Combi boilers are often seen as a heating and hot water cure all, providing more or less instantaneous hot water without the need for a storage and expansion system. However, when specifying for housing where faster flow rates are required, an unvented hot water cylinder may be a more appropriate choice. Alan Clarke, technical support manager at Heatrae Sadia, explains.
What makes combi boilers so popular?
Homeowners expect their system to provide enough hot water for their household, and this is one of the things that makes the combi boiler so attractive.
Add the fact that the combi is easy to install, has no need for hot water controls and is free of a storage tank (freeing up valuable space for the homeowner), and it’s easy to see why it’s so tempting to choose this type of boiler over anything else.
Sounds great. Why wouldn’t I specify a combi?
While combis have been developed to deliver higher flow rates for larger households, they are not always the right solution. This is especially true if a household has multiple bathrooms, if people are using a number of showers and baths simultaneously, or if luxury products like jetted air baths – which require good flow rates to work effectively – are fitted.
If faster flow rates are required a more appropriate solution can be a system boiler coupled with a high-performance unvented hot water storage cylinder.
How does an unvented hot water storage cylinder work?
Fed directly from the cold water mains, unvented systems provide powerful showers and fast filling baths, and flow rates are far faster than with a combi boiler. Our Megaflo Eco can deliver up to 72 litres per minute of hot water, and there won’t be a significant drop in performance if more than one tap or shower is used simultaneously (depending on the mains water pressure and flow rate coming into the property).
What should I look for when selecting a cylinder?
One of the key elements to consider is what an unvented cylinder is made of. Duplex stainless steel is the best option, as it has unique corrosion resistant properties. It is also lightweight, giving it a strength-to-weight ratio advantage.
For longer cylinder life look for a combination of laser and TIG welding, butt welded joints to avoid crevices, and post weld treatments. On an indirect cylinder a Superloy 825 immersion heater is a good sign of quality, and on a direct cylinder, a strong titanium immersion heater is ideal, as it will be used more frequently than on an indirect model.
Is there anything else I should know?
It’s important to check that the mains pressure is at least 1.5 bar and capable of delivering 20 litres of water per minute or more. This is a minimum requirement to provide usable performance for a small property with one bathroom – at lesser values the system will still deliver hot water, however there may some limitations, especially with multiple draw offs. If the mains supply cannot support the minimum requirements, a boosted supply or an accumulator can provide a solution.
