Well, it looks like spring is finally here in the Midwest (better late than never :>)). It is cool /cold at night and may require the heat to turn on and then turns nice / warm during the day requiring the air conditioner to run. But this cycling can also cause headaches for building owners and contractors.
A lot of commercial spaces use “auto-change over” controls or thermostats. So, at night, the space needs heat, and then in the morning it needs cooling. So, what happens if the heating system has a lockout at night — will the cooling still work in the morning? Will the heating work if the unit is in a cooling lockout? That depends on the manufacturer. A lot of manufacturers have heating as a priority so if the control board has the heating locked out, then so is the cooling.
So, here I go again, out on a job site that has just this problem. (At least the weather on the roof is a lot nicer than it has been all this winter). The units have been installed for a while and this has been an “off and on” problem since day 1, or so the contractor says. He has replaced limit controls because the control board says it is a heating problem or limit. He has replaced the main control board and ignition control board. He has replaced the gas valve and sensor. He even went as far as bypassing the primary limit (he did leave the aux limit functional thank goodness), but still has nuisance lockouts. What is he missing?
I meet him at the job site and the first thing I see is 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?
We turn on the unit, clear the lockout, turn on the heat and I hear “spark – spark” and then nothing! AHA! we have a chance at catching the actual problem.
From experience, when I hear an ignition sequence like this and I know it is “high pressure gas”, I immediately become suspicious of the regulator or vent on the regulator. 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 (maximum allowed). Then the valve tries to open, there is too much LOAD on the valve, the amperage goes up, voltage goes down and the valve can’t open. Now we are in a heating / cooling lockout due to a heating problem. That’s because, when the amperage goes up and the voltage goes down, a lot of ignition controls will react to the low voltage and lock out. This is usually due to a “high load” condition on that circuit. Almost like my last post except that was due to the contactor pulling high amperage causing a cooling lock out.
To confirm or refute my suspicion, we hook up a manometer to the inlet side of the valve. We slowly open the gas cock and it pegs my manometer (mine is good to 20 IWC ). So we have a gas valve trying to open against VERY HIGH pressure and it can’t do that. We did watch it and eventually it did drop down to under 12 IWC. We pulled the vent to make sure it is not the problem and tried it again. The problem here, though, was the regulator would allow the pressure to climb over 12 IWC every time the heating shut off. It was not a “lock down” type regulator. So, depending how long the unit was off or how quickly it cycled, the pressure could be too high for the valve to open and we were in a lockout. This probably explains why it has been happening off and on since day one as he alluded to when I met him at the site. In the mean time, a lot of part got changed again all because an EXTERNAL component to the unit was incorrect or bad.
Keep in mind that gas regulators don’t normally fail (but they can) and 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. If it is a “lock down” regulator that maintains the SET pressure when it shuts off, most problems are due to vent issues.
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.
It is also important to know that regulator have temperature ratings just as gas valves do for roof top unit applications. You must use a regulator that is capable of operating down to -40 F if exposing them to the weather on a roof.
There are also different types of regulators if you have an appliance like a water heater or unit heater that still have “standing pilots”.
You need to make sure you give all the information pertaining to the application to whoever is supplying your regulator so you get the correct one for the job.
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 taken the time to monitor the inlet pressure, he would not have unnecessarily replaced parts.
So at this site visit , if the contractor had checked the inlet gas pressure when he did the original start-up, he may not have had all the problems he feels he had and he may have made a better diagnosis than just replacing parts.
Hopefully, the next time you have a unit that has a 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, ( remember –some times they are just below the roof inside the building with just a vent tube coming out through the flashing), inlet pressure can be a very valuable diagnostic.
And remember, heating lockouts can cause cooling lockouts – depends on the manufacturer. It goes back to using any flash code diagnostics and knowing the manufacturer’s sequence of operation to get you pointed in the right direction. Then use your tools to make the proper diagnostic!