Q: I need to purchase a new pressure switch to monitor the pressure at my combustion blower. My setpoint is 18 “w.c., but when I called to order a DL 50AT, I was told that no stock was available, and that the wait could be up to five weeks. I can’t wait that long – are there any other options?
A: Absolutely. While the DL series is billed as an “air” pressure switch, many people overlook the fact that the DG pressure switch is for use with both gas and air (a widely held belief is that DGs are for gas only). Our sister companies, Thermal Products & Solutions and Olsträd Engineering, use DGs as air pressure switches for blowers, purge air, recirculation and exhaust fans on new installations and rebuilds as a standard. This is for several reasons: DG switches are more readily available in higher pressure ranges than are the DL versions, DG switches are kept in stock in higher quantities, and using like models on projects leads to a shorter list of recommended spares.
This is not to say that the DL line of switches is not necessary, as some applications require very low settings for air pressures, and the lowest range available for a DG switch (DG 6T) is 0.2-2.4 “w.c. With an operating range of 0.12-1.2 “w.c., the DL 3AT is typically a good fit where the DG 6T is not. For this reason, Combustion 911 tends toward higher stock levels of the DL 3AT and DL 3ATG than other models. Lower ranges are available from the manufacturer, but they are not as common.
Another important difference between the gas (DG) and air (DL) pressure switches is the response time of the switches when used for air. The DL switch has a larger inlet bore and therefore responds about 3x faster to a 50% change in pressure than a comparable DG switch.
Let’s compare the DG 6T with the DL 5AT:
DG 6T Pressure switch for gas and air, for positive, negative, and differential pressure, adjusting range 0.2 – 2.4 “w.c. (0.5 – 6 mbar), electrical connection via screw terminals, threaded connections ¼ NPT, with 110 V control lamp, FM and UL approved
DL 5AT Pressure switch for air and flue gas, for positive, negative and differential pressure, adjustment range 0.2 – 2 “w.c. (0.5 – 5 mbar), electrical connection via screw terminals, threaded connection RP ¼ (DN 6) and hose connection, with 110 V control lamp, FM and UL approved
As you can see, the differences between these switches are few. The threaded connection differs, and the DL 5AT comes equipped with a plastic hose connection. Otherwise, they are nearly identical.
Due to the differences between RP and NPT thread types, it is typically not recommended that they be mixed for gas connections; the standards for air lines are, understandably, not nearly as strict. And anyway, the difference between ¼” NPT and DN 6 is slight enough that an RP pipe nipple threaded into the ¼” NPT connection on a DG 6T should tighten against leaks.
DL pressure switches are manufactured with a plastic hose connection, while the DG models have a threaded connection instead. Brass barbs can be purchased from several retailers of industrial supplies, either local to your facility or online. Many customers favor a brass fitting over plastic, and the brass fittings are reusable, in the event that an existing DG is damaged or expires.
A further advantage to using DG models for both gas and air is that spares can be stocked to greater purpose. If there are DG 6Ts existent on your gas piping, and a DG 6T can be used instead of a DL 5AT on your air pressure, then why not keep a few DG 6Ts in spare stock, instead of DG 6Ts and DL 5ATs?
There are dozens of manufacturers providing the industry with components for combustion systems, and everybody does things differently. Please don’t hesitate to email questions that arise as a result of, or due to confusion regarding a pending, Kromschröder purchase to firstname.lastname@example.org.