Sizing up system savings

Published:  12 October, 2015

It is important that installers supply customers with a correctly sized appliance. David Iszchak identifies considerations necessary for installers to achieve this.

Boiler sizing matters, firstly, to ensure a home is provided with the right comfort levels and, secondly, to provide lower running costs (and sometimes installation costs). If the unit output is too low there will be disappointment with the performance and possibly even discomfort; if output is too high it can lead to low efficiency and extra cost.

The majority of new boiler installations incorporate a combination boiler, which requires relatively high heat input to deliver adequate hot water flow. These can range from around 25kW to 42kW, providing domestic hot water flow rates of between 10 and 16 litres/min at a reasonable temperature rise. However, for most of its operating life that combination boiler is supplying hot water for the heating system.

A typical three bedroom semi-detached house might require 10 to 12kW of heating when the weather is coldest, dropping to 2kW to 3kW when the property is up to temperature or the weather is chilly rather than freezing. In this scenario a high output combi of 36kW and with no hot water demand, simply maintaining a comfortable living temperature in a house already up to temperature and requiring only 3kW, would be 12 times oversized if it could not modulate its heat output.

A boiler with a modulation or turndown ratio of 4:1 will try to adapt but cannot reduce its output below 9kW, even if the heating output was range rated to 12kW. At 9kW it would still be three times the output required. This will result in short cycling, more starts and stops, and rejected energy. A similar boiler with a wider turndown ratio of 10:1 will match load to demand more accurately.

Oversizing a combi for hot water performance is similarly pointless. Why quote a customer for a boiler capable of supplying 16 litres/min when the incoming household mains supply can only achieve nine or 10 litres/min?

The situation changes slightly with a heat only boiler supplying space heating and reheating a hot water store in the typical family property. All the installer has to do is make an accurate assessment of each requirement and add them together, if heating both simultaneously is required.

Traditionally, the kW loading for the space heating is added to that required for domestic hot water reheat over an appropriate time period determined by usage. Things are evolving slowly with regards to the way we control our heating systems.

Nowadays, we fit condensing boilers that become more efficient at lower operating temperatures.

Far more energy is used in space heating than in heating domestic hot water, so if we can operate the heating system at lower temperatures we reduce operating costs. The problem arises, however, when domestic hot water needs to be reheated, which requires a high operating temperature to achieve rapid reheat. A solution is to separate the two loads so the boiler is supplying reheated domestic hot water at a high temperature, followed by water for space heating at a lower temperature. This can be achieved with the latest controls using external sensors and/or internal modulating (not on/off) thermostats.

What is important is that the hot water reheat time is kept to a minimum so as not to allow the comfort level in the property to drop significantly. The cylinder capacity can remain the same but the reheat time can be reduced by a model with a fast recovery coil. Traditionally, reheat times could be 40 to 60 minutes and an allowance was made of around 3kW. Now cylinders are available with coils having five to seven times the transfer capacity, drastically reducing reheat times.

A better spread

Take, for example, a property with a heat and hot water requirement of 15kW when both are operating at times of maximum load. If the hot water and heating requirements are separated, a lower boiler output can be chosen to satisfy the greater of the two loadings – usually the space heating – resulting in an appliance that will better adapt itself to the spread of loading. It is quite possible to purchase a cylinder with a 20kW coil to reduce the hot water heat up time even further, but then the boiler would be sized to suit the load used least and be oversized for the application it serves. Here turndown ratio becomes increasingly important.

Many systems are retrofit, with radiators often oversized on installation. Since then the property may have been improved with double glazing or loft or wall insulation.

Oversized radiators can be factored into the choice of the heating system controls and exploited by installers choosing boilers able to modulate their flow temperature in relation to demand, improving efficiency – controls uplifts under the Energy-related Products Directive (ErP) can aid sales.

Thermostatic radiator valves are not included in the ErP for a reason; they don’t improve the efficiency of the heating appliance but aim to improve room comfort. They reduce the output of oversized radiators in a system, but it is better to reduce the temperature of water going into those radiators, and thus improve efficiency.

Remember also that just because there is an 18kW boiler already installed does not mean the new one has to be of a similar size. One matched more closely to the property’s actual requirements may be cheaper and give the installer an edge in quoting for work.

There will always be customers most interested in the invoice they’re going to have to pay. Equally, there are customers who will appreciate some time spent by an installer assessing the job, giving advice, determining what the customer requires and doing that bit extra (for that bit extra on the invoice). All the installer has to do is decide what type of installer they want to be and what type of customer he or she wants to attract.

David Iszchak is technical trainer at Vokčra

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