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Condensing boilers and domestic heating controlsDecember 2009


Are controls such as TRVs still suitable when fitting condensing boilers?
Building Regulations have now effectively made condensing boilers mandatory for most domestic installations, and there is little doubt that this will have improved energy efficiency in most houses when replacing an existing conventional boiler. Seen both from the installer and householder's perspective, replacement is simple - the only change is the need for an additional pipe to drain condensate.

But is this drop-in replacement the right approach in all cases?

Historically, boilers had to be designed to avoid condensation in the flue. Traditional radiator systems were designed on the assumption that the boiler supplied a constant, quite high temperature output. As a consequence, for many years Part L focussed on the use of controls to limit the potential output of the radiators connected to the heating system. Thus there has been an escalating requirement for zone controls, often achieved using TRVs (thermostatic radiator valves), the application of domestic hot water thermostats and timers in non-combi applications, and the installation of a wall thermostat. All of these were intended to limit the possibility of overheating in the dwelling.

Condensing boilers work at their highest efficiency when they have low system temperatures, particularly in the return, but we are connecting them to existing heating and control systems that are not designed to provide the low return temperatures. The Ferrari pulling the horse cart?

It is time to radically change our approach to domestic controls. Instead of limiting the possibility of overheating by controlling the output of the radiators, we should be reducing the system temperatures. This would have the same effect in reducing overheating, but would allow the boiler to operate in a more efficient - i.e. fully condensing - mode.

The marginal increase in performance will not be huge - perhaps measured in a few percentage points - but multiplied over the millions of installed systems this represents a huge national saving.

So what is preventing a change?

The answer is pretty simple: "we just don't do it that way". Both consumers and installers are notoriously conservative, and with good reason since being at the bleeding edge of technology is often a very uncomfortable place to be. Time, though, does not stand still, and eventually the need for change will be unavoidable.

It is not just condensing boilers that will benefit from lowering system temperatures. All low temperature heating systems including heat pumps, solar thermal, and distributed heat (district heating) applications need to employ low system temperatures for maximum efficiency. We need to gear up our installers and suppliers to provide optimal controls to go with the new and growing market for low carbon heat generators.

There are a variety of possible means to achieve these aims. In Germany, it is mandatory for domestic boilers to be installed with an outside air temperature compensation control. This could work in the UK, indeed, a number of boiler manufacturers offer this as an option but the take-up is believed to be low. There are several alternative ways of adjusting system temperature to match load with varying degrees of sophistication and these have been used successfully in commercial systems for many years. With the advent of new technology "smart" communicating control devices the opportunities for innovation are considerable in what is likely to be a mass market in the future.

Even without new and clever innovations retrofitting compensation devices alongside a replacement boiler or in a newbuild installation can be cheap, simple and usually requires no pipework modification. A profit to the installer and excellent payback for the householder. What is needed is the courage to change and the training to deliver. It would be good if legislation weren't required this time to make it happen.

This article was first published in H&V News.

BSRIA provides independent testing and certification and market intelligence for building services products, including boilers and radiators.
 

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