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44 AUGUST 2014 | FUEL OIL NEWS | www.fueloilnews.com By GeorGe Lanthier Voltage Changes HVAC/ HYDRONICS T here have been a lot of changes in technology and equipment in the last 30 years, and even more so in the last 10 years. One of those changes is in the voltage present on warm air furnace high limit controls. In addition, the issue seems to be crop- ping up more and more, and I'm also hearing of some dam- age done to heating equipment simply because we make too many assumptions. Since it is a growing issue that I get a lot of questions about in seminars and also by email, I thought I would take a crack at it in an article since Lanthier's Second Law is "assume nothing." The issue is the voltage quantity that is carried across a furnace high limit switch. In addition, most of the cases I've heard about are when a 'fan center relay' has been used (Figure 1). Many new furnaces have been coming through since the 1990s with an electronic control board (Figure 2), but many furnaces are still equipped with the conventional fan and limit switch (Figure 3) and that seems to be the prob- lem with many who have to learn another fuel and the wiring changes involved. We're not just training oilmen that now want to learn gas, but many others who want to learn oil. In the traditional method of doing things, and this has gone on for ages, furnace manufacturers have used the fan and limit control and supplied line voltage, 120 Vac, to the control for both operation of the oilburner and system blower (Figure 4). With gas you'd also find similar wiring with 120 Vac on the fan side and the burner system voltage on the limit side. In Figure 5 we show a system with a 24 Vac burner. In many new furnaces the high limit and possibly an auxiliary limit may not be line voltage at all, and may be low voltage—24 Vac. For those of you who do gas, propane or natural, you may also find another safety switch called a rollout or spill switch also wired into the low or line voltage side (Figure 5). Add in an air conditioner and the wiring may become even more confusing for many. The wiring of a furnace fan and limit switch where the high limit is low voltage is allowed under the codes and standards since it is all original equipment manufacturers wiring and therefore exempt in most jurisdications. In further researching this, since furnaces are also listed it further allows this wiring to be done. The key is to evaluate your wiring before you place any type of jumper into the circuit and make sure that you check the voltage quantity with a voltmeter. When working with the elec- tronic boards, as shown in Figure 2, using a meter is essential. Using pin-probes like our PP1 (Figure 6), makes it even easier. Jumping out thermostats is becoming a place where a bit more caution should be used in the elec- tronic age, and I've always been a great believer in turning up Figure 1 Figure 2 Figure 3 Figure 4 Figure 5