of maintenance free service.
System boilers are designed to reduce installation time, they contain all the heating system components within the boiler casing, a circulating pump, pressure relief valve, and expansion vessel.
The expansion vessel is often undersized, simply to fit into the boiler casing rather than suit system water capacity.
Residual heat in the mass of an “oil fired system” boiler will cause components to fail, making system boilers expensive to maintain. This is why Hounsfield Boilers do not manufacture system boilers.
5 Year Five Point Oil Boiler Warranty…. We like to keep things simple! Terms:
✅ 1. Install the supplied oil filter kit and fire valve. ✅ 2. The life-long Teflon cored stainless flexible hose must be retained. ✅ 3. Register warranty within 30 days of installation. ✅ 4. The boiler must be serviced annually. ✅ 5. Warranty work must be approved and authorised by H.B.Ltd..
Properly engineered product does not require reams of small print or get-out clauses!
Our industry-leading warranty uniquely covers all components for five years, except for service items, rather than specific components.
For heat exchanger replacement after the warranty period, a discounted pricing structure based on age is applied, the full price is only chargeable in year eleven.
