Armstrong Integrated Systems Ltd

Integrated Low & Zero Carbon Heating Design

Getting the Best from Condensing Boilers

In order to achieve best efficiencies in modern heating systems, it’s not simply a matter of replacing a conventional boiler with a condensing boiler. For a condensing boiler to operate in the condensing mode the system conditions need to be right. Three key things influence condensing boiler efficiencies; system temperature, load and the control philosophy.

The system water passing through the boiler must be below the dew point of the flue gases if the boiler is to operate in condensing mode. So this means that the supply and return temperatures should ideally be below 55ºC. If we just stick to the UK’s tried and tested 80ºC supply, 70ºC return we’re missing out on a huge opportunity for energy savings.

Another problem the old 80/70ºC design gives us is flow rate and the ultimate pipe and pump size. Condensing boilers work best on a T of 20ºC to keep pressure drops within acceptable limits. For a load of say 840kW, a system with a 10ºC T would require a flow of 20 l/s whilst at 20ºC the flow would be halved to 10 l/s.

This can have a dramatic impact on pipe and pump size and power costs over the life of the system. Pipe size could reduce from 100mm to 80mm and pump motor size from 5.5kW to 3kW.

Next let’s consider load and control. Conventional (non-condensing) boilers are at their most efficient at high load. So, in multiple boiler installations, it made sense to bring the first boiler up to full load before bringing in the 2nd and 3rd etc. But condensing boilers are at their most efficient at low load whilst condensing boilers are at their most efficient at low load.

So if we adopt a conventional sequencing control philosophy, we’re actually switching condensing boilers at their least efficient point.

The following table shows the effect of temperature and load on the efficiency of a modern condensing boiler. For instance, boiler operating at 80ºC supply and at 100% load will have an efficiency of 87% whilst 4 boilers operating at 50ºC supply and 25% load will have an efficiency of almost 96%

So, to achieve maximum efficiencies and lowest possible running costs, we need to:

1. Design our heating systems to operate at much lower temperatures. Radiators, air handling units, and fan coils can all be selected to operate effectively well below the conventional 80/70ºC and, of course, underfloor heating systems must have low water temperatures. And, if you want a weather compensated system then compensate directly on the boiler not via a 3-way valve.

2. Design with a 20C T. This will halve the flow rate, reduce pipe sizes, reduce pump size and reduce power input.

3. Reconfigure our control philosophies so that we run all boilers at the lowest possible load to meet the system demand. So, we might run 4 boilers at 25% load rather than 1 at 100% load. As well as being energy efficient this also reduces boiler cycling, wear and tear on the boilers and start-up purge losses.

If all of this seems too much of a challenge, pre-engineered systems are available which incorporate condensing boilers and variable speed pumps with the appropriate application specific controls.

Finally, it’s worth considering the integration of condensing boilers with zero carbon solutions like solar thermal, heat pumps, micro CHP and biomass. All of these solutions, with the exception of biomass, can operate and integrate quite successfully with condensing boilers operating below the dew point. Biomass ideally needs to operate at around 80ºC supply but can be successfully integrated with condensing boilers as long as these are connected in a reduced temperature circuit after the buffer vessel. A range of Low and Zero Carbon Solution schematics are available.

Biomass System Diagram

Solar Heating System Diagram

(Click on images for larger display)

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Armstrong Integrated Systems Ltd
Wenlock Way
Manchester
M12 5JL

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