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.
York Central Tech Talk 
Mike, would you do a segment on how to test the pressure on the pressure switch on a 90+ furnace and a 2 stage furnace? Enjoying the blogs, great information. Thank you.
Whit
did you see my post
Furnace Pressure Switches
Posted on October 22, 2012
Do you need more? Let me know and I can expand that post.
what you also forgot was the types of regulators, pilot blled and 100% lock up alot of time a contractor replaces a old unit but leaves the regulator, if the old unit was a pilot like on a unit htr the regulator will cause intermitant problems also, due to the new unit being 100% shut off the regulator tend s to try and bleed and over pressurizes the gas valve
Thank you for adding that. Very good point. If they monitor the inletpressure, they should “catch” the bleed through type before it becomes a problem.
Its been my experience that 60%of service personnel and 90% of installers dont know the difference between the regulators so maybe you sbould address that issue
thanks — I’ll see about doing that in one of my next posts
Mike, I have also found the voltage setting on the transformer set incorrectly and as soon as the gas valve calls for heat it goes into lockout. John Price
I would say 80% of what look like gas valve failures on roof top units and 50% of in door units are actually the regulator. About 60% of the systems in this area are 2# systems and bugs, Ice outside are common causes for faults, indoor systems dust and debris or improperly oriented vent limiters are the common cause, that’s the first thing I check when trouble shooting a failure to ignite. An interesting thing is if you remove the inlet pressure tap, you have released the locked up pressure and if the problem was the inlet lock up pressure the unit will fire, if your pressure tap isn’t perfectly sealed it may prevent the fault from showing up while working on the unit because it can bleed the pressure between cycles as you test the equipment. Another thing is water, if the call is first thing in the morning and by the time you get there it has warmed up, the intermittent problem could be Ice when its cooler changing to water when its warmer so all day the crazy thing works all night it won’t this can be frustrating to find.
Regulator sizing is very misunderstood. I’ll use the Maxitrol 325-3 1/2″ regulator for example it has a maximum rating of 220000 btu, at first glance this should work on a 190000 btu appliance however the maximum single appliance size is 100000 btu, this is because the regulator doesn’t respond quickly enough when the diaphragm is fully deflected, (fully open), when a fast closing control shuts down the gas continues to flow as the regulator reacts and depending on how much pipe there is between the regulator and the appliance this can go as high as supply pressure, the problem is amplified by vent limiters and long vent runs. Another thing is all regulators have different capacities at different inlet pressures. If a piping system is sized for 2# and is sized for a 50% pressure drop, this means under maximum load conditions, the pressure leaving the meter is 2# however the pressure arriving at the regulator is 50% of 2# so 1 psi in the case of the maxitrol the capacity at 2# is 220000, the capacity at 1# is only 180000. Some areas allow piping systems to be sized with 1 a 1.5 psi drop so even worse with only .5 psi arriving at the regulator the capacity goes down to 120000 btu.
Sometimes a new problem develops when an appliance is changed for instance the furnace is replaced and after the pilot on the water heater keeps going out, it might not be the thermocouple. The new appliance having a faster closing gas valve the pressure in the system may be spiking on shut down and pushing the pilot off the burner in the water heater or other appliances down stream of the 2# regulator.
Thank you for all this additional information. It is appreciated.
I am a die-hard DIY’er. I have been scouring the internet for a week trying to troubleshoot an intermittent chattering gas valve (Honeywell VR8403) when there is a call for heat to our Burnham boiler. I happened to have a spare gas valve and S86 Ignition control module. I swapped out both but chattering problem persisted (intermittently). I then bypassed the Honeywell aquastat and problem still persisted. I finally realized that the chattering would only happen if ignition cycles occured close together, maybe about 5-7 minutes apart (cold in MN right now) (is this short-cycling?). That led me to suspect the pressure regulator (Maxitrol 325-3), which led me to this site when searching. Our Burnham boiler is a 165,000 input BTU boiler, so it seems the Maxitrol 325-3 is undersized for our boiler and is possibly creating high pressure when the flame cuts off. I will change it out with perhaps the 325-5A and see if the chattering goes away. Thanks for the great information here. I will update with my results.
Well, I replaced my 325-3 with a 325-5A regulator. No more chattering gas valve and all has been well since. After removing the 325-3, I pulled off it’s vent limiter and gave it a good shake. I could not hear the check ball rattling. I gave it a couple good whacks on the concrete and shook it again. Voila, I could hear the check ball rattle freely. That was the problem all along, I suspect. Stuck check ball was not allowing pressure to change on the diaphragm. But at least now I have a properly sized gas valve for my boiler.
o_o – I searched and found it. Thanks for the post. Mike McLen