Question from Sam Stephenson, via e-mail:
I like the idea of a thermal store or ‘water jacketed tube heater,’ are there many non unvented systems on the market? And why aren’t they used more?
Answer from Martyn Bridges, director of marketing and technical support and Worcester, Bosch Group:
Worcester has been making thermal store-style combi boilers for nearly 40 years, today’s models are in gas-fired products the Greenstar Highflow series and in oil-fired the Greenstar Heatslave. The primary side or heating side of the appliance is generally under a sealed primary water system however, with the Oil-Fired Heatslave this can be connected to an Open Vent system if preferred. The domestic hot water side is of course mains pressure fed and can deliver flow rates from 15 to 25 litres per minute.
The system is a great success particularly in larger homes and I would estimate there are probably 500,000 installed in peoples’ homes today. The downside of the concept is generally space and the trend for boilers to be wall-hung as opposed to floor-standing. Because of the weight and size of not only the product but the water within, it is not practical to have a wall-mounted version and also it is now possible to get instantaneous combi’s up to outputs around 40kW. With these high outputs the flow rate of combi’s can be up to comparable rates of thermal store combi’s at around 16 – 17 litres per minute.