In the past couple of weeks, I have had numerous calls about bad gas valves and bad ignition controls. The scenarios go something like this, (1) “I keep getting a lock-out in the heating section of the roof top unit. I have 24 VAC to the valve and it won’t open. I changed the valve, and it does the same thing.” OR, (2) “My ignition control sparks, but as soon as the gas valve tries to open, the spark stops and the unit locks-out. I’ve changed the gas valve and ignition control and it still locks out.” There are also other scenarios that fall into this but I give you these to think about.
So, what further information is needed here to help make a proper diagnosis? Do we need to know what the control voltage is? Do we need to know the manifold pressure is if the valve does open? Do we need to know what the inlet pressure into the gas valve is?
Actually, we need all of the above but the most important is the INLET gas pressure. One of the first things I will ask a contractor is, “Is there an external pressure regulator outside the unit?” Gas valves are designed to operate at inlet pressures below 12 IWC. In many parts of the country, the gas utilities will provide this as standard pressure because they put a regulator by the meter so the whole building is operating on “low pressure gas”. This then requires larger gas pipe sizes to get the proper amount of gas to the various appliances. But in a lot of areas, the gas utility provides medium pressure (or high pressure) gas to the building and it is the contractors responsibility to reduce the pressure to below the 12 IWC at the appliance by adding a regulator at that appliance. This helps keep installation costs down because smaller gas piping sizes can be used.
In the scenarios above, it usually turns out that the building has medium to high pressure gas and there is an external regulator at the roof top unit. Well, if there is a regulator, there should not be a problem — should there? All regulators have “vents” on them. If the vent is plugged the regulator may not work or works slowly allowing the inlet pressure to be over 12 IWC. Now the valve tries to open, there is too much LOAD on the valve so the amperage goes up, voltage goes down and the valve can’t open. Also, when the amperage goes up and the voltage goes down, a lot of ignition controls will react to the low voltage and lock out. Most contractors can’t catch this because they are using “auto-ranging digital meters” that don’t react quick enough to catch this before the unit locks out. Had they used an analog meter, they would have seen a voltage drop by watching the needle “bounce” giving them an indication that something was causing the voltage to drop. This is usually due to a “high load” condition on that circuit. So in the scenarios above, if the contractor had checked the inlet gas pressure and was able to see the voltage drop, he may have made a better diagnosis than just replacing parts.
I have also had instances where there are nuisance lock-outs and I’ll ask the contractor to leave his manometer on the inlet side of the gas valve and “cycle” the unit. It may work fine a couple of times, but, all of a sudden, when the unit shuts off, the inlet pressure goes up to 18 IWC (or higher) and may or may not bleed down before the next cycle. Of course, now the valve tries to open against the higher pressure and we have the lock-out.
Gas regulators don’t normally fail (but they can) but the most common failure is the vent can get plugged. I have seen vents plugged over the summer by bugs building nest in the tubing. I have seen the vents plugged with ICE in winter due to the exhaust vent off the unit blowing at the regulator.
On new installs, I have seen the regulators installed improperly or the wrong regulator installed. When ordering a regulator, you need to know the “building gas pressure” (inlet pressure) so you can get the proper one for the application. Sometimes, all it takes is a different internal “spring” in the regulator to set it up. Whatever the case, when you need to order a regulator, you need to know both maximum inlet pressure and maximum outlet pressure needed for the job site.
Also, some regulators are “position conscious” and the installer does not read that in the installation instructions for the regulator and puts them in vertically, or upside down, or what ever. Now they don’t work and reduce the pressure.
The point I am making here, is inlet gas pressure is probably one of the most over-looked diagnostics. Of course too little will cause problems, but TOO MUCH really causes problems. It needs to be checked and it needs to be monitored through a number of cycles when there is an external regulator for the appliance. Had the contractors in the scenarios above, taken the time to monitor the inlet pressure, they would not have unnecessarily replaced parts.
Hopefully, the next time you have a unit that has lock-out in heating and the gas pressure at the site is medium to high pressure, and there is an external regulator at the unit, you take the time to monitor the INLET pressure to that unit. Even if there is no external regulator, inlet pressure can be a very valuable diagnostic.
One last thing — this is also true for propane.