Under control?

As a result, you have to leave the job and face long delays at the wholesaler’s counter trying to get your hands on a suitable replacement. Assuming that such a part is available, the delay could cost you several hours – hours that are charged to the customer. If you had been carrying a replacement unit you could have finished the job and moved on to others, enhancing your reputation by fixing the customer’s central heating quickly and cheaply.

The problem for contractors is that most manufacturers offer a bewildering array of thermostats and controllers with differing capabilities and specifications. Any contractor that sought to meet all possible eventualities would soon need a bigger van.

If a thermostat or controller is to earn a place in the back of a contractor’s van, there must be a good chance that it will be used in the foreseeable future. The ideal situation for most contractors would be to carry a small range of products that are flexible enough to cover most situations. But in practice this is difficult to achieve because of the variety of products on offer.

Expanding ranges
Take the example of a room thermostat. Most manufacturers supply at least two models – some as many as a dozen – differentiated by a number of factors. The switch may be single-pole, single-throw or single-pole, double-throw. The switching voltage may be anything from 12-240 Vac. The thermostat may or may not support frost protection.

Thermostats are further differentiated by their operating technology. Electromechanical thermostats may be bimetallic strip or bellows models. To improve the efficacy of the temperature control, contractors may wish to upgrade bimetallic strip thermostats to bellows units, but a direct replacement might not be possible because the wiring may be different.

Many bimetallic strip models are equipped with heat shunt accelerators. These are small resistors in series with the switch in the thermostat that compensate for the inherent heat lag of the device. The accelerator stops the temperature overshooting the thermostat’s setpoint.

A bellows model is more responsive to temperature changes and does not necessarily need a heat shunt accelerator, although many manufacturers supply models with them. When it comes to replacing a bimetallic strip thermostat with a bellows model, the contractor may find that there is no neutral wire necessary to wire the heat shunt.

A replacement bellows unit without a heat shunt will be suitable for more installations than a product that does have one. Contractors will be able to improve the efficacy of the temperature control by simply changing the thermostat.

Take control
For controllers the situation is a little more complex. The extent to which a manufacturer’s range can be reduced is limited by several factors. For example, if both central heating and hot water must be controlled, a programmer with more than one output is necessary. If a single output is sufficient, say for central heating control only, a time switch can be used.

Then there is the matter of mechanical or electronic timers. Mechanical models still represent around 40% of the replacement market, but you could choose an electronic model with more flexible programming and a more informative and accurate digital display.

Finally, to complicate matters further there are three forms of controller backplate on the market. The original controller may have a proprietary backplate with its own pattern; it may be attached to one of the relics of previous attempts to introduce an industry-standard backplate; or possibly to the industry-standard backplate that appeared in 1968, driven by the success of Landis & Gyr’s products in the market.

The result is that some manufacturers’ products can easily be exchanged with those from others, and some cannot. Products from Siemens Building Technologies, for example, will integrate mechanically with any industry-standard backplate; its programmers will also integrate electrically.

To replace units on industry-standard backplates, contractors need carry only two electronic units – a time switch with a single output and a programmer with multiple outputs. Both of these should be programmable for daily (the same on/off cycles every day), weekday and weekend, or seven-day (different cycles every day) operation. Add a pair of mechanical timers, and a contractor will be able to meet almost any conceivable demand.

Another benefit is that all four units share the same footprint and simply clip onto identical backplates. They can be interchanged to upgrade or downgrade an installation should the customer so desire. It may even be possible to convince customers to upgrade while you are at their homes fixing a central heating problem (see ‘Upselling heating controllers’, below).

Wiring matters
The final element in the trouble-free replacement of central heating thermostats or controllers is accurate wiring information, particularly when replacing a device from one manufacturer with one from another. Siemens Building Technologies offers a free Professional interchange guide for contractors who plan to fit its products.

By cutting the number of thermostats and controllers in their ranges, manufacturers could make life easier for installers and homeowners. It is possible to cover almost every possibility with only a single room thermostat and four controllers.